Category Archives: Quantum Physics
UK Research and Innovation (UKRI) is investing 31 million into seven projects to show how quantum technologies could solve some of the greatest mysteries of the universe such as dark matter and black holes.
A project led by the University of Nottingham aims to provide insights to the physics of the early universe and black holes that cannot be tested in a laboratory.
The team will use quantum simulators to simulate the conditions of the early universe and black holes with sufficient accuracy to confirm some of Einsteins predictions on general relativity.
A team led by Royal Holloway, University of London, will develop new quantum sensors which can be used to search for dark matter.
The projects are supported through the Quantum Technologies for Fundamental Physics programme, delivered by the Science and Technology Facilities Council (STFC) and the Engineering and Physical Sciences Research Council (EPSRC) as part of UKRIs Strategic Priorities Fund. The programme is part of the National Quantum Technologies Programme.
STFC is proud to support these projects that utilise cutting-edge quantum technologies for novel and exciting research into fundamental physics.
Major scientific discoveries often arise from the application of new technologies and techniques. With the application of emerging quantum technologies, I believe we have an opportunity to change the way we search for answers to some of the biggest mysteries of the universe.
These include exploring what dark matter is made of, finding the absolute mass of neutrinos and establishing how quantum mechanics fits with Einsteins theory of relativity.
I believe strongly that this exciting new research programme will enable the UK to take the lead in a new way of exploring profound questions in fundamental physics.
The National Quantum Technologies Programme has successfully accelerated the first wave of quantum technologies to a maturity where they can be used to make advances in both fundamental science and industrial applications.
The investments UKRI is making through the Quantum Technologies for Fundamental Physics programme allows us to bring together the expertise of EPSRC and STFC to apply the latest advances in quantum science and technology to explore, and answer, long-standing research questions in fundamental physics.
This is a hugely exciting programme and we look forward to delivering these projects and funding further work in this area as well as exploring opportunities for exploiting quantum technologies with other UKRI partners.
As we build back better from the pandemic, its critical that we throw our weight behind new transformative technologies that could help to unearth new scientific discoveries and cement the UKs status as a science superpower.
Todays funding will enable some of the UKs most ambitious quantum researchers to develop state of art technologies that could help us solve important unanswered questions about our universe, from proving Einsteins theory of relativity to understanding the mysterious behaviour of black holes.
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Exploring the unanswered questions of our universe with quantum technologies – University of Birmingham
The University of Birmingham is a key partner in three quantum technology projects awarded funding from UK Research and Innovation (UKRI). The funding is part of a 31 million investment to demonstrate how quantum technologies could solve some of the greatest mysteries in fundamental physics.
The projects are supported through the Quantum Technologies for Fundamental Physics programme, delivered by the Science and Technology Facilities Council (STFC) and the Engineering and Physical Sciences Research Council (EPSRC) as part of UKRIs Strategic Priorities Fund.
This is a new programme which aims to demonstrate how the application of quantum technologies will advance the understanding of fundamental physics questions. It is supported by the Quantum Technology Hubs comprising the UK National Quantum Technologies Programme
The three projects awarded funding are:
Searching for variations of fundamental constants of nature
QSNET is a multi-disciplinary consortium which aims to search for spatial and temporal variations of fundamental constants of nature, using a network of quantum clocks. Led by Dr Giovanni Barontini, from the University of Birmingham, and partnered with the National Physical Laboratory; Imperial College London; University of Sussex; Max Planck Institut fuer Kernphysik; Physikalisch-Technische Bundesanstalt; Istituto Nazionale di Ricerca Metrologica; University of Delaware; University of Tokyo and the Observatoire de Paris. The project, which has received 3.7 million in funding, is also linked to three of the Quantum Technology Hubs in the UK National Quantum Technologies Programme.
QSNET proposes to build a national network of advanced atomic, molecular and highly-charged ion clocks. The network will achieve unprecedented sensitivities in testing variations of the fine structure constant and the electron-to-proton mass ratio. These are two of the parameters of the Standard Model of particle physics, which is the pillar of our understanding of the Universe, but that famously fails to describe 95% of its content: the so-called dark matter and dark energy. QSNET will test the fundamental assumption that the constants of the Standard Model are immutable, as this could be the key in solving the dark matter/dark energy enigma.
Investigating dark matter and detecting gravitational waves
The Atom Interferometer Observatory and Network (AION) is a consortium project comprising Imperial College London, Kings College London, the University of Oxford, the University of Cambridge, STFC Rutherford Appleton Laboratory, the University of Liverpool and the University of Birmingham.
This interdisciplinary team of academics will develop the science and technology to build and reap the scientific rewards from the first large-scale atom interferometer in the UK. This programme of research will enable a ground-breaking search for ultra-light dark matter and pave the way for the exploration of gravitational waves in a previously inaccessible frequency range, opening a new window on the mergers of massive black holes and novel physics in the early universe.
The University of Birmingham team, led by Dr Michael Holynski, Prof Kai Bongs, Dr Mehdi Langlois, Dr Samuel Lellouch, Sam Hedges and Dr Yeshpal Singh will bring their atom interferometry expertise to AION and focus on realising new levels of large momentum transfer to enable the exquisite sensitivity required to achieve the scientific goals of the project, while also providing leadership on the realisation of economic impact.
The AION project, which has been awarded 7.2 million in funding, will be linked to the UK National Quantum Technologies Programme through the UK Quantum Technology Hub Sensors and Timing, led by the University of Birmingham, and project work will be undertaken at the Hubs Technology Transfer Centre. This will be an opportunity for matter-wave interferometry and strontium optical clocks technology to be developed with industry through to commercialisation.
Quantum-enhanced interferometry for new physics
The Quantum Interferometry (QI) collaboration aims to search for dark matter and for quantum aspects of space-time with quantum technologies. The international QI consortium, led by Cardiff, includes the Universities of Birmingham, Glasgow, Strathclyde, and Warwick in the UK, MIT, Caltech, NIST, and Fermilab in the US, DESY and AEI Hannover in Germany.
QI will build four table-top experiments (two of them in Birmingham) to search for dark matter in the galactic halo, improve 100-m scale ALPS light-shining-through-the-wall experiment at DESY with novel single photon detectors, search for quantisation of space-time, and test models of semiclassical gravity. These experiments will allow us to explore new parameter spaces of photon dark matter interaction, and seek answers to the long-standing research question: How can gravity be united with the other fundamental forces?
The project is linked to two UK National Quantum Hubs and will apply state-of-the-art technologies, including optical cavities, quantum states of light, transition-edge sensors, and extreme-performance optical coatings, to a broad class of fundamental physics problems. Dr Vincent Boyer, Dr Haixing Miao and Dr Denis Martynov will be leading the 4 million-funded project from the University of Birmingham.Visit QI Labs for more information.
Professor Kai Bongs, Principal Investigator at the UK Quantum Technology Hub Sensors and Timing, led by the University of Birmingham, says: The UK Governments investment in these projects enables us to draw together experts in quantum physics research to explore some of the key mysteries of our universe. These projects will allow us to build on the momentum already generated through the Quantum Technology Hubs and build a pipeline feeding novel technologies into the future multi-bn Quantum Technology economy.
Science Minister Amanda Solloway said:As we build back better from the pandemic, its critical that we throw our weight behind new transformative technologies, such as quantum, that could help to unearth new scientific discoveries and cement the UKs status as a science superpower.
Todays funding will enable Birminghams most ambitious quantum researchers to use the precision of atomic clocks to help solve important unanswered questions about our universe, such as detecting dark matter and understanding the 95% of unaccounted energy content of the universe.
Announcing the awards, Professor Mark Thomson, Executive Chair of the Science and Technology Facilities Council, said: "STFC is proud to support these projects that utilise cutting-edge quantum technologies for novel and exciting research into fundamental physics.
Majorscientific discoveries often arise from the application of new technologies and techniques. With the application of emerging quantum technologies, I believe we have an opportunity to change the way we search for answers to some of the biggest mysteries of the universe.These include exploring what dark matter is made of, finding the absolute mass of neutrinos and establishing how quantum mechanics fits with Einsteins theory of relativity.
I believe strongly that this exciting new research programme will enable the UK to take the lead in a new way of exploring profound questions in fundamental physics.
For media enquiries please contact Beck Lockwood, Press Office, University of Birmingham, tel: +44 (0)781 3343348.
About the UK Quantum Technology Hub Sensors and Timing
The UK Quantum Technology Hub Sensors and Timing (led by the University of Birmingham) brings together experts from Physics and Engineering from the Universities of Birmingham, Glasgow, Imperial, Nottingham, Southampton, Strathclyde and Sussex, NPL, the British Geological Survey and over 70 industry partners. The Hub has over 100 projects, valued at approximately 100 million, and has 17 patent applications.
The UK Quantum Technology Hub Sensors and Timing is part of the National Quantum Technologies Programme (NQTP), which was established in 2014 and has EPSRC, IUK, STFC, MOD, NPL, BEIS, and GCHQ as partners. Four Quantum Technology Hubs were set up at the outset, each focussing on specific application areas with anticipated societal and economic impact. The Commercialising Quantum Technologies Challenge (funded by the Industrial Strategy Challenge Fund) is part of the NQTP and was launched to accelerate the development of quantum enabled products and services, removing barriers to productivity and competitiveness. The NQTP is set to invest 1B of public and private sector funds over its ten-year lifetime.
About the University of Birmingham
The University of Birmingham is ranked amongst the worlds top 100 institutions. Its work brings people from across the world to Birmingham, including researchers, teachers and more than 6,500 international students from over 150 countries.
About the Strategic Priorities Fund
The Strategic Priorities Fund is an 830 million investment in multi- and interdisciplinary research across 34 themes.It is funded through the governments National Productivity Investment Fund and managed by UK Research and Innovation.
The fund aims to:
About the National Quantum Technologies Programme
The National Quantum Technologies Programme (NQTP) was established in 2014 by the partners (EPSRC, STFC, IUK, Dstl, MoD, NPL, BEIS, GCHQ, NCSC2) to make the UK a global leader in the development and commercialisation of quantum technologies. World class research and dynamic innovation, as the Governments R&D Roadmap stresses, are part of an interconnected system. The NQTPs achievements to-date have been enabled by the coherent approach which brings this interconnected system together. NQTP has ambition to grow and evolve research and technology development activities within the programme to continue to ensure that the UK has a balanced portfolio, is flexible and open, so that promising quantum technologies continue to emerge.
The NQTP is set to invest 1billion of public and private sector funds over its ten-year lifetime.
Wormholes may be lurking in the universe and new studies are proposing ways of finding them – The Conversation UK
Albert Einsteins theory of general relativity profoundly changed our thinking about fundamental concepts in physics, such as space and time. But it also left us with some deep mysteries. One was black holes, which were only unequivocally detected over the past few years. Another was wormholes bridges connecting different points in spacetime, in theory providing shortcuts for space travellers.
Wormholes are still in the realm of the imagination. But some scientists think we will soon be able to find them, too. Over the past few months, several new studies have suggested intriguing ways forward.
Black holes and wormholes are special types of solutions to Einsteins equations, arising when the structure of spacetime is strongly bent by gravity. For example, when matter is extremely dense, the fabric of spacetime can become so curved that not even light can escape. This is a black hole.
As the theory allows the fabric of spacetime to be stretched and bent, one can imagine all sorts of possible configurations. In 1935, Einstein and physicist Nathan Rosen described how two sheets of spacetime can be joined together, creating a bridge between two universes. This is one kind of wormhole and since then many others have been imagined.
Some wormholes may be traversable, meaning humans may be able to travel through them. For that though, they would need to be sufficiently large and kept open against the force of gravity, which tries to close them. To push spacetime outward in this way would require huge amounts of negative energy.
Sounds like sci-fi? We know that negative energy exists, small amounts have already been produced in the lab. We also know that negative energy is behind the universes accelerated expansion. So nature may have found a way to make wormholes.
How can we ever prove that wormholes exist? In a new paper, published in the Monthly Notices of the Royal Society, Russian astronomers suggest they may exist at the centre of some very bright galaxies, and propose some observations to find them. This is based on what would happen if matter coming out of one side of the wormhole collided with matter that was falling in. The calculations show that the crash would result in a spectacular display of gamma rays that we could try to observe with telescopes.
This radiation could be the key to differentiating between a wormhole and a black hole, previously assumed to be indistinguishable from the outside. But black holes should produce fewer gamma rays and eject them in a jet, while radiation produced via a wormhole would be confined to a giant sphere. Although the kind of wormhole considered in this study is traversable, it would not make for a pleasant trip. Because it would be so close to the centre of an active galaxy, the high temperatures would burn everything to a crisp. But this wouldnt be the case for all wormholes, such as those further from the galactic centre.
The idea that galaxies can harbour wormholes at their centres is not new. Take the case of the supermassive black hole at the heart of the Milky Way. This was discovered by painstakingly tracking of the orbits of the stars near the black hole, a major achievement which was awarded the Nobel Prize in Physics in 2020. But one recent paper has suggested this gravitational pull may instead be caused by a wormhole.
Unlike a black hole, a wormhole may leak some gravity from the objects located on the other side. This spooky gravitational action would add a tiny kick to the motions of stars near the galactic centre. According to this study, the specific effect should be measurable in observations in the near future, once the sensitivity of our instruments gets a little bit more advanced.
Coincidentally, yet another recent study has reported the discovery of some odd radio circles in the sky. These circles are strange because they are enormous and yet not associated with any visible object. For now, they defy any conventional explanation, so wormholes have been advanced as a possible cause.
Wormholes hold a strong grip on our collective imagination. In a way, they are a delightful form of escapism. Unlike black holes which are a bit frightening as they trap everything that ventures in, wormholes may allow us to travel to faraway places faster than the speed of light. They may in fact even be time machines, providing a way to travel backwards as suggested by the late Stephen Hawking in his final book.
Wormholes also crop up in quantum physics, which rules the world of atoms and particles. According to quantum mechanics, particles can pop out of empty space, only to disappear a moment later. This has been seen in countless experiments. And if particles can be created, why not wormholes? Physicists believe wormholes may have formed in the early universe from a foam of quantum particles popping in and out of existence. Some of these primordial wormholes may still be around today.
Recent experiments on quantum teleportation a disembodied transfer of quantum information from one location to another have turned out to work in an eerily similar way to two black holes connected through a wormhole. These experiments appear to solve the quantum information paradox, which suggests physical information could permanently disappear in a black hole. But they also reveal a deep connection between the notoriously incompatible theories of quantum physics and gravity with wormholes being relevant to both which may be instrumental in the construction of a theory of everything.
The fact that wormholes play a role in these fascinating developments is unlikely to go unnoticed. We may not have seen them, but they could certainly be out there. They may even help us understand some of the deepest cosmic mysteries, such as whether our universe is the only one.
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University of Sheffield to lead multi-million pound project which could open up a new frontier in physics – University of Sheffield News
A collaboration of scientists from across the UK are working on a new project to detect hidden particles, the discovery of which could open up a new frontier in fundamental physics.
The project, Quantum Sensing for the Hidden Sector (QSHS), is led by scientists at the University of Sheffield and involves the Universities of Cambridge, Lancaster, Liverpool, Oxford, Royal Holloway and University College London and the National Physical Laboratory.
Funded by the Science and Technology Facilities Council (STFC), as part of UK Research and Innovation (UKRI), the project is the best supported and largest UK effort in hidden sector physics to date, and involves scientists from a range of disciplines within physics.
QSHS aims to solve some of the most fundamental mysteries in modern physics using new technologies being developed for the rapidly expanding field of quantum measurement science.
Working with the Axion Dark Matter Experiment (ADMX) collaboration in the US, but also developing pioneering quantum electronics and novel experiment designs in the UK. The group aims to shed new light on the particles of the hidden sector which could provide new insights into fundamental mysteries, most importantly the dark matter problem, which is the observation that galaxies and the observable Universe are heavier than their observed constituents - stars, planets, dust and gas.
The extra matter making up the difference could be made up wholly or partly of ultra-light particles, so-called hidden sector particles that have so far evaded detection. The signatures of these particles are signals so faint that the world's most sensitive measurement devices will be developed by our team for the search.
It's high risk, high reward science. You might see nothing or you might on the other hand make a massive discovery. Nobody knows which, but the discovery of hidden sector particles would open up a completely new frontier in fundamental physics.
Professor Ed Daw
Professor of dark matter and gravitational wave physics at the University of Sheffield
Professor Ed Daw, Professor of dark matter and gravitational wave physics at the University of Sheffield, and principal investigator for the project, said: Hidden sector particles, if they exist, may be the so-far unidentified dark matter, and may in addition solve important outstanding problems with the theory that we have developed governing quarks and the atomic nucleus. The hidden sector may even provide critical insights into the inflationary phase thought to occur very shortly after the big bang.
It's high risk, high reward science. You might see nothing or you might on the other hand make a massive discovery. Nobody knows which, but the discovery of hidden sector particles would open up a completely new frontier in fundamental physics. It would be like the invention of the particle beam accelerator, a whole new way of doing science.
Hidden sector particles may play other significant roles in physics, including in early Universe cosmology and the evolution of the Universe in the moments after it came into existence. We are excited to be embarking on this journey of discovery, and we hope the British public will share in this excitement as we start this research project.
The discovery of hidden sector particle dark matter would be a momentous event in fundamental physics. The dark matter problem is now over 50 years old, but in addition a new set of light particles would be bound to solve some of the persistent problems with the standard model of particle physics.
Professor Stafford Withington, Co-Investigator and Senior Project Scientist on QSHS from the University of Cambridge, said: In recent years, the UK has invested heavily in establishing the laboratory infrastructure needed to develop a new generation of ultra-low-noise electronics and associated control systems. The new electronics operate in a fundamentally different way to the conventional electronics with which we are all familiar. It exploits the mysterious behaviour of quantum mechanics to yield sensitivities that are limited only by the fluctuations inherent in the fundamental nature of space-time. The electronic devices are based on a range of superconducting materials, and work at temperatures of around 10mK, where thermal fluctuations are essentially eliminated.
The team will develop this technology to a high level of sophistication, and deploy it to search for the lowest-mass particles detected to date. These particles are predicted to exist theoretically, but have not yet been discovered experimentally. Our ability to probe the particulate nature of the physical world with sensitivities that push at the limits imposed by quantum uncertainty will open up a new frontier in physics.
This new window will allow physicists to to explore the nature of physical reality at the most fundamental level, and it is extremely exciting that the UK will be playing a major international role in this new generation of science.
The detection of these hidden particles requires technology of unprecedented sensitivity. The team are aiming to develop new and world-leading devices which could also be applied to make critical progress in other areas of physics such as quantum computing and quantum systems engineering.
The UK research team will form a collaboration with the US based ADMX collaboration, who operate the most sensitive detector for a particular variety of hidden sector particle, the axion.
The full QSHS team consists of The University of Sheffield (lead institution, principal investigator Prof. E Daw), University of Cambridge (co-I and senior project scientist Prof. Stafford Withington), Lancaster University (co-Is Prof. Yuri Pashkin, Dr. Ian Bailey, Dr. Ed Laird) The University of Liverpool (co-I DR. Ed Hardy), The National Physical Laboratory (co-Is Prof. Ling Hao, Prof. John Gallop), University of Oxford (co-Is Dr. Peter Leek, Prof. Gianluca Gregori, Prof. John March-Russell, Prof. Subir Sarkar, Dr. Boon-Kok Tan) , Royal Holloway - University of London (co-Is . Prof. Phil Meeson, Dr. Stephen West), and University College London (co-I Dr. Ed Romans).
With almost 29,000 of the brightest students from over 140 countries, learning alongside over 1,200 of the best academics from across the globe, the University of Sheffield is one of the worlds leading universities.
A member of the UKs prestigious Russell Group of leading research-led institutions, Sheffield offers world-class teaching and research excellence across a wide range of disciplines.
Unified by the power of discovery and understanding, staff and students at the university are committed to finding new ways to transform the world we live in.
Sheffield is the only university to feature in The Sunday Times 100 Best Not-For-Profit Organisations to Work For 2018 and for the last eight years has been ranked in the top five UK universities for Student Satisfaction by Times Higher Education.
Sheffield has six Nobel Prize winners among former staff and students and its alumni go on to hold positions of great responsibility and influence all over the world, making significant contributions in their chosen fields.
Global research partners and clients include Boeing, Rolls-Royce, Unilever, AstraZeneca, GlaxoSmithKline, Siemens and Airbus, as well as many UK and overseas government agencies and charitable foundations.
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Raytheon UK part of team transforming the Royal Navy’s technology, training and learning solutions – PRNewswire
LONDON, Jan. 13, 2021 /PRNewswire/ --Raytheon UK, a unit of Raytheon Technologies (NYSE: RTX), alongside strategic partner Capita, received a contract award to provide the Royal Navy with transformative technology, training and learning solutions over the next 12 years.
The contract, which will be led by Capita, has an initial contract value of 200 million to Raytheon UK and will ensure the Royal Navy offers best-in-class training to all its service personnel. It will accelerate the use of new technology, processes and learning solutions, aligning with the Royal Navy's transformation agenda and positioning it to thrive in the 21st century.
"This announcement allows the team to begin efforts to transform the Royal Navy's training and learning solutions, and to modernise and transform the way training is delivered across the Armed Forces," said Jeff Lewis, chief executive of Raytheon UK. "Our extensive experience in leading large and complex transformative change programmes around the world will provide the Royal Navy with tailored, digitally enabled training, fit for the future."
Raytheon UK will play a key role in modernising and transforming the Royal Navy's training analysis, design, delivery, assurance, and management/support services, helping to make the UK Armed Forces more agile and adaptable than ever to tackle future challenges.
"We are committed to investing in the UK and helping to keep the country secure and our Armed Forces equipped with the best affordable sovereign solutions, creating highly skilled jobs across the UK," Lewis said. "We look forward to delivering these solutions to the Royal Navy over the coming years."
Rear Adm. Phil Hally MBE, the Royal Navy's director of people and training, said, "The award of this 12-year contract marks a major milestone for Navy transformation. It will see the modernisation of the RN training system at scale to deliver the operational capabilities of the future, unlock more opportunities for our people, and get better trained people to the frontline, quicker."
About Raytheon UKWith facilities in Broughton, Waddington, Glenrothes, Harlow, Gloucester and Manchester, Raytheon UK is invested in the British workforce and the development of UK technology. Across the country the company employs 1,700 people. As a prime contractor and major supplier to the U.K. Ministry of Defence, Raytheon UK continues to invest in research and development, supporting innovation and technological advances across the country.
Raytheon UK is a unit of Raytheon Technologies and sits within the Raytheon Intelligence & Space business.
About Raytheon TechnologiesRaytheon Technologies Corporation is an aerospace and defense company that provides advanced systems and services for commercial, military and government customers worldwide. With four industry-leading businesses Collins Aerospace Systems, Pratt & Whitney, Raytheon Intelligence & Space and Raytheon Missiles & Defense the company delivers solutions that push the boundaries in avionics, cybersecurity, directed energy, electric propulsion, hypersonics, and quantum physics. The company, formed in 2020 through the combination of Raytheon Company and the United Technologies Corporation aerospace businesses, is headquartered inWaltham, Massachusetts.
Media ContactSen Sami+44 (0)1279 407600[emailprotected]
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Optical selection and sorting of nanoparticles according to quantum mechanical properties – Science Advances
Optical trapping and manipulation have been widely applied to biological systems, and their cutting-edge techniques are creating current trends in nanomaterial sciences. The resonant absorption of materials induces not only the energy transfer from photons to quantum mechanical motion of electrons but also the momentum transfer between them, resulting in dissipative optical forces that drive the macroscopic mechanical motion of the particles. However, optical manipulation, according to the quantum mechanical properties of individual nanoparticles, is still challenging. Here, we demonstrate selective transportation of nanodiamonds with and without nitrogen-vacancy centers by balancing resonant absorption and scattering forces induced by two different-colored lasers counterpropagating along a nanofiber. Furthermore, we propose a methodology for precisely determining the absorption cross sections for single nanoparticles by monitoring the optically driven motion, which is called as optical force spectroscopy. This method provides a novel direction in optical manipulation technology toward development of functional nanomaterials and quantum devices.
Nanoparticles and nanomaterialssuch as quantum dots, nanocrystals, carbon nanomaterials, molecular aggregates, and metal nanoparticleshave attracted great attention owing to their unique mechanical and quantum mechanical properties and have been used in various photonic, electronic, mechanical, and biomedical devices, such as light emitters, solar cells, photocatalysts, molecular electronics, structural materials, drug delivery, and bioimaging (16). Because these properties of nanoparticles/nanomaterials are strongly influenced by the surrounding environment and are significantly different from the bulk properties, such as quantum size effect, the characterization of individual nanoparticles provides important knowledge for advancing nanomaterial and quantum material sciences. Furthermore, the selection and sorting of single nanoparticles according to their characteristics are essential and desired for the precise design of functional nanostructures and development of single-quantum sensors, single-photon sources, and quantum information devices (7, 8).
Optical trapping and manipulation based on optical forces are promising tools for positioning, transporting, and aligning fine particles without mechanical contacts (9, 10). Optical tweezers proposed by Ashkin et al. have been used in various research fields, such as biophysics, cell biology, microfluidics, total analytical systems, and micromechanics (11, 12). Optical sorting of dielectric objects has been developed using holographic optics, flow cytometry, interference technology, and near field photonics (1315). Metal nanoparticles can also be separated by optical forces based on the surface plasmon resonances (16). However, these techniques are limited to the particle selection by the size and refractive index. The optical gradient and scattering forces exerted on small particles and their dependences on the diameter, wavelength, and relative refractive index are determined by the Mie theory. Furthermore, the reported methods are applicable only to the sorting of submicrometer or larger-sized dielectric particles. Trapping and manipulation of smaller-sized particles remain challenging because the optical force becomes weaker in proportion to the particle volume.
In this study, we demonstrate the optical selection and sorting of nanoparticles according to their quantum mechanical properties. Semiconductor quantum dots exhibit characteristic optoelectronic properties due to the quantum confinement of the electron-hole pairs in the nanovolume (1, 2). Diamond nanoparticles exhibit quantum resonances of point defects (17, 18). The optical forces reflect these quantum mechanical properties of nanoparticles and their optical characteristics (19, 20). The interaction between light and nanomaterials induces not only an energy transfer from the photons to the quantum mechanical motion of the electrons but also a momentum transfer between them. The change in the photon momentum give rise to optical forces, which drive the macroscopic mechanical motion of the nanoparticles. We note that there are three types of optical forces: (i) gradient force arising from the inhomogeneous intensity distribution of the electric field, (ii) dissipative scattering force caused by the real part of the refractive index, and (iii) quantum resonant absorption force exerted on nanomaterials. Therefore, we can realize the characterization and selective manipulation of single nanoparticles having various properties by monitoring and controlling the particle motions. This methodology provides a new direction in optical force technology toward advances in nanomaterial sciences.
To realize the sorting of individual nanoparticles, we use counterpropagating different-colored lasers that can extract the resonant absorption force by cancelling out the scattering forces. The counterpropagating beam systems were constructed using a pair of lenses with large numerical aperture placed opposite to each other (21) and the inversely directed evanescent waves (16). However, it is difficult to exclude the influence of the gradient force that easily negates the small effect of the quantum resonance force. Thus, we focused on tapered optical fibers, i.e., nanofibers (22, 23). We prepared a nanofiber with a diameter of several hundred nanometers and length of several millimeters (24), which exhibited the characteristics of single-mode propagation, thereby forming an intense evanescent field around the fiber and enabling long-distance propagation while maintaining a tightly focused beam of light. Using these characteristics, a uniform electric field distribution could be generated along the fiber by which the particle motion was restricted to one dimension. In addition, the optical gradient force and thermophoretic force, arising from the temperature gradient (e.g., Soret effect), were exerted in a direction perpendicular to the fiber axis such that the particle motion along the nanofiber was driven only by the resonant absorption and scattering forces. Furthermore, because the momentum of the photons in a waveguide depends on the propagation constants of the individual modes, the single-mode wave in our nanofiber had the constant photon momentum; this provides an ideal platform for analyzing the optical forces exerted on the nanoparticles. On the basis of the balance of the absorption and scattering forces induced by the different-colored lasers counterpropagating along the nanofiber, we succeeded in achieving the selective transportation of single nanoparticles according to the quantum resonant absorption (Fig. 1A).
(A) Concept of optical force absorption spectroscopy. By monitoring the mechanical motion of a single nanoparticle driven by optical forces, the resonant absorption properties can be analyzed with high sensitivity. Using two different-colored lasers counterpropagating along a nanofiber, a nanoparticle is trapped by the gradient force and transported by the absorption and scattering forces. The laser powers are adjusted to cancel out the scattering forces such that the particle moves depending on the absorption cross section. (B) Experimental setup. GR and NIR diode lasers are introduced from both ends of a nanofiber. The laser powers are measured by photodiodes (PD1 and PD2) and controlled by rotational neutral density filters to balance the forces. To record the motion of nanoparticles, a weak red laser is used, and its scattered light is monitored using a microscope-attached charge-coupled device (CCD) camera with filters to cut the strong scattered light of the GR and NIR lasers.
In addition to the selection and sorting, the proposed system can precisely determine the resonant absorption cross sections of single nanoparticles. Fluorescence and photothermal spectroscopies have been widely used for characterizing single nanoparticles and nanomaterials because of their high sensitivity at the level of single-molecule detection (25, 26). However, these methods probe the relaxation processes emitting a photon and thermal energy, which are regarded as indirect absorption measurements. When the excited states of the materials irreversibly transit to other states without undergoing relaxation processes, such as photochemical reactions, these techniques can no longer observe the resonant absorption. Absorption spectroscopy, which directly measures the excitation processes, is an indispensable tool for analyzing the interaction strengths between light and matter. In particular, the absolute values of the absorption cross sections of single nanoparticles/nanomaterials are essential for experimental physics in material science and are crucial for designing nanostructured materials at a single-quantum state level (27). However, it is still challenging to detect extremely small absorption signals of single nanoparticles and nanomaterials. In our method, accurate measurement of quantum resonant absorption is realized by precisely observing the optical forcedriven motions of the nanoparticles, called as optical force spectroscopy. This spectroscopy based on the optical momentum change instead of the energy change is conceptually different from the conventional techniques.
Figure 1B illustrates the experimental setup. A nanofiber with a diameter of 400 nm was fabricated from a commercial single-mode optical fiber (24). The diameter is constant in the waist part of the fiber over a length of several hundred micrometers. The nanofiber was soaked in an aqueous solution of diamond nanoparticles, i.e., nanodiamonds (NDs). Because nitrogen-vacancy centers (NVCs) in NDs have superior properties, such as no photobleaching, high sensitivity to the surrounding environment, and sharp zero phonon line absorption, they have been gaining attention as luminescent and magnetic-responsive nanomaterials that can be used for biological imaging, sensing, and single-photon source (17, 18). Thus, selection and sorting of NDs with and without NVCs are highly desirable. We prepared two types of NDs; one contained NVCs (>300 per particle), i.e., quantum resonant ND (r-ND), and the other was almost free from the NVCs, i.e., nonresonant ND (n-ND). The diameters of both r-NDs and n-NDs were 50 15 nm. Continuous-wave green (GR; 532 nm) and near-infrared (NIR; 1064 nm) diode lasers were launched from both ends of the nanofiber. The NVCs exhibit absorption at the GR region but not at the NIR region (28, 29). Furthermore, we introduce a weak red laser in the fiber as a probe light (690 nm, 0.1 mW) to monitor the motion of the NDs, which was recorded by an optical microscope equipped with a charge-coupled device (CCD) camera.
Figure 2A depicts the trapping and transportation of a single r-ND, where only the GR laser (70 mW) is incident from the left end of the fiber and the motion of the r-ND is observed in the waist part of the fiber. The result shows that the r-ND is attracted by the gradient force of the evanescent field and moves along the fiber because of the dissipative forces. The particle speed is constant at 110 m/s (see a trajectory in fig. S1). We evaluate the force exerted on the r-ND as 89 fN by considering the balance between the optical force and viscous drag using the Faxen formula for correcting the effect of the fiber surface [(23) and see the Supplementary Materials). When the NIR laser is simultaneously incident from the other end of the fiber (from the right), where the NIR laser power is fixed at 250 mW, and the GR laser power is varied from 70 to 0 mW, we achieve the motion control of a single r-ND (Fig. 2B). At the GR laser power of 70 mW, the r-ND moves toward the propagation direction of the GR laser (from left to right). As the GR laser power decreases, the motion decelerates and subsequently stops (~8 s). On further decreasing the GR laser power, the r-ND moves toward the opposite direction. The motion control experiment for an n-ND is illustrated in the Supplementary Materials (fig. S3).
(A) Time-sequential images of the r-ND observed at 2-s intervals. The GR laser is incident from the left end (70 mW). A single r-ND is trapped and transported along the nanofiber at the velocity of 110 m/s. (B) Time-sequential images of the r-ND observed at 4-s interval. The GR laser is incident from the left end of a nanofiber and the NIR laser from the opposite end. The power of the NIR laser is fixed at 250 mW, and the GR laser power is changed from 70 to 0 mW. At approximately 8 s, the optical forces exerted by the two lasers balance each other. The white bar indicates a scale of 100 m. The dotted line represents the nanofiber position.
The dissipative optical force exerted on an r-ND along a nanofiber is composed of two components, namely, absorption and scattering forces (Fabs, Fsca), which are represented by the absorption and scattering cross sections (abs, sca), as followsF=Fabs+Fsca=neffIc(abs+sca)(1)where I and c represent the intensity and velocity of light in a vacuum, respectively, and neff is the effective refractive index of the nanofiber (neff = 1.354 at 532 nm). The scattering cross section for Rayleigh particles is theoretically given bysca=1285n2V234(n12n22n122n22)2(2)where n1 and n2 are the refractive indices of diamond and surrounding water, respectively, is the incident laser wavelength in vacuum, and V is the volume of the particle. In the case of r-NDs including NVCs, abs is given by the transition dipole strength of an NVC and the number of NVCs in r-ND. The NIR laser induces only the scattering force, as NVCs exhibit no absorption at 1064 nm.
We perform a motion control experiment for an n-ND without NVCs to measure the balanced powers of the GR and NIR lasers for restricting the motion of the particle. The NIR laser power was fixed at 160 mW, corresponding to the intensity of 108 MW/cm2 estimated from the mode profile of the nanofiber, while the observed balanced power of the GR laser was 7.61 mW (intensity, 6.06 MW/cm2). As Fabs is not exerted on the n-ND, scattering forces (Fsca) by the GR and NIR lasers balance each other. Moreover, sca strongly depends on the wavelength (Eq. 2), which is compensated by the large difference between the intensities of the GR and NIR lasers. As sca is proportional to the square of the particle volume, the scattering force also changes significantly depending on the particle size. Fortunately, the ratio of the scattering forces at 532 and 1064 nm is constant for any particle size. This is because the volume dependence of sca is the same (V2) for both wavelengths. Thus, it is noted that the balanced powers of the two counterpropagating lasers remain unchanged for n-NDs of any size.
Furthermore, we demonstrate the selective transportation of r-NDs and n-NDs (Fig. 3 and movie S1). The same experimental setup and nanofiber were used, and the NIR laser power was 160 mW. The GR laser power was adjusted to 7.40 mW to drive different motions of the r-NDs and n-NDs. This value is slightly lower than the balanced power of the n-ND such that the scattering force exerted by the NIR laser is stronger for n-NDs than that by the GR laser, whereas the resonant absorption force on the r-NDs by the GR laser reverses the force strength relation. By switching the probe laser on and off, we can measure the emission from the NVCs and thus distinguish between the r-NDs and n-NDs. The two particles at both ends are r-NDs (numbered 1 and 4) and the other two particles are n-NDs (numbered 2 and 3). Scattered light spots of four NDs have nearly the same intensities when the probe laser is off, while the spots of r-NDs are brighter than the spots of n-NDs in Fig. 3 because the NVC emission is added to the scattered light. The r-NDs slowly move to the right (along the propagation direction of the GR laser), whereas the n-NDs move in the opposite direction (see trajectories in fig. S2). This result clearly demonstrates the selective transportation of NDs according to the quantum resonant absorption of NVCs by using the optical forces.
Time-sequential images observed at 2-s intervals. Numbers indicate individual NDs. Particles 1 and 4 represent r-NDs, and particles 2 and 3 represent n-NDs, which is confirmed by the emission of the NVCs. The powers of GR and NIR lasers are set at 7.40 and 160 mW, respectively. The r-NDs move to the right (direction of GR laser propagation), whereas the n-NDs move toward the opposite direction. The white bar indicates a scale of 100 m. The dashed line represents the nanofiber position.
Next, we analyze the absorption cross section (abs) of a single r-ND. We prepared the same experimental conditions and used the same nanofiber that was used for the balanced power measurement of an n-ND. At the NIR laser power of 160 mW, the balanced power of the GR lasers for an r-ND was measured to be 6.75 mW (intensity, 5.37 MW/cm2). Then, by turning the NIR laser off, the motion of r-NDs driven by the GR laser was observed to determine the strength of the optical force. On the basis of the balance with the viscous drag, the optical force composed of Fabs and Fsca was calculated as 6.30 fN. As a reference, the data of the n-ND were used for the present absorption analysis. By comparing the balanced powers of the GR laser for the r-ND (Pr-ND = 6.75 mW) and n-ND (Pn-ND = 7.61 mW), we obtain the ratio of Fabs and Fsca exerted on the r-ND as (Pn-ND Pr-ND):Pr-ND (Fig. 4) such that the measured optical force on the r-ND (F = 6.30 fN) can be decomposed into Fabs = 0.71 fN and Fsca = 5.59 fN. This result demonstrates that the absorption and scattering forces exerted on a single r-ND can be separately determined with subfemtonewton order accuracy. Thus, from Eq. 1, we evaluate the abs to be 2.9 1014 cm2. We repeated the measurements for 10 different r-NDs using the same nanofiber to perform the experiments under the same conditions. The average and SD of the evaluated abs were 3.3 1014 and 1.1 1014 cm2, respectively. The deviations in abs can be attributed to the variations in the number of NVCs contained in the r-NDs with different sizes and defect densities. The detailed distribution of abs and estimated number of NVCs are shown in the Supplementary Materials (see fig. S4).
The sum of the absorption and scattering forces exerted on the r-ND under GR laser irradiation (1 = 532 nm) with the power Pr-ND being balanced by the scattering force exerted by the NIR (2 = 1064 nm) laser. For the n-ND having no absorbers, the scattering forces exerted by the GR laser with Pn-ND and the NIR laser with constant power balance each other. From these balances, the ratio of the absorption and scattering forces on the ND can be determined as (Pn-ND Pr-ND):Pr-ND.
Here, we emphasize that the present method can detect the absorption cross section in the order of a square nanometer, which is close to those of single molecules (typically as large as 1015 cm2). Under the diffraction-limited illumination condition, this absorption cross section corresponds to a transmittance of ~106. Recently, Kukura et al. (30) and Celebrano et al. (31) succeeded in measuring the extremely small absorption using highly sensitive detectors, and the accuracy of their method is comparable with that of our method. However, in their technique, the Rayleigh scattering caused by nanoparticles and nanomaterials attenuates the transmitted light intensity as well such that the absorption signals cannot be extracted separately from the scattering components. In contrast, our proposed optical force spectroscopy can separately determine the absorption and scattering cross sections of single nanoparticles from the momentum change. The sensitivity is not limited by the signal-to-noise ratio of light intensity detection but restricted by the accuracy of the motion detection. Although nanometer-level position sensing techniques are available, the random thermal motion is the main factor that determines the accuracy. If the experiment is performed using superfluid helium at the cryogenic temperature, then the detection accuracy will be ultimately improved.
We demonstrated the selective transportation of single nanoparticles based on the relation between the quantum mechanical properties of nanomaterials and their macroscopic motion driven by the quantum resonant optical forces. This selective transportation is applicable to the precise sorting of nanocrystals, quantum dots, and molecular nanoparticles according to their resonant absorption properties. Optical force spectroscopy directly and sensitively measures the interaction between light and nanoparticles separately from the scattering effects based on the photon momentum change and not the energy change. It is noted that even if the reference nanoparticle having the same parent material but without absorbers is unavailable, the proposed absorption detection can still be achieved (see Materials and Methods). Although we focus on NDs as the samples for the first demonstration, note that other kinds of nanoparticles can be equally interesting targets. Size-selective optical transport of semiconductor quantum dots has been successfully demonstrated (32). Furthermore, it was reported that organic dye-doped nanoparticles have unique optical trapping characteristics according to their quantum resonance properties (33, 34). Applying the present technique to these nanomaterials will be our future endeavor. In conclusion, we believe that our scheme can enable a new class of optical force methodologies to investigate the characteristics of advanced nanomaterials and quantum materials and develop state-of-the-art nanodevices.
We used commercially available NDs having a mean diameter of 50 nm (r-NDs, FND Biotech Inc.; n-NDs, Microdiamant Japan. The absorption of NVCs appears at 532 nm after proton irradiation for fabricating r-NDs; contrarily, n-NDs exhibit no absorption at 532 nm. These were dispersed in pure water with 0.1 weight % surfactant. The concentration was adjusted such that a single ND is trapped by a nanofiber during the experiment.
A commercially available single-mode optical fiber (780HP, Thorlabs) was used to fabricate a nanofiber. It was heated with a ceramic heater at ~1400C and stretched at both ends. The waist diameter of the nanofiber used in this study was 400 nm, which remained constant (variation of <2%) over a length of several hundred micrometers. From the mode dispersion curve obtained by the fiber mode analysis, the single-mode propagation is valid when the wavelength of incident light is longer than 360 nm. The fiber was fixed on a glass slide using ultraviolet glue and soaked in a cell filled with an ND-dispersed aqueous solution.
Continuous-wave GR (532 nm) and NIR (1064 nm) diode lasers were introduced from both ends of the fabricated nanofiber. The laser powers were controlled using rotational neutral density filters. To record the motions of the NDs, we introduced a weak red laser (690 nm), and its light, scattered light from the particles, was monitored using a CCD camera. When nanoparticles other than the observed particles are trapped on the fiber, their scattering reduces the laser intensity irradiated on the particle. To avoid this disturbance, the experiments were performed after ensuring no change in the transmitted laser power.
When the reference nanoparticle having the same parent material but containing no absorbers is unavailable, the proposed absorption detection can still be realized by the following method: The measurement of the balanced laser powers for the reference particle is replaced by the calculation of the ratio of the scattering cross sections at two different wavelengths (using Eq. 2). When the refractive index of the parent material is constant at two laser wavelengths, the ratio of the scattering cross sections can be obtained using the inverse fourth power law. Using this value, we can determine the balanced laser powers for the virtual nonabsorbing particle. We analyzed the same data for 10 r-NDs as the abovementioned experiments but without using the data for n-NDs; consequently, the absorption cross sections were determined as (3.8 1.0) 1014 cm2. The variation from the above value [(3.3 1.1) 1014 cm2] would have been caused by a deviation from the Rayleigh scattering theory (Eq. 2) owing to the shape, size, and refractive index of the particles, as well as the random and systematic errors in the measurements.
Acknowledgments: Funding: The authors acknowledge the funding received from JSPS KAKENHI (grant numbers JP16H06504, JP16H06506, JP18H03882, JP18H05205, JP17K05016, and JP19H04529) and the Cooperative Research Program of Network Joint Research Center for Materials and Devices. Author contributions: H.I. and K.S. developed the concept and supervised the experiments. K.Y., H.F., and K.S. conducted the experiments. H.I. and T.W. theoretically elucidated the phenomena. H.F., K.Y., T.W., H.I., and K.S. participated in discussion of the results. H.F., H.I., and K.S. prepared the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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Birds Have a Mysterious ‘Quantum Sense’. For The First Time, Scientists Saw It in Action – ScienceAlert
Seeing our world through the eyes of a migratory bird would be a rather spooky experience. Something about their visual system allows them to 'see' our planet's magnetic field, a clever trick of quantum physics and biochemistry that helps them navigate vast distances.
Now, for the first time ever, scientists from the University of Tokyo have directly observed a key reaction hypothesised to be behind birds', and many other creatures', talents for sensing the direction of the planet's poles.
Importantly, this is evidence of quantum physics directly affecting a biochemical reaction in a cell - something we've long hypothesised but haven't seen in action before.
Using a tailor-made microscope sensitive to faint flashes of light, the team watched a culture of human cells containing a special light-sensitive material respond dynamically to changes in a magnetic field.
A cell's fluorescence dimming as a magnetic field passes over it. (Ikeya and Woodward, CC BY)
The change the researchers observed in the lab match just what would be expected if a quirky quantum effect was responsible for the illuminating reaction.
"We've not modified or added anything to these cells,"saysbiophysicist Jonathan Woodward.
"We think we have extremely strong evidence that we've observed a purely quantum mechanical process affecting chemical activity at the cellular level."
So how are cells, particularly human cells, capable of responding to magnetic fields?
While there are several hypotheses out there, many researchers think the ability is due to a unique quantum reaction involving photoreceptors called cryptochromes.
Cyrptochromes are found in the cells of many species and are involved in regulating circadian rhythms. In species of migratory birds, dogs, and other species, they're linked to the mysterious ability to sense magnetic fields.
In fact, while most of us can't see magnetic fields, our own cells definitelycontain cryptochromes.And there's evidence that even though it's not conscious, humans are actually still capable of detecting Earth's magnetism.
To see the reaction within cyrptochromes in action, the researchers bathed a culture of human cells containing cryptochromes in blue light caused them to fluoresce weakly. As they glowed, the team swept magnetic fields of various frequencies repeatedly over the cells.
They found that, each time the magnetic filed passed over the cells, their fluorescent dipped around 3.5 percent - enough to show a direct reaction.
So how can a magnetic field affect a photoreceptor?
It all comes down to something called spin - a innate property of electrons.
We already know that spin is significantly affected by magnetic fields. Arrange electrons in the right way around an atom, and collect enough of them together in one place, and the resulting mass of material can be made to move using nothing more than a weak magnetic field like the one that surrounds our planet.
This is all well and good if you want to make a needle for a navigational compass. But with no obvious signs of magnetically-sensitive chunks of material inside pigeon skulls, physicists have had to think smaller.
In 1975, a Max Planck Institute researcher named Klaus Schulten developed a theory on how magnetic fields could influence chemical reactions.
It involved something called a radical pair.
A garden-variety radical is an electron in the outer shell of an atom that isn't partnered with a second electron.
Sometimes these bachelor electrons can adopt a wingman in another atom to form a radical pair. The two stay unpaired but thanks to a shared history are considered entangled, which in quantum terms means their spins will eerily correspond no matter how far apart they are.
Since this correlation can't be explained by ongoing physical connections, it's purely a quantum activity, something even Albert Einstein considered 'spooky'.
In the hustle-bustle of a living cell, their entanglement will be fleeting. But even these briefly correlating spins should last just long enough to make a subtle difference in the way their respective parent atoms behave.
In this experiment, as the magnetic field passed over the cells, the corresponding dip in fluorescence suggests that the generation of radical pairs had been affected.
An interesting consequence of the research could be in how even weak magnetic fields could indirectly affect other biological processes. While evidence of magnetism affecting human health is weak, similar experiments as this could prove to be another avenue for investigation.
"The joyous thing about this research is to see that the relationship between the spins of two individual electrons can have a major effect on biology," says Woodward
Of course birds aren't the only animal to rely on our magnetosphere for direction. Species of fish, worms, insects, and even some mammals have a knack for it. We humans might even be cognitively affected by Earth's faint magnetic field.
Evolution of this ability could have delivered a number of vastlydifferent actionsbased on different physics.
Having evidence that at least one of them connects the weirdness of the quantum world with the behaviour of a living thing is enough to force us to wonder what other bits of biology arise from the spooky depths of fundamental physics.
This research was published in PNAS.
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Jan 7th 2021
ALBERT EINSTEIN won the 1921 Nobel prize for physics in 1922. The temporal anomaly embodied in that sentence was not, alas, one of the counterintuitive consequences of his theories of relativity, which distorted accustomed views of time and space. It was down to a stubborn Swedish ophthalmologistand the fact that Einsteins genius remade physics in more ways than one.
The eye doctor was Allvar Gullstrand, one of the five members of the Nobel Committee for Physics charged with providing an annual laureate for the Swedish Royal Academy of Sciences to approve. Gullstrand thought Einsteins work on relativity an affront to common sense (which it sort of was) and wrong (which it really wasnt). Every year from 1918 on, the committee received more nominations for Einstein than for any other candidate. And every year, Gullstrand said no.
By 1921 the rest of the committee had had enough of settling for lesser laureates: the only decision which could be made unanimously was not to award the prize at all. Amid great embarrassment the academy chose to delay the 1921 prize until the following year, when it would be awarded in tandem with that of 1922. This gave Carl Wilhelm Oseen, a Swedish physicist newly appointed to the committee, time for a cunning plan. He nominated Einstein not for relativity, but for his early work explaining lights ability to produce electric currents. Though Gullstrand was still peeved, this carried the day. In November 1922 Einstein was awarded the 1921 prize for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect.
This adroit bit of face-saving also seems, a century on, fully justified. Einsteins first paper on the nature of light, published in 1905, contained the only aspect of his work that he himself ever referred to as revolutionary. It did not explain a new experiment or discovery, nor fill a gap in established theory; physicists were quite happy treating light as waves in a luminiferous aether. It simply suggested that a new way of thinking about light might help science describe the world more consistently.
That quest for consistency led Einstein to ask whether the energy in a ray of light might usefully be thought of as divided into discrete packets; the amount of energy in each packet depended on the colour, or wavelength, of the light involved. Thus the law mentioned in his Nobel citation: the shorter the wavelength of a beam of light, the more energy is contained in each packet.
Eight years earlier, in 1897, experiments carried out by J.J. Thompson had convinced his fellow physicists that the cathode rays produced by electrodes in vacuum tubes were made up of fundamental particles which he called electrons. Over time, Einsteins energy packets came to be seen as photons. The electron showed that electric charge was concentrated into point-like particles; the photon was a way of seeing energy as being concentrated in just the same way. Work by Einstein and others showed that the two particles were intimately involved with each other. To get energy into an electron, you have to use a photon; and when an electron is induced to give up energy, the result is a photon. This mutualism is embodied in some of todays most pervasive technologies; solar cells, digital cameras, fibre-optic datalinks, LED lighting and lasers. It is used to measure the cosmos and probe the fabric of space and time. It could yet send space probes to the stars.
The settled view of light which provided a context for Einsteins work dated from 1864, when James Clerk Maxwell rolled everything physics knew about electric and magnetic forces into a theory of electromagnetic fields produced by objects carrying an electric charge. Stationary charged objects created electric fields; those moving at a constant speed created magnetic fields. Accelerating charged objects created waves composed of both fields at once: electromagnetic radiation. Light was a form of such radiation, Maxwell said. His equations suggested there could be others. In the late 1880s Heinrich Hertz showed that was true by creating radio waves in his laboratory. As well as proving Maxwell right, he added the possibility of wireless telegraphy to the range of electrical technologiesfrom streetlights to dynamos to transatlantic telegraph cablesthat were revolutionising the late 19th century.
Scientists have since detected and/or made use of electromagnetic waves at wavelengths which range from many times the diameter of Earth to a millionth the diameter of an atomic nucleus. The wavelengths of visible light380 nanometres (billionths of a metre) at the blue end of the spectrum, 700nm at the red endare special only because they are the ones to which human eyes are sensitive.
The reason Einstein found what he called Maxwells brilliant discovery incomplete was that Maxwells fields were described, mathematically, as continuous functions: the fields strength had a value at every point in space and could not jump in value from one point to the next. But the material world was not continuous. It was lumpy; its molecules, atoms and electrons were separate entities in space. Physics described the material world through statistical accounts of the behaviour of very large numbers of these microscopic lumps; heat, for example, depended on the speed with which they vibrated or bumped into each other. It was a mathematical approach quite unlike Maxwells treatment of electromagnetic fields.
Yet matter and electromagnetic radiation were intimately associated. Every object emits electromagnetic radiation just by dint of having a temperature; its temperature is a matter of the jiggling of its constituent particles, some of which are charged, and the jiggling of charged particles produces electromagnetic waves. The spread of the wavelengths seen in that radiationits spectrumis a function of the bodys temperature; the hotter the body, the shorter both the median and highest wavelengths it will emit. The reason the human eye is sensitive to wavelengths in the 380-700nm range is that those are the wavelengths that a body gives off most prolifically if it is heated to 5,500C, the temperature of the surface of the Sun. They are thus the wavelengths that dominate sunlight (see chart).
If wavelengths and temperature were so intimately involved, Einstein believed, it had to be possible to talk about them in the same mathematical language. So he invented a statistical approach to the way entropya tendency towards disordervaries when the volume of a cavity filled with electromagnetic radiation changes. He then asked, in effect, what sort of lumpiness his statistics might be explaining. The answer was lumps of energy inversely proportional to the wavelength of the light they represented.
In 1905 Einstein was willing to go only so far as suggesting that this light-as-lump point of view provided natural-seeming explanations of various phenomena. Over subsequent years he toughened his stance. His work on relativity showed that Maxwells luminiferous aether was not required for the propagation of electromagnetic fields; they existed in their own right. His work on light showed that the energy in those fields could be concentrated into the point-like particles in empty space. Light was promoted from what he called a manifestation of some hypothetical medium into an independent entity like matter.
This account was not fully satisfying, because light was now being treated as a continuous wave in some contextswhen being focused by lenses, sayand as something fundamentally lumpy in others. This was resolved by the development of quantum mechanics, in which matter and radiation are both taken to be at the same time particulate and wavy. Part of what it is to be an electron, or a photon, or anything else is to have a wave function; the probabilities calculated from these wave functions offer the only access to truth about the particles that physics can have.
Einstein was never reconciled to this. He rejected the idea that a theory which provided only probabilities could be truly fundamental. He wanted a better way for a photon to be both wave and a particle. He never found it. All these 50 years of conscious brooding, he wrote to a friend in 1951, have brought me no nearer to the answer to the question, What are light quanta? Nowadays every Tom, Dick and Harry thinks he knows it, but he is mistaken.
Though Einstein was probably not thinking of him specifically, one of those Dicks was Richard Feynman, one of four physicists who, in the late 1940s, finished off the intellectual structure of which Einstein had laid the foundations: a complete theory of light and matter called quantum electrodynamics, or QED. It is a theory in which both matter and radiation are described in terms of fields of a fundamentally quantum nature. Particleswhether of light or matterare treated as excited states of those fields. No phenomenon has been found that QED should be able to explain and cannot; no measurement has been made that does not fit with its predictions.
Feynman was happy to forgo Einsteins brooding and straightforwardly assert that light is made of particles. His reasoning was pragmatic. All machines made to detect light will, when the light is turned down low enough, provide lumpy its-there-or-its-not readings rather than continuous ones. The nature of quantum mechanics and its wave functions mean that some of those readings will play havoc with conventional conceptions of what it is for a particle to be in a given place, or to exist as an independent entity. But that is just the way of the quantum, baby.
The precise manipulation of photons has shed much light on non-locality, decoherence and other strange quantum-mechanical phenomena. It is now making their application to practical problems, through quantum computation and quantum cryptography, increasingly plausible. But this Technology Quarterly is not about such quantum weirdness (for that, see our Technology Quarterly of January 2018). It is about how photons interactions with electrons have been used to change the world through the creation of systems that can turn light directly into electricity, and electricity directly into light.
That light and electricity were linked was known long before Einstein. In the 1880s Werner von Siemens, founder of the engineering firm that bears his name, attached the most far reaching importance to the mysterious photoelectric effect which led panels of selenium to produce trickles of current. Einsteins theory was taken seriously in part because it explained why a faint short-wavelength light could produce such a current when a bright longer-wavelength light could not: what mattered was the amount of energy in each photon, not the total number of photons.
Technology built on such ideas has since allowed light to be turned into electricity on a scale that would have boggled Siemenss mind. It lets billions of phone users make digital videos and send them to each other through an infrastructure woven from whiskers of glass. It lights rooms, erases tattoos, sculpts corneas and describes the world to driverless cars. Ingenuity and happy chance, government subsidies and the search for profit have created from Einsteins suggestion a golden age of lighta burst of innovation that, a century on, is not remotely over.
This article appeared in the Technology Quarterly section of the print edition under the headline "The liberation of light"
Tokyo Institute of Technology: Quantum Mysteries: Probing an Unusual State in the Superconductor-Insulator Transition – India Education Diary
Scientists at Tokyo Institute of Technology approach the two decade-old mystery of why an anomalous metallic state appears in the superconductor-insulator transition in 2D superconductors. Through experimental measurements of a thermoelectric effect, they found that the quantum liquid state of quantum vortices causes the anomalous metallic state. The results clarify the nature of the transition and could help in the design of superconducting devices for quantum computers.
Uncovering Quantum Fluctuations Leading to an Anomalous State in 2D Superconductors
The superconducting state, in which current flows with zero electrical resistance, has fascinated physicists since its discovery in 1911. It has been extensively studied not only because of its potential applications but also to gain a better understanding of quantum phenomena. Though scientists know much more about this peculiar state now than in the 20th century, there seems to be no end to the mysteries that superconductors hold.
A famous, technologically relevant example is the superconductorinsulator transition (SIT) in two-dimensional (2D) materials. If one cools down thin films of certain materials to near absolute-zero temperature and applies an external magnetic field, the effects of thermal fluctuations are suppressed enough so that purely quantum phenomena (such as superconductivity) dominate macroscopically. Although quantum mechanics predicts that the SIT is a direct transition from one state to the other, multiple experiments have shown the existence of an anomalous metallic state intervening between both phases.
So far, the origin of this mysterious intermediate state has eluded scientists for over two decades. Thats why a team of scientists from the Department of Physics at Tokyo Institute of Technology(Tokyo Tech), Japan, recently set out to find an answer to the question in a study published in Physical Review Letters. Assistant Professor Koichiro Ienaga, who led the study, explains their motivation, There are theories that try to explain the origin of dissipative resistance at zero temperature in 2D superconductors, but no definitive experimental demonstrations using resistance measurements have been made to unambiguously clarify why the SIT differs from the expected quantum phase transition models.
The scientists employed an amorphous molybdenumgermanium (MoGe) thin film cooled down to an extremely low temperature of 0.1 K and applied an external magnetic field. They measured a traverse thermoelectric effect through the film called the Nernst effect, which can sensitively and selectively probe superconducting fluctuations caused by mobile magnetic flux. The results revealed something important about the nature of the anomalous metallic state: the quantum liquid state of quantum vortices causes the anomalous metallic state. The quantum liquid state is the peculiar state where the particles are not frozen even at zero temperature because of the quantum fluctuations.
Most importantly, the experiments uncovered that the anomalous metallic state emerges from quantum criticality; the peculiar broadened quantum critical region at zero temperature corresponds to the anomalous metallic state. This is in a sharp contrast to the quantum critical point at zero temperature in the ordinary SIT. Phase transitions mediated by purely quantum fluctuations (quantum critical points) have been long-standing puzzles in physics, and this study puts us one step closer to understanding the SIT for 2D superconductors. Excited about the overall results, Ienaga remarks, Detecting superconducting fluctuations with precision in a purely quantum regime, as we have done in this study, opens a new way to next-generation superconducting devices, including q-bits for quantum computers.
Now that this study has shed light on the two-decade old SIT mystery, further research will be required to get a more precise understanding of the contributions of the quantum vortices in the anomalous metallic state. Let us hope that the immense power of superconductivity will soon be at hand!
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