About Centre for Quantum Technologies (CQT)

The Centre for Quantum Technologies (CQT) is a research centre of excellence in Singapore. It brings together physicists, computer scientists and engineers to do basic research on quantum physics and to build devices based on quantum phenomena. Experts in this new discipline of quantum technologies are applying their discoveries in computing, communications, and sensing.CQT is hosted by the National University of Singapore and also has staff at Nanyang Technological University. With some 180 researchers and students, it offers a friendly and international work environment.Learn more about CQT atwww.quantumlah.org

Job Description

The Centre is looking for a talented and driven Research Fellow to join the research group of Prof. Valerio Scarani. The group conducts theoretical research, specialising in foundational aspects, nonlocality and device-independent certification, quantum thermodynamics, and description of quantum devices.

We are specifically looking for a candidate to lead an original research project in a broad variety of topics, but anyway centered on the theoretical description of realistic physical systems and devices. A non-exhaustive list of possible topics includes: the theory of mechanical systems in the quantum regime; the characterisation and benchmarking of large quantum devices; proposal for the detection of tabletop gravity from quantum sources; implementations of primitives in quantum networks. It is absolutely essential that the candidate proposes a convincing research plan and shows their ability to lead it.

The position is open for two years, with further extensions possible in case of exceptional performance. Starting date: January 2024.

Job Requirements

More Information

For enquiries and details about the position, please contact Prof. Valerio Scarani atphysv@nus.edu.sg.

Please include your consent by filling in theNUS Personal Data Consentfor Job Applicants.

Applications should contain your latest CV and a description of your research interests.

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Research Fellow in Theoretical Quantum Physics job with ... - Times Higher Education

By Keegan Leibrock 9/13/23 12:01am

Qimiao Si, a professor of physics and astronomy, and Jeffrey Tabor, a professor of bioengineering and biosciences, have been awarded Vannevar Bush Faculty Fellowships by the U.S. Department of Defense. The award comes with a five-year fellowship and $3 million in research funding to continue work in their respective fields.

Si studies theoretical condensed matter physics and has been a faculty member at Rice since 1995. His contributions to the field of condensed matter physics focus on strongly correlated electron systems and the theory of quantum criticality, which relates to the transition of matter from one quantum state to another.

Si said his research intends to explore the responsiveness of electrons to stimuli. With the grant money, he said he will pursue research dealing with the control of topological states of matter.

We want to establish a theoretical framework for realizing materials that have unusual properties in this case, being extremely responsive, Si said. We want to start from intuition and build up the equations to be able to make a statement about realizing new states of matter Its not the approach that one takes on a daily basis and its also highly risky. It may not work.

Tabors research focuses on bioengineering and biosciences. Since joining Rice in 2010, Tabors lab has programmed living cells to sense and respond to environmental stimuli along with developing potential methods to sense and treat intestinal disease.

Tabor said that he hopes this new research will streamline the DNA synthesis processes to open new research opportunities.

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Unfortunately, the technology thats used to make synthetic DNA is based on chemistry and has not been changed in 40 years. Its very expensive, it produces toxic chemical waste and it severely limits research, Tabor said. [Because of this], we arent able to do all the research that we want our goal is to harness the power of enzymes that come from living things to naturally make DNA in an approach that is astronomically cheaper.

Tabor said that this new process of DNA synthesis would have substantial benefits for those seeking medical treatment for cancer through cancer immunotherapy.

While there [have] been early breakthroughs [in natural DNA synthesis], theres also still a lot of challenges, Tabor said. If we were to make that process much cheaper, we could try many new designs, learn much more about how this system works and then make a new generation of these cancer immunotherapies that targeted more cancers, had less side effects and helped more people.

Si said that obstacles to success, both in seeking this grant and in his research, have driven his excitement.

Over the course of my career, Ive made many predictions, and more often than not, they turn out not to be true, Si said. To me, thats where the excitement lies, in this inevitability of unpredictability the opportunities are enormous [and] the challenges are enormous.

Si said that the grant funding will open new opportunities in quantum physics that would have otherwise not been possible.

The [$3 million grant] allows us to do two things, Si said. One is that it allows us to have a large group of postdocs and graduate students who can come together to brainstorm and formulate equations [the grant] also allows us to facilitate experimental capacity.

Si said that he would advise aspiring researchers to recognize a specific area of interest and work hard within that area.

Follow your trajectory, and youll get something great I think thats quite true, Si said.

Tabor expressed a similar sentiment, advising young researchers to be ambitious in their goals.

Dont be afraid to think big and pursue something that youre really excited about, Tabor said. Even if it seems too far away, or too risky, or too challenging, if you are excited about it, [then] that excitement will come through in your daily life, and youll be doing your best work thats a good recipe for success.

Original post:

Two professors awarded Vannevar Bush Fellowships - The Rice Thresher

When something falls into a black hole, where does it go, and will it ever come back out again? According to Einsteins General Relativity, those answers are simple: as soon as anything physical matter, antimatter, radiation, etc. crosses over the event horizon, its gone. It can add things like mass, electric charge, and angular momentum to the black hole, but little else. It goes swiftly toward and eventually into the central singularity, and will never escape again.

But our Universe isnt governed by General Relativity alone, but also by quantum physics. According to our best understanding of quantum reality, theres much more that needs to be considered. Not only are there other quantum properties inherent to the raw ingredients that go into making a black hole baryon number, lepton number, color charge, spin, lepton family number, weak isospin and hypercharge, etc. but the fabric of spacetime itself, which contains the black hole, is quantum in nature. Because of those quantum properties, black holes do not remain static, but rather evaporate over time: emitting Hawking radiation (and perhaps even more) in the process.

When black holes do evaporate, then, what happens to the information that went into creating them? Is it conserved? Is it destroyed? Is it encoded in the outgoing radiation? And if so, how? These questions are at the heart of perhaps the greatest paradox of all: the black hole information paradox. Heres both what we know and what we still need to find out.

When two particles are entangled in the quantum mechanical sense, its as though some sort of hidden, invisible connection exists between them. Many have conjectured that this connection persists even across the event horizon of a black hole, and that whatever information goes into forming a black hole will eventually emerge as the black hole evaporates.

Information

When a physicist talks about information, they dont necessarily mean what we conventionally think of as information: a string of letters, numbers, symbols, or anything else that can be encoded with bits like 0s or 1s. Conventionally, this is often described as the number of yes/no questions that must be answered to fully specify the properties of your physical system, although even that description has limitations. These are all certainly examples of information, but those examples dont encompass all the various types of information that exist. Information can also include:

That last one is tricky, because entropy an inherently thermodynamical quantity is very often misunderstood. Youll often hear statements like entropy is a measure of disorder or entropy always increases for any system and while those things are kind of true, its possible to make very ordered high-entropy systems and to decrease a systems entropy through the input of an external energy source.

As an alternative, consider this: what entropy actually measures is the number of possible arrangements of the (fully quantum) state of your system.

A system set up in the initial conditions on the left and allowed to evolve will have less entropy if the door remains closed (left) than if the door is opened (right). If the particles are allowed to mix, there are more ways to arrange twice as many particles at the same equilibrium temperature than there are to arrange half of those particles, each, at two different temperatures, resulting in a much greater entropy for the system at right than the one at left.

A classic example is to consider two systems:

Both systems have the same number of particles, the same total energy in them, but wildly different entropies from one another. The second system has a much greater amount of entropy, as there are many different ways to distribute energy among all of the particles in your system to achieve the desired configuration than there are for the first system; the number of possible arrangements of the fully quantum state of your full system is much greater for the second system than the first.

Because there is a greater number of possible arrangements, you have to provide a greater amount of information and, therefore, answer a greater number of yes/no questions to fully describe the system with a greater amount of entropy. Information and entropy arent identical, but they are proportional: a greater entropy to your system means it requires more information to fully describe it.

A wine glass, when vibrated at the right frequency, will shatter. This is a process that dramatically increases the entropy of the system, and is thermodynamically favorable. The reverse process, of shards of glass reassembling themselves into a whole, uncracked glass, is so unlikely that it never occurs spontaneously in practice. However, if the motion of the individual shards, as they fly apart, were exactly reversed, they would indeed fly back together and, at least for an instant, successfully reassemble the wine glass. Time reversal symmetry is exact in Newtonian physics.

Information and black holes

If you take a book and burn it, the books information doesnt get lost or destroyed, but merely scrambled. In principle although, maybe not in practice just yet you could trace each and every particle of paper-and-ink that went into the fire, determine where they went, and from the ash, soot, chemicals, and invisible gases they produced, keep track of every character on every page in that book. In principle, you could look at that final system of the completely burned book and reconstruct the complete information that was in the book before you burned it.

You can do this with the remnants of a shattered glass, reconstructing what the original, unbroken structure looked like. You can do this with a scrambled-and-cooked egg, reconstructing what the uncooked, unscrambled egg was like. As long as the fundamental particles that the original system was made out of were preserved, no matter what interactions they underwent in the meanwhile, that original information about the initial state of the system would be preserved as well.

But with black holes, that absolutely isnt the case any longer. In General Relativity, black holes dont have any memory about the types of particles (or the properties of those particles) that went into creating or growing the black hole. The only measurable properties a black hole can possess are mass, electric charge, and angular momentum.

One of the most important contributions of Roger Penrose to black hole physics is the demonstration of how a realistic object in our Universe, such as a star (or any collection of matter), can form an event horizon and how all the matter bound to it will inevitably encounter the central singularity. Once an event horizon forms, the development of a central singularity is not only inevitable, its extremely rapid.

In the early 1970s, this puzzle was considered by physicist Jacob Bekenstein, who recognized why this was such a problem. Whatever particles go into forming a black hole have their own properties, configuration, and amount of entropy (and information) encoded within them. According to the second law of thermodynamics, entropy can never decrease for a closed system; it can only increase or remain the same, unless some external source of energy is inputted to decrease that entropy. (And even then, the total entropy of the original system plus the external source, where the external source is where that inputted energy comes from, will continue to increase.)

But in pure General Relativity, black holes have zero entropy, and that definition simply wont work. From the perspective of an external observer, its quantum particles that go into the creation of a black hole, and as the black hole gets created and grows, the surface area of its event horizon increases. As the mass goes up, the surface area goes up, and as more particles pour in, the entropy must rise as well.

It was Bekenstein who first recognized that the information encoded by the infalling particles would, from an external observers perspective, appear to get smeared out over the surface of the event horizon, enabling a definition of entropy that was proportional to a black holes event horizons surface area. Today, this is known as the Bekenstein-Hawking entropy: the entropy of a black hole.

Encoded on the surface of the black hole can be bits of information, proportional to the event horizons surface area. When the black hole decays, it decays to a state of thermal radiation. Whether that information survives and is encoded in the radiation or not, and if so, how, is not a question that our current theories can provide the answer to.

Will that information get destroyed?

This definition was very exciting, but the notion that we had made sense of the Universe of entropy, information, and black holes was extremely short-lived. In 1974, just two years after Bekensteins earliest work on the topic, Stephen Hawking came along and not only had a spectacular realization, but performed a tremendous calculation to go with it.

His realization was that the standard way of performing quantum field theory calculations made an assumption: that space would, on tiny quantum scales, be treated as though it were flat, unaffected by the General Relativistic curvature of space. However, in the vicinity of a black hole, this wasnt just a bad approximation, it was a worse approximation than it would be under any other conditions that occurred within our physical Universe.

Instead, Hawking recognized, the calculation needed to be done in a background of curved space, where the background spatial curvature was given by Einsteins equations and the properties of the black hole in question. Hawking calculated the simplest case for a black hole with mass only, without electric charge or angular momentum in 1974, and recognized that the state of the quantum vacuum, or empty space itself, was fundamentally different in curved space, near the black holes event horizon, than the state of the quantum vacuum far away from the black hole: where space is flat.

In the far future, there will be no more matter around black holes, but instead their emitted energy will be dominated by Hawking radiation, which will cause the size of the event horizon to shrink. The transition from growing to decaying black holes will occur whenever the accretion rate drops below the mass loss rate due to Hawking radiation, an event estimated to occur some ~10^20 years in the future. How the information that went into making the black hole gets encoded into the outgoing radiation, or whether thats even the case, has not yet been determined.

That calculation revealed that black holes dont simply exist, stably, in this curved space, but that the differences in the vacuum near and far away from the event horizon lead to a continuous emission of blackbody radiation: now known as Hawking radiation. This radiation should:

This is remarkable, and is a purely quantum effect that were now realizing may apply to systems other than black holes as well.

However, it raised a new, troubling issue. If the radiation that comes out of a black hole as it evaporates, this Hawking radiation, is purely blackbody in nature, it should have no preference for:

or any other metric needed to answer a yes/no question regarding the initial quantum state of the matter that went into creating the black hole in the first place. For the first time, it seems that weve encountered a physical system where knowing and measuring all of the information about its final state doesnt, even in principle, allow you to reconstruct the initial state.

As the Universe continues to age, the last sources of light will arise from the evaporation of black holes. While the least massive black holes will complete their evaporation after only 10^67 years or so, the most massive ones will persist for over a googol (10^100) years, making them the last cosmic objects to emit light, as far as we know.

The core of the black hole information paradox

So where, then, does the information go?

Thats the puzzle: we think that information shouldnt be able to be destroyed, but if the black hole is evaporating into pure blackbody radiation, then all of that information that went into making the black hole has somehow disappeared.

The event horizon of a black hole is a spherical or spheroidal region from which nothing, not even light, can escape. But outside the event horizon, the black hole is predicted to emit radiation. Hawkings 1974 work was the first to demonstrate this, and it was arguably his greatest scientific achievement. A new study now suggests that Hawking radiation may even be emitted in the absence of black holes, with profound implications for all stars and stellar remnants in our Universe.

The truth is, despite many declarations over the years that the black hole information paradox has been resolved, that nobody knows. Nobody knows whether the information is preserved, whether its destroyed or erased, and whether it depends on what occurs in a black holes interior or whether it can be completely described from an outside observers perspective.

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We have mathematical correspondences between what happens on the inside and the outside of a black hole, including an underappreciated fact that takes us beyond the semiclassical approximation (quantum field theory calculations in a background of curved spacetime) used by Hawking: that when radiation comes out of a black hole, it should maintain a quantum mechanical entangled link to the black holes interior.

We have devised methods that allow us to map the entropy of a black holes interior onto the outgoing radiation that arises due to the Hawking mechanism, which suggests (but does not prove) that we may be approaching a mechanism for understanding how the information that went into creating a black hole gets encoded back into the Universe outside of the black holes event horizon.

Unfortunately, we dont know how to calculate individual bits of information using any of these methods; we only know how to calculate overall amounts of information as though were putting them on a scale, seeing whether they balance or not. Thats an important step, but it isnt enough to resolve this paradox.

In the final stages of a black holes evaporation, quantum gravitational effects are likely to become important. It is conceivable that these effects could play an important role when it comes to encoding the information that went into creating the black hole in the first place.

Certainly, there are other ideas that are playing a major role. String-inspired ideas like complementarity and the AdS/CfT correspondence, as well as the notion of a firewall appearing partway through the evaporation process, are considered by many working on the paradox. Others suggest that there are correlations between every quantum of radiation emitted in the Hawking process (similar to entanglement), and that the full suite of those correlations must be understood in order to resolve the paradox. Still others have suggested altering the black holes internal and external geometries over the course of the emission of Hawking radiation to attempt to preserve information, while others appeal to whatever strong quantum effects must be present at the interface of quantum physics and relativity: becoming important in the final stages of black hole evaporation.

However, we still do not understand the most important aspects of the paradox: where the information from the particles that create the black hole goes, and how that information assuming it does get out into the Universe again actually gets encoded into the outgoing radiation that results when black holes evaporate. Despite whatever claims you may have heard, make no mistake: the black hole information paradox is still an unresolved paradox, and although its still an active area of research, no one can be sure what the solution will ultimately be, or what method will eventually lead us to it.

See original here:

The grand paradox at the heart of every black hole - Big Think

September 11, 2023• Physics 16, 151

A new set of equations captures the dynamical interplay of electrons and vibrations in crystals and forms a basis for computational studies.

J. Berges/University of Bremen

J. Berges/University of Bremen

Although a crystal is a highly ordered structure, it is never at rest: its atoms are constantly vibrating about their equilibrium positionseven down to zero temperature. Such vibrations are called phonons, and their interaction with the electrons that hold the crystal together is partly responsible for the crystals optical properties, its ability to conduct heat or electricity, and even its vanishing electrical resistance if it is superconducting. Predicting, or at least understanding, such properties requires an accurate description of the interplay of electrons and phonons. This task is formidable given that the electronic problem aloneassuming that the atomic nuclei stand stillis already challenging and lacks an exact solution. Now, based on a long series of earlier milestones, Gianluca Stefanucci of the Tor Vergata University of Rome and colleagues have made an important step toward a complete theory of electrons and phonons [1].

At a low level of theory, the electronphonon problem is easily formulated. First, one considers an arrangement of massive point charges representing electrons and atomic nuclei. Second, one lets these charges evolve under Coulombs law and the Schrdinger equation, possibly introducing some perturbation from time to time. The mathematical representation of the energy of such a system, consisting of kinetic and interaction terms, is the systems Hamiltonian. However, knowing the exact theory is not enough because the corresponding equations are only formally simple. In practice, they are far too complexnot least owing to the huge number of particles involvedso that approximations are needed. Hence, at a high level, a workable theory should provide the means to make reasonable approximations yielding equations that can be solved on todays computers.

One way to reduce the complexity of the problem is to step back from the picture of individual particles in favor of one of effective quasiparticles specific to the system at hand. An early example of a quasiparticle in the literature is the phonon: instead of focusing on the atomic nuclei that could, in principle, be located anywhere in space, one considers their collective vibration about their positions in a predefined crystal structure. Scientists have studied such elastic waves for almost a century [2], often resorting to two famous approximations: the Born-Oppenheimer approximation, which assumes that the electrons respond instantaneously to displacements of the nuclei; and the harmonic approximation, which posits that this response results in restoring forces proportional to the displacements.

Stefanucci and colleagues work builds on studies made in the middle of the last century that analyzed the interaction between quasiparticles by borrowing tools from quantum field theory. In 1961, Gordon Baym published a corresponding theory of electrons and phonons, in which the phonon field assigns a displacement to points in space and time [3]. One of the aforementioned tools is the technique of Feynman diagrams, which represent interaction processes graphically (Fig. 1) and can be translated into mathematical formulas through simple rules. By combining such diagrams into sets of equations that recursively depend on each other, one can account for all possible processes occurring in physical reality. In 1965, Lars Hedin presented examples of such equations, which completely describe systems of interacting electrons [4]. In a 2017 review, Feliciano Giustino merged these approaches and coined the term Hedin-Baym equations in the context of state-of-the-art materials simulationsanswering many, but not all, open questions [5].

Stefanucci and colleagues have addressed several of the remaining issues [1]. First, they imposed requirements on the electronphonon Hamiltonian, avoiding the mistake of trying to solve a problem not properly formulated in the first place. They emphasized that the equilibrium state around which the theory is built is not known in advance, making setting up and evaluating the Hamiltonian an iterative procedure. They also stressed that this Hamiltonian cannot generally be written in terms of physical phonons, contrary to what is often supposed. Second, the team generalized Giustinos work [5] to systems driven out of equilibrium at any temperaturea key advance because this scenario reflects experimental and technological conditions. Mathematically, this generalization allows time to take on complex values. Third, the researchers carefully derived the corresponding rules for Feynman diagrams and provided the first complete set of diagrammatic Hedin-Baym equations. Such equations form the basis of systematic approximations, in which certain diagrams are neglected, and provide a criterion [3] for the resulting dynamics to respect fundamental conservation laws. Whereas the effects of electrons on phonons and vice versa are well studied separately [5], here it is crucial that both occur simultaneously.

Nowadays, parameter-free simulations of electrons and phonons rely heavily on so-called density-functional perturbation theory [6], which is based on the Born-Oppenheimer and harmonic approximations. By contrast, diagrammatic techniques are oftenbut not always [7]used in combination with parameterized model Hamiltonians. Efforts to bring both approaches together have led to so-called downfolding methods, which already exist for the electronphonon problem [8]. The insights gained by Stefanucci and colleagues will certainly help to further bridge the different strategies. Moreover, the advancements beyond thermal equilibrium will be of utmost importance because such an extension is needed to explain the latest time-resolved spectroscopy experiments and to design better photovoltaics. Finally, given that the teams results apply to any fermionboson system, such as an interacting lightmatter system, many fields will benefit from this seminal work.

Jan Berges is a postdoctoral researcher at the University of Bremen in Germany. He is working on electronphonon interactions at the interface of first principles and model calculations, with a focus on computational implementation. Since the beginning of his doctoral studies, which he completed in 2020, he has been interested in many-body instabilitiessuch as charge-density waves and superconductivityespecially in two-dimensional materials.

Researchers have demonstrated a way to sift a database of crystalline compounds for structures that can be separated into useful one-dimensional materials. Read More

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Toward a Complete Theory of Crystal Vibrations - Physics