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Qubits are a basic building block for quantum computers, but theyre also notoriously fragiletricky to observe without erasing their information in the process. Now, new research from CUBoulder and the National Institute of Standards and Technology (NIST) may be a leap forward for handling qubits with a light touch.

In the study, a team of physicists demonstrated that it could read out the signals from a type of qubit called a superconducting qubit using laser lightand without destroying the qubit at the same time.

Artist's depiction of an electro-optic transducer, an ultra-thin devicethat can capture and transform the signals coming from a superconducting qubit. (Credit: Steven Burrows/JILA)

The groups results could be a major step toward building a quantum internet, the researchers say. Such a network would link up dozens or even hundreds of quantum chips, allowing engineers to solve problems that are beyond the reach of even the fastest supercomputers around today. They could also, theoretically, use a similar set of tools to send unbreakable codes over long distances.

The study, published June 15 in the journal Nature, was led by JILA, a joint research institute between CU Boulder and NIST.

Currently, theres no way to send quantum signals between distant superconducting processors like we send signals between two classical computers, said Robert Delaney, lead author of the study and a former graduate student at JILA.

Quantum computers, which run on qubits,get their power by tapping into the properties of quantum physics, or the physics governing very small things. Delaney explained the traditional bits that run your laptop are pretty limited: They can only take on a value of zero or one, the numbers that underly most computer programming to date. Qubits, in contrast, can be zeros, ones or, through a property called superposition, exist as zeros and ones at the same time.

But working with qubits is also a bit like trying to catch a snowflake in your warm hand. Even the tiniest disturbance can collapse that superposition, causing them to look like normal bits.

In the new study, Delaney and his colleagues showed they could get around that fragility. The team uses a wafer-thin piece of silicon and nitrogen to transform the signal coming out of a superconducting qubit into visible lightthe same sort of light that already carries digital signals from city to city through fiberoptic cables.

Researchers have done experiments to extract optical light from a qubit, but not disrupting the qubit in the process is a challenge, said study co-author Cindy Regal, JILA fellow and associate professor of physics at CU Boulder.

There are a lot of different ways to make a qubit, she added.

Some scientists have assembled qubits by trapping an atom in laser light. Others have experimented with embedding qubits into diamonds and other crystals. Companies like IBM and Google have begun designing quantum computer chips using qubits made from superconductors.

A quantum computer chip designed by IBM that includes four superconducting qubits. (Credit: npj Quantum Information,2017)

Superconductors are materials that electrons can speed around without resistance. Under the right circumstances, superconductors will emit quantum signals in the form of tiny particles of light, or photons, that oscillate at microwave frequencies.

And thats where the problem starts, Delaney said.

To send those kinds of quantum signals over long distances, researchers would first need to convert microwave photons into visible light, or optical, photonswhich can whiz in relative safety through networks fiberoptic cables across town or even between cities. But when it comes to quantum computers, achieving that transformation is tricky, said study co-author Konrad Lehnert.

In part, thats because one of the main tools you need to turn microwave photons into optical photons is laser light, and lasers are the nemesis of superconducting qubits. If even one stray photon from a laser beam hits your qubit, it will erase completely.

The fragility of qubits and the essential incompatibility between superconductors and laser light makes usually prevents this kind of readout, said Lehnert, a NIST and JILA fellow.

To get around that obstacle, the team turned to a go-between: a thin piece of material called an electro-optic transducer.

Delaney explained the team begins by zapping that wafer, which is too small to see without a microscope, with laser light. When microwave photons from a qubit bump into the device, it wobbles and spits out more photonsbut these ones now oscillate at a completely different frequency. Microwave light goes in, and visible light comes out

In the latest study, the researchers tested their transducer using a real superconducting qubit. They discovered the thin material could achieve this switcheroo while also effectively keeping those mortal enemies, qubits and lasers, isolated from each other. In other words, none of the photons from the laser light leaked back to disrupt the superconductor.

Our electro-optic transducer does not have much effect on the qubit, Delaney said.

The team hasnt gotten to the point where it can transmit actual quantum information through its microscopic telephone booth. Among other issues, the device isnt particularly efficient yet. It takes about 500 microwave photons, on average, to produce a single visible light photon.

The researchers are currently working to improve that rate. Once they do, new possibilities may emerge in the quantum realm. Scientists could, theoretically, use a similar set of tools to send quantum signals over cables that would automatically erase their information when someone was trying to listen in. Mission Impossible made real, in other words, and all thanks to the sensitive qubit.

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What quantum information and snowflakes have in common, and what we can do about it - CU Boulder Today

The nature of time is a tricky notion to pin down. But whether it is a fundamental part of our universe or just an illusion has huge implications

By Richard Webb

Skizzomat

WE ARE BORN; we live; at some point, we die. The notion that our existence is limited by time is fundamental to human experience. We cant fight it and truth be told, we dont know what we are fighting against. Time is a universal whose nature we all and physicists especially fail to grasp. But why is time so problematic? If we had a really good answer to that question, says Astrid Eichhorn, a theoretical physicist at the University of Southern Denmark in Odense, then it wouldnt be so problematic.

On a certain level, time is simple: it is what stops everything happening at once. That might seem flippant, but it is at least something people can agree on. The causal order of things is really what time is all about, says Eichhorn.

Viewed this way, the existence of time can be interpreted as a necessary precondition for the sort of universe where things lead to other things, among them intelligent life that can ask questions, such as what is time?. Beyond that, times essence is mysterious. For instance, why can things only influence other things in one direction in time, but in multiple directions in the three dimensions of space.

Most physical theories, from Isaac Newtons laws of motion to quantum mechanics, skirt such questions. In these theories, time is an independent variable against which other things change, but which cant be changed by anything else. In that sense, time exists outside physics, like the beat of a metronome outside the universe to which everything inside it plays out.

Albert Einsteins theories of relativity, developed in the early 20th century, threw such ethereal notions over a barrel. In relativity, time is a physical, dynamic thing, fused with space to form space-time the fabric of the universe itself. And space-time isnt absolute, but relative, warped by motion and gravity. If you travel fast, or if you are in a strong gravitational field, it slows down.

The relativity of time has wide-ranging consequences. Because there is no unique way of defining its passage, there is no unique way of defining now. Einstein concluded that all nows past, present and future must exist simultaneously, a picture known as the block universe that is completely at odds with our intuitions.

That mismatch occurs because, in our universe, the speed of light is finite. We can only reach certain times within a certain, well, time, so we can never achieve that God-like block-universe view. In practice, causality limits what we can perceive in a very strict way, and our experience and anything that affects us is limited strongly by causality, says cosmologist Katie Mack at North Carolina State University.

The mysteries dont stop there. By making time part of the physical fabric of a universe that, as far as we can tell, began in a big bang some 13.8 billion years ago, Einsteins theory of general relativity implies that time itself had a beginning and perhaps an end, too. There can be no eternal metronome ticking outside the universe as quantum theory implies, because such a source would have to exist outside space and time itself. This sets up a currently unbridgeable divide between relativity and quantum theory. In attempting to cross it, researchers such as Eichhorn hope to make progress towards a more unified picture of physics one that would have to have a very different conception of time.

Many quantum gravity theories propose that if you could zoom in very close to the fabric of Einsteins space-time, to a fine-grained level known as the Planck scale, you would discover a substructure a kind of quantum pixelation. That would open up entirely new possibilities. It may very well be that the quantum structure of space and time is different in the presence of matter than it is if youre just thinking of sort of a universe which contains just space and time, says Eichhorn.

Not everyone thinks we need to go that far. Some see an avenue to finding the nature of time in a better understanding of quantum theory. Or perhaps time is itself a mirage. Like the colour or pattern of a tree leaf, time might be something of no significance, says Mack, the passage of which we invent to make sense of local patterns around us and our own lives.

After all, we never measure time itself, but rather regular changes be it the passage of the seasons, the swing of a pendulum or the oscillation of a caesium atom that we reverse-engineer into some mysterious thing we call time. Its something that we see, and that appears to be there, says Mack. It may not matter to the cosmos.

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How can we better hold environmental polluters accountable? How can we enhance the efficiency of qubits?

These questions, which loom large for the researchers who study them, are the type of big-issue topics UC Santa Barbara graduate students are encouraged to tackle. And theyre the central themes of the dissertations that won the 2021-2022 Winifred and Louis Lancaster Dissertation Awards.

This years recipients are Emily Williams and Mark Turiansky, selected by the awards committee for dissertations with significant impact on the field in terms of methodological and substantive contributions.

As global temperatures rise and communities feel the effects of climate change, how do we as a global society address the uneven distribution of harms and gains?

The tropics, for instance, are already bearing the brunt of sea level rise and ocean acidification, yet they are not the places that have generated the magnitude of carbon emissions that cause these events, nor do they benefit in a proportionate way from the activities that cause these emissions.

Elsewhere around the world, weather events of disastrous proportions are increasing in severity and frequency, clearly caused by anthropogenic activity, yet who exactly do we hold accountable?

Inequalities and blind spots such as these are the type of thing that spark Emily Williams curiosity and activist drive. A long-time environmentalist, she got her first taste of the discipline of environmental studies as an undergraduate at UCSB under the tutelage of the late Professor William Freudenburg.

He opened my eyes to thinking about the causes of climate change, Williams said. She became conscious of the strategies corporations use to justify their actions and their methods of deflection from their outsized contribution to the problem.

Around that time, Typhoon Haiyan, then the most powerful typhoon on record, struck the central Philippines, becoming a strong and real reminder of global warmings effects. But even more compelling for Williams who had become part of a civil delegation to the UN Framework Convention on Climate Change (the international climate negotiations space) was the maddening slowness to address these impacts.

Fast-forward several years, and Williams desire to illuminate the gaps in climate accountability resulted in her dissertation, Interrogating the science of climate accountability: Allocating responsibility for climate impacts within a frame of climate justice. In it, she builds a best practices conceptual framework to identify responsibility for climate impacts.

She then tests it using an empirical case study involving the drought in the greater Four Corners region and the Zuni people who live there.

I had the opportunity to work with very diverse mentors, meaning I got to do the attribution science, engage ethnographic methods, organizational sociology and some science and technology studies-related work, she said. Its certainly hard to do interdisciplinary work, but if you find a group of mentors that will support you in this effort, its fascinating.

Among the things she uncovered in her research is the meteorological concept of vapor pressure deficit and its role on droughts, as a result of increased temperatures.

By linking this fundamental principle to vegetation, Williams and her co-authors were able to estimate what the Four Corners region would look like without climate change, and identify the human fingerprint in this whodunit of global warming.

This ability to definitively attribute effects to human activity can help build a case toward holding polluters accountable, advancing the field of climate justice. Its also what earned Williams the Lancaster Award.

Emilys outstanding integration of theory with qualitative and quantitative methods and her passionate commitment to climate justice truly set her apart, said her adviser, geography professor David Lpez-Carr.

Her dissertation makes a significant contribution to the nascent climate accountability literature by being the first to identify the human contribution to regional climate change and to follow those climate change impacts on vulnerable populations at the local level," Lpez-Carr said.

Her work provides a framework for future researchers and practitioners to advance the important area of climate accountability, with real-world implications for holding those responsible for climate change emissions and for mitigating impacts on vulnerable populations, he said.

I feel so honored and so humbled to have received this award, said Williams, who plans to complete a short post-doc before moving into the nonprofit world for more advocacy work. I know for certain that anyone who gets through a Ph.D. program, with all the challenges and opportunities the program presents, deserves such an award.

"I chose my dissertation topic because I believe so deeply in the importance of ensuring climate accountability work is done within principles of justice. I am just so happy that the selection committee thinks this topic is important, too.

The quantum world holds much potential for those who learn to wield it. This space of subatomic particles and their behaviors, interactions and emergent properties can open the door to new materials and technologies with capabilities we have yet to even dream of.

Mark Turiansky is among those at the forefront of this discipline at UCSB, joining some of the finest minds in the quantum sciences as a fellow at the NSF-supported UCSB Quantum Foundry.

The field of quantum information science is rapidly developing and has garnered a ton of interest, said Turiansky, who developed an abiding interest in physics as a child. In the past few years, billions of dollars of funding have been allocated to quantum information science.

Enabled by relatively recent technologies that allow for the study of the universeat its smallest scales, quantum researchers like Turiansky are still just scratching the surface as they work to nail down the fundamentals of the strange yet powerful reality that is quantum physics.

At the heart of some of these investigations is the quantum defect imperfections in a semiconductor crystal that can be harnessed for quantum information science.

One common example is the nitrogen-vacancy center in a diamond: In an otherwise uniform crystalline carbon lattice, an NV center is a defect wherein one carbon atom is replaced with a nitrogen atom, and an adjacent spot in the lattice is vacant. These defects can be used for sensing, quantum networking and long-range entanglement.

The NV center is only one such type of quantum defect, and though well-studied, has its limitations. For Turiansky, this underlined the need to gain a better understanding of quantum defects and to find ways to predict and possibly generate more ideal defects.

These needs became the basis of his dissertation, Quantum Defects from First Principles, an investigation into the fundamental concepts of quantum defects, which could lead to the design of a more robust qubit the basic unit of a quantum computer.

To explore his subject, Turiansky turned his attentions to hexagonal boron nitride.

Hexagonal boron nitride is an interesting material because it is two-dimensional, which means that you can isolate a plane of the material that is just one atom thick, he said. By shining light on this material, it is possible to detect quantum defects called single-photon emitters by the bright spots that shine back. These single photons, he said, are inherently quantum objects that can be used for quantum information science.

The main feat was identifying the defect that was responsible for single-photon emission, Turiansky said. He accomplished it with computational methodologies that he worked to develop in his research.

One methodology that Ive worked on a lot is for nonradiative recombination, he said, describing it in his paper as fundamental to the understanding of quantum defects, dictating the efficiency and operation of a given qubit.

By applying his methodology, Turiansky was able to determine the origin of these single photon emitters a topic of much debate in the community. Its a feat that could be applied to examine other quantum defects, and one that was deemed worthy of the Lancaster Award.

Marks work has moved the field forward by systematically identifying promising quantum defects, and providing an unambiguous identification of the microscopic nature of the most promising quantum emitter in hexagonal boron nitride, said Turianskys adviser, materials professor Chris Van de Walle. He accomplished this by creatively applying the computational approaches he developed and fruitfully collaborating with experimentalists.

Its really an exceptional honor to receive such a prestigious award for my research efforts over the last five years, Turiansky said. Its even more meaningful knowing the high quality of research turned out at UCSB and the fierce competition of my peers.

"Im incredibly grateful to my adviser, group members, collaborators, friends and family who helped make this achievement possible.

The two Lancaster dissertations are enteres into a national competition sponsored by the Council of Graduate Schools. A check for $1,000 and a plaque will be awarded upon completion of entry for the national competition.

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Emily Williams, Mark Turiansky Win 2021-22 Winifred and Louis Lancaster Dissertation Awards - Noozhawk