Aedan Gardill, pictured left, and Megan Tabbutt, both second-year students in the Kolkowitz group, will receive up to three years of funding to pursue their research projects. Below, Tabbutt and Gardill summarize their research projects.
Megan Tabbutt
Optical atomic clocks are now the most precise time keepers in the world, keeping time to better than one second over the age of the universe. With support from the NDSEG, I will work with my collaborators to build a new “multiplexed” strontium optical lattice atomic clock, which will consist of two clocks in one vacuum vessel. We will use this new kind of clock to perform tests of Einstein’s theory of relativity, such as measuring the relativistic effects of gravity on the passage of time at the millimeter scale, which may one day have applications ranging from the prediction of volcanic eruptions to water resource management and flood prevention. We will also engineer strong interactions between the atoms that make up the clock to generate entangled states for quantum enhanced clock performance, among other pursuits.
Aedan Gardill
Superconducting qubits are a promising system for quantum computing, but external sources of “noise” currently limit their usefulness. A better understanding of the sources of this noise in the qubits should help advance quantum computing efforts. With the NDSEG fellowship, my research will focus on using nitrogen vacancy centers in diamond as sensors with nanometer-scale resolution. We will develop and apply novel sensing techniques to study interesting solid state systems, such as investigating the origins of noise that currently limit superconducting qubit performance.
Progress in neutral atom quantum computing with a 2D array of qubits
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WQI Director Mark Saffman’s group recently published their manuscript, “Rydberg mediated entanglement in a two-dimensional neutral atom qubit array,” on arXiv.
The three-year, $4 million funding will allow the researchers to apply emerging tools to identify new materials and fabrication methods that can improve the performance of these systems.
“All of physics is quantum on some level, and quantum systems let you understand how physics works when you get to the cleanest, smallest, most isolated systems,” says Shimon Kolkowitz, assistant professor of physics at the University of Wisconsin–Madison and lead investigator of the grant. “We think that quantum computing, and quantum technologies more generally, are a really promising area of technological development and research.”
Quantum systems — which make use of single atoms or electrons and the quantum mechanical properties that govern them — have the potential to push boundaries in such areas as computing, precision sensing, and secure communications.
Quantum computers, for example, allow scientists to simulate quantum mechanics in ways that classical computers cannot. But, the computing power of quantum computers has not yet exceeded classical ones.
A scanning electron microscope image of a gate-defined double quantum dot qubit fabricated by the Eriksson group.
A limiting factor in quantum computing power is the number of qubits, or quantum bits, that can be strung together. Like bits in a classic computer, the more qubits in a quantum computer, the more the computing power. And the limiting factor in how many qubits can be connected with each other while remaining in the fragile quantum states required to perform a computation — called “coherence” in quantum lingo — is their resistance to external environmental factors, or “noise” that may cause them to “decohere.”
However, researchers have found that the materials used to make the qubits themselves generate a lot of this noise.
“People for quite a while have seen this noise, treated it as a fact of nature, and tried to design around it. But no one really knows what it is or how to get rid of it,” Kolkowitz says. “Even more fundamentally than just understanding or reducing this noise, we think that if you can reduce or ultimately eliminate this noise, it actually opens up the design space for the kinds of qubits you can build, and that will make it much easier to wire qubits together.”
With the DOE funding, Kolkowitz, along with colleagues at WQI and the Livermore National Laboratory, seeks to first identify the nature of the noise and how specific materials contribute to it, and then to develop ways to reduce it.
Atom-size quantum defects in a diamond, imaged using a confocal microscope in the Kolkowitz lab.
Work in Kolkowitz’s group, as well as that of Victor Brar, assistant professor of physics and co-investigator on the grant, has led to the development of quantum sensors that allow the researchers to characterize things like magnetic fields at the nanometer scale, or to see how single atoms are arranged in various materials. Part of the DOE funding will be used to continue improving these sensors.
Kolkowitz and Brar then want to use their sensors to identify the noise affecting qubits designed by UW quantum computing researchers Mark Eriksson and Robert McDermott.
“And then we can work in a feedback loop, where, for example, Robert McDermott makes samples and characterizes their performance, then we study the noise limiting that performance with these quantum and nanoscale probes to figure out what’s happening on the microscopic scale,” Kolkowitz says. “Then, we give that information to our theory collaborators here and at Livermore who build models and simulations based on what we’ve measured. And then Robert can use what we’ve learned to design and make new samples to see if we’ve improved on these issues.”
An image taken with a scanning tunneling microscope of a single atomic defect in graphene
Trying to identify sources of this noise is nothing new, but what Kolkowitz finds most promising about the work funded through this grant is the development and application of new sensing technologies.
“These emerging tools that use quantum states and quantum systems themselves should give us access to the origins and behavior of noise in quantum platforms on scales that haven’t been accessible before,” Kolkowitz says.
Not so defective, after all: Demystifying advanced quantum materials
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Sometimes, flaws are what makes a thing special.
That’s the case for a type of material called optical quantum emitters, which send out light in an exceptionally precise manner, one photon at a time, often due to tiny imperfections in a crystal’s structure.
The ability to emit light one photon at a time could allow optical quantum emitters to become the backbones of ultrafast computers, super high-resolution sensors and uncrackable long-range secure communication technologies.
Recently, buzz has been building about a newly discovered variety of quantum emitters consisting of two-dimensional materials (think flat sheets only as thick as a single molecule, similar to graphene). But there’s a hitch: No one truly understands the exact natures of the tiny flaws, called defects, that cause these two-di materials to become optical quantum emitters. And that’s been a major obstacle in obtaining these potentially useful materials.
Measurement of a superconducting qubit with a microwave photon counter
The superconducting qubit group at WQI introduced an approach to measurement based on a microwave photon counter demonstrating raw single-shot measurement fidelity of 92% [Science, 361, 1239 (2018)]. This scheme provides access to the classical outcome of projective quantum measurement at the millikelvin stage and could form the basis for a scalable quantum-to-classical interface, see this article for broader perspective on this technology.
Effects of charge noise on a pulse-gated singlet-triplet qubit
We study the dynamics of a pulse-gated semiconductor double quantum dot qubit. In our experiments, the qubit coherence times are relatively long, but the visibility of the quantum oscillations is low. We show that these observations are consistent with a theory that incorporates decoherence arising from charge noise that gives rise to detuning fluctuations of the double dot.
Quasiparticle poisoning of superconducting microwave resonators
Nonequilibrium quasiparticles represent a significant source of decoherence in superconducting quantum circuits. Here we investigate the mechanism of quasiparticle poisoning in devices subjected to local quasiparticle injection.
