Surprising communication between atoms could improve quantum computing

A dark room with pink-hued lasers reflecting off of mirrors
In their experiments, UW–Madison physicists led by Deniz Yavuz immobilized a group of rubidium atoms by laser-cooling them to just slightly above absolute zero. Then, they shined a laser at rubidium’s excitation wavelength to energize electrons. | Photo provided by Yuvuz Lab

A group of University of Wisconsin­–Madison physicists has identified conditions under which relatively distant atoms communicate with each other in ways that had previously only been seen in atoms closer together — a development that could have applications to quantum computing.

The physicists’ findings, published Oct. 14 in the journal Physical Review A, open up new prospects for generating entangled atoms, the term given to atoms that share information at large distances, which are important for quantum communications and the development of quantum computers.

“Building a quantum computer is very tough, so one approach is that you build smaller modules that can talk to each other,” says Deniz Yavuz, a UW–Madison physics professor and senior author of the study. “This effect we’re seeing could be used to increase the communication between these modules.”

The scenario at hand depends on the interplay between light and the electrons that orbit atoms. An electron that has been hit with a photon of light can be excited to a higher energy state. But electrons loathe excess energy, so they quickly shed it by emitting a photon in a process known as decay. The photons atoms release have less energy than the ones that boosted the electron up — the same phenomenon that causes some chemicals to fluoresce, or some jellyfish to have a green-glowing ring.

“Now, the problem gets very interesting if you have more than one atom,” says Yavuz. “The presence of other atoms modifies the decay of each atom; they talk to each other.”

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Two WQI students named to QISE-NET’s Fall 2020 cohort

split screen profile photo of Vincent Liu and Cecilia Vollbrecht

Two WQI graduate students, Chuanhong (Vincent) Liu (McDermott Group) and Cecilia Vollbrecht (Goldsmith Group), have had their projects awarded funding through QISE-NET, the Quantum Information Science and Engineering Network. Run through the University of Chicago, QISE-NET is open to any student pursuing an advanced degree in any field of quantum science. Liu, Vollbrecht, and other students in their cohort earn up to three years of support, including funding, mentoring and training at annual workshops. All awardees are paired with a mentoring QISE company or national lab, at which they will complete part of their projects. Liu and Vollbrecht explain their projects below.

Profile photo of Vincent Liu
Chuanhong (Vincent) Liu
Chuanhong (Vincent) Liu | McDermott Group | Mentoring partner: NIST

“The Single Flux Quantum (SFQ) digital logic family has been proposed as a scalable approach for the control of next-generation multiqubit arrays. With NIST’s strong track record in the field of SFQ digital logic and the expertise of McDermott’s lab in the superconducting qubit area, we expect to achieve high fidelity SFQ-based qubit control. The successful completion of this research program will represent a major step forward in the development of a scalable quantum-classical interface, a critical component of a fully error-corrected fault-tolerant quantum computer.”

profile photo of Cecilia Vollbrecht
Cecilia Vollbrecht
Cecilia Vollbrecht | Goldsmith Group | Mentoring Partner: NIST

“The goal of my proposal is to develop a coupled cavity array that will allow us to simulate complex quantum phenomena. With the partnership between NIST and Prof. Goldsmith’s group I can combine the expertise of both groups to create an array where we characterize energy transfer and loss pathways, couplings, and coherence. The knowledge gained from these experiments will help to make a highly controlled cavity quantum electrodynamics platform.”

Q-NEXT collaboration awarded National Quantum Initiative funding

The University of Wisconsin–Madison solidified its standing as a leader in the field of quantum information science when the U.S. Department of Energy (DOE) and the White House announced the Q-NEXT collaboration as a funded Quantum Information Science Research Center through the National Quantum Initiative Act. The five-year, $115 million collaboration was one of five Centers announced today.

Q-NEXT, a next-generation quantum science and engineering collaboration led by the DOE’s Argonne National Laboratory, brings together nearly 100 world-class researchers from three national laboratories, 10 universities including UW–Madison, and 10 leading U.S. technology companies to develop the science and technology to control and distribute quantum information.

“The main goals for Q-NEXT are first to deliver quantum interconnects — to find ways to quantum mechanically connect distant objects,” says Mark Eriksson, the John Bardeen Professor of Physics at UW–Madison and a Q-NEXT thrust lead. “And next, to establish a national resource to both develop and provide pristine materials for quantum science and technology.”

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New study expands types of physics, engineering problems that can be solved by quantum computers

cartoon image of a mirror machine

A well-known quantum algorithm that is useful in studying and solving problems in quantum physics can be applied to problems in classical physics, according to a new study in the journal Physical Review A from University of Wisconsin–Madison assistant professor of physics Jeff Parker.

Quantum algorithms – a set of calculations that are run on a quantum computer as opposed to a classical computer – used for solving problems in physics have mainly focused on questions in quantum physics. The new applications include a range of problems common to physics and engineering, and expands on the types of questions that can be asked in those fields.

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First cohort of students dives into new quantum computing master’s degree

a group of students in a physics lab being shown a quantum computer

The inaugural MS in Physics–Quantum Computing is the first program of its kind in the U.S. It addresses an emerging workforce need by preparing students to enter this rapidly growing and highly complex field. Most of the students will complete their degrees August. We checked in with them after their first semester to see how their studies were going.

Interested in earning a Master’s in Quantum Computing? Visit go.wisc.edu/MSPQC for more info.

Chicago Quantum Summit to gather international experts

abstract graphic of atoms

Top experts in quantum technology from around the globe — including experts from the Wisconsin Quantum Institute — will gather at the University of Chicago on Oct. 25 to discuss the future of quantum information science and strategies to build a quantum workforce.

The second annual Chicago Quantum Summit, hosted by the Chicago Quantum Exchange, will engage scientific and government leaders and the industries that will drive the applications of emerging quantum information science.

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WQI scientists earn grant to improve materials for quantum sensing, computing

four panels of images showing quantum sensors and qubits used in the lab and to develop quantum computers

Researchers at the Wisconsin Quantum Institute (WQI) have been awarded a US Department of Energy grant to study the noise that hampers advances in quantum systems, including quantum computers.

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 grayscale image of a scanning electron micrograph of one of the double quantum dot qubits
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.

small white dots, representing single atom defects, are visible in a dark purplish background that is from a diamond
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.”

scanning tunneling micrograph showing graphene with a single atomic defect as a white dot in the center.
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.

Other WQI members who are co-investigators on the grant include Jennifer Choy, Laura Faoro, Mark Friesen, and Alex Levchenko.

Effects of charge noise on a pulse-gated singlet-triplet qubit

schematic of a quantum dot qubit and the available electron paths

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.

 

Read the full article at: https://arxiv.org/abs/1701.06971