Quantum Computing

New Ways to Count Photons Could Lead to Better Quantum Computing

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Insider Summary

  • Researchers have developed a photon number solving system to accurately resolve more than 100 photons.
  • The team reports that this is a step forward in the capabilities of the development effort of quantum computing, which enables the generation of truly quantum random numbers.
  • Critical Quote: “They were able to estimate that they resolved 100 photons impacting this detector with very accurate resolution.” — Robert Edwards, senior staff scientist and deputy associate director for theoretical and computational physics at Jefferson Lab

PRESS RELEASE — Experts in nuclear physics and quantum information have demonstrated applying a photon number solving system to accurately resolve to more than 100 photons. The feat represents a major step forward in capabilities for the quantum computing development effort. It also enables the generation of truly quantum random numbers, a long-sought goal for developing unbreakable encryption techniques for applications in, for example, military communications and financial transactions. The detector was recently reported in Nature Photonics.

Physicists around the world are passionately pursuing the promise of reliable and powerful quantum computing. Harnessing quantum computing will not only be a giant leap forward for science, it will boost the economy and enhance national security. But getting there has eluded the best brains on the planet so far.

A pair of engineers at the US Department of Energy’s Thomas Jefferson National Accelerator Facility have designed a critical part of a photon detection system that has brought physicists one step closer to fully operational photonic-based quantum computing — that is, a quantum computer built entirely with light. The engineers are part of an interdisciplinary team of federal and academic researchers led by the Jefferson Lab working to advance quantum computing in nuclear physics.

There are many different ways to try to build a fully functional quantum computer. For photonic-based computing, the detection of quantum particles of light, or photons, is critical. Currently, each detector can resolve up to about 10 photons, but that number is too small for many methods of quantum state generation. No one has yet demonstrated detection of more than 16 photons, but simulations suggest that quantum computing will require detecting large numbers of photons — 50 or more.

Crossing the 50 photon threshold means being able to implement “cubic gates” — a milestone towards building a complete set of gates for universal quantum computing, explains Amr Hossameldin, team member and graduate research assistant in quantum computing and quantum optics at the University of Virginia.

The team broke the record by 16 photons and demonstrated the number of photons was around 35 per single detector and up to 100 with a three-detector system.

“They were able to estimate that they resolved 100 photons impacting the detector with this very accurate resolution,” said Robert Edwards, senior staff scientist and deputy associate director for theoretical and computational physics at Jefferson Lab. “It’s incredibly accurate — and it’s never been achieved.

“Lack of detection has been a major limitation to this approach to quantum computing. The new photon number resolution is a necessary step towards implementing a universal instruction set.”

The new detection system also has another very valuable secondary benefit: generation of truly random quantum numbers — a boon for unbreakable secret codes or encryption in fields such as military communications and financial transactions.

The so-called random numbers generated by classical computer algorithms are not truly random. The algorithm by which they are generated can be compromised with some effort by playing the numbers game — looking for which numbers appear more frequently than others. The generation of true random numbers using quantum physics has no such drawbacks or biases.

“There is intrinsic randomness in quantum mechanics, where you can have a physical system that is in two states at once,” explains Olivier Pfister, a professor of physics at UVa who specializes in quantum and quantum information and served as the external team lead for the project. “And when you want to know which is which, it’s random.

“Einstein was bugged by this. He called it ‘The Old One playing dice with the universe.’ And we don’t know any better than Einstein.”

Pfister and Hossameldin are co-authors of the paper presenting the team’s research in the peer-reviewed journal Nature Photonics. The other authors are Chris Cuevas and Hai Dong of the Jefferson Lab, Richard Birrittella and Paul Alsing of the Air Force Research Laboratory in New York, Miller Eaton of UVa, and Christopher C. Gerry of City University of New York.

Signal not seen before

The team’s efforts were inspired by an announcement in 2019 by the DOE Office of Science offering funding opportunities for quantum information science research for nuclear physics under the Quantum Horizons program. Edwards secured a small grant to fund a series of lectures featuring experts in quantum computing.

Pfister was the first lecturer in March 2020. A week later, the COVID-19 pandemic shut down the lab, but the seeds for co-research with photonic-based quantum computing have been planted.

A large team of physicists, engineers and postdoctorals was formed. Funding was obtained through the Laboratory Directed Research & Development (LDRD) program. The LDRD program supports projects at the forefront of science and technology. The collaboration was initiated with the goal of using quantum photonics for computations relevant to the Jefferson Lab science program.

UVa already had a photon-based system for making quantum calculations using pulsed lasers, but lacked the means to detect with high speed and accuracy the number of photons impacting its detector before the signal decayed.

Meanwhile, detecting particles with speed and accuracy is Jefferson Lab’s forte. The Continuous Electron Beam Accelerator Facility, or CEBAF, has been used for decades in experiments that rely on ultra-sensitive detectors to measure fleeting cascades of subatomic particles that are created when the particle beam strikes a target at nearly the speed of light. CEBAF is a user facility of the DOE Office of Science that more than 1,850 nuclear physicists access for their research.

In the team’s experiment at Pfister’s Quantum Optics Lab at UVa, Hossameldin connected three superconducting edge transition sensor (TES) devices to create a single detector, with each TES device capable of seeing 35 photons, and placing them in front of the laser and turned around. on the beam.

A high-speed digitizer designed and developed by Dong at Jefferson Lab is an essential part of the detector’s electronics.

“The original TES digitizer lacked the high-speed capabilities that came with our design,” said Cuevas. “Our digitizer has 12-bit accuracy with a sampling time of 4 seconds, so this allows us to capture signals from TES that have never been seen before.”

Quantum computing research is advancing at an exponential rate, and Cuevas predicts new technologies will soon replace their systems. But larger collaborations to build light-based quantum computers are continuing.

“This project is an excellent example of where design can be reused and applied to completely different scientific applications,” said Cuevas. “Sharing technology is a core foundation for the scientific community and, as electronics engineers, it’s exciting to know that our designs can advance important discoveries.”

Jefferson Lab — By Tamara Dietrich

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