*In this series, researchers answer three questions about their latest results in the fields of quantum, AI, and data science. Editor’s note: Responses may be edited for length or clarity.*

**Summary**

Researchers at the University of Virginia, in collaboration with team members from Thomas Jefferson National Accelerator Facility (Jefferson Lab), the Air Force Research Laboratory, and the City University of New York, developed a method using transition-edge sensors (TES) to detect larger numbers of photons than the current standard for quantum systems—at the mesoscopic rather than microscopic scale. Resolving large photon numbers is crucial in key quantum applications, such as computation, sensing, and cryptography. In a *Nature Photonics *paper, the team demonstrates its detection scheme can resolve 0 to 100 photons. They also demonstrate an application of this technology: a quantum random-number generator, which is relevant to cryptography.

**Citation**

Eaton, M., Hossameldin, A., Birrittella, R.J. *et al.* (2023) “Resolution of 100 photons and quantum generation of unbiased random numbers.” *Nature Photonics ***17**, 106–111. DOI: 10.1038/s41566-022-01105-9

**Q&A with co-author Amr Hossameldin of the University of Virginia **

*(1) What was the problem you set out to solve? *

We wanted to push the limit of what photon number resolving measurements can do—beyond what was previously possible.

We were able to resolve up to 100 photons. The previous record was 16 photons [1]. The ability to resolve photons is highly desirable for a variety of quantum information applications, including computation, sensing and cryptography. In the paper, we demonstrate the utility of this new improvement in detection capability by implementing a quantum random number generator with no inherent bias. We show how it’s robust against environmental noise, phase and amplitude fluctuations in the laser, loss, detector inefficiency, and eavesdropping.

*(2) What was the solution, and how did you reach it? *** **

We sent laser pulses through a system made of three multiplexed, highly quantum efficient, superconducting TES detection channels, each capable of resolving a maximum of approximately 37 photons. We then used a field-programmable gate array designed and fabricated by Jefferson Lab to process the output signal in real time and resolve photons from 0 to 100.

When sampling photons from a laser beam, the parity (even/odd character) of the photon count is an increasingly perfect quantum coin toss with the photon number. We used this to generate truly random numbers (as opposed to usual pseudo random numbers) [2].

*(3) How will your results impact the field going forward?*

These results open avenues in quantum sensing, such as reaching the Heisenberg limit [*a limit to measurement accuracy in quantum mechanics*] with large photon-number parity detection [3] and can enable photonic quantum computation by generating cubic field gates—key to universal continuous-variable quantum computation and to quantum simulation of hard-to-calculate physical systems. In the preparation of a cubic phase, one must detect a large number of photons (simulations suggest 50 or more) [4]. The detection scheme we demonstrate here now easily surpasses this previously unreachable milestone.

**References**

[1] Morais, L. A. et al. Precisely determining photon-number in real-time. Preprint at https://arxiv.org/abs/2012.10158 (2020).

[2] Gerry, C. C. et al. Proposal for a quantum random number generator using coherent light and a non-classical observable. J. Opt. Soc. Am. B 39, 1068–1074 (2022).

[3] Gerry, C. C. Heisenberg-limit interferometry with four-wave mixers operating in a nonlinear regime. Phys. Rev. A 61, 043811 (2000).

[4] Ghose, S. & Sanders, B. C. Non-Gaussian ancilla states for continuous variable quantum computation via Gaussian maps. J. Mod. Opt. 54, 855–869 (2007).