Quantum sensors

Quantum sensors refer to a technical field that uses microscopic particles such as electrons to efficiently detect small magnetic fields with high spatial resolution. In this article, I would like to write about the research on quantum sensors that I was conducting when I was a Ph. D. student at the University of Oxford.

Around 2010, quantum sensors were not widely popular in Japan; but a boom in this field was emerging in the United States and Europe. Experiments utilizing electron spins in diamonds enabled highly precise measurements, sparking both academic interest and practical application prospects. At the time, I was at Oxford University, where my supervisors, Simon Benjamin and Joe Fitzsimons, also sensed the advent of the quantum sensor era and started theoretical research on quantum sensors in our laboratory. In particular, Simon and Joe were theoretically investigating the possibility of further improving the sensitivity of quantum sensors by using a special property called quantum entanglement. I participated in these discussions and explored research themes related to quantum sensors.

Quantum sensors using quantum entanglement

As I progressed with literature research on quantum sensors involving quantum entanglement, I discovered an unexpected fact. In 1997, Susana and her colleagues had already published a theoretical paper on quantum sensors using quantum entanglement, which contained extremely pessimistic results. Their conclusions were that quantum sensors using quantum entanglement (1) have improved sensitivity in ideal situations where there is no noise, and (2) do not improve sensitivity in situations where noise is present. In other words, in a realistic situation, the use of quantum entanglement cannot be expected to improve sensitivity. Of course, it was believed that quantum sensors could produce a sensitivity sufficient for practical use even without the use of quantum entanglement. Accordingly, it is unlikely that the existence of the research paper by Susanna will diminish the field of quantum sensors. However, many researchers started to consider that the study of sensitivity enhancement using quantum entanglement was impractical.

Progress in research through internship at NTT

An unexpected turning point came when I had the opportunity to intern at the NTT Basic Research Laboratories while studying at Oxford. For about one month, I engaged in research in a group conducting quantum device experiments at NTT. This experience taught me that many quantum devices were affected by what is called non-Markovian noise. Although details are omitted here, "non-Markovian" noise means that the environment causing the noise retains memory of the past. However, the theoretical analysis conducted by Susana et al. in 1997 assumed a different type of noise, called Markovian noise. In other words, Susana et al. performed their calculations using a model different from the noise that actually exists. After returning to Oxford, I adopted a model of "non-Markovian" noise and theoretically investigated whether quantum entanglement could improve the performance of quantum sensors. Astonishingly, I demonstrated that under the influence of non-Markovian noise, the performance of quantum sensors significantly improves with quantum entanglement! Convinced that this was a groundbreaking result, I wrote the manuscript with the help of Simon and Joe, uploaded it to arXiv, an internet site, in 2011 to share it with the world, and submitted it to an academic journal.

Papers published around the same time

Here, an unexpected twist of fate occurred. About two months after we uploaded our manuscript online (arXiv), a paper with almost the same content was uploaded on the same site. The paper, which was written by Alex et al. (one of the co-authors was Susana, who wrote^[1]), also showed that the sensitivity of quantum sensors could be improved by using quantum entanglement under the influence of non-Markovian noise. There are two intriguing points about this episode. One point is that from 1997 to 2011 (until I published my paper), no researchers had investigated quantum sensors using quantum entanglement under the influence of non-Markovian noise. The other is that in the short period of two months in 2011, two independent groups simultaneously uploaded papers on this theme on the internet. In a way, I was lucky. If I had procrastinated and published my paper two months later, I wouldn't have been the first to make this discovery.

Fortunately, my paper was published in a journal called Physical Review A in 2011^[2], and Alex's paper was published in Physical Review Letters in 2012^[3]. These papers had a significant impact, leading to numerous subsequent papers. Experimental demonstrations have also been conducted in recent years^[4].

Conclusion

Lastly, I would like to share my reflections. Writing this paper was made possible by two key points. First, I had the opportunity to study at Oxford University, where I was part of a research group working on quantum sensors, a field that was scarcely popular in Japan at the time. Second, I did not limit myself to Oxford University but participated in an internship at NTT, where I conducted research in a different setting and learned about the nature of noise in actual quantum devices. These experiences highlight that groundbreaking ideas in research can emerge from working in various places and encountering different people. To those readers aspiring to become researchers, I encourage you to engage with diverse people in many different settings.

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[1] S. F. Huelga, et al., "Improvement of frequency standards with quantum entanglement." Physical Review Letters 79.20 (1997): 3865.
[2] Y. Matsuzaki, et al., "Magnetic field sensing beyond the standard quantum limit under the effect of decoherence." Physical Review A 84.1 (2011): 012103.
[3] A. W. Chin, S. F. Huelga, et al., "Quantum metrology in non-Markovian environments." Physical Review Letters 109.23 (2012): 233601.
[4] X. Long, et al., "Entanglement-enhanced quantum metrology in colored noise by quantum Zeno effect." Physical Review Letters 129.7 (2022): 070502.

Yuichiro Matsuzaki／Associate Professor, Faculty of Science and Engineering, Chuo University
Areas of Specialization: Quantum Information Theory and Quantum Metrology

His field of expertise is quantum information theory.

His career background is as follows:
April 2001 to March 2005: He earned a bachelor’s degree from the Department of Physics in the Faculty of Science and Engineering, Waseda University.
April 2005 to January 2008: He earned a Master’s Degree from the Department of Multidisciplinary Sciences in the Graduate School of Arts and Sciences, the University of Tokyo.
January 2008 to February 2011: He earned a Ph.D. from the Department of Materials, the University of Oxford.
February 2011 to March 2011: He was a post-doctorate fellow at Aalto University.
April 2017 to March 2018: He served as a Visiting Associate Professor in the Institute for Chemical Research, Kyoto University.
April 2011 to December 2018: He was a full-time employee in the NTT Basic Research Laboratories.
January 2019 to March 2023: He was a principal researcher in the National Institute of Advanced Industrial Science and Technology.
April 2023 until present: He serves as Associate Professor in the Faculty of Science and Engineering, Chuo University.

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