Friday, November 02, 2018

Theoretical Way to Overcome Photon Shot Noise Limits

Nature publishes a fairly theoretical paper by Hoi-Kwan Lau & Aashish A. Clerk from University of Chicago "Fundamental limits and non-reciprocal approaches in non-Hermitian quantum sensing." The paper points to a way to overcome what is considered to be a fundamental limit - photon shot noise. The authors also plan to attack thermal noise in their future research.

"In a quantum setting, optical sensors are typically limited because light is made up of particles, and this discreteness leads to unavoidable noise. But this study revealed an unexpected method to combat that limitation... We think we’ve uncovered a new strategy for building extremely powerful quantum sensors."

Unfortunately, this revolution will not happen overnight. Everything in this paper is highly theoretical. A sensing system nonreciprocity is said to be the key factor in increasing the signal while keeping the same noise:

In conclusion, the authors say: "We... discussed a new method for enhancing dispersive measurement using effective non-Hermitian physics, namely the use of nonreciprocity to enhance sensing. We show that nonreciprocity allows one to arbitrarily exceed the fundamental bound on the measurement rate of a reciprocal sensor, and discussed a simple implementation that does not require any amplification processes. We also show that nonreciprocity can enhance the sensitivity of mode-splitting type sensor.

Finally, we note that the general theory developed in this work could be easily applied to more general kinds of sensing problems. For example, the same formalism could be used to understand the performance of non-Hermitian sensors when thermal noise dominates (as would be the case for systems deep in the classical limit).



  1. In quantum optics people work since a long time on non-Poissonian ways of detecting light (light squeezing, bunching and anti-bunching etc). Problem is that the use case is very limited. You'd need to control emission, channel and reception. Useful e.g. for quantum encryption but I don't see the use case for "regular" imaging where e.g. scattering and diffusive reflection cause you to lose your phase relations.

  2. I think that the authors are using macroscopic theory to explain something that is fundamentally a quantum effect.

    1. They talk about quantum sensing, even in the title. The only place they talk about the classical physics approach is their future work on thermal noise.

    2. This is true but, without doing a detailed analysis of the work, seems that the treatment is like continuous waves, but the photons are discrete and cannot be treated as a continuous flow but when its number is very high. And in this case the shot noise is not too much significant.

      Maybe I'm wrong because I'm not an expert in the mathematical treatment used. However only see possible to overcome the shot noise if the photons or electrons aren't the behavior of single particles, like when are used using quantum entanglement. This is possible to do in laboratory conditions but not in practical sensors and in particular in image sensors.

    3. I fully agree with you. The very reason that I posted this article is that it goes against my and, probably, everybody else's common sense.


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