Optical phase estimation approaching the quantum limit in a single shotOptical phase estimation approaching the quantum limit in a single shot

Optical phase estimation approaching the quantum limit in a single shot

Optical phase estimation is at the center of many metrological tasks where the value of a physical parameter of a system is mapped to the phase of an electromagnetic field, and single-shot measurements of this phase retrieve the information of this parameter. In this article, we demonstrate optimized estimation strategies for single-shot measurements for the optical phase of coherent states, which achieve sensitivities surpassing the heterodyne limit and potentially approaching the quantum limit, the Cramer-Rao lower bound (CRLB). These strategies are based on optimized photon counting, coherent displacement operations, and fast feedback. Our demonstration uses fast processing for optimizing the single-shot measurement during the optical mode, and enables surpassing the heterodyne limit for a wide range of optical powers without correcting for detection efficiency of our system. This is, to our knowledge, the most sensitive single-shot measurement of an unknown phase encoded in optical coherent states

This research has been published in Physical Review Letters, and can be accessed in https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.120505

Optical phase estimation approaching the quantum limit in a single shot

(a) The input state and optimized local oscillator (LO) interfere on a beam splitter such that the input is displaced in phase space. The probability distribution for the unknown phase is then updated according to Bayes rule and new optimal values for the LO are applied. (b) Experimental data (points with error bars) shows that adaptive non-Gaussian measurements surpass the limits of ideal heterodyne detection (dashed red) and approach the fundamental sensitivity limit given by the CRLB for our detection efficiency of 70% (solid black)

Phase tracking for sub-shot-noise-limited receivers

Phase trackingNonconventional receivers for phase-coherent states based on non-Gaussian measurements such as photon counting surpass the sensitivity limits of shot-noise-limited coherent receivers, the quantum noise limit (QNL). These non-Gaussian receivers can have a significant impact in future coherent communication technologies.

However, random phase changes in realistic communication channels, such as optical fibers, present serious challenges for extracting the information encoded in coherent states.

While there are methods for correcting random phase noise with conventional heterodyne detection, phase tracking for non-Gaussian receivers surpassing the QNL is still an open problem.

Here we demonstrate phase tracking for non-Gaussian receivers to correct for time-varying phase noise while allowing for decoding beyond the QNL.
The phase-tracking method performs real-time parameter estimation and correction of phase drifts using the data from the non-Gaussian discrimination measurement, without relying on phase reference pilot fields.

This method enables non-Gaussian receivers to achieve higher sensitivities and rates of information transfer than ideal coherent receivers in realistic channels with time-varying phase noise.

This demonstration makes sub-QNL receivers a more robust, feasible, and practical quantum technology for classical and quantum communications.

See the article

Becerra Chavez publishes Low Power Light article in Nature Partner Journals

Elohim Becerra Chavez recently published Quantum measurements: surpassing conventional sensitivity limits at low powers in Nature Partner Journals

Light has intrinsic quantum noise, which limits how well we can measure it, especially at low powers, and bounds how much information we can communicate. A team led by F. Elohim Becerra at the University of New Mexico demonstrated optimized measurements for light pulses with different phases at low powers, such as those used in coherent optical communication. These optimized measurements can surpass the ultimate sensitivity limits of ideal conventional detectors, even in the presence of loss and noise encountered in realistic situations. The measurements are based on combining the input pulse with a reference field, and counting single photons in a fraction of the pulse. By analyzing the detection outcome, the reference field can be optimized to enhance the measurement’s sensitivity. Optimized measurements at low powers may lead to more-efficient optical communication in realistic environments.

The full article can be found online at npj Quantum Information 3, Article number: 43 (2017) .

Becerra’s group publishes article on robust measurements at low powers by counting photons

Dr. Elohim Becerra Chavez’s research group recently published Robust Measurement for the Discrimination of Binary Coherent States in Physical Review Letters.

Optical communication uses light to encode and transmit information over long distances, such as in optical fibers with losses, and requires reliable detection schemes to read out information from low levels of light. However, noise and imperfections in real-world devices and detectors severely affect the measurement fidelity and ultimately limits the amount of information that we can communicate. Recent work in the quantum-optics group led by Elohim Becerra at UNM demonstrated an optimized measurement capable of overcoming these imperfections by counting the number of photons in pulses of light carrying information in their optical phase. This measurement reads off the information contained in the light by first combining the light pulse with a reference pulse to compare their relative phase, and then counting the number of photons in the combined pulse. Previous work in ultra-sensitive measurements has focused instead on detecting no light vs. any amount of light in the combined pulse, and their sensitivities have been limited by noise and imperfections. This novel measurement scheme allows for a high degree of robustness to these imperfections while reaching high measurement sensitivities. Moreover, due to its simplicity, this measurement is inherently compatible with high-bandwidth communication technologies to accommodate the high rates of information transfer in today’s optical communication networks.

This work was published in the July 2 issue of Physical Review Letters:

Robust Measurement for the Discrimination of Binary Coherent States
M. T. DiMario and F. E. Becerra
Phys. Rev. Lett. 121, 023603 (2018)

The full article can be found online.