It is now well appreciated that quantum physics can be used to build better sensors. Such sensors can be based on unitary systems like various types of interferometers or open systems based on scattering and lossy transmission channels. I would highlight not only the importance of the quantum states used as probes, but also the importance of the quantum-ness of the measurement schemes. I would bring out the especial importance of the squeezed states of matter and light for sensing studies. I would illustrate the results with experiments.
It is now well appreciated that quantum physics can be used to build better sensors. Such sensors can be based on unitary systems like various types of interferometers or open systems based on scattering and lossy transmission channels. I would highlight not only the importance of the quantum states used as probes, but also the importance of the quantum-ness of the measurement schemes. I would bring out the especial importance of the squeezed states of matter and light for sensing studies. I would illustrate the results with experiments.
It is now well appreciated that quantum physics can be used to build better sensors. Such sensors can be based on unitary systems [1,2] like interferometers or open systems based on scattering and lossy transmission channels [3-5]. The framework of the quantum Fisher information enables one to obtain best estimates of the parameters and then one can design possible experiments that can reach Cramer- Rao bounds. I would bring out not only the importance of the quantum states used as probes, but also the importance of the ‘quantum’ measurement schemes especially the ones that depend on time reversed arrangements. I would illustrate the great usefulness of squeezed states of matter and light for metrology. [1] S. C. Burd et al., Quantum amplification of mechanical oscillator motion, Science 364, 1163 (2019). [2] G. S. Agarwal, and L. Davidovich, Quantifying quantum-amplified metrology via Fisher information, Phys. Rev. Res. 4, L 012014 (2022). [3] J. Wang, L. Davidovich, and G. S. Agarwal, Quantum sensing of open systems: Estimation of damping constants and temperature, Phys. Rev. Res. 2, 033389 (2020). [4] F. Li, T. Li, M. O. Scully, and G. S. Agarwal, Quantum advantage with seeded squeezed light for absorption measurement, Phys. Rev. Applied 15, 044030 (2021). [5] T.Li, F. Li, X. Liu, V. Yakovlev and G. S. Agarwal, Quantum-enhanced stimulated Brillouin scattering spectroscopy and imaging, OPTICA 9, 959 (2022).
An important part of optical sensing is interferometry, where direct or homodyne detection at the output of an interferometer yields information about small phase shifts. Classical interferometry operates at the shot-noise limit (SNL), caused by the photon structure of light and reached when an interferometer is fed with coherent light. The use of nonclassical light at the input of an interferometer enables overcoming the SNL. In particular, squeezed light is known to be less noisy than coherent light. An interferometer fed with strongly squeezed light can, in principle, provide the phase sensitivity at the Heisenberg limit, the ultimate limit allowed by the laws of physics. However, this supersensitive operation is strongly degraded by loss – both inside the interferometer and outside it, at the stage of detection. While the former (internal loss) seems to be inevitable, the latter (detection loss) can be overcome, and we demonstrate it in our recent experiment. The clue is to amplify light at the interferometer output using a phase-sensitive optical parametric amplifier. This device does not introduce additional noise, but it amplifies the fragile quantum signal and thus makes it tolerant to loss at the detection stage. In a proof-of-principle experiment, we build a polarization Mach-Zehnder interferometer and feed it with coherent light in one port and a squeezed vacuum in another port. At one output port, we amplify light with an optical parametric amplifier and measure a phase sensitivity of 13 mrad for 1500 photons in the interferometer. This result, achieved with 50% detection efficiency, overcomes the SNL by 3 dB. Moreover, by using stronger amplification, we can overcome the SNL under detection efficiency as low as 13%. Our results outline a way to use squeezed light for real-life sensing, where the detection efficiency cannot be very high, especially in spectral ranges like middle-infrared or terahertz.
It can be said that all of humanity's efforts can be understood as a problem of optimization. We each have a natural sense of what is ``good'' or ``bad'' and thus our actions tend towards maximizing -- or optimizing -- some notion of good and minimizing those things we perceive as bad or undesirable. Within the sciences, the greatest form of good is knowledge. It is this pursuit of knowledge that leads to not only life-saving innovations and technology, but also to furthering our understanding of our natural world and driving our philosophical pursuits. The principle method of obtaining knowledge in the sciences is by performing measurements; the simple act of comparing one attribute of a system to a known standard and recording the observed value is how all scientific progress is made. The act of performing measurements is in fact so important that there is an entire field of study surrounding it: metrology. One critical component of metrology is the development of new techniques to perform measurements, or alternative measurement schemes that are more optimal in some way. This is where there is room to exploit quantum physics to improve our techniques \--- we can perform quantum metrology. In quantum mechanics we routinely deal with the smallest, weakest, most delicate of systems. Quantum properties are inherently very sensitive to their environment; this of course makes them highly intolerant of noise but also makes them great resources to perform sensitive measurements. Quantum metrology concerns itself with utilizing quantum phenomena to extract more information from the natural world than is possible by conventional, or classical, means. To perform optimal measurements, these quantum systems must of course be optimal by some metric. Performing the ``optimal'' measurement requires several ingredients. First, we need the optimal tools or instrumentation. In quantum mechanical language, this means we need the optimal probe state. Then, we need to optimize the interaction of our instrumentation with the system we which to interrogate so that we can extract the desired information. This translates to needing the best possible interaction between the probe state and the system in question -- in other words, we need to optimize the evolution of the probe. Finally, we must take care to extract the most information as possible at the output; we must not neglect any information present in the evolved probe state. The entire quantum metrology process can be summarized as thus: probe
Quantum optical sensors are ubiquitous in various fields of research, from biological or medical sensors to large-scale experiments searching for dark matter or gravitational waves. Gravitational-wave detectors have been very successful in implementing cavities and quantum squeezed light for enhancing sensitivity to signals from black hole or neutron star mergers. However, the sensitivity to weak forces is limited by the available energy and the optical decoherence in the system. Here, we derive the fundamental sensitivity limit of cavity- and squeezed-light-enhanced interferometers with optical loss. This limit is attained by the optimal use of an additional internal squeeze operation, which allows the mitigation of readout loss. We demonstrate the application of internal squeezing to various scenarios and confirm that it indeed allows one to reach the best sensitivity in cavity- and squeezed-light-enhanced linear force sensors. Our work establishes the groundwork for the future development of optimal sensors in real-world scenarios where, up until now, the application of squeezed light was curtailed by various sources of decoherence.
A fundamental aspect of quantum systems lies in their extreme sensitivity to external perturbations. This characteristic, which generally represents a considerable challenge for physicists seeking to isolate these systems in order to observe their quantum properties, can actually be transformed into an advantage. Indeed, this intrinsic sensitivity makes quantum systems ideal candidates for high-precision metrology. Quantum sensors exploit this susceptibility to measure physical quantities with precision difficult to match with classical approaches.