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Physics World

Spectroscopy squeezed beyond the shot-noise limit

Philippe Grangier

Published Aug 1, 1992• 1 citation
Publisher

Abstract

For many years the "shot-noise limit" has been considered as the fundamental limitation on measurements made with light beams. Shot noise is due to the quantum nature of light: the number of photons detected in a given time from a "stable" light beam (for instance, a laser beam) is an intrinsically random number which has fluctuations of order N1/2 if the average result is N. So if the number of photons, N, is limited by time or power considerations, shot noise places a fundamental limitation on the precision to which intensity, and changes in intensity, can be measured. This is particularly critical for "one-shot" ultra-sensitive experiments, such as the detection of a gravitational burst using large-scale interferometers.

DOI
10.1088/2058-7058/5/8/23

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•Jan 1, 2017•1 citation
For many years the "shot-noise limit" has been considered as the fundamental limitation on measurements made with light beams. Shot noise is due to the quantum nature of light: the number of photons detected in a given time from a "stable" light beam (for instance, a laser beam) is an intrinsically random number which has fluctuations of order N1/2 if the average result is N. So if the number of photons, N, is limited by time or power considerations, shot noise places a fundamental limitation on the precision to which intensity, and changes in intensity, can be measured. This is particularly critical for "one-shot" ultra-sensitive experiments, such as the detection of a gravitational burst using large-scale interferometers.
Optimizing the sensitivity for interferometric measurement of optical phase involves maximizing the intensity of the illuminating light source and minimizing any noise sources. Limits to the light source intensity may arise from the availability of a stable high-power laser or from damage to the system being measured. For example, we describe here a scanning confocal polarization microscope designed to image the dynamics of nerve networks. In this case the light intensity limit is set by several factors, including the scanning rate, the required response time and damage limits of the biological system. Assuming a stable experimental configuration for the interferometer and light source, the limiting noise source for visible light interferometry is shot-noise associated with the quantum state of the light inputs to the interferometer. Shot-noise can be thought of as arising from vacuum fluctuations entering the dark input port of the interferometer.(1) These vacuum field fluctuations combine with the light entering the other port to cause shot-noise in the detected photocurrent at the output ports of the interferometer. This shot-noise current normally limits the measured phase variance Δϕ to a value of 1/N where N is the average number of photons detected in a measurement interval. We describe an experiment which demonstrates that this shot-noise limit can be improved by injecting squeezed vacuum into the normally dark input port.(2) In this case the phase variance is reduced below the 1/N limit by the degree of squeezing. In contrast to early experiments,(2) we are now operating the interferometer in a dark fringe mode(3–5) in order to avoid setting a limit to the light level caused by detector saturation (near 1 mW).
As a direct result of the quantum nature of electromagnetic radiation, all forms of light exhibit inherent and unavoidable random fluctuations. Because these fluctuations reveal information about the nature of light and the underlying processes that generate it, they have received considerable attention since the time of Einstein. But they are also a source of noise that limits the accuracy with which information can be transmitted by a beam of light. Although experiments often still struggle with instrumental noise, some are quickly approaching the measurement limits set by the quantum-statistical nature of light. With the help of new techniques to “squeeze” the uncertainty of light, researchers can now conduct experiments with greater precision than possible with laser light. The generation of various forms of squeezed light in recent years has raised hopes for its application to such diverse areas as gravity-wave detection based on optical interferometry and reduced-error lightwave communications.
The optical shot-noise limit of a laser beam represents the classical limit in precision and signal to noise ratio achievable in optical sensors and imaging systems. It is known that quantum states of light, such as squeezed light and quantum correlated twin beams can be used to enhance precision and signal to noise ratio to beyond this limit. This promises optical sensors with fundamentally superior performance to the state of the art. While quantum-enhanced signal to noise ratio has been demonstrated with macroscopic powers of squeezed probe beam, demonstrations of quantum enhanced precision have remained at extremely low powers of light (femto-Watts). Here we will give examples where increasing SNR with quantum light is not sufficient to increase precision and we will review proof of principle experiments that have demonstrated sub shot noise precision, including for absorption spectroscopy and imaging. We will also report our efforts to achieve this sub-shot-noise precision performance for transmission measurement with ~0.1mW of average probe power, equating to ~10W peak power in a pulsed experiment. Time permitting, we will discuss how integrated optics and integrated electronics enables 10s GHz bandwidth in the detection of squeezed light, for integrated quantum sensor technology.
Optical techniques are widely used in many areas of science and technology to make accurate measurements and diagnostics, from microscopy, spectroscopy, chemical analysis, to gravitational wave detection and ranging. There are many reasons for this: light allows us to extract information in a remote and non-destructive way, it carries information in a massively parallel way, and perhaps more importantly, optical measurements can reach very high precision and sensitivity levels. It is therefore important to know what is the ultimate limit of sensitivity that can be possibly achieved in the estimation of a physical parameter p that is encoded in a light beam of fixed mean photon number N. This limit is imposed by the unavoidable quantum fluctuations of light and depends on the quantum state of light that conveys the information about p. When the light is in a coherent state, this limit is called ‘standard quantum limit’ and scales as 1/N1/2.
An improvement in precision beyond the limit set by the vacuum-state or zero-point fluctuations of the electromagnetic field is reported for the measurement of phase modulation in an optical interferometer. The experiment makes use of squeezed light to reduce the level of fluctuations below the shot-noise limit. An increase in the signal-to-noise ratio of 3.0 dB relative to the shot-noise limit is demonstrated, with the improvement currently limited by losses in propagation and detection and not by the degree of available squeezing.
Without light, there would be no sight. Light acts as a probe in various measurement techniques ranging from imaging to spectroscopy, from small scale cantilever displace- ment measurement in atomic force microscopy to large scale mirror motion in interfer- ometry, light detection and ranging (LIDAR) and many other fields. As the light probe intensity rises, it not only allows reducing the background noise effect, but it also aids precision to the measurement by reducing the photon noise (shot-noise) contribution. However, increasing intensity beyond certain threshold level is not always advantageous for ultra sensitive measurements. For example in the detection of gravitational waves, the current power circulating in the large scale interferometers can not be increased further without introducing other noise sources like thermal effects on the mirrors, un- wanted scattered photons, and back action due to radiation pressure. In the imaging of delicate photo sensitive sample, high power can causes cell damage or it may lead to disturb the regular process under investigation, viz. favouring certain biochemical reaction, which do not correspond to natural in vitro behaviour. At low intensity, pho- ton noise is an important concern and quantum states of light with correlated photon fluctuation can ideally represent a fruitful way to build specific measurement strategies to surpass the limitation of standard approach based on classical light sources, offering an avenue of solutions for ultra sensitive measurements. This thesis work focuses on two application of quantum enhanced measurement strate- gies. The first part of the thesis work has been dedicated to the realization of the first wide-field microscope with Sub Shot Noise (SSN) sensitivity. This is based on the ex- ploitation of quantum correlations in the Squeezed Vacuum state. In the second part, the investigations have made on the role of the quantum correlated beams in an unusual interferometric scheme, in which two identical optical interferometers are subject to the same phase fluctuation. The scheme, in the classical regime, has been already imple- mented in a large scale experiment devoted to the search of possible quantum gravity (QG) effects at Fermilab.