Nano- and micromechanical oscillators act as great sensors of a wide variety of signals, but their sensitivity and bandwidth can be limited by quantum backaction imposed by optomechanical displacement measurement. We experimentally demonstrate a new paradigm for optomechanical measurement and control based on strong interactions with short light pulses. Using unique nanophotonic optomechanical cavities, we show that single pulsed measurements can achieve sub-quantum-limit resolution. Moreover, we demonstrate a new protocol to deterministically produce squeezed mechanical states, which can reduce single-quadrature fluctuations to arbitrarily small magnitudes. We discuss the application of the resulting squeezing and entanglement for mechanical quantum sensing.
Spin noise spectroscopy (SNS) allows optically detect the fluctuations of a spin ensemble. Such fluctuations induce noise in the birefringence of the medium, which can be probed by recording polarization fluctuations of a laser beam after its propagation through the sample. As spin fluctuations are centered on the zero frequency, a transverse magnetic field is applied: spin precession then shifts the spin noise resonance at the Larmor frequency, so that it is not hidden by technical noises. The first SNS experiment was reported in the early 1980s by Alexandrov and Zapasskii, but it is only in 2004 that advances in narrow linewidth lasers and low noise electronics allow using this technique to probe various systems, such as thermal atomic vapors, semiconductors, or quantum wells. It was also proposed for magnetic field sensing, and possible measurements of correlations beyond the second-order raise a lot of interest, as they can give access to new quantum phenomena.
We have performed spin noise spectroscopy in a metastable helium gas cell, and show that we can get two polarization dependent noise peaks when we record the linear instead of the circular birefringence fluctuations. Moreover, the relatively simple structure of helium allows us to show that it depends on the closest optical transition even when we are detuned by more than the Doppler broadening. We can also show that the behavior of the polarization dependence is strongly affected by time dependent B-field fluctuations.
We performed simulations using a density matrix time evolution model, with added random fluctuations. It reproduces quite well the experimental results, including the transition dependence and the time dependent B-field noise effect, which have not been reported yet.
Due to its simple level structure, helium 4 is particularly attractive to investigate original scenario for nonlinear light-atom interaction. We illustrate this feature by two examples: i) the demonstration of an original CPT-enhanced phase sensitive amplifier for light, which has been used to generate a squeezed vacuum and, ii) the originality of spin noise spectroscopy in metastable helium 4.
The minimum resolvable signal in optical metrology and sensing applications is usually limited by the so-called standard quantum limit. One way to improve the signal-to-noise ratio is to use squeezed states of light. Squeezed light can be generated using different types of nonlinear interactions either in solid-state nonlinear crystals or in atomic systems. One way to generate squeezed light is to use a phase sensitive amplifier that will amplify one quadrature of the considered mode and deamplify the other one. We have recently shown that metastable helium vapor at room temperature can exhibit strong four-wave mixing effects and behave like a perfect phase sensitive amplifier when it is prepared in coherent population trapping situation. In this talk, we will present the application of this phase sensitive amplification to the generation of squeezed vacuum states of light and detail the performances and limitations of the system
Noise is usually something that one would like to avoid when performing measurements. However, the information contained in fundamental noises might be of great interest for physicists. A recent example is the development of spin noise spectroscopy (SNS). In magnetic systems, the spectroscopy of the fundamental noise due to random spin fluctuations can be optically performed, by measuring the associated fluctuations of the Faraday rotation experienced by a linearly polarized probe beam, which propagates through the sample in the presence of a dc magnetic field [1]. Although a first experimental effort was initially reported in the early 1980s [2], only recently this method has seen a renewed interest due to advances in narrow line-width lasers and development in low noise electronics required for spectrum analysis [3]: it is now used to probe different properties of various media such as thermal atomic vapors, semi-conductors or quantum wells or defects in diamond [4]. In this talk, we will report our efforts to use this technique to probe the transitions of metastable helium and to understand the differences between the SNS spectra obtained along the different transitions.
[1] V. S. Zapasskii, Adv. Opt. and Phot. 5 131 (2013)
[2] E. B. Aleksandrov and V. S. Zapasskil, JETP 54 64 (1981)
[3] S. A. Crooker, D. G. Rickel, A. V. Balatsky, and D. L. Smith, Nature 431 49 (2004)
[4] N. A. Sinitsyn and Y. V. Pershin, Rep. on Prog. in Phys. 79 , 106501 (2016)
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