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arXiv:2305.02904 · Advanced Quantum Technologies

Quantum Enhanced Probes of Magnetic Circular Dichroism

Chengyun Hua, Claire E. Marvinney, Seongjin Hong, Matthew Feldman, Yun‐Yi Pai, Michael Chilcote, Joshua D. Rabinowitz, Raphael C. Pooser, Alberto M. Marino, Benjamin J. Lawrie

Published Sep 19, 2023• 7 citations
arXivHTMLPDF

Abstract

Abstract Magneto‐optical microscopies, including optical measurements of magnetic circular dichroism, are increasingly ubiquitous tools for probing spin‐orbit coupling, charge‐carrier g‐factors, and chiral excitations in matter, but the minimum detectable signal in classical magnetic circular dichroism measurements is fundamentally limited by the shot‐noise limit of the optical readout field. Here, a two‐mode squeezed light source is used to improve the minimum detectable signal in magnetic circular dichroism measurements by 3 decibels compared with state‐of‐the‐art classical measurements, even with relatively lossy samples like terbium gallium garnet. These results provide a framework for new quantum‐enhanced magneto‐optical microscopies that are particularly critical for environmentally sensitive materials and for low temperature measurements where increased optical power can introduce unacceptable thermal perturbations.

DOI
10.1002/qute.202300126

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•Feb 10, 2022•0 citations
Abstract Magneto‐optical microscopies, including optical measurements of magnetic circular dichroism, are increasingly ubiquitous tools for probing spin‐orbit coupling, charge‐carrier g‐factors, and chiral excitations in matter, but the minimum detectable signal in classical magnetic circular dichroism measurements is fundamentally limited by the shot‐noise limit of the optical readout field. Here, a two‐mode squeezed light source is used to improve the minimum detectable signal in magnetic circular dichroism measurements by 3 decibels compared with state‐of‐the‐art classical measurements, even with relatively lossy samples like terbium gallium garnet. These results provide a framework for new quantum‐enhanced magneto‐optical microscopies that are particularly critical for environmentally sensitive materials and for low temperature measurements where increased optical power can introduce unacceptable thermal perturbations.
Abstract Magneto‐optical sensors including spin noise spectroscopies and magneto‐optical Kerr effect microscopies are now ubiquitous tools for materials characterization that can provide new understanding of spin dynamics, hyperfine interactions, spin‐orbit interactions, and charge‐carrier g‐factors. Both interferometric and intensity‐difference measurements can provide photon‐shot‐noise‐limited sensitivity, but further improvements in sensitivity with classical resources require either increased laser power that can induce unwanted heating and electronic perturbations or increased measurement times that can obscure out‐of‐equilibrium dynamics and slow experimental throughput. Proof‐of‐principle measurements have already demonstrated quantum enhanced spin noise measurements with a squeezed readout field that are likely to be critical to the nonperturbative characterization of spin excitations in quantum materials that emerge at low temperatures. Here, a truncated nonlinear interferometric readout for low‐temperature magneto‐optical Kerr effect and related magneto‐optical microscopies that is accessible with today's quantum optical resources is proposed. 10 sensitivity is achievable with optical power as small as 1 µW. As a result, measurements may be performed at temperatures as low as 83 mK in commercially available dilution refrigerators. This combination of high sensitivity and low operating temperature is impossible to achieve with any classical measurement. The quantum advantage for the proposed measurements persists even in the limit of large loss and small squeezing parameters.
The measurement of circular dichroism (CD) has widely been exploited to distinguish the different enantiomers of chiral structures. It has been applied to natural materials (e.g., molecules) as well as to artificial materials (e.g., nanophotonic structures). However, especially for chiral molecules the signal level is very low and increasing the signal-to-noise ratio is of paramount importance to either shorten the necessary measurement time or to lower the minimum detectable molecule concentration. As one solution to this problem, we propose here to use quantum states of light in CD sensing to reduce the noise below the shot noise level encountered when using coherent states of light. Through a multiparameter estimation approach, we identify the ultimate quantum limit of the precision in CD sensing, allowing for general schemes including additional ancillary modes. We show that the ultimate quantum limit can be achieved by various optimal schemes. These include using a pair of Fock-state probes in a direct sensing configuration and pairs of twin beams in an ancilla-assisted sensing configuration, for both of which photon number-resolved detection is shown to be the optimal measurement. These optimal schemes offer a significant quantum enhancement even in the presence of some additional system loss. The near optimality of a scheme using a single twin beam in a direct sensing configuration is also shown for cases where the actual CD signal is very small. Alternative optimal schemes involving single-photon sources and detectors are also proposed. This work paves the way for further investigations of quantum metrological techniques in chirality sensing.
An atomic magnetometer detects atomic responses to the magnetic field, and its sensitivity is ultimately limited by quantum noise fluctuations. For magnetometers based on nonlinear magneto-optical rotation (NMOR), the possible concurrent generation of light squeezing due to polarization self-rotation complicates the optimization for magnetometer sensitivity. Here, we study NMOR magnetometers with frequency-modulated light in a paraffin coated 87Rb vapor cell in the low and high power regimes corresponding to situations with and without light squeezing, respectively, with detection observables being different Stokes components reflecting the magnetic-field-induced atomic circular dichroism or birefringence. We found that the overall best sensitivity is achieved in the low power regime when there is no light squeezing and for circular dichroism measurement. We provide a general insight on parameter optimization and the choice of detection observables, from the delicate trade-off between the atomic responses and the noises including the technical and quantum optical noises. Our results could have practical significance in optical atomic magnetometry.
Optical atomic magnetometry is a powerful tool for continuous sensing applications, yet, in the absence of quantum correlations, its sensitivity is limited by the standard quantum limit (SQL) stemming from a trade-off between optical probe imprecision and quantum measurement backaction. Beyond-SQL sensitivity requires quantum correlations that modify these measurement noise sources. Here we demonstrate such sensitivity by using entangled state of the probe light and by engineering correlations between measurement imprecision and backaction. Having first explored SQL in a broad range of frequencies, we demonstrate overcoming the limit by combining variational readout with coupling the magnetometer to one mode of a bipartite entangled light state and conditioning the results on the other entangled mode. Tuning the detected light quadratures and combining the signals from the two measurement channels, we achieve sensitivity beyond the SQL in a broad range of acoustic frequencies which has so far remained inaccessible to quantum-noise-limited optical magnetometry.
An experimental scheme is proposed for squeezing-enhanced simultaneous measurement of the polarization angle and ellipticity of laser beam beyond the shot noise limit. In the scheme, a two-mode squeezed light is exploited to enhance the sensitivity of the measurement of polarization rotation angle, and a single-mode squeezed light is used to improve the sensitivity of ellipticity measurement. Noise analysis is performed to show that the signal-to-noise ratios in both the measurements of polarization angle and ellipticity can exceed the shot noise limit. This work is expected to be of considerable interest for experiments such as the observation of vacuum magnetic birefringence, gravitational wave detection, spin–orbital interaction, material characterization, polarization microscopy, atomic magnetometer, or biomedical optics.
Abstract In the last few decades, the atomic magnetometry [1–7] has experienced remarkable development and reached to an outstanding sensitivity. This exciting technology is finding applications in various of fields [8–18]. The last two obstacles to further improve the sensitivity of magnetometry are quantum fluctuations and ambient magnetic field. For magnetometry based on optical readout of atomic ensemble’s spin precession, the fundamental limitation of sensitivity is restricted by spin projection noise (SPN) and photon shot noise (PSN). Meanwhile, in practical applications, ambient magnetic field noise which is regarded as a common-mode magnetic-field (CMM) noise is also limiting the sensitivity. Several techniques for reducing PSN [19–25], SPN [26, 27], and CMM noise [7, 28–31] have been proposed, respectively. To achieve the best sensitivity, it is essential to find an efficacious way to eliminate the noises from different sources, simultaneously. For this purpose, here we demonstrate a sub-shot-noise magnetic gradiometer utilizing entangled optical detection with quantum-correlated twin beams. This leads to the simultaneous suppression of PSN and CMM noise. The quantum enhancement spans a frequency range from 7 Hz to 6 MHz with maximum squeezing of 5.5 dB below the standard quantum limit (SQL). The sensitivity of gradient magnetic field reaches 18 fT/cm√Hz at 20 Hz with these sophisticated techniques. Our study opens up new possibilities to use quantum-enhanced technology in developing sensitive magnetometry for practical applications with noisy and physically demanding environments.