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Optical and Quantum Sensing and Precision Metrology II

Revealing invisible spatial structure of quantum squeezed vacuum

Eugeniy E. Mikhailov, Charris Gabaldon, Pratik J. Barge, Lior Cohen, Hwang Lee, Irina V. Novikova

Published Mar 19, 2025• 0 citations
Publisher

Abstract

Quantum enhanced measurements depend on generation of squeezed light with quantum fluctuations below standard quantum limit. Precise spatial information about squeezed modes is crucial to reach maximum quantum advantage in sensing applications. Yet the mixed mode squeezed vacuum (the extreme case of squeezed light) is particularly elusive for such characterization. We will present our method, based on the structured light homodyning, to extract squeezing levels and mode shapes from a multimode quantum squeezed vacuum beam.

DOI
10.1117/12.3053436

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Quantum enhanced measurements depend on generation of squeezed light with quantum fluctuations below standard quantum limit. Precise spatial information about squeezed modes is crucial to reach maximum quantum advantage in sensing applications. Yet the mixed mode squeezed vacuum (the extreme case of squeezed light) is particularly elusive for such characterization. We will present our method, based on the structured light homodyning, to extract squeezing levels and mode shapes from a multimode quantum squeezed vacuum beam.
Squeezed states of light, with field fluctuations smaller than the coherent state fluctuations (or shot noise), are used for improving accuracy of quantum-noise limited measurements, like the detection of gravitational waves. They are also essential resources for quantum information transfer protocols. We studied a squeezed vacuum field generated in hot Rb vapor via the polarization self-rotation effect. We studied the mode structure of the squeezed field by spatially-masking the laser beam after its interaction with the Rb atomic vapor. From analysis of the data we developed a multi-mode theory to simulate the mode composition of the squeezed vacuum field. Our experiments showed that the amount of observed squeezing may be limited by the complex mode structure due to the excitement of higher order spatial modes during the nonlinear atom-light interaction. We demonstrated that optimization of the spatial profile of the beam led to higher detected squeezing. Our studies are useful for enhancing precision metrology and quantum memory applications.
Multimode squeezed light is an increasingly popular tool in photonic quantum technologies, including sensing, imaging, and computation. Meanwhile, the existing methods of its characterization are technically complicated, and in the best case, deal with a single mode at a time. Here, we demonstrate experimentally how the squeezing can be measured in multiple spatial modes simultaneously, using optical parametric amplification and direct detection followed by modal decomposition based on spatial intensity correlations. We apply this method to a multimode squeezed vacuum generated via high-gain parametric down-conversion. We measure the degrees of squeezing and anti-squeezing for eight strongest spatial modes, obtaining highest squeezing and anti-squeezing values of −5.2±0.2-5.2 \pm 0.2−5.2±0.2 dB and 8.6±0.38.6 \pm 0.38.6±0.3 dB, respectively.
The spatial mode is an essential component of an electromagnetic field description, yet it is challenging to characterize it for optical fields with the low average photon number, such as in a squeezed vacuum. We present a method for the reconstruction of the spatial modes of such fields based on the homodyne measurements of their quadrature noise variance performed with a set of structured masks. We show theoretically that under certain conditions, we can recover individual spatial mode distributions by using the weighted sum of the basis masks, where weights are determined using measured variance values and phases. We apply this approach to analyze the spatial structure of a squeezed vacuum field with various amount of excess thermal noise generated in Rb vapor.
Nowadays, the realization of quantum computations and communications based on continuous variables has attracted a significant attention due to a substantial expansion of the system dimensionality. The main progress in this area is attributed to the implementation of multimode systems based on squeezed states of light. One of the simplest ways to generate such states relies upon their producing in a single-pass optical parametric amplifier (OPA) using ultrafast pumping. However, for homodyne detection of such multimode states, the profile of the local oscillator (LO) must perfectly match the profile of the measured mode. Usually, this is not the case; therefore a proper treatment of multimode squeezing is required. In this work, we study both theoretically and experimentally the multimode squeezed light generated in type-0 and type-II OPA. We characterize such sources and investigate the degree of squeezing in dependence on the LO spectral profile, employing a pulse shaping technique. The theoretical analysis is performed using the Schmidt-mode theory. This work might have a significant impact on the realization of multimode quantum protocols
Multimode squeezed light is a key resource for high-dimensional quantum technologies, enhancing metrological sensitivity, boosting communication security, and enabling parallel processing in computation. Its practical potential, however, remains constrained by the inherent single-mode operation of homodyne detection, necessitating post-processing for multimode characterization. Here, we overcome this long-standing challenge by employing multimode optical parametric amplification, enabling loss-tolerant direct detection of squeezing in each mode, which in turn permits mode sorting after amplification. As a result, we demonstrate, for the first time to the best of our knowledge, the real-time monitoring of multimode squeezing. With a spatial light modulator sorting the modes, we simultaneously measure squeezing in nine spatial modes co-propagating within one beam. Although mode sorting and filtering reduce the detection efficiency to less than 0.3%, we observe high-purity squeezing of up to - 7.9 ± 0.6 dB - to the best of our knowledge, the highest squeezing recorded for pulsed light. Furthermore, we demonstrate real-time, loss-tolerant characterization of continuous-variable entanglement and extend it to the detection of cluster states. Similar methods can be applied in the frequency domain, facilitating a crucial capability for scalable quantum technologies.