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arXiv:2405.15612 · iScience

Dual opposing quadrature-PT symmetry

Wencong Wang, Jacob Kokinda, Jiazhen Li, Qing Gu, Dongmei Liu, Jianming Wen

Published Dec 25, 2024• 2 citations
arXivHTMLPDF

Abstract

Our recent research on type-I quadrature parity-time (PT) symmetry, utilizing an open twin-beam system, not only enables observing genuine quantum photonic PT symmetry amid phase-sensitive amplification (PSA) and loss in the presence of Langevin noise but also reveals an additional classical-to-quantum (C2Q) transition in noise fluctuations. In contrast to the previous setup, our exploration of an alternative system assuming no loss involves a type-II PSA-only scheme. This scheme facilitates dual opposing quadrature-PT symmetry, offering a comprehensive and complementary comprehension of C2Q transitions and PT-enhanced quantum sensing with optimal performance in the symmetry unbroken region. Furthermore, our investigation into the quantum correlation with the Einstein-Podolsky-Rosen criteria uncovers previously unexplored connections between PT symmetry and nonclassicality, as well as quantum entanglement within the continuous-variable framework.

DOI
10.1016/j.isci.2024.111655

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Our recent research on type-I quadrature parity-time (PT) symmetry, utilizing an open twin-beam system, not only enables observing genuine quantum photonic PT symmetry amid phase-sensitive amplification (PSA) and loss in the presence of Langevin noise but also reveals an additional classical-to-quantum (C2Q) transition in noise fluctuations. In contrast to the previous setup, our exploration of an alternative system assuming no loss involves a type-II PSA-only scheme. This scheme facilitates dual opposing quadrature-PT symmetry, offering a comprehensive and complementary comprehension of C2Q transitions and PT-enhanced quantum sensing with optimal performance in the symmetry unbroken region. Furthermore, our investigation into the quantum correlation with the Einstein-Podolsky-Rosen criteria uncovers previously unexplored connections between PT symmetry and nonclassicality, as well as quantum entanglement within the continuous-variable framework.
Abstract While gain-loss-coupled photonic platforms have achieved significant success in studying classical parity-time (PT) symmetry, they encounter challenges in demonstrating pure quantum effects due to incompatible operator transformations and Langevin noise. Here, we present compelling evidence that a non-Hermitian (NH) twin-beam system, undergoing phase-sensitive amplification and balanced loss, not only enables observing the usual eigenvalue-associated PT phase transition but also exhibits distinctive features absent in classical NH or traditional Hermitian quantum scenarios, encompassing quadrature PT symmetry, anomalous loss-induced quadrature squeezing, and classical-to-quantum (C2Q) transitions in noise fluctuations. Furthermore, our proposed bipartite open system promises optimal sensing for various quantum observable measurements, showcasing an improved signal-to-noise ratio and sensitivity, constrained by quantum Cramér–Rao bound or Fisher information. These findings deepen the comprehension of authentic quantum optical PT symmetry involving both gain and loss, addressing contentious issues and illuminating new facets of the subject.
Quantum Langevin noise makes the observation of quantum-optical parity-time (PT) symmetry in an open system with both gain and loss elusive. Here, we challenge this problem by exploiting twin beams produced from a nonlinear parametric process, one undergoing phase-sensitive linear quantum amplification (PSA) and the other engaging balanced loss merely. Unlike previous studies involving phase-insensitive linear quantum amplification (PIA), our PSA-loss scheme enables only one pair of quadratures to evolve PT-symmetrically with variances transiting from periodic oscillations to exponential growths when crossing an exceptional point (EP), while tailors the conjugate pair with PT-adjusted quadrature squeezing. We further investigate such asymmetric PT-quadrature squeezing for quantum sensing by evaluating the quantum Cramer–Rao bound with distinct features beyond existing protocols. The proposed quadrature PT sheds new light on continuous-variable based quantum information and technology.
Exceptional-point (EP) sensing has attracted considerable interest because of its anomalous response scaling. However, recent studies have shown that the enhanced response near an EP is inevitably accompanied by amplified quantum noise, fundamentally limiting the achievable signalto-noise ratio (SNR). Here, we propose a fundamentally different route toward non-Hermitian quantum sensing based on symmetry-protected noise suppression rather than response amplification. We develop a fully quantum continuous-variable model that unifies parity-time (PT) and anti-paritytime(APT) symmetries within a single framework. Exploiting the incompatibility between these two symmetries, we uncover a non-Hermitian Dirac eigenspectrum and reveal a hidden symmetryprotected phase transition embedded in the Hamiltonian spectrum. Remarkably, this hidden phasetransition simultaneously constitutes a symmetry-protected vacuum-noise fixed point, where collective three-mode quadratures exhibit suppressed quantum fluctuations despite the absence of any anomalous spectral response. As a consequence, quantum sensing is enhanced through the suppression of excess quantum noise while maintaining a finite response sensitivity, establishing a sensing mechanism fundamentally different from conventional EP-based approaches. These results reveal an unexpected connection between hidden symmetry, quantum fluctuations, and non-Hermitian quantum metrology, and establish noise suppression as an alternative paradigm for non-Hermitian quantum sensing.
The emergence of parity-time (PT) symmetry has greatly enriched our study of symmetry-enabled non-Hermitian physics, but the realization of quantum PT symmetry faces an intrinsic issue of unavoidable symmetry-breaking Langevin noises. Here we construct a quantum pseudo-anti-PT (pseudo-APT) symmetry in a two-mode bosonic system without involving Langevin noises. We show that the spontaneous pseudo-APT symmetry breaking leads to an exceptional point, across which there is a transition between different types of quantum squeezing dynamics; i.e., the squeezing factor increases exponentially (oscillates periodically) with time in the pseudo-APT-symmetric (broken) region. Such dramatic changes of squeezing factors and quantum dynamics near the exceptional point are utilized for ultraprecision quantum sensing. These exotic quantum phenomena and sensing applications can be experimentally observed in two physical systems: spontaneous wave mixing nonlinear optics and atomic Bose-Einstein condensates. Our Letter offers a physical platform for investigating exciting APT symmetry physics in the quantum realm, paving the way for exploring fundamental quantum non-Hermitian effects and their quantum technological applications.
Quasi-parametric amplification (QPA) is a nonlinear interaction in which the idler wave is depleted through a loss mechanism. QPA plays an important role in signal amplification in ultrafast photonics and quantum light generation. In this report, we explore the generalized features characterized by non-Hermitian parity-time ( PT ) symmetry in the QPA process and uncover multiple PT -symmetric phase transitions in such a QPA process in which transitions are ultrasensitive to external parameters. In particular, we demonstrate the feasibility of detection of 10 –11 inhomogeneities of the doped absorber, which is several orders of magnitude more sensitive than similar measurements performed in a linear absorption regime. In doing so, we reveal a family of PT -symmetric phase transitions appearing in the QPA process and provide a distinctive nonlinear optical sensing mechanism for precise optical measurements.
Quantum entanglement and intensity correlations in continuous variable (CV) regimes have investigated in the time domain in addition to many analogous investigations performed in the frequency domain. The specific signatures of time-resolved CV quadrature entanglement and intensity difference squeezing have been analyzed for periodically-pulsed optical parametric oscillator (OPO). An application to quantum key distribution scheme based on intensity correlation of twin beams has also been considered.