On-chip non-Hermitian dynamic systems based on dissipative engineering have achieved remarkable results, but intrinsic losses and Langevin noise limit the further development of on-chip non-Hermitian devices. In this article, we propose a simple and feasible on-chip gain-gain coupling fully quantum non-Hermitian sensing model based on quasi-phase-matched second-order degenerate parametric downconversion in periodically poled LiNbO 3 waveguides and demonstrate the feasibility of the theory through finite element simulations. Our theory suggests that this model exhibits hidden quadrature pseudo-PT (parity-time) symmetry behavior and gives rise to the phenomenon of competitive amplification between downconverted photons. Additionally, based on quantum Fisher information, our proposed scheme can achieve quantum-sensing enhancement in the full PT-symmetric domain and super-Heisenberg scaling close to the exceptional point (EP). For detuning disturbances, our system exhibits excellent robustness in the nonspecial EP region. These findings will introduce a new perspective for the development of on-chip non-Hermitian technologies and promote the development of non-Hermitian quantum-sensing theories.
On-chip non-Hermitian dynamic systems based on dissipative engineering have achieved remarkable results, but intrinsic losses and Langevin noise limit the further development of on-chip non-Hermitian devices. In this article, we propose a simple and feasible on-chip gain-gain coupling fully quantum non-Hermitian sensing model based on quasi-phase-matched second-order degenerate parametric downconversion in periodically poled LiNbO 3 waveguides and demonstrate the feasibility of the theory through finite element simulations. Our theory suggests that this model exhibits hidden quadrature pseudo-PT (parity-time) symmetry behavior and gives rise to the phenomenon of competitive amplification between downconverted photons. Additionally, based on quantum Fisher information, our proposed scheme can achieve quantum-sensing enhancement in the full PT-symmetric domain and super-Heisenberg scaling close to the exceptional point (EP). For detuning disturbances, our system exhibits excellent robustness in the nonspecial EP region. These findings will introduce a new perspective for the development of on-chip non-Hermitian technologies and promote the development of non-Hermitian quantum-sensing theories.
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.
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.
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.
The efficient sensing of weak environmental perturbations via special degeneracies called exceptional points in non-Hermitian systems has attracted enormous attention in the last few decades. However, in contrast to the extensive literature on parity-time (PT) symmetric systems, the exotic hallmarks of anti-PT symmetric systems are only beginning to be realized now. Very recently, a characteristic resonance of vanishing linewidth in anti-PT symmetric systems was shown to exhibit tremendous sensitivity to intrinsic nonlinearities. Given the primacy of sensing in non-Hermitian systems, in general, and the immense topicality of anti-PT symmetry, we investigate the statistical bound to the measurement sensitivity for any arbitrary perturbation in a dissipatively coupled, anti-PT symmetric system. Using the framework of quantum Fisher information and the long-time solution to the full master equation, we analytically compute the Cram\'er-Rao bound for the system properties such as the detunings and the couplings. As an illustrative example of this formulation, we inspect and reaffirm the role of a long-lived resonance in dissipatively interacting systems for sensing applications.
Non-Hermitian systems exhibit phenomena absent from Hermitian systems, including exceptional points (EPs), at which two or more eigenvectors coalesce. Conventional implementations rely on gain and loss, which strongly limit quantum coherence. Here, following a proposal by Wang and Clerk (PRA 2019), we realize a closed four-mode quantum system that emulates the dynamics of a PT dimer - two coupled resonators with balanced gain and loss - without engineered dissipation. The four modes are implemented as harmonics of a superconducting coplanar-waveguide resonator, with parametric couplings engineered using a current-pumped SNAIL. We use this device as a sensor for small variations in the PT dimer coupling strength. From signal-to-noise-ratio measurements, we observe enhanced sensitivity near the EP in a non-quantum-limited regime.