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arXiv:2203.08703 · Photonics Research

Nonlinear quantum spectroscopy with parity–time-symmetric integrated circuits

Pawan Kumar, Sina Saravi, Thomas Pertsch, Frank Setzpfandt, Andrey A. Sukhorukov

Published Apr 26, 2022• 4 citations
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Abstract

We propose a novel quantum nonlinear interferometer design that incorporates a passive parity–time (PT)-symmetric coupler sandwiched between two nonlinear sections where signal–idler photon pairs are generated. The PT symmetry enables efficient coupling of the longer-wavelength idler photons and facilitates the sensing of losses in the second waveguide exposed to analyte under investigation, whose absorption can be inferred by measuring only the signal intensity at a shorter wavelength where efficient detectors are readily available. Remarkably, we identify a new phenomenon of sharp signal intensity fringe shift at critical idler loss values, which is distinct from the previously studied PT symmetry breaking. We discuss how such unconventional properties arising from quantum interference can provide a route to enhancing the sensing of analytes and facilitate broadband spectroscopy applications in integrated photonic platforms.

DOI
10.1364/prj.450410

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We propose a novel quantum nonlinear interferometer design that incorporates a passive parity–time (PT)-symmetric coupler sandwiched between two nonlinear sections where signal–idler photon pairs are generated. The PT symmetry enables efficient coupling of the longer-wavelength idler photons and facilitates the sensing of losses in the second waveguide exposed to analyte under investigation, whose absorption can be inferred by measuring only the signal intensity at a shorter wavelength where efficient detectors are readily available. Remarkably, we identify a new phenomenon of sharp signal intensity fringe shift at critical idler loss values, which is distinct from the previously studied PT symmetry breaking. We discuss how such unconventional properties arising from quantum interference can provide a route to enhancing the sensing of analytes and facilitate broadband spectroscopy applications in integrated photonic platforms.
Integrated nonlinear waveguide structures enable generation of quantum entangled photons. We describe theoretically the effects of spatially inhomogeneous loss on the creation of photon pairs through spontaneous parametric down-conversion in quadratically nonlinear directional couplers, where photons experience effective parity-time (PT) symmetric potential due to the presence of optical loss in one of the waveguides. We show that for losses below the PT-breaking threshold, the quantum photon states can be flexibly tuned similarly to conservative couplers, whereas for stronger losses, the correlations between two waveguide modes are suppressed. We also formulate a quantum-classical correspondence with sum-frequency generation for fast evaluation of device performance. These results can be applied for the design of quantum plasmonic circuits.
Quantum spectroscopy based on nonlinear interferometers attracts increasing attention due to its unique practical advantages [1]. By tailoring spontaneous parametric down conversion (SPDC) for generation of frequency-correlated photons and their nonlocal quantum interference, it becomes possible to perform sensing and imaging across broad spectral ranges through simple detection of paired photons in the visible [2,3]. It was recently demonstrated that nonlinear integrated waveguides can improve the performance of quantum spectroscopy [4]. However, while fabrication of integrated circuits with multiple waveguides is well established, their applications for quantum spectroscopy remain largely undeveloped.
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.
Integrated optical quantum circuits based on photonic waveguiding structures are increasingly gaining attention as a possible solution for scalable quantum technologies with important applications to quantum simulations. Quantum communication provides secure information transmission, but the distance over which quantum states of light can be distributed without significant disturbance is limited due to inescapable losses and noise in optical elements. Loss is the greatest challenge facing the implementation of integrated photonic technologies, and it is inescapable in experimental reality. In recent years there is a rise of interest in structures with spatially inhomogeneous losses. Light propagation in waveguiding structures with spatially distributed sections of loss can be used for implementation of quantum plasmonic circuits, which are able to strongly confine light to sub-wavelength dimensions, as well as for parity-time (PT) symmetric structures, with phase transition associated with PT-symmetry breaking, which opens new possibilities for light manipulation. The PhD thesis contains research on the controllable classical and quantum dynamics of optical frequency conversion processes in quadratically nonlinear photonic integrated circuites in the presence of losses. Namely, I discuss spontaneous parametric down-conversion (SPDC), sum-frequency generation (SFG) and optical parametric amplification (OPA) in nonlinear structures governed by non-Hermition Hamiltonians. I explore the fundamental features of multi-photon generation in integrated nonlinear waveguides. I have been shown that arrays of coupled nonlinear waveguides can serve as a robust integrated platform for the generation of entangled photon states with nonclassical spatial correlations through spontaneous parametric down-conversion (SPDC), and that the operation of such quantum circuit is tolerant even to relatively high losses. Furthermore, I have studied the bi-photon multimode quantum emission tomography in waveguide structures with spatially inhomegeneous losses. The PhD thesis also covers the research on the effect of these losses in waveguide couplers possessing parity-time (PT) symmetry. I have identified an anti-PT spectral symmetry of a parametric amplifier based on those couplers. Finally, I describe the single-photon conversion to a photon pair, and identify opportunities for the efficient enhancement of this process.
With their conceptual description of Parity-Time symmetry in 1998 [1], Bender and Boettcher inaugurated a new field of studies with fascinating implications on topology [2] and exceptional point dynamics [3], to name a few. Typically, PT-symmetric Hamiltonians exhibit real eigenvalues below a certain threshold, whereas above, the PT-symmetry is spontaneously broken as the eigenvalue spectrum becomes complex. Entirely passive configurations based solely on differential losses [7] allow the physics of PT symmetry to be carried over to a quantum-optical context [6] and explored therein [4]. Here we study the ways in which the degree of loss systematically impacts the characteristics of quantum correlations between interfering photons by analyzing the two-photon correlations in PT-symmetric directional couplers.
ABSTRACT Exceptional points (EPs), singularities in non‐Hermitian systems where eigenvalues and eigenstates coalesce, exhibit unique spectral responses and enhanced sensitivity. However, in quantum‐interference settings such as two‐photon measurements, the interference transition generally does not occur at the EPs, limiting their applications for quantum sensing. To address this issue, we develop an anti‐parity‐time‐symmetric model based on indirectly coupled waveguides and introduce a cascaded‐waveguide configuration that produces an effective rotation of the system's eigenbasis. This modification shifts the two‐photon bunching‐antibunching transition to coincide with the EP. Near the EP, a small refractive‐index perturbation leads to a sharp change in visibility, which becomes more pronounced by increasing waveguide length. This passive, gain‐free design may minimize noise and offer a feasible route toward EP‐enhanced quantum phase sensing on integrated platforms.