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Single-Mode, Narrow Band Photon Source Using SPDC for Hybrid Quantum Systems

Lisa Mueller, Jeremiah Hashley

Published Jun 21, 2024• 0 citations
Publisher

Abstract

Generating single-mode, narrow-band photons with high purity and brightness has proven to be difficult. Researchers at the University of Vienna have designed a photon source using Spontaneous Parametric Down-Conversion (SPDC) with a doubly resonant Optical Parametric Oscillator (OPO) utilizing an additional birefringent crystal to tune clusters independent of SPDC phase-matching. Achieving the doubly resonant condition in the OPO that enables a single mode is the challenge when creating a photon source with both single-mode and narrow-bandwidth. This is resolved here, as the resulting photon source has a bandwidth of 10.9 MHz to match the linewidths of atomic transitions. The source also produces single mode photon pairs at a rate of 47.5 Hz without mode filters reducing loss in the system. This novel design enables research and applications in the quantum realm using hybrid light-matter interactions.

DOI
10.1364/opticaopen.26069659.v1

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Generating single-mode, narrow-band photons with high purity and brightness has proven to be difficult. Researchers at the University of Vienna have designed a photon source using Spontaneous Parametric Down-Conversion (SPDC) with a doubly resonant Optical Parametric Oscillator (OPO) utilizing an additional birefringent crystal to tune clusters independent of SPDC phase-matching. Achieving the doubly resonant condition in the OPO that enables a single mode is the challenge when creating a photon source with both single-mode and narrow-bandwidth. This is resolved here, as the resulting photon source has a bandwidth of 10.9 MHz to match the linewidths of atomic transitions. The source also produces single mode photon pairs at a rate of 47.5 Hz without mode filters reducing loss in the system. This novel design enables research and applications in the quantum realm using hybrid light-matter interactions.
Generating single-mode, narrow-band photons with high purity and brightness has proven to be difficult. Researchers at the University of Vienna have designed a photon source using Spontaneous Parametric Down-Conversion (SPDC) with a doubly resonant Optical Parametric Oscillator (OPO) utilizing an additional birefringent crystal to tune clusters independent of SPDC phase-matching. Achieving the doubly resonant condition in the OPO that enables a single mode is the challenge when creating a photon source with both single-mode and narrow-bandwidth. This is resolved here, as the resulting photon source has a bandwidth of 10.9 MHz to match the linewidths of atomic transitions. The source also produces single mode photon pairs at a rate of 47.5 Hz without mode filters reducing loss in the system. This novel design enables research and applications in the quantum realm using hybrid light-matter interactions.
There has been increasing interest in the generation of such photon pairs using a spontaneous parametric down-conversion (SPDC) source inside an optical cavity to enhance the down-conversion into modes resonant to the cavity, each of the modes having a MHz spectrum. These optical parametric oscillators (OPO) operated far below threshold also provide a well defined spatial mode. Our setup consists of a bow-tie cavity with a periodically poled KTP crystal as nonlinear medium, phase-matched for parametric down-conversion. This photon-pair source is designed to provide photons suitable for interaction with atoms. These photons could be directly used in applications towards a quantum memory of light, like in experiments on slow light or the storage of single photons in EIT media. In this way an optical polarization rotation in an atomic sample could be measured achieving super-resolution.
Cavity-enhanced spontaneous parametric down-conversion (SPDC) can be used to implement heralded single-photon sources with narrow bandwidth down to tens of megahertz, thus compatible with atom-based quantum memories. In this work, we propose and study the cavities that are either singly resonant at the frequency of only one of the generated photons or doubly resonant at the frequency of one generated photon and for the pump field. We derive analytical expressions for the generation rate and the spectral brightness as a function of the main structure parameters. Our analysis shows that by exploiting counterpropagating SPDC inside a cavity with properly designed distributed Bragg reflectors, pure narrow-band heralded single photons can be generated. In particular, when a pulsed pump illuminates a 1-mm-long singly resonant cavity, the bandwidth of the resonant and the nonresonant photons will be 120 MHz and 5.7 GHz, respectively, with heralded single-photon purity as high as 0.993. Due to its small absorption probability inside the cavity, the heralding efficiency can approach unity if the nonresonant partner is used as a heralded single photon. Finally, we also show that continuous-wave excitation of a 1-cm-long cavity resonant for the pump and copropagating fields can provide photon pairs in a narrow bandwidth of 10 MHz with spectral brightness as high as $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $({\text{s}\phantom{\rule{4.pt}{0ex}}\text{mW}\phantom{\rule{4.pt}{0ex}}\text{MHz})}^{\ensuremath{-}1}$. This source can efficiently couple with quantum memories and can find applications in quantum information processing and communication.
Abstract We demonstrate an ultrabright, narrow-band and frequency-tunable photon-pair source based on cavity-enhanced spontaneous parametric down conversion (SPDC) which is compatible with atomic transition of rubidium D 2 -line (780 nm) or cesium D 2 -line (852 nm). With the pump beam alternating between a high and a low power phase, the output is switching between the optical parametric oscillator (OPO) and photon-pair generation mode. We utilize the OPO output light to lock the cavity length to maintain the double resonances of signal and idler, as well as to lock the signal frequency to cesium atomic transition. With a type-II phase matching and a double-passed pump scheme such that the cluster frequency spacing is larger than the SPDC bandwidth, the photon-pair output is in a nearly single-mode operation as confirmed by a scanning Fabry–Perot interferometer with its output detected by a photomultiplier. The achieved generation and detection rates are 7.24 × 10 5 and 6142 s −1 mW −1 , respectively. The correlation time of the photon pair is 21.6(2.2) ns, corresponding to a bandwidth of 2 π × 6.6(6) MHz. The spectral brightness is 1.06 × 10 5 s −1 mW −1 MHz −1 . This is a relatively high value under a single-mode operation with the cavity-SPDC scheme. The generated single photons can be readily used in experiments related to atomic quantum memories.
Cavity-enhanced spontaneous parametric down-conversion is capable of efficient generation of single photons with suitable spectral properties for interfacing with the atoms. However, beside the remarkable progress of this technique, multimode longitudinal emission remains a major drawback. Here, we demonstrate a bright source of single photons that overcomes this limitation by a novel mode-selection technique based on the introduction of an additional birefringent element to the cavity. This enables us to tune the double resonance condition independent of the phase matching and thus to achieve single-mode operation without mode filters. Our source emits single-frequency-mode photons at 852 nm, which is compatible to the Cs D2 line, with a bandwidth of 10.9 MHz and a detected photon-pair rate of 2.5 kHz at 10 mW of pump power, while maintaining g(2)(0) = 0.13. The efficiency of our source is further underlined by detecting a four-photon rate of 0.28 Hz at 20 mW of pump power. These detected rates correspond to a photon-pair generation rate of 47.5 Hz, and a four-photon generation rate of 37 Hz. Such photon generation rates open up a variety of new applications reaching from hybrid light-matter interactions to optical quantum information tasks based on long temporal coherence.
Efficiently generating photon pairs with high heralding efficiency and high single photon purity that are bandwidth matched to quantum emitters, quantum memories, and other matter-based qubits is critical for quantum networking applications. However, nonlinear optics-based sources require substantial spectral engineering to overcome the orders of magnitude bandwidth mismatch between those sources and qubit systems. A popular solution is cavity-enhanced spontaneous parametric down conversion (SPDC) where the cavity sets the photon bandwidth and simultaneously enhances the spectral brightness of the SPDC. Bulk, free-space configurations are generally required to achieve the MHz-scale bandwidths required to interface with most qubit systems. Replicating these in scalable integrated photonic architectures is an ongoing challenge due to the much higher propagation losses that limit the size and linewidth of chip-based resonators. We show here how an intra-cavity slow light medium, acting as an ultra-narrow filter, would enable narrowband photon pair generation in broadband cavities with high single photon purity and without compromising the heralding efficiency. We show that such metrics can be readily realized in erbium doped thin-film lithium niobate microrings using realistic design parameters.