Cavity-enhanced Spontaneous Parametric Down-conversion Research Articles

Cavity-enhanced induced coherence without induced emission

This paper presents a theoretical study of the enhancement of Zou-Wang-Mandel (ZWM) interferometry through cavity-enhanced spontaneous parametric down-conversion (SPDC) processes producing frequency-entangled biphotons. The ZWM interferometry shows the capability to generate interference effects between single signal photons via indistinguishability between the entangled idler photons. This paper extends the foundational principles of ZWM interferometry by integrating cavity-enhanced SPDCs, aiming to narrow photon bandwidths for improved coherence and photon pair generation efficiency, which is critical for applications in quantum information technologies, quantum encryption, and quantum imaging. This work explores the theoretical implication of employing singly resonant optical parametric oscillators within the ZWM interferometer to produce narrow-band single photons. By combining cavity-enhanced SPDCs with ZWM interferometry, this study fills a gap in current theoretical proposals, offering significant advancements in quantum cryptography and network applications that require reliable, narrow-band single photons.

Narrow-band photon pair generation through cavity-enhanced spontaneous parametric down-conversion

Narrow-band photon pair generation through cavity-enhanced spontaneous parametric down-conversion

Telecom quantum photonic interface for a 40Ca+ single-ion quantum memory

Entanglement-based quantum networks require quantum photonic interfaces between stationary quantum memories and photons, enabling entanglement distribution. Here we present such a photonic interface, designed for connecting a 40Ca+ single-ion quantum memory to the telecom C-band. The interface combines a memory-resonant, cavity-enhanced spontaneous parametric down-conversion photon pair source with bi-directional polarization-conserving quantum frequency conversion. We demonstrate preservation of high-fidelity entanglement during conversion, fiber transmission over up to 40 km and back-conversion to the memory wavelength. Even for the longest distance and bi-directional conversion the entanglement fidelity remains larger than 95% (98%) without (with) background correction.

Bright narrowband photon pairs generated in an intra-cavity 2N walk-off compensating configuration

We studied theoretically the influence of the walk-off effect on the spectral brightness of the type-I cavity-enhanced spontaneous parametric down-conversion. The results show that the spectral brightness of the generated entangled photon pairs can be improved significantly by using a 2N walk-off compensating configuration, in which N pairs of nonlinear crystals with their walk-off angles of opposite signs are stacked and bonded in sequence along the propagation direction. The spectral brightness of the entangled photon pairs are expected theoretically to be 1.27×105 pairs/(smWMHz) for N = 4 with a submegahertz bandwidth in a triple-resonant cavity configuration. Our results provide a solid theoretical basis for the generation of entangled photon-pair sources of high spectral brightness and narrow bandwidth, which may have potential applications in quantum technology.

Birefringence compensation utilizing quarter-wave plates in cavity-enhanced spontaneous parametric down-conversion process

Single-photon source is an essential element in quantum information processing, and extensively used in the proof-in-principle demonstration in quantum physics, quantum imaging, quantum cryptography, etc. Considering the operating temperature and system complexity, it is a favorable option to choose spontaneous parametric down-conversion (SPDC) combined with the enhancement effect of a cavity. When generating significant single-photon source via the cavity-enhanced type-II spontaneous parametric down-conversion method, there appears inevitable birefringence effect which obviously influences the resonance condition. In order to compensate for birefringence effect, different approaches have been used such as introducing compensating crystal, placing a half-wave plate, tuning the temperature of the nonlinear crystal, customized conjoined double-cavity structure, and cluster effect. In this work, two quarter-wave plates, with an angle of 45° between the optical axis and the crystal axis, are placed in the cavity to ensure the double resonance of signal photon and idler photon. In the process, the signal photon and idler photon generated simultaneously have different polarizations perpendicular to each other through the type-II nonlinear crystal. Considering horizontally polarized photon, its polarization is changed into left circular polarization by the first quarter-wave plate and then returns as vertical polarization. After traversing a long optical path, it shifts to right circular polarization through the second quarter-wave plate. When the photon passes through the same quarter-wave plate again, the polarization state is originally converted into horizontal polarization state. Then the photon completes a round-trip. The other photon with vertical polarization experiences the same process. As a result, the signal photon and idler photon travel identical optical path. The general explanation is described by the Jones matrices, with the emphasis on the transformation of the polarizations of photons. This method can effectively compensate for birefringence effect, achieving double resonance by using a relatively simple device under the condition of smaller intra-cavity loss and more flexible for adjustment. The signal (idler) photon has a sub-natural linewidth of <inline-formula><tex-math id="Z-20230611154134">\begin{document}$1.01( 1.08 )\;{\rm{MHz}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20230422_Z-20230611154134.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20230422_Z-20230611154134.png"/></alternatives></inline-formula>, demonstrating the feasibility of the proposed technique. This introduced compensating method paves the way to the realization of single-photon quantum source applied to the research of single-photon-single-atom quantum information processing, quantum interface and quantum network node with a single cesium atom confined in the strongly coupled cavity quantum electrodynamics system.

Single-Photon Storage in a Ground-State Vapor Cell Quantum Memory

Interfaced single-photon sources and quantum memories for photons together form a foundational component of quantum technology. Achieving compatibility between heterogeneous, state-of-the-art devices is a long-standing challenge. We built and successfully interfaced a heralded single-photon source based on cavity-enhanced spontaneous parametric down-conversion in ppKTP and a matched memory based on electromagnetically induced transparency in warm $^{87}$Rb vapor. The bandwidth of the photons emitted by the source is 370 MHz, placing its speed in the technologically relevant regime while remaining well within the acceptance bandwidth of the memory. Simultaneously, the experimental complexity is kept low, with all components operating at or above room temperature. Read-out noise of the memory is considerably reduced by exploiting polarization selection rules in the hyperfine structure of spin-polarized atoms. For the first time, we demonstrate single-photon storage and retrieval in a ground-state vapor cell memory, with $g_{c,\text{ret}}^{(2)}=0.177(23)$ demonstrating the single-photon character of the retrieved light. Our platform of single-photon source and atomic memory is attractive for future experiments on room-temperature quantum networks operating at high bandwidth.

Efficient quantum memory for photonic polarization qubits generated by cavity-enhanced spontaneous parametric downconversion.

Quantum memories, for storing then retrieving photonic quantum states on demand, are crucial components for scalable quantum technologies. Spontaneous parametric downconversion (SPDC) with a nonlinear crystal is the most widely used process for generating entangled photon pairs or heralded single photons. Despite the desirability of efficient quantum memories for SPDC-generated single photons, the storage and retrieval efficiencies achieved with this approach still fall below 50%, a threshold value for practical applications. Here, we report an efficiency of > 70% for the storage of heralded single photons generated by cavity-enhanced SPDC using atomic quantum memories based on electromagnetically induced transparency (EIT). In addition, we demonstrate the quantum memory for single-photon polarization qubits with a fidelity of ∼96%. This result paves the way towards the development of large-scale quantum networks.

Towards On-Demand Heralded Single-Photon Sources via Photon Blockade

Spontaneous parametric down-conversion (SPDC) in a laser pumped optical nonlinear medium can produce heralded single photons with a high purity but a very low yield. Improving the yield by increasing the pump power in SPDC inevitably reduces the purity due to excitation of multi-photon events. We propose a scheme to overcome this purity-yield trade-off by suppressing multi-photon events in a cavity-enhanced SPDC via the photon blockade effect. By introducing a strong photon-photon interaction into the intracavity medium and increasing the pump power, we can improve the available single-photon yield to larger than $90\%$, while maintaining a high purity of $99\%$, towards on-demand generation of single photons through the SPDC process. Our quasi-on-demand SPDC sources may boost single-photon-based quantum information technology.

General and complete description of temporal photon correlations in cavity-enhanced spontaneous parametric down-conversion

Heralded single-photon sources are the most commonly used sources for optical quantum technology applications. There is strong demand for accurate prediction of their spectral features and temporal correlations with ever-increasing precision. This is particularly important in connection with the intrinsically stochastic photon-pair generation process in heralded sources. Here we present a complete theoretical description of the temporal correlation of a signal-idler, signal-signal, and signal-signal-idler coincidences of photons generated by continuous-wave pumped cavity-enhanced spontaneous parametric down-conversion. The theory excellently predicts the measurements, which has been experimentally confirmed in our setup utilizing single-photon detectors with high temporal resolution. This enables us to resolve and analyze the multiphoton correlation functions in great detail.

An efficient, tunable, and robust source of narrow-band photon pairs at the 87Rb D1 line

We present an efficient and robust source of photons at the 87Rb D1-line (795 nm) with a narrow bandwidth of δ = 226(1) MHz. The source is based on non-degenerate, cavity-enhanced spontaneous parametric down-conversion in a monolithic optical parametric oscillator far below threshold. The setup allows for efficient coupling to single mode fibers. A heralding efficiency of ηheralded = 45(5) % is achieved, and the uncorrected number of detected photon pairs is 3.8 × 103/(s mW). For pair generation rates up to 5 × 105/s, the source emits heralded single photons with a normalized, heralded, second-order correlation function g c ( 2 ) < 0.01 . The source is intrinsically stable due to the monolithic configuration. Frequency drifts are on the order of δ/20 per hour without active feedback on the emission frequency. We achieved fine-tuning of the source frequency within a range of >2 GHz by applying mechanical strain.

Narrowband photon pairs with independent frequency tuning for quantum light-matter interactions.

We describe a cavity-enhanced spontaneous parametric down-conversion (CE-SPDC) source for narrowband photon pairs with filters designed such that 97.7% of the correlated photons are in a single mode of 4.3(4) MHz bandwidth. Type-II phase matching, a tuneable-birefringence resonator, MHz-resolution pump tuning, and tuneable Fabry-Perot filters are used to achieve independent signal and idler tuning. We map the CE-SPDC spectrum using difference frequency generation to precisely locate the emission clusters, demonstrate CE-SPDC driven atomic spectroscopy, and measure a contribution from unwanted modes of 7.7%. The generated photon pairs efficiently interact with neutral rubidium, a well-developed system for quantum networking and quantum simulation. The techniques are readily extensible to other material systems.

Stabilization of Transverse Modes for a High Finesse Near-Unstable Cavity

We develop a method to lock a high-finesse near-unstable Fabry–Perot (FP) cavity (F = 7330) to a frequency stable dye laser operating at 605.78 nm using the Pound–Drever–Hall technique. The experimental results show the feasibility of locking this cavity to different transverse modes. This method links the external FP cavity to the dye laser cavity, and a 379 kHz final linewidth of the FP cavity is achieved. Such a near-unstable cavity is potentially useful for cavity-enhanced spontaneous parametric down-conversion to generate narrow-band single photon or photon pairs in different transverse modes.

Storage of telecom-C-band heralded single photons with orbital-angular-momentum encoding in a crystal

A memory-based quantum repeater architecture provides a solution to distribute quantum information to an arbitrary long distance. Practical quantum repeaters are likely to be built in optical-fiber networks which take advantage of the low-loss transmission between quantum memory nodes. Most quantum memory platforms have characteristic atomic transitions away from the telecommunication band. A nondegenerate photon pair source is therefore useful for connection of a quantum memory to optical fibers. Here, we report a high-brightness narrowband photon-pair source which is compatible with a rare-earth-ion-doped crystal Pr3+:Y2SiO5. The photon-pair source is generated through a cavity-enhanced spontaneous parametric down-conversion process with the signal photon at 606 nm and the idler photon at 1540 nm. Moreover, using the telecom C-band idler photons for heralding, we demonstrate the reversible transfer of orbital-angular-momentum qubit between the signal photon and the quantum memory based on Pr3+:Y2SiO5.

Novel single-mode narrow-band photon source of high brightness tuned to cesium D2 line

Cavity-enhanced Spontaneous parametric down-conversion (SPDC) 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, multi-mode longitudinal emission remains as 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 photon-pair generation rate exceeding 47 KHz at 10 mW of pump power, while maintaining a low $g^{\left(2\right)}(0) =$ 0.13. The efficiency of our source is further underlined by measuring a four-photon generation rate of 37 Hz at 20 mW of pump power. This brightness opens up a variety of new applications reaching from hybrid light-matter interactions to optical quantum information tasks based on long temporal coherence.

Background and Review of Cavity-Enhanced Spontaneous Parametric Down-Conversion.

Spontaneous parametric down-conversion (SPDC) in a nonlinear crystal has been a workhorse for the generation of entangled and correlated single-photon pairs used for quantum communications applications for nearly three decades. However, as a naturally broadband process, the ability of SPDC to interface with the very narrow energy transitions in atomic ensembles for implementing quantum memories, which are needed for quantum repeaters to extend the reach of quantum communications, was initially limited. To overcome this limitation, the process was enhanced by placing the nonlinear crystal inside a resonating cavity. This modified process has some important advantages, including narrowing the spectral linewidth of generated photons into brighter resonant modes of the cavity, and the ability to lock the desired mode of the cavity to the targeted transition frequency of the atomic ensemble. This paper presents an overview of the principle of cavity-enhanced SPDC, a review of works to date using this technique, and an example of one of these implementations.

Background and Review of Cavity-Enhanced Spontaneous Parametric Down-Conversion

Background and Review of Cavity-Enhanced Spontaneous Parametric Down-Conversion

Background and Review of Cavity-Enhanced Spontaneous Parametric Down-Conversion

Background and Review of Cavity-Enhanced Spontaneous Parametric Down-Conversion

Generation of narrow-band single-photon states via spontaneous parametric down-conversion for quantum memories in doped crystals

We present experimental results on the generation of narrow-band single-photon states via cavity-enhanced spontaneous parametric down-conversion in a PPLN crystal and demonstrate the feasibility of conditional preparation of single 867-nm photons with a 70-MHz linewidth, compatible with optical quantum memory devices based on Nd3+-doped isototopically pure Y7LiF4 crystals.

Quantum Correlations between Single Telecom Photons and a Multimode On-Demand Solid-State Quantum Memory

Quantum correlations between long-lived quantum memories and telecom photons that can propagate with low loss in optical fibers are an essential resource for the realization of large-scale quantum information networks. Significant progress has been realized in this direction with atomic and solid-state systems. Here, we demonstrate quantum correlations between a telecom photon and a multimode on-demand solid state quantum memory. This is achieved by mapping a correlated single photon onto a spin collective excitation in a Pr3+:Y2SiO5 crystal for a controllable time. The stored single photons are generated by cavity-enhanced spontaneous parametric down-conversion and heralded by their partner photons at telecom wavelength. These results represent the first demonstration of a multimode on-demand solid state quantum memory for external quantum states of light. They provide an important resource for quantum repeaters and pave the way for the implementation of quantum information networks with distant solid state quantum nodes.Received 9 December 2016DOI:https://doi.org/10.1103/PhysRevX.7.021028Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum communicationQuantum information architectures & platformsQuantum memoriesQuantum Information

Generation of Narrow-Band Polarization-Entangled Photon Pairs at a Rubidium D1 Line

Using the process of cavity-enhanced spontaneous parametric down-conversion (SPDC), we generate a narrow-band polarization-entangled photon pair resonant on the rubidium (Rb) D1 line (795 nm). The degenerate single-mode photon pair is selected by multiple temperature controlled etalons. The linewidth of generated polarization-entangled photon pairs is 15 MHz which matches the typical atomic memory bandwidth. The measured Bell parameter for the polarization-entangled photons S=2.73+-0.04 which violates the Bell-CHSH inequality by ~18 standard deviations. The presented entangled photon pair source could be utilized in quantum communication and quantum computing based on quantum memories in atomic ensemble.