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Optical and Quantum Sensing and Precision Metrology II

Unlocking quantum potential with widely tunable OPO lasers: from frequency conversion to atomic transitions in hard-to-reach infrared

Siamak Dadras, Andrew Ross, Pei-Wen Tsai, Christopher Haimberger, Adam T. Heiniger

Published Mar 19, 2025• 0 citations
Publisher

Abstract

Widely tunable optical parametric oscillator (OPO) lasers have recently shown great potential in quantum applications. Their ability to generate low noise, Watt-level optical power and access arbitrary wavelengths in the near-IR and mid-IR makes them ideal for nonlinear frequency conversion of non-telecom photons from atomic or solid-state systems into telecom photons, enabling photonic interconnects in quantum computing, sensing, and communication. Additionally, OPOs have been used for inducing high-Rabi-frequency optical excitations, such as in Rydberg sensing, by providing low-noise, high-power coherent light at hard-to-reach wavelengths. This presentation highlights the design of two OPO systems by TOPTICA Photonics, focusing on their quantum applications, including photon frequency conversion and atomic transitions at exotic wavelengths in the near-IR and mid-IR.

DOI
10.1117/12.3051900

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Widely tunable optical parametric oscillator (OPO) lasers have recently shown great potential in quantum applications. Their ability to generate low noise, Watt-level optical power and access arbitrary wavelengths in the near-IR and mid-IR makes them ideal for nonlinear frequency conversion of non-telecom photons from atomic or solid-state systems into telecom photons, enabling photonic interconnects in quantum computing, sensing, and communication. Additionally, OPOs have been used for inducing high-Rabi-frequency optical excitations, such as in Rydberg sensing, by providing low-noise, high-power coherent light at hard-to-reach wavelengths. This presentation highlights the design of two OPO systems by TOPTICA Photonics, focusing on their quantum applications, including photon frequency conversion and atomic transitions at exotic wavelengths in the near-IR and mid-IR.
A frequency-tunable source of squeezed light would allow investigations of the atom-field interaction in a regime where fundamental new phenomena have been predicted.1 There are also opportunities for spectroscopy with sensitivity beyond the shot-noise limit. Toward these objectives, in our experiment we employ a single-frequency Ti-sapphire laser with tuning optimized around 852 nm (cesium resonance transition). The infrared output of the laser is frequency doubled in an external buildup cavity with a KNbO3 crystal as the nonlinear element. Efficient doubling provides 0.65 W of tunable blue output for 1.35 W of infrared input. This blue output is then used to pump an optical parametric oscillator (OPO) containing a second KNbO3 crystal for downconversion back to the infrared. The high power of the blue pump that we have achieved and the large nonlinearity of KNbO3 allows us to employ an OPO that is resonant only for the subharmonic infrared light. This design facilitates tuning of the OPO compared with the case in which the blue pump and the infrared are both resonant. The doubling cavity, OPO, and laser itself are locked relative to a stable reference cavity by an rf sideband technique, thus providing the possibility for tunability of the system with a single knob.
SummaryScalable quantum communication relies on transferring entangled photons (flying qubits) between distant nodes via fiber networks in the telecom C-band to minimize fiber loss. Quantum frequency conversion bridges VIS/NIR photon emission from stationary qubits to telecom wavelengths, with laser synchronization via optical frequency combs ensuring precise wavelength matching. Using an absolutely referenced, tunable OPO and coherent frequency conversion, we demonstrate a setup for matching the photon wavelengths of Ca⁺ ions and SnV centers, transmitted over a 14 km fiber link.
Photons as flying qubits are ideal carriers of quantum information. However, efficient quantum interfaces which transfer flying into stationary qubits and vice versa are needed to realize complex quantum networks. Stationary qubit systems (atoms, ions, or solid state implementations) couple to photons via characteristic optical transitions. In order to establish key quantum operations, such as coherent photon storage, entanglement swapping, etc., high-quality photons with a tailored spectral, spatial, and temporal mode are needed. One way to generate these photons is to use the stationary qubit systems themselves. Another approach which does not impose limitations in terms of the photon’s wavelength or bandwidth relies on all-optical single photon sources. An example is spontaneous parametric down conversion (SPDC) inside an optical cavity with well-defined linewidth. In SPDC, a pump photon incident on a nonlinear medium is split into two photons of lower energy that are often referred to as signal and idler. Resonating the fluorescent fields, the setup becomes an optical parametric oscillator (OPO) far below the threshold. The usually THz broad emission spectrum of SPDC then follows the Airy function of the resonator. The spectral bandwidth of the generated photons can easily reach values of several MHz, which is comparable to atom resonance linewidths. By introducing additional losses or simply changing the cavity mirror reflectivities the bandwidth is tunable up to several 100 MHz. This is comparable to the bandwidth of single photons generated by e.g. quantum dots.
In a typical quantum repeater scenario, remote quantum memories have to be interconnected by flying qubits transmitted in quantum channels. e.g. single photons in photonic fiber links. To establish long-distance communication, photons in the low-loss telecom wavlength regime are desirable. However, the majority of the candidates for quantum memories such as trapped atoms or ions, rare earth ensembles, color centers in diamond, atomic ensembles etc. possess optical transitions merely in the visible or near-infrared spectral regime below 1000 nm. One possible solution is the transduction of these wavelengths to the telecom region via quantum frequency conversion (QFC) based on three-wave-mixing processes in a nonlinear χ(2)-medium. Technical achievements in recent years enabled high conversion efficiencies and low conversion-induced noise contributions resulting in signal-to-background ratios high enough to demonstrate the preservation of several classical and quantum properties of the photons, e.g. first- and second-order coherence [1], time-bin entanglement [2], polarization entanglement [3] or indistinguishability [4]. QFC thus provides a powerful toolbox to interface quantum systems to the telecom bands and beyond that yet to interconnect dissimilar quantum systems.
A single-frequency optical parametric oscillator (OPO) operating in the wavelength range around 2 µm is reported. The OPO comprises a periodically poled LiNbO 3 (PPLN) crystal for quasi-phase matching and a four-mirror ring-cavity resonant to the signal. The OPO signal and idler are tuned from 1.89 to 2.05 µm and from 2.21 to 2.42 µm, respectively, by changing the crystal temperature. Output powers in excess of 2 W are demonstrated at a pump power of 9 W for both the signal and idler over all of the respective tuning ranges. A detailed characterization of the intensity and frequency noise is presented. The OPO exhibits an integrated intensity noise below 0.1% and a linewidth of ∼58 kHz for an observation time of 1 ms. The reported performance makes this source an ideal candidate for the generation of squeezed light around 2 µm via cascaded second-harmonic generation (SHG) and parametric downconversion (PDC).