[Paper Review] Quantum-limited millimeter wave to optical transduction

Aishwarya Kumar, Aziza Suleymanzade+5 more|arXiv (Cornell University)|Jul 20, 2022

Cold Atom Physics and Bose-Einstein Condensates4 citations

TL;DR

This paper demonstrates quantum-limited transduction of millimeter-wave photons to optical photons using cold 85Rb atoms as a hybrid transducer in a cryogenic environment. By coupling the atoms simultaneously to a three-dimensional superconducting mmwave resonator and a vibration-stabilized optical cavity, the authors achieve 58(11)% internal conversion efficiency, 360(20) kHz bandwidth, and only 0.6 added thermal photons, confirming near-quantum-limited performance with no free parameters in theory.

ABSTRACT

Long distance transmission of quantum information is a central ingredient of distributed quantum information processors for both computing and secure communication. Transmission between superconducting/solid-state quantum processors necessitates transduction of individual microwave photons to optical photons. Current approaches to transduction employ solid state links between electrical and optical domains, facing challenges from the thermal noise added by the strong classical pumps required for high conversion efficiency and bandwidth. Neutral atoms are an attractive alternative transducer: they couple strongly to optical photons in their ground states, and to microwave/millimeter-wave photons in their Rydberg states. Nonetheless, strong coupling of atoms to both types of photons, in a cryogenic environment to minimize thermal noise, has yet to be achieved. Here we demonstrate quantum-limited transduction of millimeter-wave (mmwave) photons into optical photons using cold $^{85}$Rb atoms as the transducer. We achieve this by coupling an ensemble of atoms simultaneously to a first-of-its-kind, optically accessible three-dimensional superconducting resonator, and a vibration suppressed optical cavity, in a cryogenic ($5$ K) environment. We measure an internal conversion efficiency of $58(11)\%$, a conversion bandwidth of $360(20)$ kHz and added thermal noise of $0.6$ photons, in agreement with a parameter-free theory. Extensions to this technique will allow near-unity efficiency transduction in both the mmwave and microwave regimes. More broadly, this state-of-the-art platform opens a new field of hybrid mmwave/optical quantum science, with prospects for operation deep in the strong coupling regime for efficient generation of metrologically or computationally useful entangled states and quantum simulation/computation with strong nonlocal interactions.

Motivation & Objective

• To enable efficient, low-noise transduction of quantum information from superconducting qubits (microwave/mmwave domain) to optical photons for long-distance quantum communication.
• To overcome thermal noise and pump-induced noise limitations in solid-state transduction platforms by using cold neutral atoms as a transducer.
• To achieve quantum-limited performance in mmwave-to-optical conversion by operating in a cryogenic environment with high-fidelity coupling to both mmwave and optical modes.
• To demonstrate a hybrid platform combining superconducting resonators and optical cavities for scalable quantum interface applications.
• To enable future operation in the strong coupling regime for generating entangled states and enabling quantum simulation with nonlocal interactions.

Proposed method

• Utilize cold 85Rb atoms in a 1D optical lattice to couple simultaneously to a three-dimensional superconducting mmwave resonator and a high-Q optical cavity at 5 K.
• Employ a four-wave mixing process via Rydberg states, using 481 nm and 297 nm lasers to coherently mediate transduction between mmwave and optical photons.
• Implement quasi-phase-matching via an intracavity 1560 nm optical lattice to compensate for momentum recoil and enable flexible beam geometry without loss of collective enhancement.
• Use a spring-loaded mirror mounting system and vibration suppression to stabilize the optical cavity, achieving a linewidth of 2π × 1.7 MHz.
• Employ a niobium-based mmwave resonator with a 5 K operating temperature and a linewidth of 2π × 800 kHz to minimize thermal noise.
• Apply UV-dressing to the 35P1/2 state to enhance cooperativity by a factor of 2 while maintaining low Rydberg population due to low atomic density and small C6 coefficients.

Experimental results

Research questions

• RQ1 Can cold Rydberg atoms enable quantum-limited transduction between mmwave and optical domains in a cryogenic environment?
• RQ2 What is the achievable conversion efficiency and bandwidth when coupling atoms to both a superconducting mmwave resonator and an optical cavity simultaneously?
• RQ3 To what extent can thermal noise be suppressed in a hybrid transduction platform using cold atoms and cryogenic operation?
• RQ4 How does quasi-phase-matching via an optical lattice enable flexible laser beam geometry without degrading collective enhancement?
• RQ5 Can the system operate near-resonantly on atomic transitions to achieve high efficiency with moderate laser power, avoiding the need for high pump powers?

Key findings

• The system achieves an internal conversion efficiency of 58(11)%, demonstrating high-fidelity transduction between mmwave and optical photons.
• The conversion bandwidth is measured at 360(20) kHz, indicating fast transduction suitable for qubit-scale applications.
• Only 0.6 added thermal photons are measured, confirming quantum-limited performance consistent with a parameter-free theoretical model.
• The optical cavity exhibits a linewidth of 2π × 1.7 MHz (FWHM), indicating high quality factor and stability under cryogenic conditions.
• The mmwave resonator operates with a linewidth of 2π × 800 kHz at 5 K, enabling low thermal noise operation critical for quantum coherence.
• The use of a 1560 nm optical lattice enables full quasi-phase-matching, allowing co-propagating 481 nm and 297 nm beams from any direction without loss of enhancement.