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Coherent conversion between optical and microwave photons in Rydberg gases

Martin Kiffner, Amir Feizpour, K. T. Kaczmarek, Dieter Jaksch, Joshua Nunn

Published Jan 26, 2016• 2 citations
OpenAlex

Abstract

Quantum information encoded in optical photons can be transmitted over long distances with very high information density, and suffers from negligible thermal noise at room temperature. On the other hand, microwave photons at cryogenic temperatures can be confined in high quality resonators and strongly coupled to solid-state qubits, providing a quantum bus to connect qubits and a route to deterministic photonic non-linearities. The coherent interconversion of microwave and optical photons has therefore recently emerged as a highly desirable capability that would enable freely-scalable networks of optically-linked qubits, or large-scale photonic information processing with multi-photon interactions mediated by microwaves. Here, we propose a route to efficient and coherent microwave-optical conversion based on frequency mixing in Rydberg atoms. The interaction requires no microfabricated components or cavities, and is tunable, broadband, and both spatially and spectrally multimode.

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Quantum information encoded in optical photons can be transmitted over long distances with very high information density, and suffers from negligible thermal noise at room temperature. On the other hand, microwave photons at cryogenic temperatures can be confined in high quality resonators and strongly coupled to solid-state qubits, providing a quantum bus to connect qubits and a route to deterministic photonic non-linearities. The coherent interconversion of microwave and optical photons has therefore recently emerged as a highly desirable capability that would enable freely-scalable networks of optically-linked qubits, or large-scale photonic information processing with multi-photon interactions mediated by microwaves. Here, we propose a route to efficient and coherent microwave-optical conversion based on frequency mixing in Rydberg atoms. The interaction requires no microfabricated components or cavities, and is tunable, broadband, and both spatially and spectrally multimode.
Abstract We propose an efficient scheme for microwave-to-optical quantum frequency conversion mediated by a single atom simultaneously coupled to a superconducting microwave resonator and an optical cavity. The atom mediates efficient interactions between the two electromagnetic modes via non-linear frequency mixing owing to the presence of huge microwave transition dipole moments between nearby Rydberg states as well as suitable optical transitions between low-lying states. In the regime of four-wave mixing with off-resonant fields, the excited states can be adiabatically eliminated, leading to an effective Hamiltonian that directly describes the two-mode interaction responsible for the frequency conversion. However, in this regime, the converter is slow, hence sensitive to losses due to cavity decays. To enhance the performance of the conversion process, we employ pulse engineering to tailor the atom-field couplings. The resulting protocol exhibits high robustness against decoherence and parameter fluctuations, making it well suited for realistic experimental implementations. Our scheme provides a promising route toward high-fidelity and scalable hybrid quantum interfaces connecting superconducting and optical quantum networks.
Abstract The coupling of microwave and optical systems presents an immense challenge due to the natural incompatibility of energies, but potential applications range from optical interconnects for quantum computers to next-generation quantum microwave sensors, detectors and coherent imagers. Several of the engineered platforms that have emerged are constrained by specific conditions, such as cryogenic environments, impulse protocols or narrowband fields. Here we employ Rydberg atoms that allow the wideband coupling of optical and microwave photons at room temperature with the use of a modest set-up. We present continuous-wave conversion of a 13.9 GHz field to a near-infrared optical signal using an ensemble of Rydberg atoms via a free-space six-wave mixing process designed to minimize noise interference from any nearby frequencies. The Rydberg photonic converter exhibits a conversion dynamic range of 57 dB and a wide conversion bandwidth of 16 MHz. Using photon counting, we demonstrate the readout of photons of free-space 300 K thermal background radiation at 1.59 nV cm −1 rad −1/2 s −1/2 (3.98 nV cm −1 Hz −1/2 ) with a sensitivity down to 3.8 K of noise-equivalent temperature, allowing us to observe Hanbury Brown and Twiss interference of microwave photons.
Quantum frequency conversion between superconducting (SC) microwave qubits and telecom optical photons is critical for long distance communication of networked SC quantum processors. While SC qubits operate at cryogenic temperatures to sustain their quantum coherence, converting them to the optical domain enables transferring the quantum states to room temperature and over long distances. For such a quantum state transduction process, several schemes have been investigated, including optomechanics, magnons, piezomechanics, and Pockels electro-optics (EO). The EO conversion approach is particularly attractive since it is broadband, low noise, mechanically and thermally stable (i.e., does not rely on freestanding structures), scalable (largescale integration of EO devices with superconducting circuits is possible), and tunable (e.g., using bias voltages). In this presentation we review our progress in an electro-optic based quantum transduction and discuss the promises and challenges.
We show that cold Rydberg gases enable an efficient six-wave mixing process where microwave or terahertz fields are coherently converted into optical fields and vice versa. This process is made possible by the long lifetime of Rydberg states, the strong coupling of millimeter waves to Rydberg transitions and a quantum interference effect related to Electromagnetically Induced Transparency. We show that conversion efficiencies within an independent atom approach are of the order of 95% and analyse the impact of dipole-dipole interactions on our scheme. We find that effective conversion efficiencies in the presence of Rydberg-Rydberg interactions can still be as high as 85% based on an implementation with Rubidium atoms. Our frequency conversion scheme does not require cavities and can be implemented for a broad spectrum of terahertz and microwave fields due to the abundance of transitions within the Rydberg manifold.
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^{85}85Rb 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 (555 K) environment. We measure an internal conversion efficiency of 58(11)%58(11)\%58(11)%, a conversion bandwidth of 360(20)360(20)360(20) kHz and added thermal noise of 0.60.60.6 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.