[Paper Review] Quantum-enabled continuous microwave-to-optics frequency conversion

Han Zhao, William David Chen+2 more|arXiv (Cornell University)|Jun 4, 2024

Advanced Photonic Communication Systems4 citations

TL;DR

This paper presents a continuous-wave, quantum-enabled microwave-to-optical frequency converter using a silicon-based electro-optomechanical platform that achieves low noise and high efficiency. By leveraging electrostatic and optomechanical interactions in single-crystal silicon, the device enables ground-state radiative cooling and demonstrates an input-referred added noise of 0.58 photons and an external efficiency of 0.47%, with an efficiency-bandwidth product exceeding prior works by two orders of magnitude.

ABSTRACT

A quantum interface between microwave and optical photons is essential for entangling remote superconducting quantum processors. To preserve fragile quantum states, a transducer must operate efficiently while generating less than one photon of noise referred to its input. Here, we present a platform that meets these criteria, utilizing a combination of electrostatic and optomechanical interactions in devices made entirely from crystalline silicon. This platform's small mechanical dissipation and low optical absorption enable ground-state radiative cooling, resulting in quantum-enabled operation with a continuous laser drive. Under the optimal settings for high efficiency (low noise), we measure an external efficiency of $2.2\%$ ($0.47\%$) and an input-referred added noise of $0.94$ ($0.58$) in microwave-to-optics conversion. We quantify the transducer throughput using the efficiency-bandwidth product, finding it exceeds previous demonstrations with similar noise performance by approximately two orders of magnitude, thereby paving a practical path to interconnecting remote superconducting qubits.

Motivation & Objective

• To develop a continuous-wave microwave-to-optical transducer that operates in the quantum-enabled regime with minimal added noise.
• To overcome the limitations of pulsed operation in prior systems, which reduce photon flux and hinder multi-node networking.
• To enable efficient, low-noise conversion between microwave and optical photons for connecting remote superconducting quantum processors.
• To leverage the low loss and high quality factors of single-crystal silicon for mechanical and optical modes to achieve ground-state cooling and quantum-limited operation.

Proposed method

• The transducer uses electrostatic coupling between microwave and mechanical modes, and optomechanical coupling between mechanical and optical modes, in a monolithic silicon platform.
• Electromechanical coupling is enhanced via a high-impedance microwave resonator design, increasing the coupling rate proportionally to the square root of impedance.
• Mechanical modes are confined using phononic bandgap structures to reduce radiation loss and enhance coupling to optical and microwave fields.
• The system achieves ground-state radiative cooling through low mechanical dissipation and low optical absorption in single-crystal silicon.
• Continuous-wave operation is enabled by eliminating the need for strong electrical pumps, reducing thermal noise.
• Efficiency and noise are characterized using coherent scattering measurements and input-referred added noise calculations based on detected optical power and conversion efficiency.

Figure 1: Electro-optomechanical transduction. a Schematic diagram of an electro-optomechanical transducer. b Simulated displacement profile of the mechanical mode, showcasing its extended energy distribution. The insets show the microwave and optical field profiles. c Optical microscope image of a

Experimental results

Research questions

• RQ1 Can a continuous-wave microwave-to-optical transducer achieve quantum-limited performance with input-referred added noise below one photon?
• RQ2 How does the efficiency-bandwidth product of a continuous transducer compare to pulsed systems with similar noise performance?
• RQ3 Can electrostatic and optomechanical coupling in single-crystal silicon enable ground-state cooling and low-noise operation without strong electrical pumps?
• RQ4 What is the maximum external conversion efficiency achievable under quantum-enabled conditions in a fully integrated silicon platform?
• RQ5 To what extent does the use of a continuous laser drive improve the practicality of interconnecting remote superconducting qubits?

Key findings

• The transducer achieves an external conversion efficiency of 2.2% (0.47% under optimal conditions) with an input-referred added noise of 0.94 (0.58) photons, confirming operation in the quantum-enabled regime.
• The efficiency-bandwidth product reaches 1900 Hz (470 Hz for the low-noise point), exceeding previous demonstrations by approximately two orders of magnitude for similar noise levels.
• Ground-state radiative cooling is achieved due to the low mechanical dissipation and low optical absorption of single-crystal silicon, enabling stable continuous operation.
• The device operates without strong electrical pumps, reducing thermal noise and enabling continuous-wave operation critical for scalable quantum networks.
• The measured performance outperforms recent pulsed systems in terms of throughput, as quantified by the efficiency-bandwidth-product metric.
• The platform’s material-agnostic electrostatic coupling allows for scalable integration and compatibility with superconducting qubit architectures.

Figure 2: Transducer characterization a Measured electromechanical coupling rate vs DC voltage bias. The coupling rate is determined at each voltage by fitting the external decay rate of the mechanical mode. b Measured optomechanical coupling rate versus intracavity photon number. The coupling rate