[Paper Review] Quantum entanglement between optical and microwave photonic qubits

Srujan Meesala, David P. Lake+7 more|arXiv (Cornell University)|Dec 21, 2023

Mechanical and Optical Resonators4 citations

TL;DR

This paper demonstrates a chip-scale source of entangled optical and microwave photonic qubits using a piezo-optomechanical transducer integrated with a superconducting resonator. By driving spontaneous parametric down-conversion in the transducer and employing dual-rail time-bin encoding, the authors achieve a lower-bound Bell state fidelity of 0.794, exceeding the classical limit and enabling scalable quantum network interconnects between superconducting processors and optical communication channels.

ABSTRACT

Entanglement is an extraordinary feature of quantum mechanics. Sources of entangled optical photons were essential to test the foundations of quantum physics through violations of Bell's inequalities. More recently, entangled many-body states have been realized via strong non-linear interactions in microwave circuits with superconducting qubits. Here we demonstrate a chip-scale source of entangled optical and microwave photonic qubits. Our device platform integrates a piezo-optomechanical transducer with a superconducting resonator which is robust under optical illumination. We drive a photon-pair generation process and employ a dual-rail encoding intrinsic to our system to prepare entangled states of microwave and optical photons. We place a lower bound on the fidelity of the entangled state by measuring microwave and optical photons in two orthogonal bases. This entanglement source can directly interface telecom wavelength time-bin qubits and GHz frequency superconducting qubits, two well-established platforms for quantum communication and computation, respectively.

Motivation & Objective

• To enable scalable interconnection between superconducting quantum processors and optical quantum networks by generating entangled microwave-optical photonic qubits on a single chip.
• To overcome challenges in microwave-optical entanglement generation, including weak nonlinearities, optical absorption, and integration losses.
• To implement a loss-insensitive, discrete-variable entanglement protocol using dual-rail encoding for both microwave and optical qubits.
• To verify entanglement fidelity via joint measurement in multiple bases, ensuring robustness against detection and collection losses.
• To enable direct mapping of microwave photonic qubits onto superconducting qubits for future quantum information processing.

Proposed method

• Utilizes a piezo-optomechanical transducer integrated with a superconducting kinetic inductance resonator to enable strong coupling between microwave and optical modes via mechanical phonons.
• Drives spontaneous parametric down-conversion (SPDC) in the transducer to generate correlated microwave and optical photon pairs.
• Employs dual-rail encoding: time-bin encoding for optical qubits (early/late time bins) and orthogonal microwave modes (labeled + and −) arising from hybridized acoustic-electrical resonances.
• Performs post-selection on single-photon events in the optical channel to isolate the computational subspace and reduce noise contributions.
• Uses maximum likelihood state tomography on conditional microwave heterodyne voltages to reconstruct density matrices and extract fidelity metrics.
• Applies a fidelity lower bound formula based on conditional probabilities in Z- and X-bases: $ F_{\text{lb}} = \frac{1}{2}(p_{\text{ee}}+p_{\text{ll}}-p_{\text{el}}-p_{\text{le}} + p_{{++}}+p_{{--}}-2\sqrt{p_{{+-}}p_{{-+}}}) $.

Experimental results

Research questions

• RQ1 Can a chip-scale device generate entangled microwave and optical photonic qubits with high fidelity using a piezo-optomechanical transducer?
• RQ2 Is the entanglement fidelity robust against optical and microwave detection losses, as required for realistic quantum networks?
• RQ3 Can dual-rail encoding in both microwave and optical domains preserve entanglement under experimental noise and finite conversion efficiency?
• RQ4 To what extent does pump-induced thermal noise limit the achievable Bell state fidelity in such a system?
• RQ5 Can the generated microwave photonic qubit be coherently mapped onto a superconducting qubit for future quantum information processing?

Key findings

• The experiment achieves a lower-bound Bell state fidelity of $ F_{\text{lb}} = 0.794^{+0.048}_{-0.071} $, exceeding the classical limit of 0.5 by more than four standard deviations.
• The measured fidelity is consistent with a theoretical model that accounts for pump-induced thermal noise, predicting a fidelity above 0.83.
• Conditional microwave state tomography confirms strong correlations in both Z- and X-bases, with entries in the computational subspace matching expected values for a Bell state.
• The fidelity remains robust against losses due to the use of post-selection and dual-rail encoding, which suppresses contributions from non-computational subspaces.
• The microwave-to-photonic conversion efficiency is measured at $ \eta_{\text{mw}} = 0.59 $, with the main fidelity degradation attributed to vacuum components from finite conversion efficiency.
• The system enables direct mapping of microwave photonic qubits onto superconducting qubits, such as transmons or qutrits, with minimal optical pump crosstalk.