[Paper Review] High-fidelity parametric beamsplitting with a parity-protected converter

Yao Lu, Aniket Maiti+7 more|arXiv (Cornell University)|Mar 2, 2023

Quantum Information and Cryptography69 references7 citations

TL;DR

The paper demonstrates a fast, high-fidelity parametric beamsplitter between two high-Q microwave cavities using a differentially-driven parity-protected DC-SQUID converter, achieving over 99.98% beamsplitter fidelity with suppressed coupler-induced decoherence.

ABSTRACT

Fast, high-fidelity operations between microwave resonators are an important tool for bosonic quantum computation and simulation with superconducting circuits. An attractive approach for implementing these operations is to couple these resonators via a nonlinear converter and actuate parametric processes with RF drives. It can be challenging to make these processes simultaneously fast and high fidelity, since this requires introducing strong drives without activating parasitic processes or introducing additional decoherence channels. We show that in addition to a careful management of drive frequencies and the spectrum of environmental noise, leveraging the inbuilt symmetries of the converter Hamiltonian can suppress unwanted nonlinear interactions, preventing converter-induced decoherence. We demonstrate these principles using a differentially-driven DC-SQUID as our converter, coupled to two high-Q microwave cavities. Using this architecture, we engineer a highly-coherent beamsplitter and fast ($\sim$ 100 ns) swaps between the cavities, limited primarily by their intrinsic single-photon loss. We characterize this beamsplitter in the cavities' joint single-photon subspace, and show that we can detect and post-select photon loss events to achieve a beamsplitter gate fidelity exceeding 99.98$\%$, which to our knowledge far surpasses the current state of the art.

Motivation & Objective

• Motivate fast, high-fidelity two-mode interactions between microwave resonators for bosonic quantum computation and simulation.
• Address parasitic processes and drive-induced decoherence that limit prior beamsplitters.
• Leverage symmetry (parity protection) and differential driving to suppress unwanted nonlinear interactions.
• Demonstrate a beamsplitter between two high-Q cavities with coherence limited primarily by intrinsic photon loss.

Proposed method

• Use a symmetric DC-SQUID (parity-protected) as a nonlinear converter to couple two high-Q cavities.
• Drive the converter differentially via a buffer (λ/4 mode) to implement a resonant, tunable beamsplitter between cavities.
• Derive a beamsplitter Hamiltonian with g_BS proportional to J1(|φ_d1|)J1(|φ_d2|) and Δ_BS = Δ_ab − Δ_d + Δ_Z,ab.
• Engineer a differential drive to keep the coupler in its ground state and suppress drive-induced excitations.
• Characterize performance in the joint single-photon subspace (dual-rail qubit) and perform randomized benchmarking with leakage detection.
• Estimate decoherence-limited fidelity from κ_BS and g_BS to assess gate performance.

Figure 1: The differentially driven SQUID as a parity protected converter. a , The symmetric DC-SQUID contains two orthogonal modes SQUIDModesLecocq2011 ; KamalSQUIDModes , the common mode (coupler) and the differential mode (actuator). We selectively couple the former to two bosonic modes and the l

Experimental results

Research questions

• RQ1 Can a parity-protected, differential-drive converter enable fast, high-fidelity beamsplitting between two high-Q cavities?
• RQ2 What are the dominant decoherence channels when performing parametric beamsplitting, and can symmetry suppression mitigate coupler-induced errors?
• RQ3 How does the beamsplitter perform in the single-photon (dual-rail) subspace under realistic drive strengths and losses?
• RQ4 To what extent can post-selection on photon loss events improve effective gate fidelity?
• RQ5 Is the approach scalable to other bosonic modes or multi-mode operations beyond the demonstrated dual-rail beamsplitter?

Key findings

• Beamsplitter achieves g_BS/2π > 5 MHz at the operating point.
• Decoherence-limited fidelity exceeds 99.9% across a range of drive strengths.
• Coupler excitation remains negligible with drive amplitudes up to the operating point, showing no monotonic increase with drive.
• Heating of the coupler is suppressed to below ~4×10^−5 excitations per swap, consistent with undriven background levels.
• Raw randomized benchmarking yields an effective unselected beamsplitter fidelity of 99.941(1)%, with leakage-detected fidelity 99.985(1)%.
• Fidelity is ultimately limited by cavity photon loss, preserving the noise bias beneficial for bosonic encodings.

Figure 2: Beamsplitting with the differentially-driven SQUID. a , Beamsplitting implements an effective driven Rabi evolution in the Bloch sphere of the dual-rail qubit formed by the single photon subspace Alice and Bob, where decay can be detected by monitoring the vacuum state. b , Resonant evolut