[Paper Review] Benchmarking Gate Fidelities in a Si/SiGe Two-Qubit Device

Xiao Xue, Thomas F. Watson+8 more|arXiv (Cornell University)|Nov 9, 2018

Advancements in Semiconductor Devices and Circuit DesignEngineering42 citations

TL;DR

This paper performs a comprehensive fidelity characterization of single-qubit and two-qubit gates in silicon spin qubits using standard RB, interleaved RB variants, and introducing character randomized benchmarking (CRB) to reliably extract the two-qubit CPhase gate fidelity and analyze cross-talk and error correlations.

ABSTRACT

We report the first complete characterization of single-qubit and two-qubit gate fidelities in silicon-based spin qubits, including cross-talk and error correlations between the two qubits. To do so, we use a combination of standard randomized benchmarking and a recently introduced method called character randomized benchmarking, which allows for more reliable estimates of the two-qubit fidelity in this system. Interestingly, with character randomized benchmarking, the two-qubit CPhase gate fidelity can be obtained by studying the additional decay induced by interleaving the CPhase gate in a reference sequence of single-qubit gates only. This work sets the stage for further improvements in all the relevant gate fidelities in silicon spin qubits beyond the error threshold for fault-tolerant quantum computation.

Motivation & Objective

• Characterize single-qubit and two-qubit gate fidelities in a Si/SiGe spin-qubit device.
• Assess cross-talk and error correlations between the two qubits.
• Develop and apply character randomized benchmarking (CRB) to obtain reliable two-qubit gate fidelities.
• Evaluate interleaved benchmarking approaches to isolate the CPhase gate fidelity.
• Provide performance benchmarks relative to fault-tolerance thresholds for silicon spin qubits.

Proposed method

• Combine standard randomized benchmarking on individual qubits and simultaneous RB to assess cross-talk.
• Use interleaved RB to estimate fidelities of particular gates within reference sequences.
• Apply character randomized benchmarking (CRB) to extract decoupled decay channels and gate fidelities beyond the multi-qubit Clifford group.
• Interleave the CPhase gate in RB sequences and compute gate fidelity from depolarizing parameters using established relations.
• Analyze three decay channels via CRB: two single-qubit subspaces and the two-qubit parity subspace.
• Infer whether two-qubit errors are independent or correlated from CRB fits.

Experimental results

Research questions

• RQ1 What are the single-qubit gate fidelities for Q1 and Q2 in a Si/SiGe double-dot device?
• RQ2 How does simultaneous driving (cross-talk) affect single-qubit gate fidelities?
• RQ3 What is the fidelity of the two-qubit CPhase gate in this silicon spin-qubit system?
• RQ4 Can character randomized benchmarking reliably separate and quantify distinct error channels in a two-qubit system?
• RQ5 Are the two-qubit errors predominantly independent on each qubit or correlated across qubits?

Key findings

• Single-qubit gate fidelities are around 99% in isolation: Q1 = 99.20±0.03% and Q2 = 98.79±0.02%.
• Simultaneous single-qubit RB reduces fidelities to 97.67±0.04% for Q1 and 94.26±0.10% for Q2, indicating cross-talk effects.
• Two-qubit Clifford fidelity is 82.10±2.75%.
• CRB reference fidelity (simultaneous single-qubit Clifford operations) is 91.9±0.1%.
• CRB interleaved with CPhase yields a CPhase fidelity of 92.0±0.5% (two-qubit space).
• Projected CPhase fidelities in single-qubit space range from 91% to 95% depending on control/target configuration and eigenstate.