Tunable Inductive Coupler for High-Fidelity Gates Between Fluxonium Qubits
Helin Zhang, Chunyang Ding, Daniel Weiss, Ziwen Huang, Yuwei Ma, Charles Guinn, Sara Sussman, Sai Pavan Chitta, Danyang Chen, Andrew Houck, Jens Koch, David Schuster
Abstract
The fluxonium qubit is a promising candidate for quantum computation due to its long coherence times and large anharmonicity. We present a tunable coupler that realizes strong inductive coupling between two heavy-fluxonium qubits, each with approximately <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mn>50</a:mn></a:math>-MHz frequencies and approximately <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:mn>5</d:mn></d:math>-GHz anharmonicities. The coupler enables the qubits to have a large tuning range of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><g:mi>X</g:mi><g:mi>X</g:mi></g:math> coupling strengths (<j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><j:mo>−</j:mo><j:mn>35</j:mn></j:math> to 75 MHz). The <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><m:mi>Z</m:mi><m:mi>Z</m:mi></m:math> coupling strength is <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><p:mo><</p:mo><p:mn>3</p:mn></p:math> kHz across the entire coupler bias range and <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><s:mo><</s:mo><s:mn>100</s:mn></s:math> Hz at the coupler off position. These qualities lead to fast high-fidelity single- and two-qubit gates. By driving at the difference frequency of the two qubits, we realize a <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><v:msqrt><v:mi>i</v:mi><v:mrow><v:mstyle mathsize="0.85em"><v:mi>SWAP</v:mi></v:mstyle></v:mrow></v:msqrt></v:math> gate in 258 ns with fidelity 99.72%, and by driving at the sum frequency of the two qubits, we achieve a <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><z:msqrt><z:mi>b</z:mi><z:mrow><z:mstyle mathsize="0.85em"><z:mi>SWAP</z:mi></z:mstyle></z:mrow></z:msqrt></z:math> gate in 102 ns with fidelity 99.91%. This latter gate is only five qubit Larmor periods in length. We run cross-entropy benchmarking for over 20 consecutive hours and measure stable gate fidelities, with <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><db:msqrt><db:mi>b</db:mi><db:mrow><db:mstyle mathsize="0.85em"><db:mi>SWAP</db:mi></db:mstyle></db:mrow></db:msqrt></db:math> drift (<hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><hb:mn>2</hb:mn><hb:mi>σ</hb:mi></hb:math>) <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><kb:mo><</kb:mo><kb:mn>0.02</kb:mn><kb:mi mathvariant="normal">%</kb:mi></kb:math> and <ob:math xmlns:ob="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><ob:msqrt><ob:mi>i</ob:mi><ob:mrow><ob:mstyle mathsize="0.85em"><ob:mi>SWAP</ob:mi></ob:mstyle></ob:mrow></ob:msqrt></ob:math> drift <sb:math xmlns:sb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><sb:mo><</sb:mo><sb:mn>0.08</sb:mn><sb:mi mathvariant="normal">%</sb:mi></sb:math>. Published by the American Physical Society 2024