Tunable Inductive Coupler for High-Fidelity Gates Between Fluxonium Qubits

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>&lt;</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>&lt;</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>&lt;</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>&lt;</sb:mo><sb:mn>0.08</sb:mn><sb:mi mathvariant="normal">%</sb:mi></sb:math>. Published by the American Physical Society 2024