Quantum limits of superconducting-photonic links and their extension to millimeter waves

Photonic addressing of superconducting circuits has been proposed to overcome wiring complexity and heat-load challenges. However, such superconducting-photonic links suffer from an efficiency-noise trade-off that limits scalability. This trade-off arises because increasing power conversion efficiency entails reducing optical power, which makes the converted signal susceptible to shot noise. We analyze this trade-off and find that the laser-driven qubit gate infidelity scales inversely with the number of photons used. While methods such as nonlinear detection or squeezed light could mitigate this effect, we consider generating higher-frequency electrical signals, such as millimeter waves (100 GHz), using laser light. At these higher frequencies, circuits have higher operating temperatures and cooling power budgets and alleviate constraints posed by the trade-off. Therefore, we demonstrate an optically driven cryogenic millimeter-wave source with a maximum power efficiency of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mn>8</a:mn><a:mo>×</a:mo><a:msup><a:mn>10</a:mn><a:mrow><a:mo>−</a:mo><a:mn>5</a:mn></a:mrow></a:msup></a:math> that can generate a maximum of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:mn>0.7</d:mn><d:mspace width="0.1em"/><d:mtext fontfamily="times">μ</d:mtext><d:mrow><d:mi mathvariant="normal">W</d:mi></d:mrow></d:math> of 80 GHz power, with a <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><j:mn>1200</j:mn></j:math>-thermal-photon equivalent of added noise at <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><m:mn>4</m:mn><m:mspace width="0.1em"/><m:mrow><m:mi mathvariant="normal">K</m:mi></m:mrow></m:math>. Using this source, we perform frequency-domain spectroscopy of superconducting <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><r:mi>Nb</r:mi></r:math><u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><u:mi>Ti</u:mi></u:math><x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><x:mrow><x:mrow><x:mi mathvariant="normal">N</x:mi></x:mrow></x:mrow></x:math> resonators at 80–90 GHz. Our results show a promising approach to lessen the efficiency-noise constraints on superconducting-photonic links, while leveraging the benefits of photonic signal delivery. Further optimization of power efficiency and noise at high frequencies could make scalable photonic control of superconducting qubits viable at temperatures exceeding <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><bb:mn>1</bb:mn><bb:mspace width="0.1em"/><bb:mrow><bb:mi mathvariant="normal">K</bb:mi></bb:mrow></bb:math>. Published by the American Physical Society 2024