Quantum Microwave Radiometry with a Superconducting Qubit

Zhixin Wang, Mingrui Xu, Xu Han, Wei Fu, Shruti Puri, S. M. Girvin, Hong X. Tang, S. Shankar, and M. H. Devoret

Phys. Rev. Lett. 126, 180501 – Published 5 May 2021

Abstract

The interaction of photons and coherent quantum systems can be employed to detect electromagnetic radiation with remarkable sensitivity. We introduce a quantum radiometer based on the photon-induced dephasing process of a superconducting qubit for sensing microwave radiation at the subunit photon level. Using this radiometer, we demonstrate the radiative cooling of a 1 K microwave resonator and measure its mode temperature with an uncertainty $\sim 0.01 K$ . We thus develop a precise tool for studying the thermodynamics of quantum microwave circuits, which provides new solutions for calibrating hybrid quantum systems and detecting candidate particles for dark matter.

Received 9 November 2019
Accepted 29 March 2021

DOI:https://doi.org/10.1103/PhysRevLett.126.180501

Physics Subject Headings (PhySH)

Cavity quantum electrodynamics Photon statistics Quantum fluctuations & noise Quantum optics with artificial atoms Quantum sensing Radio, microwave, & sub-mm astronomy Superconducting quantum optics

Radiation detectors

Atomic, Molecular & OpticalStatistical Physics & ThermodynamicsQuantum Information, Science & Technology

Authors & Affiliations

Zhixin Wang ^1,*, Mingrui Xu², Xu Han^2,‡, Wei Fu², Shruti Puri¹, S. M. Girvin ¹, Hong X. Tang ², S. Shankar^1,§, and M. H. Devoret ^1,†

¹Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
²Department of Electrical Engineering, Yale University, New Haven, Connecticut 06520, USA

^*Corresponding author. zhixin.wang@yale.edu
^†Corresponding author. michel.devoret@yale.edu
^‡Present address: Argonne National Laboratory, Lemont, Illinois 60439, USA.
^§Present address: Department of Electrical and Computer Engineering, University of Texas, Austin, Texas 78758, USA.

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Vol. 126, Iss. 18 — 7 May 2021

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Images

Figure 1
(a) Diagram of an archetypal radio telescope. Blue box: radio receiver. Orange box: Earth’s atmosphere. Green trident: receiving antenna. Blue arrow: extraterrestrial radiation. Red box: reference black-body radiator for temperature calibration. (b) Setup of our quantum microwave radiometric experiment. Blue area: mixing-chamber stage (MXC) of a dilution refrigerator. Orange box: variable-temperature stage (VTS), which is attached to the still stage of the same dilution refrigerator through a weak thermal link. Green transmission line: a niobium–titanium superconducting coaxial cable inductively coupled to a tunable antenna resonator (green). Blue arrow: residual thermal noise from the input line impinging on the resonator. Orange arrow: thermal leakage from the VTS to the MXC circuit due to the finite thermal conductivity along the interstage microwave wiring. Red box: tunable broadband noise generator. JPC: Josephson parametric converter with three ports: signal (S), idler (I), and pump. cQED: a transmon qubit (purple) dispersively coupled to a readout cavity (blue). SPA: superconducting nonlinear asymmetric inductive element parametric amplifier [41, 42, 43], which enables high-fidelity single-shot qubit readout.
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Figure 2
(a) Pulse sequences for noise-induced dephasing measurements. For all datasets reported in this Letter, qubit rotations (purple) are completed in 200 ns; the second $π / 2$ pulse is applied after the JPC pump (gray) by $τ - τ_{p} = 1 μ s$ ; the single-shot qubit readout (blue) lasts for $1.08 μ s$ . (b) Frequency landscape of the experiment. The solid green peak (right) belongs to the antenna resonator. The light green arrow indicates its frequency tunability. Along the frequency sweep, its down-converted image (dashed green peak) at $f_{a} - f_{p}$ at crosses the two resonances of the qubit-readout cavity (blue, $f_{r}^{g (e)} = f_{r} \pm χ / 4 π$ ) separated by $χ / 2 π$ . The red floor refers to the added white noise. (c) Qubit-dephasing spectra at different added white-noise powers (from top to bottom, ${\bar{n}}_{add}$ equal to 3.77, 1.66, and 0) showing the radiative heating, equilibrium, and cooling regimes, respectively. The blue ribbon stands for the range $f_{r}^{e} < f_{a} - f_{p} < f_{r}^{g}$ . The accompanying sketches illustrate these regimes through an analogy based on pottery in a kiln—the color (temperature) contrasts between the amphorae and their backgrounds. For data in this subfigure, $τ_{p} = 0.54 μ s$ and $T_{VTS} = 1.03 K$ .
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Figure 3
(a) Noise model of the radiometer. Left beam splitter (green): tunable antenna resonator. Right beam splitter (gray): ohmic loss along the coaxial cables and circulators between the resonator and the cQED module. Their energy transmissivities are denoted as $t_{a}$ and $t_{loss}$ . Four colored boxes represent the thermal baths. Yellow: internal bath in equilibrium with the VTS, whose thermal leakage to the MXC-stage circuits is represented by the transmissivity $t_{leak}$ . Red: white-noise source. Blue: external bath in the transmission line. Gray: dissipation between the antenna and the cQED cavity. (b) $1 - η_{a}$ and (c) ${\bar{n}}_{r}^{eff}$ in the absence of added white noise at different antenna frequencies with $T_{VTS} = 1.03 K$ . The solid red curve in (b) is the theoretical prediction based on the experimental values of $κ_{a, c}$ , $κ_{a, i}$ , $κ_{r}$ , and $χ$ , with no fitting parameter. In (c), contributions of different baths are indicated using the same color code as that in (a). The orange line separates the VTS leakage (below) and the transmitted radiation from the resonator (above). For all data in this figure, $τ_{p} = 1.08 μ s$ .
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