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arXiv:2102.01415 · Bulletin of the American Physical Society

Detecting spins with a microwave photon counter

Emanuele Albertinale, Léo Balembois, Eric Billaud, V. Ranjan, D. Flanigan, Thomas Schenkel, Estève, Daniel, D. Vion, Patrice Bertet, Emmanuel Flurin

Published Feb 2, 2021• 0 citations
arXivHTMLPDF

Abstract

Quantum emitters respond to resonant illumination by radiating electromagnetic fields. A component of these fields is phase-coherent with the driving tone, while another one is incoherent, consisting of spontaneously emitted photons and forming the fluorescence signal. Atoms and molecules are routinely detected by their fluorescence at optical frequencies, with important applications in quantum technology and microscopy. Spins, on the other hand, are usually detected by {their coherent response} at radio- or microwave frequencies, either in continuous-wave or pulsed magnetic resonance. Indeed, fluorescence detection of spins is hampered {by their low spontaneous emission rate} and by the lack of single-photon detectors in this frequency range. Here, using superconducting quantum devices, we demonstrate the detection of a small ensemble of donor spins in silicon by their fluorescence at microwave frequency and millikelvin temperatures. We enhance the spin radiative decay rate by coupling them to a high-quality-factor and small-mode-volume superconducting resonator, and we connect the device output to a newly-developed microwave single-photon counter based on a superconducting qubit. We discuss the potential of fluorescence detection as a novel method for magnetic resonance spectroscopy of small numbers of spins.

DOI
10.48550/arxiv.2102.01415

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By consistently applying the formalism of quantum electrodynamics, we developed a comprehensive theoretical framework describing the interaction of single microwave photons with an array of superconducting transmon qubits in a waveguide cavity resonator. In particular, we analyze the effects of microwave photons on the array’s response to a weak probe signal exciting the resonator. The study reveals that high quality factor cavities provide a better spectral resolution of the response, while cavities with moderate quality factors allow better sensitivity for a single-photon detection. Remarkably, our analysis showed that a single-photon signal can be detected by even a sole qubit in a cavity under the realistic range of system parameters. We also discuss how the quantum properties of the microwave radiation and electrodynamical properties of resonators affect the response of qubits’ array. Our results provide an efficient theoretical background for informing the development and design of quantum devices consisting of arrays of qubits, especially for those using a cavity where an explicit expression for the transmission or reflection is required.
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Superconducting circuits have recently become a major contender for the implementation of quantum computers owing to their compatibility with existing microchip fabrication technologies. Their utility stems largely from their ability to be coupled with resonant cavities for the purposes of trapping and relaying microwave radiation. Because of this feature, scientists are able to transmit quantum information from a variety of qubits. Though quantum-limited amplifiers exist, unfortunately there currently exists no microwave photon counter capable of single shot quantum detection. Hence, we use superconducting circuits with Josephson junctions to design a microwave photon detector based on a modified phase qubit that exhibits a bandwidth of 4GHz, and a detection fidelity of 98%. We use metastable barrier transition (driven by incident photons) to create an avalanche effect analogous to current photo-diodes. Linear coupling of the junction flux with the radiation permits photodetection from an arbitrary quantum source in the GHz range. We show the device to be robust to changes in drive frequency, temporal photon width, and resonator quality factor, and we optimize our device with respect to these parameters. We show the device to be stable over the necessary time scales, and yet sensitive enough to accurately measure photons on demand.