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IEEE Transactions on Applied Superconductivity

Superconducting Qubit Network as a Single Microwave Photon Detector for Galactic Axion Search

Emanuele Enrico, C. Bonavolontà, G. Brida, G. Coda, Evgeni V. Il’ichev, Luca Fasolo, M. V. Fistul, Alice Meda, L. Oberto, G. Oelsner, M. Rajteri, B. Ruggiero, P. Silvestrini, M. Valentino, Paolo Vanacore, Mikhail Lisitskiy

Published Dec 8, 2023• 1 citation
Publisher

Abstract

Experimental search of galactic axions requires detection of single photons in the microwave range. We work on a novel approach to detect single microwave photons based on a coherent collective response of quantum states occurring in a superconducting qubit network (SQN) embedded in a lowdissipative superconducting resonator. We propose a two resonators detector configuration with two parallel resonators without common part and with separated input and output terminals. The device consists of a low-dissipative resonator with embedded SQN in which microwave photons arrive (“signal resonator”), and a transmission line for measuring the frequency dependent transmission coefficient demonstrating resonant drops at the qubit frequencies (“readout resonator”). In comparison with T-type three terminal device recently proposed and investigated by us, the device with two resonators with separated input and output terminals doesn't contain common part of both resonators and exclude an unwanted noise from measurement readout circuits to the signal resonator. A layout of two resonators four terminal SQN detectors containing 5 flux qubits weakly coupled to a low-dissipative signal and readout resonator was developed and optimized. The samples were fabricated by Manhattan Al-based technology with Nb resonator circuits. The SQN detector was experimentally tested in terms of microwave measurements of scattering parameters of both resonators, and crosstalk properties. Comparison of experimental data with results of the simulations permits one to conclude that the electromagnetic conditions of the fundamental resonant peak of 8.5 GHz of both resonators aren't affected by the crosstalk phenomenon and their performances provided by the design remain not altered for correct device operation

DOI
10.1109/tasc.2023.3340640

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Experimental search of galactic axions requires detection of single photons in the microwave range. We work on a novel approach to detect single microwave photons based on a coherent collective response of quantum states occurring in a superconducting qubit network (SQN) embedded in a lowdissipative superconducting resonator. We propose a two resonators detector configuration with two parallel resonators without common part and with separated input and output terminals. The device consists of a low-dissipative resonator with embedded SQN in which microwave photons arrive (“signal resonator”), and a transmission line for measuring the frequency dependent transmission coefficient demonstrating resonant drops at the qubit frequencies (“readout resonator”). In comparison with T-type three terminal device recently proposed and investigated by us, the device with two resonators with separated input and output terminals doesn't contain common part of both resonators and exclude an unwanted noise from measurement readout circuits to the signal resonator. A layout of two resonators four terminal SQN detectors containing 5 flux qubits weakly coupled to a low-dissipative signal and readout resonator was developed and optimized. The samples were fabricated by Manhattan Al-based technology with Nb resonator circuits. The SQN detector was experimentally tested in terms of microwave measurements of scattering parameters of both resonators, and crosstalk properties. Comparison of experimental data with results of the simulations permits one to conclude that the electromagnetic conditions of the fundamental resonant peak of 8.5 GHz of both resonators aren't affected by the crosstalk phenomenon and their performances provided by the design remain not altered for correct device operation
We propose a novel approach to detect a low power microwave signal with a frequency of the order of several GHz based on a coherent collective response of quantum states occurring in a superconducting qubits network (SQN). An SQN composes of a large number of superconducting qubits embedded in a low-dissipative superconducting resonator. Our theory predicts that an SQN interacting with the off-resonance microwave radiation, demonstrates the collective alternating current Stark effect that can be measured even in the limit of single photon counting. A design of the layout of three terminals SQN detectors containing 10 flux qubits weakly coupled to a low-dissipative R-resonator and T-transmission line was developed. The samples were fabricated by Al-based technology with Nb resonator. The SQN detector was tested in terms of microwave measurements of scattering parameters and two-tone spectroscopy. A substantial shift of the frequency position of the transmission coefficient drop induced by a second tone pump signal was observed, and this effect clearly manifests a nonlinear multiphoton interaction between the second-tone microwave pump signal and an array of qubits.
Axions detection requires the ultimate sensitivity down to the single-photon limit. In the microwave region, this corresponds to energies in the yJ range. This extreme sensitivity has to be combined with an extremely low dark-count rate since the probability of axions conversion into microwave photons is supposed to be very low. To face this complicated task, we followed two promising approaches that both rely on the use of superconducting devices based on the Josephson effect. The first one is to use a single Josephson junction (JJ) as a switching detector (i.e., exploiting the superconducting to normal state transition in the presence of microwave photons). We designed a device composed of a coplanar waveguide terminated on a current-biased JJ. We tested its efficiency to pulsed (pulse duration 10 ns) microwave signals since this configuration is closer to an actual axions search experiment. We show how our device is able to reach detection capability of the order of ten photons with the frequency of 8 GHz. The second approach is based on an intrinsically quantum device formed by two resonators coupled only via a superconducting qubit network. This approach relies on quantum nondemolition measurements of the resonator photons. We show that by injecting radiofrequency power into the resonator, the frequency position of the resonant drop in the transmission coefficient (S21) can be modulated up to 4 MHz. We anticipate that, once optimized, both the devices have the potential to reach single-photon sensitivity.
Quantum sensing is a rapidly expanding research field that finds one of its applications in fundamental physics, as the detection of light Dark Matter. Qubit-based superconducting devices have already been successfully applied in detecting few-GHz single photons via Quantum Non-Demolition measurements. The optimization and new design schemes of circuits embedding qubits will yield notable enhancements in sensitivity and suppression of dark count rates in experiments involving high-precision microwave photon detection, particularly in the search for Axions and Dark Photons. The Qub-It collaboration is developing a novel microwave photon detection scheme based on two qubits coupled to the same resonator, both in 2D and 3D formats.
Superconducting qubit networks (SQNs) embedded in a low-dissipative resonator is a promising device allowing one not only to establish the collective quantum dynamics on a macroscopic scale but also to greatly enhance the sensitivity of detectors of microwave photons. A quantum ac Stark effect provided by coupling between an SQN and microwave photons of a resonator, leads to a strong nonlinear interaction between photons. Here, we present a two-tone spectroscopy experiment in which a set of 10 superconducting flux qubits is coupled to the input R- resonator and the output T- transmission line. An external microwave pump field close to the resonance frequency populates macroscopically the resonator mode as a Bose-Einstein condensate, while a second probe beam scans the resonances referred also as Bogoliubov-like excitations. The corresponding excitation frequency measured from the transmission coefficient, |S21(f)| displays an abrupt change of the resonant dip position once the power of the pump field overcomes a critical value Pcr. This sharp shift occurs in a narrow region of pump frequencies, and can be tuned by an applied magnetic field. It is a signature of bistability of the photon number inside the resonator, in agreement with theory.
A growing interest in the detection of single microwave-photons has been stimulated by the search for light dark matter, such as Axions and Axion-like particles, together with the fast development of quantum technologies based on superconducting devices. Many solutions have been proposed in literature but most of them still fail to satisfy the tight requirements imposed by Dark Matter experiments. For instance, many have a large dark-count rate that still make parametric amplifiers a preferable choice. On the contrary, a device based on a current biased Josephson junction resonantly activated by the absorption of a microwave photon may have dark-counts rate down to the mHz level. Here, we report the experimental study of the resonant activation of an Al Josephson junction excited by continuous radiofrequency. The device under study consists of a chip with a transmission line terminated by an Al Josephson-junction, fabricated by shadow-mask evaporation technique. The sample-holder with the chip inside is hosted in a superconducting box and it is thermally anchored to the mixing-chamber plate of a dilution refrigerator at a temperature of about 15 mK. We measured the escape rates of the junction in different configurations of current bias and radiofrequency excitations. Response to continuous wave has been measured and interpreted within the RCSJ model. These results are used to optimize the design of a single microwave-photon detector.
We propose an on-chip superconducting microwave frequency divider based on the simultaneous excitation of two superconducting qubits by a single photon. The proposed circuit consists of two flux qubits and three superconducting resonators. A high-frequency photon stored in the input resonator simultaneously excites two qubits through an effective three-body interaction, and the excitation energy is subsequently transferred into two low-frequency resonators. As a result, one high-frequency microwave signal can be divided into two spatially separated low-frequency output signals. The continuous-wave output, conversion efficiency, and controllable pulse outputs of the proposed device are analyzed numerically using the master-equation approach. Compared with conventional nonlinear frequency-conversion schemes, the proposed device naturally generates spatially separated output channels, which may simplify microwave signal routing in superconducting quantum processors.