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IEEE Ukrainian Microwave Week (UkrMW)

Scheme for Flux-Qubit-Based Microwave Single-Photon Counter with Weak Continuous Measurement

V. I. Shnyrkov, Wu Yangcao, Oleg G. Turutanov, V. Yu. Lyakhno, A. A. Soroka

Published Sep 21, 2020• 0 citations
Publisher

Abstract

Single-photon detectors are crucial elements of prospective quantum devices and techniques such as quantum communication systems, quantum computers, quantum radars, astrophysical detectors, etc. The detection of individual photons in microwave range is a much larger challenge as compared to that of in optics due to the quanta energy difference of 4 to 5 orders of magnitude which requires a quite different approach. So "artificial atoms" with discrete energy spectrum such as superconducting qubits are used here exploiting resonance transitions between their quantum states. A scheme incorporating a flux qubit with controllable spectrum as a single photon counter for 3-12 GHz range and weak continuous readout circuit based on rf superconducting quantum interferometer (SQUID) with UHF pumping is proposed and analyzed. High SQUID pumping frequency (about 500 MHz) along with using cooled microwatt HEMT preamplifier provides the measurement frequency band of up to 30 MHz while substantially diminishing the back-action effect.

DOI
10.1109/ukrmw49653.2020.9252799

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Single-photon detectors are crucial elements of prospective quantum devices and techniques such as quantum communication systems, quantum computers, quantum radars, astrophysical detectors, etc. The detection of individual photons in microwave range is a much larger challenge as compared to that of in optics due to the quanta energy difference of 4 to 5 orders of magnitude which requires a quite different approach. So "artificial atoms" with discrete energy spectrum such as superconducting qubits are used here exploiting resonance transitions between their quantum states. A scheme incorporating a flux qubit with controllable spectrum as a single photon counter for 3-12 GHz range and weak continuous readout circuit based on rf superconducting quantum interferometer (SQUID) with UHF pumping is proposed and analyzed. High SQUID pumping frequency (about 500 MHz) along with using cooled microwatt HEMT preamplifier provides the measurement frequency band of up to 30 MHz while substantially diminishing the back-action effect.
Various types of single-photon counters operating in infrared, ultraviolet, and optical wavelength ranges are successfully used to study electromagnetic fields, analyze radiation sources, and solve problems in quantum informatics. However, their operating principles become ineffective at millimeter band, S-band, and ultra-high frequency bands of wavelengths due to the decrease in quantum energy by 4–5 orders of magnitude. Josephson circuits with discrete Hamiltonians and qubits are a good foundation for the construction of single-photon counters at these frequencies. This paper presents a frequency-tuned microwave photon counter based on a single-junction superconducting quantum interferometer and flux qutrit. The control pulse converts the interferometer into a two-level system for resonance absorption of photons. Decay of the photon-induced excited state changes the magnetic flux in the interferometer, which is measured by a SQUID magnetometer. Schemes for recording the magnetic flux using a DC SQUID or ideal parametric detector, based on a qutrit with high-frequency excitation, are discussed. It is shown that the counter consisting of an interferometer with a Josephson junction and a parametric detector demonstrates high performance and is capable of detecting single photons in a microwave band.
Abstract An analysis of the measurement of the magnetic flux in superconducting qubits based on RF SQUIDs was carried out with an 800 MHz bandwidth low-power-consumption cryogenic high-electron-mobility transistor amplifier. The preliminary experimental results obtained at temperatures 2 K and 4 K for RF SQUIDs in hysteretic, and in two non-hysteretic, regimes with a pump frequency of about 30 MHz are discussed. Parameters of RF SQUIDs in the hysteretic and non-hysteretic modes are analyzed within the framework of the resistively and capacitively shunted junction model for Josephson junctions. Its sensitivity at a temperature of 30 mK and frequency band (speed) are calculated and optimized to read the states of a flux qubit used as a single microwave photon counter. It is shown that an RF SQUID, operated in an adiabatic non-hysteretic mode for qubit readout, allows us to minimize its back-action effect and the dark count rate. This is due to the absence of Josephson generation, the small amplitude of the resonator electromagnetic field, and the selection of the pump frequency that does not coincide with the characteristic frequencies of the flux qubit.
A superconducting flux qubit is inductively coupled to a superconducting quantum interference device (SQUID) magnetometer, capacitively shunted to form a 1.294-GHz resonator. The qubit-state-dependent resonator frequency is weakly probed with a microwave signal and detected with a microstrip SQUID amplifier. At a mean resonator occupation n‾=1.5\overline{n}=1.5n=1.5 photons, the readout visibility is increased by a factor of 4.5 over that using a cryogenic semiconductor amplifier. As n‾\overline{n}n is increased from 0.008 to 0.1, no reduction in T1{T}_{1}T1​ is observed, potentially enabling continuous monitoring of the qubit state.
Superconducting qubits demonstrate good perspectives as scalable blocks for quantum information processing. These systems can be individually readout, addressed, and controlled, making them some of the most attractive thin-film qubits. Quantum measurements should be performed in order to readout the final state of flux qubits. To observe the dynamics of such systems, one requires a fast magnetometer with exceptional sensitivity and low back-action. We address the problem of reducing the back-action of an RF SQUID readout circuit, which may significantly destroy a Josephson flux qubit in the case of the circuits integration. The main sources of such back-action in conventional RF SQUID and DC SQUID circuits are low-ohmic shunting resistors ensuring high damping. The advantage of flux qubits is the presence of an optimal bias point at which, in the first-order approximation, the device is immune to fluctuations in readout and control lines. Nonetheless, even such systems have characteristics limited by a second-order dissipation. The dissipation can be eliminated by using an “ideal parametric readout” based on an RF SQUID acting in non-hysteretic adiabatic mode. In this paper, we propose an RF SQUID in a non-hysteretic regime as a fast, sensitive, and scalable readout system with low back-action for measurements of a single MW photon counter based on a flux qubit.
Single-photon detection played an important role in the development of quantum optics. Its implementation in the microwave domain is challenging because the photon energy is five orders of magnitude smaller. In recent years, significant progress has been made in developing single microwave photon detectors (SMPDs) based on superconducting quantum bits or bolometers. In this paper we present a practical SMPD based on the irreversible transfer of an incoming photon to the excited state of a transmon qubit by a four-wave mixing process. This device achieves a detection efficiency $\ensuremath{\eta}=0.43$ and an operational dark count rate $\ensuremath{\alpha}=85\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, mainly due to the out-of-equilibrium microwave photons in the input line. The corresponding power sensitivity is $\mathcal{S}={10}^{\ensuremath{-}22}\phantom{\rule{0.2em}{0ex}}\mathrm{W}/\sqrt{\mathrm{Hz}}$, one order of magnitude lower than the state of the art. The detector operates continuously over hour time-scales with a duty cycle ${\ensuremath{\eta}}_{D}=0.84$, and offers frequency tunability of at least 50 MHz around 7 GHz.
Cyclically operated Single Microwave Photon Counter with 10−2210^\mathrm{-22}10−22 W/Hz\mathrm{W/\sqrt{Hz}}W/Hz​ sensitivity
Single photon detection played an important role in the development of quantum optics. Its implementation in the microwave domain is challenging because the photon energy is 5 orders of magnitude smaller. In recent years, significant progress has been made in developing single microwave photon detectors (SMPDs) based on superconducting quantum bits or bolometers. In this paper we present a practical SMPD based on the irreversible transfer of an incoming photon to the excited state of a transmon qubit by a four-wave mixing process. This device achieves a detection efficiency η=0.43η= 0.43η=0.43 and an operational dark count rate α=85α= 85α=85 s−1\mathrm{s^{-1}}s−1, mainly due to the out-of-equilibrium microwave photons in the input line. The corresponding power sensitivity is S=10−22\mathcal{S} = 10^{-22}S=10−22 W/Hz\mathrm{W/\sqrt{Hz}}W/Hz​, one order of magnitude lower than the state of the art. The detector operates continuously over hour timescales with a duty cycle ηD=0.84η_\mathrm{D}=0.84ηD​=0.84, and offers frequency tunability of at least 50 MHz around 7 GHz.