Real-time detection of microwave pulses with a superconducting proximity bolometer

L. S. Kuzmin

Axel Stockhausen, K. Ilin, M. Siegel +5 more

T. I. Novikova, K. Shein, Sergey V. Seliverstov +2 more

Kohvakka, Kassius

Sensitive and reliable photon detectors are ubiquitous tools in countless fields of engineering and the sciences. One of the most mature and widely applied approaches to photodetection is bolometry, where the absorption of radiation causes a measurable change in the temperature of the absorber material. Applications of bolometry include fields such as astrophysics, medicine, particle physics, and metrology, all of which greatly benefit from the development of more versatile and efficient detectors. In the field of superconducting quantum circuits, considerable progress has recently been made in developing ultrasensitive superconducting bolometers. Operating in the single-photon regime, these devices hold great potential for several applications within quantum technology, including qubit state measurement and characterization of microwave components and cryogenic environments. State-of-the-art bolometers based on graphene have been shown to reach sufficient energy resolution and operation speed to be applicable in the context of circuit quantum electrodynamics. In this thesis, a superconducting bolometer based on superconductor-normal-metal-superconductor Josephson junctions is studied. In particular, a detection protocol based on optimal filtering of the noisy detector output signal is implemented to detect microwave pulses during continuous operation of the device. The fidelity of the implemented real-time detection scheme is experimentally tested and compared to the theoretical energy resolution of the studied bolometer, computed from noise equivalent power. The device is found to reach 50% detection probability with minimal dark-count rate at pulse energies of approximately 2.2 zJ, exceeding the predicted resolution of 0.75 zJ by only a factor of three. In addition, the characteristic energy of the detector is determined to be approximately 1.5 zJ based on the total energy of pulses detected with fidelity F=0.5. The results give hope of future applications of the detection scheme to more sensitive bolometer samples.

Runfeng Su, Y D Zhang, Xuecou Tu +8 more

Abstract Relaxation oscillations related to terahertz (THz) power detection in superconducting niobium nitride hot electron bolometers (HEBs) have been studied using microwaves as a probe. The reflected microwave signal, which is related to the impedance changes of the HEB irradiated by an incident THz signal, can be used to characterize relaxation oscillations. The frequency of relaxation oscillation switching between the superconducting and resistive states increases linearly with incident THz power but nonlinearly with bias voltage or bath temperature. The observed strong periodic pulse trains of relaxation oscillations have a maximum duty cycle of ∼50%. These strong switching pulses indicate that DC Joule heating, THz radiation or thermal energy is absorbed in the bi-stable region of the HEB intermittently. In addition to relaxation oscillations, we also see weaker oscillations when the device is in the resistive state, which we call intrinsic oscillations. These have a constant frequency at the order of 10 kHz which is independent of external stimuli. The weak periodic pulse trains at a constant frequency can be divided into mainly two types—one is the pulse train with a small duty cycle and the other is the quasi-square wave. Measurement of these oscillations in HEBs has some practical applications. Digital THz power measurement with frequencies below and above the gap frequency can be made based on the linearity between relaxation oscillation frequency and incident THz power. The relaxation oscillation frequency can be used to estimate the maximum speed of a THz direct detection system especially for a multi-pixel HEB array. In particular, extra noise induced by relaxation oscillations and intrinsic oscillations can be determined for the HEB operating as a THz direct detector.

Eugene M. Gershenzon, Murray Gershenzon, Gregory N Gol'tsman +2 more