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arXiv:0906.1205 · Applied Physics Letters

Energy resolution of terahertz single-photon-sensitive bolometric detectors

Daniel F. Santavicca, Bertrand Reulet, B. S. Karasik, S. V. Pereverzev, D. Olaya, M. E. Gershenson, Luigi Frunzio, D. E. Prober

Published Feb 22, 2010• 35 citations
arXivHTMLPDF

Abstract

We report measurements of the energy resolution of ultrasensitive superconducting bolometric detectors. The device is a superconducting titanium nanobridge with niobium contacts. A fast microwave pulse is used to simulate a single higher-frequency photon, where the absorbed energy of the pulse is equal to the photon energy. This technique allows precise calibration of the input coupling and avoids problems with unwanted background photons. Present devices have an intrinsic full-width at half-maximum energy resolution of approximately 23 THz, near the predicted value due to intrinsic thermal fluctuation noise.

DOI
10.1063/1.3336008

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IEEE Transactions on Applied Superconductivity•Jun 1, 2007•40 citations
We report measurements of the energy resolution of ultrasensitive superconducting bolometric detectors. The device is a superconducting titanium nanobridge with niobium contacts. A fast microwave pulse is used to simulate a single higher-frequency photon, where the absorbed energy of the pulse is equal to the photon energy. This technique allows precise calibration of the input coupling and avoids problems with unwanted background photons. Present devices have an intrinsic full-width at half-maximum energy resolution of approximately 23 THz, near the predicted value due to intrinsic thermal fluctuation noise.
The measurement of energy is a fundamental tool used in quantum technology and computing. Some of the most sensitive energy detectors-bolometers and calorimeters-are thermal, meaning that they operate by absorbing incoming energy, converting it into heat and reading out the resulting temperature change electrically using a thermometer. Recently, superconductor-normal-conductor-superconductor radiation sensors with metallic and graphene absorbers haven been predicted to be capable of full-width at half-maximum energy resolutions of 0.75 zJ and 0.05 zJ, respectively. However, these estimates are only mathematically extracted from steady-state noise and responsivity measurements. Here we show that a metallic superconductor-normal-conductor-superconductor sensor can be used for zeptojoule calorimetry. With the approach, we measure the energy of 1-μs-long 8.4-GHz microwave pulses with a full-width at half-maximum energy resolution finer than 0.95 ± 0.02 zJ (=5.9 ± 0.12 meV) corresponding to 170 photons at 8.4 GHz. The technique provides a potential path to real-time calorimetric detection of single photons in the 10-GHz range.
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.
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.
Terahertz (THz) direct detectors based on superconducting niobium nitride (NbN) hot electron bolometers (HEBs) with thermal, microwave (MW) and THz biasing have been studied at 0.65 THz and 4.2 K in detail. The current responsivity and noise equivalent power (NEP) have been measured and compared respectively. The detectors with the MW and THz biasing have about one order higher current responsivity than that of the thermal one. With the MW biasing, an NEP at the order of pW/√Hz is obtained by choosing the MW frequency and power appropriately.
Terahertz (THz) direct detectors based on superconducting niobium nitride hot electron bolometers with thermal, microwave and THz biasing methods have been studied at a frequency of 0.65 THz and at an operating temperature of 4.2 K. A low noise amplifier at room temperature is used to read out the current change of the detectors modulated by the chopping frequency at 732 Hz. The noise spectral density, current responsivity and noise equivalent power (NEP) have been measured and compared respectively. The detectors with the microwave and THz biasing methods have about one order higher current responsivity than that of the thermal one. With the microwave biasing, an optical NEP at the order of pW/√Hz which shows no deterioration is obtained by choosing the microwave frequency and power appropriately.
We are developing hot-electron superconducting transition-edge sensors (TES) capable of counting THz photons and operating at . We fabricated superconducting Ti nanosensors with Nb contacts with a volume of on planar Si substrates and have measured the thermal conductance in the material, G=4times10-3W/K at 0.3 K, caused predominantly by the weak electron-phonon coupling. The corresponding phonon-noise NEP=3times10-19W/Hz1/2. Detection of single optical photons (1550 nm and 670 nm wavelength) has been demonstrated for larger devices and yielded the thermal time constants of 30 mus at 145 mK and of 25 mus at 190 mK. This hot-electron direct detector (HEDD) is expected to have a small enough energy fluctuation noise for detecting individual photons with v>THz where NEP~3times10-20W/Hz1/2is needed for spectroscopy in space.