Photon counting detectors in terahertz frequencies are developed for future astronomical intensity interferometry. Superconducting tunnel junction detectors with extremely low leakage current can be used for fast photon counting, more than a GHz bandwidth, in terahertz frequencies. We plan to demonstrate intensity correlation using antenna coupled SIS terahertz photon counters to measure the statistical behavior of thermal photons. This could bring a new measurement opportunity in the field of terahertz quantum optics.
Photon counting detectors in terahertz frequencies are developed for future astronomical intensity interferometry. Superconducting tunnel junction detectors with extremely low leakage current can be used for fast photon counting, more than a GHz bandwidth, in terahertz frequencies. We plan to demonstrate intensity correlation using antenna coupled SIS terahertz photon counters to measure the statistical behavior of thermal photons. This could bring a new measurement opportunity in the field of terahertz quantum optics.
The terahertz band, a unique segment of the electromagnetic spectrum, is crucial for observing the cold, dark universe and plays a pivotal role in cutting-edge scientific research, including the study of cosmic environments that support life and imaging black holes. High-sensitivity superconductor–insulator–superconductor (SIS) mixers are essential detectors for terahertz astronomical telescopes and interferometric arrays. Compared to the commonly used classical Nb/AlO x /Nb superconducting tunnel junction, the Nb/AlN/NbN hybrid superconducting tunnel junction has a higher energy gap voltage and can achieve a higher critical current density. This makes it particularly promising for the development of ultra-wideband, high-sensitivity coherent detectors or mixers in various scientific research fields. In this paper, we present a superconducting SIS mixer based on Nb/AlN/NbN parallel-connected twin junctions (PCTJ), which has a bandwidth extending up to 490 GHz–720 GHz. The best achieved double-sideband (DSB) noise temperature (sensitivity) is below three times the quantum noise level.
The terahertz region of the electromagnetic spectrum [0.3-10THz] has, so far, not been exploited fully due to the lack of suitable sources and detectors. Indeed, THz frequency lies between the frequency range of traditional electronics and photonics where the existing technology cannot be simply extended. SIS Niobium tunnel junctions that are currently used as mixing element in heterodyne receivers are intrinsically limited in frequency by the energy gap of Nb and operate only at low temperature (4.2K). An alternative to these devices consists of using High-Tc superconducting receivers. Over the past years, we have developed a new approach based on ion irradiation to make Josephson nano-junctions and SQUIDs with YBCO thin films [1,2]. In this talk, we will present a study of the high-frequency mixing properties of such junctions up to 400 GHz [3]. Conversion gain has been measured at frequencies spanning the range below and above the characteristic frequency fc = (2e/h)IcRn of the junctions. The transition between two distinct mixing regimes has been clearly evidenced, in good agreement with the prediction of the three ports model.[1] N. Bergeal et al., Appl. Phys. Lett. 87, 102502 (2005)[2] N. Bergeal et al., Appl. Phys. Lett. 89, 1112515 (2006)[3] Luo et al, arXiv:1203.1734 (2012)
The terahertz regime, as a last radio window, remains to be fully explored, and astronomical and atmospheric observations in this regime are scientifically important. Like other frequency regimes, developing high-sensitivity detectors (coherent and incoherent) is of particular significance for both ground-based and space-borne facilities. As the coherent detector of choice below 1.4 THz, superconductor-insulator-superconductor (SIS) heterodyne mixers have achieved as high a sensitivity as five times the quantum limit around 1.4 THz. It is, however, still a challenge to developing SIS mixers at frequencies beyond 1.4 THz with considerable transmission loss in superconducting circuits due to the Cooper-pair breaking by energetic photons and increased many difficulties in designing and fabricating. So far, superconducting hot electron bolometer (HEB) mixers have been the most sensitive heterodyne detectors at frequencies above 1.5 THz, and successfully used to detect molecular spectral lines up to 2.5 THz from ground-based and space telescopes. Although spiral-antenna coupled NbN HEB mixers show a good sensitivity in the whole THz frequency range, the directly measured spectral response with Fourier transform spectrometer falls quickly as frequency increases, especially above 3 THz. The terahertz band is also of particular importance to observe astronomical objects such as cosmic microwave background, early distant objects, cold objects and dusty objects. Aiming at such objects, we develop a terahertz imaging array system by combining advanced superconducting detectors such as transition edge sensor (TES) and microwave kinetic inductance detectors (MKIDs), thus the system has a frequency band centred at 350 m, an operational temperature of 0.3 K, and a sensitivity reaching background limit performance for ground-based applications. In addition, it is expected to have some breakthroughs in ultra-sensitive superconducting TES and MKID, low noise multi-channel readout and multiplexing, efficient terahertz-wave coupling technology, and large-scale array system integration. The developed terahertz imaging array system will serve as the next-generation instrument of Dome A 5 m terahertz telescope, conducting a 350 m-band legacy survey for studying the planets, stars, galaxies and cosmology. Besides the application in astronomy, the developed terahertz imaging array system can also be applied to some areas requiring rapid detection such as security, deep space exploration, and biomedical imaging. In this paper, we mainly introduce the superconducting detectors developed at Purple Mountain Observatory and those for international collaborative projects.
Recent coupled Josephson junction experiments in our laboratory have demonstrated that high critical current density tunnel junctions can serve as effective local oscillators at frequencies up to and in excess of the gap sum frequency of the junction, i.e. well above 1 Terahertz for a niobium or niobium compound tunnel junction. While the details of the behavior of such a THz. oscillator were found not to be in accord with the predictions of the accepted theory of the A.C. Josephson effect in the gap region significant radiation could be capacitively coupled from the oscillator junction to an adjacent junction, sufficient for SIS mixer experiments at Terahertz frequencies. Research efforts are now under way to further extend and ex-pand these studies. A high critical current density all NbN tunnel junction system is now under development for Terahertz applications and a new set of coupled Josephson oscillator - SIS detector experiments is being initiated using NbN tunnel junc-tions. In this paper we will review the original coupled junction high frequency experiments and report on the recent progress of the current NbN tunnel junction experiments.
We demonstrate a new type of terahertz detector with superconducting tunnel junctions. The detector has two long junctions integrated on both wings of a log-periodic antenna. In this type of detector, the long junction is a lossy transmission line working based on the Cooper-pair breaking, as well as a standing-wave resonance line working based on the photon-assisted tunneling. A prototype detector using Nb/Al/AlOx/Al/Nb junctions was fabricated, and the principle of the detector was verified. The prototype shows a gradual increase in sensitivity starting from 0.35 THz. We have also identified a resonance signal at 0.25 THz below the effective gap frequency.
The strong interaction of superconducting integrated circuits with microwave photons forms the basis of circuit quantum electrodynamics (cQED), an attractive paradigm for scalable quantum computing and a test bed for quantum optics in the strong coupling regime [1–6]. While singlephoton detectors are an important component of the standard quantum optics toolbox [7], they have not played a role in cQED, as there are no microwave photon counters currently available. In the search for a microwave component that will emulate the nonlinearity of traditional optical photon counters, it is natural to consider the Josephson junction—a nonlinear, nondissipative superconducting circuit element with tunable microwave-frequency transitions. There has been recent interest in the development of on-chip detectors to probe the microwave radiation emitted by mesoscopic quantum circuits [8–14], including proposals to utilize Josephson circuits as photon counters in the context of cQED [8,9]. This study represents the first experimental measurement of the coincidence counting statistics of microwave photons. We exploit microwaveinduced transitions between discrete energy levels in a Josephson junction to realize an efficient, versatile microwave detector. Moreover, we use a parallelized detector circuit to probe the counting statistics of both coherent and thermal microwave sources. Acircuitdiagramofthesingle-channeldetectorisshown in Fig. 1(a). The Josephson junction is current biased near thecriticalcurrent I0.Thejunctionpotentialenergyisgiven by Uð� Þ¼� I