Superconducting materials are key for technologies enabling a large number of devices, including THz wave mixers and single-photon detectors, though they are limited at very low temperatures for conventional superconductors. High-temperature operation could, in principle, be offered using cuprate superconductors. However, the complexity of the material in thin film form, the extremely short coherence length, and material stability have hindered the realization of THz devices working at liquid nitrogen temperatures. YBa2Cu3O7-delta (YBCO) nanodevices have demonstrated nonlinear properties typical of Josephson-like behavior, which have the potential for mixing of AC signals in the THz range due to the large superconducting energy gap. Here, we present AC Josephson functionalities for terahertz waves utilizing Abrikosov vortex motion in nanoscale-confined, fully planar YBCO thin film bridges. We observe Shapiro step-like features in the current-voltage characteristics when irradiating the device with monochromatic subterahertz waves (100-215 GHz) at 77 K. We further explore these nonlinear effects by detecting THz high-order harmonic mixing for signals from 200 GHz up to 1.4 THz using a local oscillator at 100 GHz. Our results open a path to an easily fabricated HTS nonlinear nanodevice based on dimensional confinement, suitable for terahertz applications.

Detection of light quanta in superconducting nano- and microwires is the key enabling technology for fields ranging from quantum optics and quantum photonics to emerging applications like dark matter searches. However, recent progress in accessing lower photon energies or utilizing high-temperature superconductors reveals substantial gaps in understanding quantum detection physics and calibrating photonic quantum systems. To bridge these gaps, we develop a universal model that incorporates spatially and energy-resolved detection physics, essential for photonic quantum sensors. We validate our approach using modern MgB2 nanowire detectors, retrieving their detection threshold and its intrinsic energy blur, by disentangling the complex statistics of single- and multi-photon detection. Our model can augment quantum detector tomography by embedding physical constraints, and it offers a practical tool for modeling and engineering a broad class of detectors under diverse operating conditions.

Abstract Ultra-fast single-photon detectors with high current density and operating temperature can benefit space and ground applications, including quantum optical communication systems, lightweight cryogenics for space crafts, and medical use. Here we demonstrate magnesium diboride (MgB2) thin-film superconducting microwires capable of single-photon detection at 1.55 μm optical wavelength. We used helium ions to alter the properties of MgB2, resulting in microwire-based detectors exhibiting single-photon sensitivity across a broad temperature range of up to 20 K, and detection efficiency saturation for 1 μm wide microwires at 3.7 K. Linearity of detection rate vs incident power was preserved up to at least 100 Mcps. Despite the large active area of up to 400 × 400 μm2, the reset time was found to be as low as ~ 1 ns. Our research provides possibilities for breaking the operating temperature limit and maximum single-pixel count rate, expanding the detector area, and raises inquiries about the fundamental mechanisms of single-photon detection in high-critical-temperature superconductors.

Improving the scalability, reproducibility, and operating temperature of superconducting nanowire single photon detectors (SNSPDs) has been a major research goal since the devices were first proposed. The recent innovation of helium-ion irradiation as a postprocessing technique for SNSPDs could enable high detection efficiencies to be more easily reproducible, but is still poorly understood. In addition, fabricating detectors at micron-wide scales from high-T-c materials could improve scalability and operating temperature, respectively. At the same time, fabrication of successful devices in wide wires and from higher-T-c materials like magnesium diboride has proven challenging. In this work, we compare helium ion irradiation in niobium nitride and magnesium diboride detectors with different material stacks in order to better understand the mechanics of irradiation and practical implications of encapsulating layers on effective dose. We examine the effects of experimental effective dose tests and compare these results to the damage per ion predicted by simulations in corresponding material stacks. In both materials, irradiation results in an increase in count rate, though for niobium nitride this increase has not fully saturated even at the highest tested dose of 2.6 x 10(17 )ions/cm(2), while for resist-encapsulated magnesium diboride even the lowest tested dose of 1 x 10(15) ions/cm(2) appears higher than optimal. Our results demonstrate the general applicability of helium ion irradiation to vastly different devices and material stacks, albeit with differing optimal doses, and show the reproducibility and effectiveness of this postprocessing technique in significantly improving SNSPD efficiency.

We have studied THz heterodyne detection in sub-micrometer MgB2 hot electron bolometer (HEB) mixers based on superconducting MgB2 films of ∼5nm (HEB-A), corresponding to a critical temperature (Tc) of 33.9 K, and ∼7nm (HEB-B), corresponding to a Tc of 38.4 K. We have measured a double sideband (DSB) receiver noise temperature of 2590 K for HEB-A and 2160 K for HEB-B at 1.6 THz and 5 K. By correcting for optical losses, both HEBs show receiver noise temperatures of ∼1600 K referenced to the front of anti-reflection (AR)-coated Si lenses. An intermediate frequency (IF) noise bandwidth of 11 GHz has been measured for both devices. The required local oscillator (LO) power is about 13 μW for both HEBs. We have also measured a DSB receiver noise temperature of 3290 K at 2.5 THz and 5 K but with an AR-coated lens optimized for 1.6 THz. Besides, we have observed a step-like structure in current voltage (IV) curves, which becomes weaker when the LO power increases and observable only in their differential resistance. Such a correlated structure appears also in the receiver output power as a function of voltage, which is likely due to electronic inhomogeneities intrinsic to the variations in the thickness of the MgB2 films. Different behavior in the IV curves around the low bias voltages, pumped with the same LO power at 1.6 and 5.3 THz, was observed for HEB-B, suggesting the presence of a high-energy σ-gap in the MgB2 film.

Heterodyne receivers combining a NbN HEB mixer with a local oscillator (LO) are the work horse for high resolution ( ≥106 ) spectroscopic observations at supra-terahertz frequencies. We report an MgB2 HEB mixer working at 5.3 THz with 20 K operation temperature based on a previously published paper [Y. Gan et al, Appl. Phys. Lett., 119, 202601 (2021)]. The HEB consists of a 7 nm thick MgB2 submicron-bridge contacted with a spiral antenna. It has a Tc of 38.4 K. By using hot/cold blackbody loads and a Mylar beam splitter all in vacuum, and applying a 5.25 THz FIR gas laser as the LO, we measured a minimal DSB receiver noise temperature of 3960 K. The latter gives a DSB mixer noise temperature of 1470 K. This sensitivity is 28 times better than a room temperature Schottky mixer at 4.7 THz, but about 2.5 times less sensitive than an NbN HEB mixer. The latter must be operated around 4 K. The IF noise bandwidth is about 10 GHz, which is 2.5-3 times larger than an NbN HEB. With further optimization, such MgB2 HEBs are expected to reach a better sensitivity. That the low noise, wide IF bandwidth MgB2 HEB mixers can be operated in a compact, low dissipation 20 K Stirling cooler can significantly reduce the cost and complexity of heterodyne instruments and therefore facilitate new space missions.

We have demonstrated a low noise superconducting MgB2 hot electron bolometer (HEB) mixer working at the frequency of 5.3 terahertz (THz) with 20 K operation temperature. The bolometer consists of a 7 nm thick MgB2 submicrometer bridge contacted with a spiral antenna to couple THz radiation through a high resistive Si lens, and it has a superconducting critical temperature of 38 K. By using hot/cold blackbody loads and a Mylar beam splitter all in vacuum and applying a 5.25 THz far-infrared gas laser as a local oscillator, we measured a minimal double sideband receiver noise temperature of 3960 K at the LO power of 9.5 μW. This can be further reduced to 2920 K if a Si lens with an antireflection coating optimized at this frequency and a 3 μm beam splitter are used. The measured intermediate frequency (IF) noise bandwidth is 9.5 GHz. The low noise, wide IF bandwidth mixers, which can be operated in a compact, low dissipation Stirling cooler, are more suitable for space applications than the existing HEB mixers. Furthermore, we likely observed a signature of the double-gap in MgB2 by comparing current–voltage curves pumped at 5.3 and 1.6 THz.

Properties of superconducting nanowires set the performance level for superconducting nanowire single photon detectors (SNSPDs). Reset time in commonly employed large area SNSPDs, 1-10 ns, is known to be limited by the nanowire's kinetic inductance to the load impedance ratio. On the other hand, reduction of the kinetic inductance in small area (waveguide integrated) SNSPDs prevents biasing them close to the critical current due to latching into a permanent resistive state. In order to reduce the reset time in SNSPDs, superconducting nanowires with both low kinetic inductance and fast electron energy relaxation are required. In this paper, we report on a study of kinetic inductance in narrow (15-100 nm) and long (up to 120 mu m) superconducting MgB2 nanowires made from 5 nm thick films, offering such combination of properties. Such films were grown using hybrid physical chemical vapor deposition, resulting in a critical temperature of similar to 32 K, and a switch current density of 5 x 10(7) A cm(-2) (at 4.8 K). Using microwave reflectometry, we measured a kinetic inductance of L-k0(4.8 K) = 1.3-1.6 pH/ regardless of the nanowire width, which results in a magnetic field penetration depth of similar to 90 nm. These values are very close to those in pristine MgB2. We showed that after excitations by a 50 fs pulsed laser the reset time in 35 nm x 120 mu m MgB2 nanowires is 130 ps, which is more than a factor of 10 shorter than in NbN nanowires of similar length-to-width ratios. Depending on the bias current, such MgB2 nanowires function as single-, double, or triple-photon detectors for both visible (lambda = 630 nm) and infrared (lambda = 1550 nm) photons, with a dark count rate of <10 cps. Although the apparent photon detection efficiency seems so far to be low, further technological advances (uniform nanowire width, smaller thickness, increasing the switching current closer to the pair-breaking current) may improve this figure of merit.

We present an experimental study of gain and noise bandwidths in superconducting MgB2 hot-electron bolometer (HEB) terahertz mixers in a 0.1-20 GHz intermediate frequency (IF) range. At an elevated temperature and with a 90-GHz local oscillator (LO), we measure a gain bandwidth of 13-14 GHz, which is the first accurate data for ultrathin MgB2 films. The output noise spectrum has its maximum in the 100-200 K range, depending on the temperature (or the LO power) and the bias point, and its spectrum also confirms the gain bandwidth data obtained with the mixing experiment. Using both the gain and the output noise spectra, we obtain the mixer input noise temperature, which is nearly constant up to 20 GHz. Using the measured data and the HEB mixer theory, we argue that noise bandwidth in the current MgB2 HEB mixers is similar to 30 GHz.

State-of-the-art Superconducting Nanowire Single Photon Detectors based on low-Tc materials reach 100% quantum efficiency. However, the response time is limited to >1-10 ns. Recently, it has been shown that due to a much lower kinetic inductance, a 100 ps response rate can be achieved in 120 $\mu$m-long $MgB_{2}$ nanowires. In this work, we demonstrate experimentally that such $MgB_{2}$ nanowires function as single-photon detectors for both visible ($\lambda$= 630 nm) and infrared ($\lambda$= 1550 nm) photons when biased close to the critical current, with a dark count rate of <10 cps. $MgB_{2}$ photodetectors over-perform NbN SNSPDs in speed by at least an order of magnitude for similar nanowire lengths. Such photodetectors offer a platform for single-photon detectors with a long thought-after combination of a large detector area and a response rate of up to 10 GHz with a single readout line.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Further leaps in astronomy demand new detector materials and devices reaching the fundamental detection limit1. Superconducting hot-electron bolometer (S-HEB) mixers form the baseline for modern astronomical receivers above 1 THz. In these, the wave beating between the Local Oscillator (LO) and the THz signal causes temperature oscillations in a metal around the transition temperature, at the Intermediate Frequency (IF), enabling read-out through changes in electrical resistance R (resistive read-out) as long as the temperature can follow the signal modulation. Despite huge efforts, the instantaneous bandwidth in practical niobium nitride (NbN)-based S-HEB mixers does not exceed 4-5GHz, limited by the electron temperature relaxation rates. The search for new materials lead to MgB2 devices,2 where 11 GHz bandwidths and a 1000K noise temperature are possible but at the expense of high LO power requirements, which is particularly detrimental for array applications. Beyond superconducting materials, charge-neutral graphene has been discussed as an ideal platform for terahertz bolometric direct detectors due to its small heat capacity and weak electron-phonon coupling. However, absence of large-area graphene homogeneously doped to Dirac point hinders any prospects for practical detectors in astronomy and other sensing applications. Furthermore, negligible temperature dependent resistance has kept this approach as not acceptable for bolometric mixers where voltage read-out is required. Here we investigate graphene that is doped to the Dirac point by assembly of molecular dopants on its surface with a high uniformity across the wafer2. With the resistance dominated by quantum localization, and thermal relaxation of carriers governed by electron diffusion, we demonstrate a graphene bolometric terahertz mixer with a gain bandwidth (presently) of 9 GHz (relaxation time 20 ps) and a mixer noise temperature of 475 K. We conclude that with the present quality of graphene, optimization of the device layout will result in a mixer noise temperature as low as 36 K and a gain bandwidth exceeding 20 GHz, with a Local Oscillator power of < 100 pW for operation temperatures <1K. Given the scalability of the material and in conjunction with emerging quantum-limited amplifiers in the GHz domain, we envisage large arrays of quantum–limited sensors in the THz domain for radio astronomy, potentially surpassing superconductor-based heterodyne detectors. References [1] M. Rowan-Robinson, “Astronomy. Probing the cold universe” Science 325, 546–7 (2009). [2] E. Novoselov and S. Cherednichenko, “Low noise terahertz MgB2 hot-electron bolometer mixers with an 11 GHz bandwidth” Appl. Phys. Lett. 110, 032601 (2017).

Superconducting Hot electron Bolometer (HEB) mixers are used in many terahertz astronomical receivers. At frequencies above 1THz, HEB mixers have noise temperature lower than any other mixers working at that range. There have been some studies conducted to investigate THz performance for HEB mixers made of both NbN and MgB2 thin films. However, experimental data for HEB mixer characteristics at microwave (intermediate frequency) frequencies are very scarce and noisy. There are no accurate and systematic IF impedance data. The problem is explained by difficulty of system calibration at cryogenic environment, and device fixture de-embedding. From theoretical models and some experiments, it is expected that IF impedance varies strongly with IF, with a significant imaginary part. With the lack of such data, accurate HEB-to-LNA impedance matching is rarely conducted. As a consequence, electrical standing waves are observed, leading to ripples in the receiver gain and the system noise temperature. In this work we studied IF impedance of MgB2 HEB mixers from 20MHz to 67GHz in a continuous sweep, using an advanced cryogenic microwave/millimeter wave probing station and Vector Network Analyzer. HEB devices were fabricated from high quality 5-10nm MgB2 thin films with many bridge dimensions, integrated with one-port CPW contacts. Device temperature was adjusted from 5K to 40K (normal state). On-wafer calibration kit was mounted next to HEB wafers, and allowed for accurate system calibration up to 67GHz using a 100μm GroundSignal-Ground (GSG) probe. This way, impedance was measured directly, without any need for de-embedding, in a frequency range far exceeding any other previous studies. HEB devices were set on different temperatures, and different bias voltages were applied. We clearly observe correlation of the measured impedance with dc characteristics of HEB devices. Using measured complex-S11 data, we analyze scenarios for HEB-LNA matching. 29th IEEE International Symposium on Space THz Technology (ISSTT2018), Pasadena, CA, USA, March 26-28, 2018

Terahertz (THz) hot-electron bolometer mixers reach a unique combination of low noise, wide noise bandwidth, and high operation temperature when 6 nm thick superconducting MgB2 films are used. We obtained a noise bandwidth of 11 GHz with a minimum receiver noise temperature of 930 K with a 1.63 THz Local Oscillator (LO), and a 5 K operation temperature. At 15 K and 20 K, the noise temperature is 1100 K and 1600 K, respectively. From 0.69 THz to 1.63 THz, the receiver noise increases by only 12%. Device current-voltage characteristics are identical when pumped with LOs from 0.69 THz up to 2.56 THz, and match well with IVs at elevated temperatures. Therefore, the effect of the THz waves on the mixer is totally thermal, due to absorption in the π conduction band of MgB2.

We discuss a custom built hybrid physical chemical vapor deposition system for MgB 2 ultrathin film deposition: construction, deposition process development, and optimization. Achieved films on SiC substrates have a critical temperature (T c ) ranging from 35 K (10-nm-thick films) to 41 K (40-nm-thick films). The 20-nm-thick unpatterned film had a room temperature resistivity of 13 μ Ω·cm, whereas it becomes 50 μΩ·cm in submicrometer size bridges with a critical current density J c (4.2 K) up to 1.2 × 10 8 A/cm 2 . The lower value of resistivity corresponds to the higher of both T c and J c . The surface roughness, measured with an atomic force microscope, is approximately 1.5 nm.

In this paper, we study variation of the MgB2 hot-electron bolometer mixer characteristics such as noise temperature, gain, output noise, and local oscillator (LO) power at 5-, 15-, and 20-K bath temperatures, and at 0.69- and 1.63-THz LO frequencies. The main reason for the noise temperature rising at higher temperatures is a reduction of the mixer gain, which occurs proportionally to the LO power reduction. Contrary to this, the output noise remains constant (for the same bias point).

We discuss performance of submicron size hot-electron bolometer mixers made from thin MgB 2 superconducting films. With a superconducting transition temperature of ~30 K, such terahertz (THz) mixers can operate with high sensitivity at temperatures up to 20 K. Due to very small dimensions local oscillator power requirements are rather low. In the intermediate frequency band of 1-3 GHz, the double sideband receiver noise temperature is 1600 K at 10 K operation temperature, 2000 K at 15 K, 2500-3000 K at 20 K. The gain bandwidth of such devices is 6 GHz and the noise bandwidth is estimated to be 6-8 GHz.

We present results of experimental study of the gain bandwidth (GBW) of MgB2 hot electron-bolometer (HEB) mixers at 0.1THz and 0.4THz. Antenna integrated 0.25-1.5um(2) area devices were made from thin MgB2 films deposited with a custom made HPCVD system. Film as thin as 15-45nm had a T-c from 35K to 40K. The GBW was found to be independent on the bias conditions, the bath temperature, and the LO frequency. The maximum GBW of 6GHz was observed for 15nm thick HEBs. At an 0.7THz LO and a 23K bath temperature the receiver noise temperature of this mixer was 3000K (corrected for optical losses).