M. Siegel

Terahertz Performance of Integrated Lens Antennas With a Hot-Electron Bolometer

Radiation coupling efficiency and directive properties of integrated lens antennas with log-spiral, log-periodic, and double-slot planar feeds coupled to a hot-electron bolometer were experimentally studied at frequencies from 1 to 6 THz and compared with simulations based on the method of moments and physical-optics ray tracing. For all studied antennas, the modeled spectral dependence of the coupling efficiency fits to the experimental data obtained with both Fourier transform spectroscopy and noise temperature measurements only if the complex impedance of the bolometer is explicitly taken into account. At high frequencies, the radiation pattern of integrated antennas exhibits sidelobes, which are higher than those predicted by the antenna model

Anisotropic rare-earth spin ensemble strongly coupled to a superconducting resonator.

Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. Ideal candidates for coherent qubit interconversion are optically active spins, magnetically coupled to a superconducting resonator. We report on an on-chip cavity QED experiment with magnetically anisotropic Er(3+)∶Y2SiO5 crystals and demonstrate collective strong coupling of rare-earth spins to a lumped element resonator. Moreover, the electron spin resonance and relaxation dynamics of the erbium spins are detected via direct microwave absorption, without the aid of a cavity.

Spectral cut-off in the efficiency of the resistive state formation caused by absorption of a single-photon in current-carrying superconducting nano-strips

Abstract.We have studied supercurrent-assisted formation of the resistive state in nano-structured disordered superconducting Nb(N) films after absorption of a single optical to near-infrared photon. The efficiency of the resistive state formation has a pronounced spectral cut-off; corresponding threshold photon energy decreases with the bias current. Analysis of the experimental data in the framework of the refined hot-spot model suggests that the quantum yield for near-infrared photons increases with the photon energy. Relaxation of the resistive state depends on the photon energy making the phenomena feasible for the development of energy resolving single-photon detectors.

Terahertz heterodyne receiver with quantum cascade laser and hot electron bolometer mixer in a pulse tube cooler

A liquid cryogen-free terahertz heterodyne receiver in a pulse tube cooler has been realized. The receiver operates at 2.5 THz. It is based on a quantum cascade laser (QCL) as local oscillator and a hot electron bolometric mixer. A detailed study of the QCL beam quality yielded a beam propagation factor of 1.1–1.2. The double sideband noise temperature of the system is 2000 K and when corrected for optical losses in the signal path it is ∼800 K.

Double-barrier Josephson structures as the novel elements for superconducting large-scale integrated circuits

An overview of the current status of different types of non-hysteretic Josephson junctions is given with emphasis on double-barrier structures. The results of theoretical work on double-barrier SIS′IS Josephson junctions (I is a tunnel barrier, S′ is a thin film with TC′<TC) are presented. The microscopic model for the supercurrent is developed for two cases: the S′ interlayer in the clean and in the dirty limit. The model describes the cross-over from direct Josephson coupling of the external S electrodes to the regime of two serially connected SIS′ junctions. We calculate the ICRN product as a function of the TC′/TC ratio, the interlayer thickness and the barrier strengths and compare the theory with experimental data for Nb/AlOx/Al/AlOx/Nb junctions. We argue that these junctions are very promising in rapid single flux quantum (RSFQ) and programmable voltage standard applications, since they are intrinsically shunted and have controllable interfaces. We formulate the requirements for materials and interface barriers in order to increase critical current densities and ICRN products in double-barrier junctions.

Intrinsic detection efficiency of superconducting nanowire single-photon detectors with different thicknesses

We evaluate experimentally the intrinsic detection efficiency (IDE) of superconducting NbN nanowire single-photon detectors in the range of wire thicknesses from 4 to 12 nm. The study is performed in the broad spectral interval between near-ultraviolet (wavelength 400 nm) and near-infrared (wavelength 2000 nm) light with plane waves at normal incidence. For visible light the IDE of the thinnest detectors reaches 70%. We use numerically computed absorptance of the nanowire-structures for the analysis of the experimental data. Variations in the detection efficiency with both the wire thickness and the wavelength evidence the red boundary of the hot-spot photon-detection mechanism. We explain the detection at larger wavelengths invoking thermal excitation of magnetic Pearl vortices over the potential barrier at the edges of the wire.

Josephson junctions, interconnects, and crossovers on chemically etched edges of YBa2Cu3O7−x

A technique of ultraviolet photolithography of YBa2Cu3O7−x and PrBa2Cu3O7−x films combined with nonaqueous Br‐ethanol chemical etching was developed. Josephson junctions, interconnects, and crossovers on the basis of chemically etched edges of c‐axis oriented YBa2Cu3O7−x thin films were prepared and investigated. For the Josephson junctions with a PrBa2Cu3O7−x barrier, the IcRn product values of about 10 mV at 4.2 K and up to about 0.6 mV at 77 K were achieved. Shapiro steps were observed in the temperature range up to about 89 K. Critical current spreads of about ±10% were observed. At 77 K, the electrodes of crossovers carried more than 106 A/cm2. For a 160‐nm‐thick SrTiO3 intermediate layer in crossovers, a resistivity of more than 109 Ω cm was achieved at T≤100 K.

Tantalum nitride superconducting single-photon detectors with low cut-off energy

Materials with a small superconducting energy gap are expected to favor a high detection efficiency of low-energy photons in superconducting nanowire single-photon detectors. We developed a TaN detector with smaller gap and lower density of states at the Fermi energy than in comparable NbN devices, while other relevant parameters remain essentially unchanged. The observed reduction of the minimum photon energy required for direct detection is in line with model predictions of ≈1/3 as compared to NbN.

Fluctuation effects in superconducting nanostrips

Superconducting fluctuations in long and narrow strips made from ultrathin NbN films, have been investigated. For large bias currents close to the critical current fluctuations led to localized, temporary transitions into the normal conducting state, which were detected as voltage transients developing between the strip ends. We present models based on fluctuations in the Cooper pair density and current-assisted thermal-unbinding of vortex-antivortex pairs, which explain the current and temperature dependence of the experimental fluctuation rates.

Demonstration of digital readout circuit for superconducting nanowire single photon detector.

We demonstrate the transfer of single photon triggered electrical pulses from a superconducting nanowire single photon detector (SNSPD) to a single flux quantum (SFQ) pulse. We describe design and test of a digital SFQ based SNSPD readout circuit and demonstrate its correct operation. Both circuits (SNSPD and SFQ) operate under the same cryogenic conditions and are directly connected by wire bonds. A future integration of the present multi-chip configuration seems feasible because both fabrication process and materials are very similar. In contrast to commonly used semiconductor amplifiers, SFQ circuits combine very low power dissipation (a few microwatts) with very high operation speed, thus enabling count-rates of several gigahertz. The SFQ interface circuit simplifies the SNSPD readout and enables large numbers of detectors for future compact multi-pixel systems with single photon counting resolution. The demonstrated circuit has great potential for scaling the present interface solution to 1,000 detectors by using a single SFQ chip.