Roman Sobolewski

Ultrafast superconducting single-photon detectors for near-infrared-wavelength quantum communications

Abstract The paper reports progress on the design and development of niobium-nitride, superconducting single-photon detectors (SSPDs) for ultrafast counting of near-infrared photons for secure quantum communications. The SSPDs operate in the quantum detection mode, based on photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-width superconducting stripe. The devices are fabricated from 3.5 nm thick NbN films and kept at cryogenic (liquid helium) temperatures inside a cryostat. The detector experimental quantum efficiency in the photon-counting mode reaches above 20% in the visible radiation range and up to 10% at the 1.3–1.55 μn infrared range. The dark counts are below 0.01 per second. The measured real-time counting rate is above 2 GHz and is limited by readout electronics (the intrinsic response time is below 30 ps). The SSPD jitter is below 18 ps, and the best-measured value of the noise-equivalent power (NEP) is 2 × 10−18 W/Hz1/2. at 1.3 μm. In terms of photon-counting efficiency and speed, these NbN SSPDs significantly outperform semiconductor avalanche photodiodes and photomultipliers.

Femtosecond response of a free-standing LT-GaAs photoconductive switch

We present a novel, free-standing low-temperature GaAs (LT-GaAs) photoconductive switch and demonstrate its femtosecond performance. A 1-microm-thick layer of a single-crystal LT-GaAs was patterned into 5-10-microm-wide and 15-30-microm-long bars, separated from their GaAs substrate and, subsequently, placed across gold coplanar transmission lines deposited on a Si substrate, forming a photoconductive switch. The switch was excited with 110-fs-wide optical pulses, and its photoresponse was measured with an electro-optic sampling system. Using 810-nm optical radiation, we recorded an electrical transient as short as 360 fs (1.25 THz, 3-dB bandwidth) and established that the photo-carrier lifetime in our LT-GaAs was 150 fs. Our free-standing devices exhibited quantum efficiency of the order of approximately 7%, and their photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the excitation power, below a well-defined saturation threshold.

Microwave detection and mixing in Y-Ba-Cu-O thin films at liquid-nitrogen temperatures

Microwave properties of an Y‐Ba‐Cu‐O microstrip detector have been studied at 24 GHz frequency band at the liquid‐nitrogen temperature. The amplitude of the video detection signal was found to be proportional to the incident microwave power. Direct mixing action was also demonstrated, setting the upper limit for the film response time as short as 40 ps. Occurrence of microwave mixing in the absence of dc current bias was observed, suggesting the presence of the inverse Josephson effect. Physical properties of the detector response were very similar to the enhanced mode of detection, characteristic for granular superconducting films.

Simple patterning of spray-deposited Y-Ba-Cu-O films

A new patterning method suitable for spray-deposited Y-Ba-Cu-O films has been demonstrated. The method is a single-step process, based on the application of metal stencil masks to define a desired pattern. Lines less than 50 mu m wide, with superconducting properties identical to these of unpatterned films, have been successfully deposited.

Time-resolved photoresponse in the resistive flux-flow state in Y–Ba–Cu–O superconducting microbridges

We report our studies on ultrafast voltage transients of optically thick superconducting YBa2Cu3O7−x microbridges biased with nanosecond supercritical current pulses (I > Ic) and, simultaneously, illuminated with 100 fs optical pulses (810 nm wavelength) from a Ti:sapphire laser. The pulsed current bias created a resistive flux-flow state in the superconductor, while the laser pulse transferred it into a state of more intensive flow of vortices within a time of less than 100 ps. The light-enhanced flux state remained constant until the end of the biasing pulse. The amplitude of the photoresponse signal increased rapidly with the increase of laser fluence in the range from 8 × 106 to 2 × 108 photons per pulse, as well as with the current-bias increase up to 2Ic, exhibiting the maximum, when the light illuminated the entire area of the microbridge. Maximal repetition rate of optically excited YBa2Cu3O7−x photoresponse signals was found to be in the GHz range, appropriate for applications of YBa2Cu3O7−x thin-film microbridges as high-power ultrafast switches.

Deposition and properties of high critical temperature superconducting ceramic thin films

We report our studies on the fabrication and superconducting properties of thin films in the YBaCuO and BiSrCaCuO systems. The films were manufactured on ZrO2:Y, ZrO2:Gd, MgO, and LaAlO3 substrates, by chemical deposition of properly mixed nitrate precursors, as well as by d.c. magnetron sputtering from a single oxide target. In all cases, as-deposited films required a high temperature heat treatment to become superconducting. After annealing, our films were granular with the thickness ranging from 0.5 to 5 μm, and exhibited sharp superconducting transitions with the zero resistivity state reached at 89 K and 77 K for YBaCuO and BiSrCaCuO films respectively. Patterning techniques, most suitable for our deposition methods, were also developed and tested.

Microwave absorption in YBa2Cu3O7−y thin films

Abstract We report our measurements of high frequency properties of the YBa2Cu3O7−y thin films prepared on heated sapphire substrates using a standard dc magnetron deposition process. The film was exposed to a uniform illumination of the millimeter-wavelength microwave radiation. We observed an abnormal microwave absorption, which manifested itself in a substantial reduction of the film supercurrent. Our results are explained assuming a granular nature of the film.

Statistical properties of three-level atom resonance fluorescence

Abstract Statistical properties of resonance fluorescence of a three-level atom, interacting with a monochromatic, strong laser field are examined. Analytical formulas, describing first- and second-order correlation functions are presented. The effect of antibunching is predicted and the nontrivial difference between the statistical properties of differently polarized fluorescence is discussed.

Josephson effects and microwave response of HTS thin films

Abstract The response of thin films YBCO and BSCCO to microwave fields has been investigated. Some unexpected features have been observed and an explanation is given in terms of a robust fluxon lattice whose motion is correlated with the applied microwave field.

Fabrication and properties of high-Tc films deposited on LaGaO3 and LaAlO3 substrates☆

Abstract We report on our progress in the fabrication and characterization of high-T c thin films prepared on the in-house grown single crystal LaGaO 3 and LaAlO 3 substrates. Y Ba Cu O and Bi Sr Ca Cu O films were deposited using a chemical-spray technique and a dc-magnetron-sputtering process, followed by an ex-situ high temperature annealing. In the case of LaAlO 3 substrates, both fabrication methods resulted in highly oriented films, characterized by very good superconducting properties. On the LaGaO 3 substrates we succeeded in fabricating only Bi Sr Ca Cu O superconducting films.