Leonid S. Revin

Underdamped Josephson junction as a switching current detector

We demonstrate the narrow switching distribution of an underdamped Josephson junction from the zero to the finite voltage state at millikelvin temperatures. We argue that such junctions can be used as ultrasensitive detectors of the single photons in the GHz range, operating close to the quantum limit: a given initial (zero voltage) state can be driven by an incoming signal to the finite voltage state. The width of the switching distribution at a nominal temperature of about T = 10 mK was 4.5 nA, which corresponds to an effective noise temperature of the device below 60 mK.

Soliton scattering as a measurement tool for weak signals

We have considered relativistic soliton dynamics governed by the sine-Gordon equation and affected by short spatial inhomogeneities of the driving force and thermal noise. Developed analytical and numerical methods for calculation of soliton scattering at the inhomogeneities allowed us to examine the scattering as a measurement tool for sensitive detection of polarity of the inhomogeneities. We have considered the superconducting fluxonic ballistic detector as an example of the device in which the soliton scattering is utilized for quantum measurements of superconducting flux qubits. We optimized the soliton dynamics for the measurement process varying the starting and the stationary soliton velocity as well as configuration of the inhomogeneities. For experimentally relevant parameters we obtained the signal-to-noise ratio above 100 reflecting good practical usability of the measurement concept.

Observation of photon noise by cold-electron bolometers

We have observed the photon noise by measuring a response to the black body 350 GHz radiation and noise of the cold-electron bolometers (CEBs). The experimental results have been fit to the theoretical model of CEBs with two heat-balance equations. The measured noise has been decomposed into several terms with the help of theory. It is demonstrated that the photon noise exceeds any other noise components, which allows us to conclude that the bolometers measure the photon noise. Moreover, a peculiar shape of the noise dependence on the absorbed power originates completely from the photonic component according to the theory. In the additional experiment on heating of the cryostat plate together with the sample holder, we have observed the near independence of the noise on the electron temperature of the absorber, which has provided another proof of the presence of the photon noise in the first experiment. The least ratio between internal and photon noise equivalent powers, observed in our experiments, is 1.1 for the absorbed power of 1-2 pW.

The effect of bias current asymmetry on the flux-flow steps in the grain boundary YBaCuO long Josephson junctions

The current-voltage characteristics and critical current versus magnetic field dependence of long 24 degrees[001]-tilt YBa2Cu3O7-delta bicrystal grain-boundary junctions are studied both experimentally and theoretically. For the opposite magnetic field directions, the flux-flow steps with significantly different height and slope are observed. It is demonstrated that the most probable reason of this discrepancy is recently predicted asymmetry of spatial bias current distribution due to crystallographic anisotropy of bicrystal substrates [Kupriyanov et al. JETP Lett, 95, 289 (2012)].

The effect of bias feed profile on the linewidth of noisy Josephson flux flow oscillator

For creation of a noisy non-stationary spectrometer operating in the sub-THz frequency range, the emitting power and spectral characteristics of a flux-flow oscillator based on a long Josephson junction are considered. The effect of bias current profiles on the spectral linewidth of a long overlap Josephson junction is investigated and compared with the inline junction geometry. It has been demonstrated that the spectral linewidth can be increased by a factor of three with a moderate reduction of the emitted power.

Fine tuning of phase qubit parameters for optimization of fast single-pulse readout

We analyze a two-level quantum system, describing the phase qubit, during a single-pulse readout process by a numerical solution of the time-dependent Schroedinger equation. It has been demonstrated that the readout error has a minimum for certain values of the system’s basic parameters. In particular, the optimization of the qubit capacitance and the readout pulse shape leads to significant reduction in the readout error. It is shown that in an ideal case the fidelity can be increased to almost 97% for 2 ns pulse duration and to 96% for 1 ns pulse duration.

Strong Electron Self-Cooling in the Cold-Electron Bolometers Designed for CMB Measurements

We have realized cold-electron bolometers (CEB) with direct electron self-cooling of the nanoabsorber by SIN (Superconductor-Insulator-Normal metal) tunnel junctions. This electron self-cooling acts as a strong negative electrothermal feedback, improving noise and dynamic properties. Due to this cooling the photon-noise-limited operation of CEBs was realized in array of bolometers developed for the 345 GHz channel of the OLIMPO Balloon Telescope in the power range from 10 pW to 20 pW at phonon temperature T ph =310 mK. The negative electrothermal feedback in CEB is analogous to TES but instead of artificial heating we use cooling of the absorber. The high efficiency of the electron self-cooling to T e =100 mK without power load and to Te=160 mK under power load is achieved by:-a very small volume of the nanoabsorber (0.02 μm 3 ) and a large area of the SIN tunnel junctions,-effective removal of hot quasiparticles by arranging double stock at both sides of the junctions and close position of the normal metal traps,-self-protection of the 2D array of CEBs against interferences by dividing them between N series CEBs (for voltage interferences) and M parallel CEBs (for current interferences),-suppression of Andreev reflection by a thin layer of Fe in the AlFe absorber. As a result even under high power load the CEBs are working at electron temperature T e less than T ph . To our knowledge, there is no analogue in the bolometers technology in the world for bolometers working at electron temperature colder than phonon temperature.

Quick and reliable technology for fabrication of stand-alone BSCCO mesas

The technology of wet etching allowing fabrication of stand-alone BSCCO mesa structures was proposed. The produced mesas can be made much thicker than ones usually being studied. The time required for the fabrication is much smaller in comparison with the standard method of ion milling. The process used is controllable which provides acceptable precision of mesa fabrication. The IV characteristics of the sample showing Josephson nature were obtained. The qualitative comparison with characteristics of similar structures fabricated by other groups was carried out.