Geometrical jitter and bolometric regime in photon detection by straight superconducting nanowire

We present a direct observation of the geometrical jitter in single photon detection by a straight superconducting nanowire. Differential measurement technique was applied to the 180-{\mu}m long nanowire similar to those commonly used in the technology of superconducting nanowire single photon detectors (SNSPD). A non-gaussian geometrical jitter appears as a wide almost uniform probability distribution (histogram) of the delay time (latency) of the nanowire response to detected photon. White electrical noise of the readout electronics causes broadened, Gaussian shaped edges of the histogram. Subtracting noise contribution, we found for the geometrical jitter a standard deviation of 8.5 ps and the full width at half maximum (FWHM) of the distribution of 29 ps. FWHM corresponds to the propagation speed of the electrical signal along the nanowire of $6.2\times10^{6}$ m/s or 0.02 of the speed of light. Alternatively the propagation speed was estimated from the central frequency of the measured first order self-resonance of the nanowire. Both values agree well with each other and with previously reported values. As the intensity of the incident photon flux increases, the wide probability distribution collapses into a much narrower Gaussian distribution with a standard deviation dominated by the noise of electronics. We associate the collapse of the histogram with the transition from the discrete, single photon detection to the uniform bolometric regim

Andreev interferometer with three superconducting electrodes

We develop a quasiclassical theory of Andreev interferometers with three superconducting electrodes. Provided tunneling interface resistance between one superconducting electrode and the normal metal strongly exceeds two others, significant current sensitivity to the external magnetic flux is observed only at subgap voltages. If all barrier conductances are comparable, multiple Andreev reflection comes into play and substantial current modulation can be achieved in both subgap and overgap voltage regimes. Our analysis reveals a large variety of interesting features which can be used for performance optimization of Andreev interferometers.Comment: 9 pages, 13 figure

Timing jitter in photon detection by straight superconducting nanowires: Effect of magnetic field and photon flux

We studied the effect of the external magnetic field and photon flux on timing jitter in photon detection by straight superconducting NbN nanowires. At two wavelengths 800 and 1560 nm, statistical distribution in the appearance time of the photon count exhibits Gaussian shape at small times and exponential tail at large times. The characteristic exponential time is larger for photons with smaller energy and increases with external magnetic field while variations in the Gaussian part of the distribution are less pronounced. Increasing photon flux drives the nanowire from quantum detection mode to the bolometric mode that averages out fluctuations of the total number of nonequilibrium electrons created by the photon and drastically reduces jitter. The difference between Gaussian parts of distributions for these two modes provides the measure for the electron-number fluctuations. Corresponding standard deviation increases with the photon energy. We show that the two-dimensional hot-spot detection model explains qualitatively the effect of magnetic field

Design and Comparison of Diamond‐ and Sapphire‐Based NbN KIDs for Fusion Plasma Polarimetric Diagnostics

In this paper we present the design and characterization measurements performed on Kinetic Inductance Detectors produced on sapphire and policrystalline diamond substrates. Designed to be part of a nuclear fusion polarimetric diagnostic instrument, the foreseen plasma probing frequency of the final devices is 1.3 THz with a maximum response time under 10 ms and cross-polarization target accuracy of 1$\%$, in accordance with the guidelines of the upcoming ITER reactor \cite{zeeland2013} \cite{Donne2007}. These detectors are based on superconducting micro-resonators that undergo de-tuning upon absorption of radiation. The main characteristics of the devices include polarization sensitiveness and lumped elements multi-pixel configuration produced from photo-lithographed Niobium Nitride (NbN) thin films. The relatively high $T_C$ of bulk NbN ($\sim$16~K) enables operation at liquid helium temperatures, simplifying the setup of the final detection system when compared to the sub-kelvin temperatures employed in astrophysical KID arrays. The DC characterization measurements performed on test strips patterned on the two substrate materials highlighted large differences in the thin films quality, with the superconductor deposited on diamond showing reduced critical temperature, lower \textcolor{red}{critical} current density and increased values of the kinetic inductance compared to the strips on sapphire. This difference is believed to be mainly due to the higher lattice constant and thermal expansion coefficient mismatch between film and substrate in the case of diamond, \textcolor{red}{with the} different surface finish quality of the crystalline samples at our disposal \textcolor{red}{also likely playing a role.} The response to microwave read-out tones of the prototypes obtained from the same films follows a similar behavior, with the devices produced on sapphire generally outperforming those on diamond. Nevertheless, diamond is a promising candidate for this kind of application, \textcolor{red}{especially considering} the advantages given by its radiation hardness. The devices on both substrates showed a response to THz radiation, bolometric in nature, that fulfills the requirement guidelines and represent a good starting point to optimize the design for the application at hand

Optical response of a titanium-based cold-electron bolometer

We present experimental results on the testing of cold-electron bolometer (CEB) detectors comprised of a thin Ti film absorber and two SIN junctions integrated with a planar antenna. The CEB performance was tested in a He-3 sorption cryostat HELIOX-AC-V at bath temperatures of 280-305 mK. The optical response was measured using the hot/cold load method by flipping a Cu reflector opposite a blackbody surface inside a 3 K shield and using a thermal source with variable temperature. In the first experiment, the detector chip was mounted in an optical sample-holder whose aperture was switched towards or away from a blackbody source changing the incident radiation temperature from 3 K to 270 mK. As a result, we measured the optical response to a 3 K/270 mK radiation temperature change. The measured voltage response value for the detector integrated in a double-dipole antenna was Delta V-out = 120 mu V. This corresponds to a noise equivalent power of NEP = V-n/(dV/dP) = 3.5 x 10(-17) W Hz(-1/2), where dV/dP is the voltage to power response obtained from the incoming power estimation based on the Planck formula

Local thermal fluctuations in current-carrying superconducting nanowires

We analyze the effect of different types of fluctuations in internal electron energy on the rates of dark and photon counts in straight current-carrying superconducting nanowires. Dark counts appear due to thermal fluctuations in statistically independent cells with the effective size of the order of the coherence length; each count corresponds to an escape from the equilibrium state through an appropriate saddle point. For photon counts, spectral broadening of the deterministic cut off in the spectra of the detection efficiency can be phenomenologically explained by local thermal fluctuations in the electron energy within cells with the same effective volume as for dark counts