Two-Level System as a Quantum Sensor for Absolute Calibration of Power

A two-level quantum system can absorb or emit not more than one photon at a time. Using this fundamental property, we demonstrate how a superconducting quantum system strongly coupled to a transmission line can be used as a sensor of the photon flux. We propose four methods of sensing the photon flux and analyse them for the absolute calibration of power by measuring spectra of scattered radiation from the two-level system. This type of sensor can be tuned to operate in a wide frequency range, and does not disturb the propagating waves when not in use. Using a two-level system as a power sensor enables a range of applications in quantum technologies, here in particular applied to calibrate the attenuation of transmission lines inside dilution refrigerators

Development of a Superconducting Differential Double Contour Interferometer

We study operation of a new device, the superconducting differential double contour interferometer (DDCI), in application for the ultra sensitive detection of magnetic flux and for digital read out of the state of the superconducting flux qubit. DDCI consists of two superconducting contours weakly coupled by Josephson Junctions. In such a device a change of the critical current and the voltage happens in a step-like manner when the angular momentum quantum number changes in one of the two contours. The DDCI may outperform traditional Superconducting Quantum Interference Devices when the change of the quantum number occurs in a narrow magnetic field region near the half of the flux quantum due to thermal fluctuations, quantum fluctuations, or the switching a loop segment in the normal state for a while by short pulse of an external current.Comment: 11 pages, 8 figures, A version of the article has been accepted for publication in Nano Letter

Quantum Regime of a Two-Dimensional Phonon Cavity

The quantum regime in acoustic systems is a focus of recent fundamental research in the new field of Quantum Acoustodynamics (QAD). Systems based on surface acoustic waves having an advantage of easy integration in two-dimensions are particularly promising for the demonstration of novel effects in QAD and development of novel devices of quantum acousto-electronics. We demonstrate the vacuum mode of the surface acoustic wave resonator by coupling it to a superconducting artificial atom. The artificial atom is implemented into the resonator formed by two Brag mirrors. The results are consistent with expectations supported by the system model and our calculations. This work opens the way to map analogues of quantum optical effects into acoustic systems

A phononic crystal coupled to a transmission line via an artificial atom

We study a phononic crystal interacting with an artificial atom { a superconducting quantum system { in the quantum regime. The phononic crystal is made of a long lattice of narrow metallic stripes on a quatz surface. The artificial atom in turn interacts with a transmission line therefore two degrees of freedom of different nature, acoustic and electromagnetic, are coupled with a single quantum object. A scattering spectrum of propagating electromagnetic waves on the artificial atom visualizes acoustic modes of the phononic crystal. We simulate the system and found quasinormal modes of our phononic crystal and their properties. The calculations are consistent with the experimentally found modes, which are fitted to the dispersion branches of the phononic crystal near the first Brillouin zone edge. Our geometry allows to realize effects of quantum acoustics on a simple and compact phononic crystal

Enhancement of Superconductivity by Amorphizing Molybdenum Silicide Films Using a Focused Ion Beam

We have used focused ion beam irradiation to progressively cause defects in annealed molybdenum silicide thin films. Without the treatment, the films are superconducting with critical temperature of about 1 K. We observe that both resistivity and critical temperature increase as the ion dose is increased. For resistivity, the increase is almost linear, whereas critical temperature changes abruptly at the smallest doses and then remains almost constant at 4 K. We believe that our results originate from amorphization of the polycrystalline molybdenum silicide films

Detection of Coherent Terahertz Radiation from a High-Temperature Superconductor Josephson Junction by a Semiconductor Quantum-Dot Detector

We examine the application of Josephson radiation emitters to spectral calibration of single-photon-resolving detectors. Josephson junctions are patterned in a YBCO film on a bicrystal sapphire substrate and are voltage controlled to generate radiation in the frequency range of 0.05-1 THz. The detector used in this work consists of a gate-defined quantum-dot photon-to-charge transducer coupled to a single-electron transistor. Both the emitter and the detector are equipped with a matching on-chip wide-band antenna. The combination of a tuneable emitter and detector allows us to determine the efficacy of the YBCO emitter and also to analyze the elementary processes involved in the detection

ZnO tetrapod p-n junction diodes

ZnO nanocrystals hold the potential for use in a wide range of applications particularly in optoelectronics. We report on the fabrication of a highly sensitive p-n junction diode structure based on a single ZnO tetrapod shaped nanocrystal. This device shows a noted response to ultraviolet light with high internal gain. The high responsivities we have observed exceed 104 A/W and are likely due to impact-ionization effects at the p-n junction interface

Andreev Interferometers in a Strong Radio-Frequency Field

We experimentally study the influence of 1-40 GHz radiation on the resistance of normal (N) mesoscopic conductors coupled to superconducting (S) loops (Andreev interferometers). At low RF amplitudes we observe the usual h/2e superconducting-phase-periodic resistance oscillations as a function of applied magnetic flux. We find that the oscillations acquire a pi-shift with increasing RF amplitude, and consistent with this result the resistance at fixed phase is an oscillating function of the RF amplitude. The results are explained qualitatively as a consequence of two processes. The first is the modulation of the phase difference between the N/S interfaces by the RF field, with the resistance adiabatically following the phase. The second process is the change in the electron temperature caused by the RF field. From the data the response time of the Andreev interferometer is estimated to be <40ps. However there are a number of experimental features which remain unexplained; these include the drastic difference in the behaviour of the resistance at different phases as a function of RF frequency and amplitude, and the existence of a "window of transparency" where heating effects are weak enough to allow for the pi-shift. A microscopic theory describing the influence of RF radiation on Andreev interferometers is required

Capacitive coupling of coherent quantum phase slip qubits to a resonator

We demonstrate capacitive coupling of coherent quantum phase slip (CQPS) flux qubits to a resonator patterned on a highly disordered TiN film. We are able to detect and characterise CQPS flux qubits with linewidths down to $\Delta\omega = 12\pm1\,\text{MHz}$ on several resonator modes, and show that, unlike inductive coupling, here the coupling strength does not depend on the qubit’s energy. Since the qubit is galvanically decoupled from the resonator, our approach provides flexibility in material, design and fabrication choices for CQPS-based devices. Our results are two-fold: we report CQPS in TiN and demonstrate, to our knowledge for the first time, capacitive coupling of a CQPS flux qubit