R. Shaikhaidarov

Evidence for interacting two-level systems from the 1/f noise of a superconducting resonator

The performance of a great variety of electronic devices--ranging from semiconductor transistors to superconducting qubits--is hampered by low-frequency noise with spectra proportional to 1/f. The ubiquity and negative impact of 1/f noise has motivated intensive research into its cause, and it is now believed to originate from a bath of fluctuating two-level defect states (TLSs) embedded in the material. This phenomenon is commonly described by the long-established standard tunnelling model (STM) of independent TLS. A key prediction of STM is that the noise should vanish at low temperatures. Here we report measurements on superconducting microresonators over previously unattainable, very long time scales that show an increase in 1/f noise at low temperatures and low microwave power, contrary to the STM. We propose a new generalised tunnelling model that includes significant interaction between multiple TLSs, which fully describes these observations, as well as recent studies of individual TLS lifetimes in superconducting qubits.

Quantum wave mixing and visualisation of coherent and superposed photonic states in a waveguide

Superconducting quantum systems (artificial atoms) have been recently successfully used to demonstrate on-chip effects of quantum optics with single atoms in the microwave range. In particular, a well-known effect of four wave mixing could reveal a series of features beyond classical physics, when a non-linear medium is scaled down to a single quantum scatterer. Here we demonstrate the phenomenon of quantum wave mixing (QWM) on a single superconducting artificial atom. In the QWM, the spectrum of elastically scattered radiation is a direct map of the interacting superposed and coherent photonic states. Moreover, the artificial atom visualises photon-state statistics, distinguishing coherent, one- and two-photon superposed states with the finite (quantised) number of peaks in the quantum regime. Our results may give a new insight into nonlinear quantum effects in microwave optics with artificial atoms.The phenomenon of wave mixing is expected to show peculiar features when scaled down to the quantum level. Here, the authors show how coherent electromagnetic waves propagating in a 1D transmission line with an embedded two-level artificial atom are mapped into a quantised spectrum of narrow peaks.

Electrostatic effects in coupled quantum dot-point contact-single electron transistor devices

We study the operation of a system where quantum dot (QD) and point contact (PC) defined in a two-dimensional electron gas of a high-mobility GaAs/AlGaAs heterostructure are capacitively coupled to each other and to metallic single electron transistor (SET). The charge state of the quantum dot can be probed by the point contact or single electron transistor. These can be used for sensitive detection of terahertz radiation. In this work, we explore an electrostatic model of the system. From the model, we determine the sensitivity of the point contact and the single electron transistor to the charge excitation of the quantum dot. Nearly periodic oscillations of the point contact conductance are observed in the vicinity of pinch-off voltage. They can be attributed to Coulomb blockade effect in a quasi-1D channel because of unintentional formation of small quantum dot. The latter can be a result of fluctuations in GaAs quantum well thickness.

Quantum Wave Mixing and Probing of Photonic States in 1D space

DOI:10.31868/RFL2018.28-29 The wave mixing is well revealed and theoretically described phenomenon of a nonlinear optics. It has applications in phase conjugation, generation of squeezed states, parametric frequency conversion, signal regeneration schemes and exploited significantly for spectroscopic study of various systems. The wave mixing was thoroughly investigated in a medium such as fibre, atomic beams and vapours, with various number of mixed waves, exploiting two or more levels of a system. However, any medium represents a huge ensemble of atoms, so one needs many photons to drive the medium efficiently. Also, energy levels are broadened in homogeneously and hence what is accessible in wave mixing experiment is collective response of an ensemble of atoms. Quantum Wave Mixing (QWM) reveals itself as an elastic scattering of coherent classical and non-classical photonic states of electromagnetic waves on a single atom. We show a spectrum, corresponding to four-wave mixing of non-classical photonic states with a fingerprint of interacting photon states: the number of frequency peaks due to stimulated emission

Development of a Superconducting Differential Double Contour Interferometer

We study operation of a new device, the superconducting differential double contour interferometer (DDCI), in the application for the ultrasensitive 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, caused by an external magnetic flux or a nearby electric current, happens in a step-like manner when the angular momentum quantum number changes by one in one of the two contours. With a choice of parameters, the DDCI may outperform traditional superconducting quantum interference devices.

Single photon detection of the coherent THz radiation of HTS Josephson Junctions

The voltage biased individual Josephson Junction made of high temperature superconductor (HTS JJ) can potentially serve as the tuneable spectral source of terahertz radiation. The power is however relatively small, and one needs a sensitive detector in order to study and to optimise such a source. We use a single photon semiconductor quantum dot detector to probe the radiation of the HTS JJ. The detector is a resonant device with a spectral resolution of 30% at resonance frequency. Despite a rather wide resonance line the detector allows to discriminate between the coherent and the thermal radiation of HTS JJ. We found that HTS JJ emits to vacuum only 10-18W in a form of the coherent radiation, which is only ~ 4 × 10-10 part of the total dissipated power. Partially a small conversion factor can be attributed to the non optimal coupling of the HTS JJ to the planar metallic antenna. The experiments reveal also a specific of the operation of the quantum dot detector itself. We observe two resonances at 0.3 THz and 0.8 THz. They are likely related to excitation of the plasma mode in the patterned two dimensional electron gas.