V. I. Shnyrkov

Stochastic resonance in superconducting loops containing Josephson junctions. Numerical simulation

A numerical simulation of the stochastic resonance is carried out in the adiabatic approximation in overdamped systems based on superconducting loops closed by a weak link. The systems under consideration include a single-ring rf SQUID, two rings coupled by a common magnetic flux, and a ring closed by a 4-terminal Josephson junction. It is shown that coupling of single SQUID rings enhances the gain and the signal-to-noise ratio. These effects can be used to create new stochastic SQUID antennas for measurements of harmonic and quasi-harmonic signals. The stochastic resonance in 4-terminal SQUIDS exists even at values of the dimensionless inductance l<1.

Superposition of states in flux qubits with a Josephson junction of the ScS type (Review Article)

The consequences of the transition to a quantum description of magnetic flux motion in the superconducting ring closed by an ScS type Josephson junction are considered. Here we review the principal results regarding macroscopic quantum tunneling (MQT) of Bose condensate consisting of a macroscopically large number of Cooper electron pairs. These phenomena are illustrated by the original data obtained from the study of MQT and coherent states in a modified flux qubit with energy level depletion ΔE01 ≈ 2·10–23 J (ΔE01/hu2009≈u200930 GHz). State superposition properties in a two-well potential and the issues associated with quantum measurements of local curvature of qubits’ superposition energy levels are analyzed.

Quantum detector based on a superposition of macroscopic states in a phase qubit

A quantum detector whose working principle is based on magnetic-field modulation of a circulating supercurrent in the quantum ground state of a macroscopic superconducting loop with a Josephson junction. Under the influence of an external magnetic flux equal to Φ0∕2 (or Φ0), two (or three) classical states are coupled to each other by quantum tunneling through a potential barrier, and therefore the detector is a two-level (or three-level) system. In the low-temperature region and under the condition of very weak damping, the mean value of the circulating supercurrent reflects the character of the variation of the quantum superposition of macroscopic states, which is sensitive to the symmetry of the potential. The variations of the current are amplified and detected in a measurement scheme similar to the signal registration in a nonhysteretic rf SQUID. It is shown by a numerical analysis that in comparison with a qubit detector based on an SIS junction, a detector with an ScS junction is faster and has muc...

Stochastic resonance in an RF SQUID with shunted ScS junction

Using a point (superconductor–constriction–superconductor, ScS) contact in a single-Josephson-junction superconducting quantum interference device (RF SQUID) provides stochastic resonance conditions at any arbitrary small value of loop inductance and contact critical current, unlike SQUIDs with more traditional tunnel (superconductor–insulator–superconductor, SIS) junctions. This is due to the unusual potential energy of the ScS RF SQUID which always has a barrier between two wells, thus making the device bistable. This paper presents the results of a numerical simulation of the stochastic dynamics of the magnetic flux in an ScS RF SQUID loop affected by band-limited white Gaussian noise and low-frequency sine signals of small and moderate amplitudes. The difference in stochastic amplification of RF SQUID loops incorporating ScS and SIS junctions is discussed.

Signal characteristics of charge-phase qubit detector with parametric energy conversion

The characteristics of a partially coherent quantum detector based on a charge-phase qubit, coupled with a classical (ωT<kBT∕ħ) resonant circuit, are analyzed. It is shown that in an electromagnetic field signal characteristics with the maximum coefficient of conversion arise when the effective quantum inductance of the qubit assumes positive and negative values periodically with the frequency of low-frequency oscillations of the occupation probability of the energy levels (Rabi type) ΩR≈ωT. The physical nature of parametric energy conversion (regeneration) in a qubit detector with a periodic change of the sign of the effective inductance and its possible application in quantum informatics for detecting weak signals is discussed.

The two-Josephson-junction flux qubit with large tunneling amplitude

In this paper we discuss solid-state nanoelectronic realizations of Josephson flux qubits with large tunneling amplitude between the two macroscopic states. The latter can be controlled via the height and form of the potential barrier, which is determined by quantum-state engineering of the flux qubit circuit. The simplest circuit of the flux qubit is a superconducting loop interrupted by a Josephson nanoscale tunnel junction. The tunneling amplitude between two macroscopically different states can be increased substantially by engineering of the qubit circuit if the tunnel junction is replaced by a ScS contact. However, only Josephson tunnel junctions are particularly suitable for large-scale integration circuits and quantum detectors with present-day technology. To overcome this difficulty we consider here a flux qubit with high energy-level separation between the “ground” and “excited” states, consisting of a superconducting loop with two low-capacitance Josephson tunnel junctions in series. We demonst...

Isolation of a Josephson qubit from the electromagnetic environment

We consider two aspects of isolation of a Josephson flux (charge-flux) qubit from the external dissipative electromagnetic environment: (i) selecting an optimal topology of the superconducting qubit circuit and (ii) passive filtering of Planck radiation at the input of the qubit-state detection circuit. When reading the state of a macroscopic quantum object (“Schrodingers cat”) with the weak continuous measurement technique, the coupling to the environment, both direct and through the connected circuits, is the cause of the rapid loss of coherence of the superposition states. The coefficients of coupling to the external electromagnetic environment are discussed, as well as the problem of their minimization for flat (2D) and bulk (3D) designs of the qubit quantization loops. The analysis of the characteristics of low-temperature combined broadband filters designed to effectively reduce the electromagnetic noise in the control and measurement circuits is carried out. It is shown experimentally that a cryog...

Charge-flux qubit coupled to a tank circuit in a strong low-frequency electromagnetic field

A superconducting charge-flux qubit coupled to a high-Q tank circuit was studied in a low-frequency electric field. A fine structure of the multiphoton resonance lines and quantum interference effects associated with the excitation of a quasi-two-level system due to the Landau–Zener–Stuckelberg tunneling was observed. The results obtained for multiphoton resonant excitations and low-frequency oscillations of the average occupation of quantum levels were compared using different parameters of the measuring circuit. The mechanism responsible for the fine structure of resonance lines was considered. The method to measure the impedance arising in the tank circuit due to the oscillations of the superconducting current in the qubit and the main sources of decoherence were discussed.

Stochastic-parametric amplification of narrow-band signals in a single-junction SQUID interferometer

The features of the response of a single-junction superconducting quantum interferometer to a low-frequency harmonic signal in the presence of noise and a high-frequency electromagnetic field are investigated through numerical solution of the equations of motion. It is shown that in this situation a system described by a double-well potential will display stochastic-parametric amplification of weak harmonic signals owing to the cooperative effects of noise and the high-frequency field. The gain is a nonmonotonic function of the amplitude of the high-frequency field and the variance of the noise flux and passes through a maximum. A detailed numerical analysis of the dependence of the gain on the noise intensity and on the frequency and amplitude of the high-frequency field is carried out in the stochastic, parametric, and stochastic-parametric amplification regimes. It is shown that at optimal amplitude of the high-frequency field the gain for a weak harmonic signal reaches rather high values (10–30). The ...

High-Tc rf SQUID for magnetic microscopy

The spectral density of magnetic flux noise SΦ1/2(fu200a) is investigated for high-Tc rf SQUIDs with a pumping frequency of 390–457 MHz, placed inside three-layer Permalloy and superconducting shields. A superconducting interferometer with inner dimensions of the pickup loop of 100×100u2009μm is prepared by thin-film technology with a YBaCuO–PrBaCuO–YBaCuO Josephson junction of the ramp-edge type. It is shown that with the use of a cooled preamplifier, the energy sensitivity of SQUIDs in the “white” noise region (at frequencies above 1 kHz) is 4×10−30u2009J/Hz and is mainly determined by the intrinsic noise of the high-Tc superconducting interferometer and the shields. At low frequencies the predominant noise is from external fields penetrating directly into the shields. The addition of a ferromagnetic antenna to the SQUID microscope increases the intrinsic noise of the magnetometer to 8×10−30u2009J/Hz at frequencies above 1 kHz.