arXiv subjects

A. B. Zorin

Publications and source records attributed to A. B. Zorin.

At least 37 records · Page 2Linked to original sources

arXiv2008

Quantum dynamics in a camel-back potential of a dc SQUID

We investigate the quantum dynamics of a quadratic-quartic anharmonic oscillator formed by a potential well between two potential barriers. We realize this novel potential shape with a superconducting circuit comprised of a loop interrupted by two Josephson junctions, with near-zero current bias and flux bias near half a flux quantum. We investigate escape out of the central well, which can occur via tunneling through either of the two barriers, and find good agreement with a generalized double-path macroscopic quantum tunneling theory. We also demonstrate that this system exhibits an "optimal line" in current and flux bias space along which the oscillator, which can be operated as a phase qubit, is insensitive to decoherence due to low-frequency current fluctuations.

cond-mat.supr-con↗

arXiv2008

Modeling of quasiparticle-induced excitations of a Josephson charge-phase qubit

We have analyzed quasiparticle transitions in an Al charge-phase qubit inducing a dynamic change of the qubit states. The time-averaged mixed state is related to the strong coupling of the qubit to an ensemble of non-equilibrium quasiparticles in the leads. Such quasiparticles tunnel stochastically on and off the island and can excite the qubit. Continuous monitoring of the qubit impedance at a frequency of 80 MHz shows the admixture of the excited state. We present a numerical description of these cyclic transitions and compare it with our experimental data.

cond-mat.supr-con↗

arXiv2008

An Optimal Tunable Josephson Element for Quantum Computing

We introduce a three-junction SQUID that can be effectively used as an optimal tunable element in Josephson quantum computing applications. This device can replace the simple dc SQUID generally used as tunable element in this kind of applications, with a series of advantages for the coherence time and for the tolerance to small errors. We study the device both theoretically and experimentally at 4.2 K, obtaining a good agreement between the results.

quant-ph↗

arXiv2008

Single flux quantum circuits with damping based on dissipative transmission lines

We propose and demonstrate the functioning of a special Rapid Single Flux Quantum (RSFQ) circuit with frequency-dependent damping. This damping is achieved by shunting individual Josephson junctions by pieces of open-ended RC transmission lines. Our circuit includes a toggle flip-flop cell, Josephson transmission lines transferring single flux quantum pulses to and from this cell, as well as DC/SFQ and SFQ/DC converters. Due to the desired frequency-dispersion in the RC line shunts which ensures sufficiently low noise at low frequencies, such circuits are well-suited for integrating with the flux/phase Josephson qubit and enable its efficient control.

cond-mat.supr-con↗

arXiv2007

Quasi-particle transitions in charge-phase qubits probed by rf oscillations

We investigated transitions in Al charge-phase qubits of SQUID-configuration inductively coupled to an rf tank circuit that made it possible to read out the state by measuring the inductance of the qubit. Depending on the flux and charge bias and the amplitude of the rf oscillations we probed either the ground state or a dynamic change of the qubit states which we attributed to stochastic single quasi-particle tunneling onto and off the qubit island and exchange of energy with the qubit. Within this model, a selection rule for quasi-particle-induced transitions in the qubit is discussed.

cond-mat.other↗

arXiv2006

Self-shunted Al/AlOx/Al Josephson junctions

Self-shunted aluminum Josephson junctions with high-transparency barriers were fabricated using the shadow-evaporation technique and measured at low temperatures, T=25 mK. Due to high junction transparency, the IV-characteristics were found to be of only small hysteresis with retrapping-to-switching current ratio of up to 80%. The observed critical currents were close to the Ambegaokar-Baratoff values (up to 80-100%). Good barrier quality was confirmed by the low subgap leakage currents in the quasiparticle branches, which makes the self-shunted Al junctions promising for application in integrated RSFQ-qubit circuitry.

cond-mat.mes-hall↗

arXiv2006

Subgap conductivity in SIN-junctions of high barrier transparency

We investigate the current-voltage characteristics of high-transparency superconductor-insulator-normal metal (SIN) junctions with the specific tunnel resistance below 30 kOhm per square micron. The junctions were fabricated from different superconducting and normal conducting materials, including Nb, Al, AuPd and Cu. The subgap leakage currents were found to be appreciably larger than those given by the standard tunnelling model. We explain our results using the model of two-electron tunnelling in the coherent diffusive transport regime. We demonstrate that even in the high-transparency SIN-junctions, a noticeable reduction of the subgap current can be achieved by splitting a junction into several submicron sub-junctions. These structures can be used as nonlinear low-noise shunts in Rapid-Single-Flux-Quantum (RSFQ) circuitry for controlling Josephson qubits.

cond-mat.mes-hall↗

arXiv2005

Dynamics of Josephson junctions and single-flux-quantum networks with superconductor-insulator-normal metal junction shunts

Within the framework of the microscopic model of tunneling, we modelled the behavior of the Josephson junction shunted by the Superconductor-Insulator-Normal metal (SIN) tunnel junction. We found that the electromagnetic impedance of the SIN junction yields both the frequency-dependent damping and dynamic reactance which leads to an increase in the effective capacitance of the circuit. We calculated the dc I-V curves and transient characteristics of these circuits and explained their quantitative differences to the curves obtained within the resistively shunted junction model. The correct operation of the basic single-flux-quanta circuits with such SIN-shunted junctions, i.e. the Josephson transmission line and the toggle flip-flop, have also been modelled.

cond-mat.supr-con↗

arXiv2005

Bloch inductance in small-capacitance Josephson junctions

We show that the electrical impedance of a small-capacitance Josephson junction includes besides the capacitive term $-i/ωC_B$ also an inductive term $iωL_B$. Similar to the known Bloch capacitance $C_B(q)$, the Bloch inductance $L_B(q)$ also depends periodically on the quasicharge $q$, and its maximum value achieved at $q=e (\textrm{mod} 2e)$ always exceeds the value of the Josephson inductance of this junction $L_J(ϕ)$ at fixed $ϕ=0$. The effect of the Bloch inductance on the dynamics of a single junction and a one-dimensional array is described.

cond-mat.supr-con↗

arXiv2004

Steps towards a capacitance standard based on single-electron counting at PTB

A capacitance standard based on the definition of capacitance C = Ne/U is realized when a capacitor is charged with a known number N of electrons (e is the elementary charge) and the voltage U across the capacitor is measured. If U is measured by means of a Josephson voltage standard, C is solely determined by quantum numbers and fundamental constants. Presently, PTB is setting up this experiment using a special type of single-electron pump, the so-called r-pump, for transferring electrons one by one onto a cryogenic vacuum capacitor with the decadic capacitance C = 1 pF. We report on the progress in the setup with emphasis on the characterization of the single-electron tunneling elements. We describe the modifications of the single-electron circuit that are necessary to implement the capacitor charging experiment by using a four- or five-junction r-pump.

cond-mat.mes-hall↗

arXiv2004

Josephson tunnel junctions with nonlinear damping for RSFQ-qubit circuit applications

We demonstrate that shunting of Superconductor-Insulator-Superconductor Josephson junctions by Superconductor-Insulator-Normal metal (S-I-N) structures having pronounced non-linear I-V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlO${_x}$/Nb junction shunted by Nb/AlO${_x}$/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited non-hysteretic I-V characteristics at temperatures down to at least 1.4 K.

cond-mat.supr-con↗

arXiv2004

Single-charge devices with ultrasmall Nb/AlOx/Nb trilayer Josephson junctions

Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200 nm down to 70 nm were fabricated from standard Nb/AlOx/Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25 mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of 1.35 meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5 fF to 4 fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100 nm by 100 nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.

cond-mat.mes-hall↗

arXiv2004

Coupling and dephasing in Josephson charge-phase qubit with radio frequency readout

The Cooper pair box qubit of SQUID configuration enables the readout of the qubit states by probing the effective Josephson inductance of the SQUID. This is realized by coupling the qubit to a tank circuit which induces a small alternating supercurrent in the SQUID loop. The effect of a small (but finite) geometrical inductance of the loop on the eigenstates of the system is figured out. The effect of qubit dephasing due to quadratic coupling to the tank circuit is evaluated. It is shown that the rate of dephasing in the vicinity of the magic points is relatively low unless the Josephson junctions forming the qubit are rather dissimilar. In the vicinity of the avoided level-crossing point such dephasing is always significant.

cond-mat.supr-con↗

arXiv2003

Josephson charge-phase qubit with radio frequency readout: coupling and decoherence

The charge-phase Josephson qubit based on a superconducting single charge transistor inserted in a low-inductance superconducting loop is considered. The loop is inductively coupled to a radio-frequency driven tank circuit enabling the readout of the qubit states by measuring the effective Josephson inductance of the transistor. The effect of qubit dephasing and relaxation due to electric and magnetic control lines as well as the measuring system is evaluated. Recommendations for operation of the qubit in magic points producing minimum decoherence are given.

cond-mat.mes-hall↗

arXiv2003

Rf-induced transport of Cooper pairs in superconducting single electron transistors in a dissipative environment

We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In our samples with relatively large charging energy values Ec > EJ, where EJ is the energy of the Josephson coupling, two transport mechanisms were found to be dominating, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage Vg, either sequential tunneling of pairs via the transistor island (in the open state of the transistor around the points Qg = CgVg = e mod(2e), where Cg is the gate capacitance) or their cotunneling through the transistor (for Qg away of these points) was found to prevail in the net current. As the open states of our transistors had been found to be unstable with respect to quasiparticle poisoning, high-frequency gate cycling (at f ~ 1 MHz) was applied to study the sequential tunneling mechanism. A simple model based on the master equation was found to be in a good agreement with the experimental data.

cond-mat.mes-hall↗

arXiv2003

Cooper pair cotunneling in single charge transistors with dissipative electromagnetic environment

We observed current-voltage characteristics of superconducting single charge transistors with on-chip resistors of R about R_Q = h/4e^2 = 6.45 kOhm, which are explained in terms of Cooper-pair cotunneling. Both the effective strength of Josephson coupling and the cotunneling current are modulated by the gate-induced charge on the transistor island. For increasing values of the resistance R we found the Cooper pair current at small transport voltages to be dramatically suppressed.

cond-mat.mes-hall↗

arXiv2001

Cooper-pair qubit and Cooper-pair electrometer in one device

An all-superconductor charge qubit enabling a radio-frequency readout of its quantum state is described. The core element of the setup is a superconducting loop which includes the single-Cooper-pair (Bloch) transistor. This circuit has two functions: First, it operates as a charge qubit with magnetic control of Josephson coupling and electrostatic control of the charge on the transistor island. Secondly, it acts as the transducer of the rf electrometer, which probes the qubit state by measuring the Josephson inductance of the transistor. The evaluation of the basic parameters of this device shows its superiority over the rf-SET-based qubit setup.

cond-mat.supr-con↗

arXiv2001

Single electron transistors with high quality superconducting niobium islands

Deep submicron Al-AlOx-Nb tunnel junctions and single electron transistors with niobium islands were fabricated by electron beam gun shadow evaporation. Using stencil masks consisting of the thermostable polymer polyethersulfone (PES) and germanium, high quality niobium patterns with good superconducting properties and a gap energy of up to 2Delta = 2.5 meV for the niobium were achieved. The I(U) characteristics of the transistors show special features due to tunneling of single Cooper pairs and significant gate modulation in both the superconducting and the normal state.

cond-mat.mes-hall↗