arXiv subjects

P. Delsing

Publications and source records attributed to P. Delsing.

49 records · Page 3Linked to original sources

arXiv2004

Coherent dynamics of a Josephson charge qubit

We have fabricated a Josephson charge qubit by capacitively coupling a single-Cooper-pair box (SCB) to an electrometer based upon a single-electron transistor configured for radio-frequency readout (RF-SET). Charge quantization of 2e is observed and microwave spectroscopy is used to extract the Josephson and charging energies of the box. We perform coherent manipulation of the SCB by using very fast DC pulses and observe quantum oscillations in time of the charge that persist to ~=10ns. The observed contrast of the oscillations is high and agrees with that expected from the finite E_J/E_C ratio and finite rise-time of the DC pulses. In addition, we are able to demonstrate nearly 100% initial charge state polarization. We also present a method to determine the relaxation time T_1 when it is shorter than the measurement time T_{meas}.

cond-mat.supr-con↗

arXiv2003

Free Evolution of Superposition States in a Single Cooper Pair Box

We have fabricated a single Cooper-pair box (SCB) in close proximity to a single electron transistor (SET) operated in the radio-frequency mode (RF-SET) with an inductor and capacitor lithographed directly on chip. The RF-SET was used to measure the charge state of the SCB revealing a 2e periodic charge quantization. We performed spectroscopy measurements to extract the charging energy (EC) and the Josephson coupling energy (EJ). Control of the temporal evolution of the quantum charge state was achieved by applying fast DC pulses to the SCB gate. The dephasing and relaxation times were extracted from these measurements

cond-mat↗

arXiv2002

Self-heating in small mesa structures

We study analytically and numerically a problem of self-heating in small mesa structures. Our results show that the self-heating is proportional to a characteristic in-plane size of the mesa. Experimental data for small high-$T_c$ superconductor Bi2212 mesas are in qualitative agreement with our calculations. We estimate the self-heating in Bi2212 mesas with different sizes and demonstrate that the self-heating can effectively be obviated in small mesa structures.

cond-mat.supr-con↗

arXiv2002

Noise in the Single Electron Transistor and its back action during measurement

Single electron transistors (SETs) are very sensitive electrometers and they can be used in a range of applications. In this paper we give an introduction to the SET and present a full quantum mechanical calculation of how noise is generated in the SET over the full frequency range, including a new formula for the quantum current noise. The calculation agrees well with the shot noise result in the low frequency limit, and with the Nyquist noise in the high frequency limit. We discuss how the SET and in particular the radio-frequency SET can be used to read out charge based qubits such as the single Cooper pair box. We also discuss the backaction which the SET will have on the qubit. The back action is determined by the spectral power of voltage fluctuations on the SET island. We will mainly treat the normal state SET but many of the results are also valid for superconducting SETs.

cond-mat.mes-hall↗

arXiv2001

Universal Quantum Gates for Single Cooper Pair Box Based Quantum Computing

We describe a method for achieving arbitrary 1-qubit gates and controlled-NOT gates within the context of the Single Cooper Pair Box (SCB) approach to quantum computing. Such gates are sufficient to support universal quantum computation. Quantum gate operations are achieved by applying sequences of voltages and magnetic fluxes to single qubits or pairs of qubits. Neither the temporal duration, nor the starting time, of a gate operation is used as a control parameter. As a result, the quantum gates have a constant and known duration, and depend upon standard control parameter sequences regardless of when the gate operation begins. This simplifies the integration of quantum gates into parallel, synchronous, quantum circuits. In addition, we demonstrate the ability to fabricate such gates, and large-scale quantum circuits, using current e-beam lithography technology. These features make the SCB-based scheme a credible contender for practical quantum computer hardware.

quant-ph↗

arXiv2000

Josephson effects in a superconductor-normal metal mesoscopic structure with a dangling superconducting arm

We studied a mesoscopic cross-like normal metal structure connected to two superconducting (S) and two normal (N) reservoirs. We observed the Josephson effect under unusual conditions when there is no current through one of the two S/N interfaces. The potential difference between the S reservoirs was zero unless the voltage applied between S and N reservoirs exceeded a critical value although the electric potential in the N wire connecting the superconductors varied in a nonmonotonic way. The observed effects are discussed theoretically.

cond-mat.supr-con↗

arXiv2000

First experimental evidence of one-dimensional plasma modes in superconducting thin wires

We have studied niobium superconducting thin wires deposited onto a SrTiO$_{3}$ substrate. By measuring the reflection coefficient of the wires, resonances are observed in the superconducting state in the 130 MHz to 4 GHz range. They are interpreted as standing wave resonances of one-dimensional plasma modes propagating along the superconducting wire. The experimental dispersion law, $ω$ versus $q$, presents a linear dependence over the entire wave vector range. The modes are softened as the temperature increases close the superconducting transition temperature. Very good agreement are observed between our data and the dispersion relation predicted by Kulik and Mooij and Schön.

cond-mat.mes-hall↗

arXiv2000

Evidence for coexistence of the superconducting gap and the pseudo - gap in Bi-2212 from intrinsic tunneling spectroscopy

We present intrinsic tunneling spectroscopy measurements on small Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ mesas. The tunnel conductance curves show both sharp peaks at the superconducting gap voltage and broad humps representing the $c$-axis pseudo-gap. The superconducting gap vanishes at $T_c$, while the pseudo-gap exists both above and below $T_c$. Our observation implies that the superconducting and pseudo-gaps represent different coexisting phenomena.

cond-mat.supr-con↗

arXiv2000

Intrinsic tunneling spectroscopy in small Bi2212 mesas

We present intrinsic tunneling spectroscopy measurements on small Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ mesas. The tunnel conductance curves show both sharp peaks at the superconducting gap voltage and broad humps representing the $c$-axis pseudo-gap. The superconducting gap vanishes at $T_c$, while the pseudo-gap exists well above $T_c$. Our observation implies that the superconducting and pseudo-gaps represent different coexisting phenomena.

cond-mat.supr-con↗

arXiv1998

Bias and temperature dependence of the noise in a single electron transistor

A single electron transistor based on Al-AlO_x-Nb tunnel junctions was fabricated by shadow evaporation and in situ barrier formation. Its output current noise was measured, using a transimpedance amplifier setup, as a function of bias voltage, gain, and temperature, in the frequency range 1...300 Hz. The spot noise at 10 Hz is dominated by a gain dependent component, indicating that the main noise contribution comes from fluctuations at the input of the transistor. Deviations from ideal input charge noise behaviour are found in the form of a bias dependence of the differential charge equivalent noise, i. e. the derivative of current noise with respect to gain. The temperature dependence of this effect could indicate that heating is activating the noise sources, and that they are located inside or in the near vicinity of the junctions.

cond-mat.mes-hall↗

arXiv1998

Gain Dependence of the Noise in the Single Electron Transistor

An extensive investigation of low frequency noise in single electron transistors as a function of gain is presented. Comparing the output noise with gain for a large number of bias points, it is found that the noise is dominated by external charge noise. For low gains we find an additional noise contribution which is compared to a model including resistance fluctuations. We conclude that this excess noise is not only due to resistance fluctuations. For one sample, we find a record low minimum charge noise of qn = 9*10^-6 e/sqrt(Hz) in the superconducting state and qn = 9*10^-6 e/sqrt(Hz) in the normal state at a frequency of 4.4 kHz.

cond-mat.mes-hall↗

arXiv1998

Two-dimensional arrays of low capacitance tunnel junctions: general properties, phase transitions and Hall effect

We describe transport properties of two-dimensional arrays of low capacitance tunnel junctions, such as the current voltage characteristic and its dependence on external magnetic field and temperature. We discuss several experiments in which the small capacitance of the junctions plays an important role. In arrays where the junctions have a relatively large charging energy, (i.e. when they have a low capacitance) and a high normal state resistance, the low bias resistance increases with decreasing temperature and eventually at very low temperature the array becomes insulating even though the electrodes in the array are superconducting. This transition to the insulating state can be described by thermal activation. In an intermediate region where the junction resistance is of the order of the quantum resistance and the charging energy is of the order of the Josephson coupling energy, the arrays can be tuned between a superconducting and an insulating state with a magnetic field. We describe measurements of this magnetic-field-tuned superconductor insulator transition, and we show that the resistance data can be scaled over several orders of magnitude. Four arrays follow the same universal function. At the transition the transverse (Hall) resistance is found to be very small in comparison with the longitudinal resistance. However, for magnetic field values larger than the critical value.we observe a substantial Hall resistance. The Hall resistance of these arrays oscillates with the applied magnetic field. features in the magnetic field dependence of the Hall resistance can qualitatively be correlated to features in the derivative of the longitudinal resistance, similar to what is found in the quantum Hall effect.

cond-mat.mes-hall↗

arXiv1997

Coulomb blockade effects in anodised niobium nanostructures

Niobium thin film wires were fabricated using electron beam lithography with a four layer liftoff mask system, and subsequently thinned by anodisation. The resistance along the wire was monitored in situ and trimmed by controlling the anodisation voltage. Depending on the room temperature sheet resistance, samples showed either superconducting or insulating behaviour at low temperatures. A Coulomb blockade was observed for samples exceeding 6 kOhm per square. Samples were also made in a single electron transistor-like geometry with two weak links made by combined angular evaporation and anodisation. Their current-voltage characteristics could be modulated by a voltage applied to an overlapping gate.

cond-mat.mes-hall↗