AIP conference proceedings

Quasiparticle Poisoning in a Single Cooper-Pair Box

Justin F. Schneiderman, Per Delsing, Göran Johansson, Matthew D. Shaw, Hans M. Bozler, P. M. Echternach

Published Jan 1, 2006• 5 citations

Abstract

We investigate the pheonomenon of quasiparticle poisoning in a single Cooper‐pair box (SCB). We have designed, fabricated, and tested an SCB that demonstrates a transition between poisoned and unpoisoned Coulomb staircases, depending on the speed with which the gate charge is swept. Poisoning is shown to be suppressed at moderately high sweep rates. Coulomb staircases were measured for a variety of sweep rates, and quasiparticle tunneling rates were extracted from this data.

DOI: 10.1063/1.2355009

Related papers

Progress in measurement techniques for the single Cooper-pair box qubit

Justin F. Schneiderman

The aluminum single Cooper-pair box (SCB) qubit has demonstrated potential as a candidate for quantum computing. Measurements of coherent manipulation of the relevant SCB states have been demonstrated with various charge and quantum capacitance readout methods. We performed studies of quasiparticle dynamics in an SCB that yielded poisoning probabilities and rates of quasiparticle tunneling processes. Attempts to prevent poisoning with a differential SCB isolated from ground and other quasiparticle sources revealed the possibility for quasiparticle generation on superconducting islands of diminutive size. We have gone on to conduct more advanced measurements of coupled qubits as an important step towards the realization of multi-qubit systems and other precursors that are pivotal in the evolution of reliable quantum computing.

Jan 1, 20151 citation

The single Cooper-pair box as a charge qubit

Kevin Bladh, T. Duty, David Gunnarsson +1 more

We present a series of measurements on nine single Cooper-pair boxes (SCBs), where the charging energy, E C , and the Josephson coupling energy, E J , have been varied. We have investigated both the ground state properties of the SCBs and their quantum coherent properties. The state of the SCBs could be manipulated by an external gate voltage and the charge was measured by coupling it capacitatively to a radio-frequency single-electron-transistor (RF-SET). By ramping the gate voltage and simultaneously measuring the charge of the SCBs using the RF-SET, we could measure the Coulomb staircases of the SCBs. For sufficiently low E C the SCBs showed a fully 2 e periodic Coulomb staircase. For samples with higher E C the staircase showed a short step for odd number of charges indicating quasi-particle 'poisoning'. However, if E C was not too large, the short step could be removed by applying a parallel magnetic field. We attribute this effect to a stronger suppression of the superconducting energy gap in the reservoir than in the box. Using microwave spectroscopy we have determined E C and E J for the SCBs. These values agree well with the shape of the Coulomb staircases which we measure. For a limited range of gate voltage, the SCBs were found to behave as model two-level quantum-mechanical systems. A non-adiabatic change in the induced island charge was used to bring two charge states into resonance. The resulting time evolution showed clear charge oscillations between the ground and excited state, with an amplitude above 70% and a frequency given by the energy level separation divided by Planck's constant. These oscillations had a longest coherence time of T 2 = 9 ns, at a point where the pure charge states are degenerate. The coherence time at this point was found to be limited by the relaxation rate. Away from the charge degeneracy point, the coherence time was limited by the pure dephasing rate. The dependence of T 2 on gate charge suggested that low frequency fluctuators were the main source of dephasing away from the degeneracy point.

New Journal of PhysicsAug 26, 200537 citations

Quasiparticle Poisoning and Quantum Coherence in a Differential Charge Qubit

Justin F. Schneiderman, Matthew D. Shaw, B. S. Palmer +2 more

We demonstrate the operation of a differential single Cooper-pair box, a charge qubit consisting of two aluminum islands, isolated from ground, coupled by a pair of small-area Josephson junctions, and read out with a superconducting differential radio-frequency single electron transistor. We have tested four devices, all of which show evidence of quasiparticle poisoning. The devices are characterized with microwave spectroscopy and temperature dependence studies, and Coulomb staircases are shown to be e-periodic in all samples. However, coherent control is still possible with non-adiabatic voltage pulses. Coherent oscillation experiments and a relaxation time measurement were performed using a charge derivative readout technique.

arXiv:0705.0695May 4, 20071 citation

Quasiparticle Poisoning in a Cooper Pair Box caused by a measuring SET

Carlos Sánchez‐Somolinos, B. S. Palmer, M. A. Manheimer +1 more

box in the presence of quasiparticle poisoning. The model uses a master equation approach to flnd the probabilities for the box to be in the even or odd state. The transition rates between the two states are calculated assuming a flxed number of non-equilibrium quasiparticles in the leads and island of the box. We fabricated Al/AlOx/Al devices with a Cooper pair box capacitively coupled to an SET and measured the charge on the box for SET bias currents ranging from about 1 pA to 1 nA. We flnd good agreement between the theory and measurements in the temperature range from 60 mK to 300 mK. For large SET bias the poisoning in the Cooper pair box increases and the charge staircase develops additional features. Our model is capable of qualitatively reproducing the features induced by the measuring SET.

Bulletin of the American Physical SocietyMar 16, 20060 citations

Microsecond Resolution of Quasiparticle Tunneling in the Single-Cooper-Pair Transistor

Andrew J. Ferguson, Nadia Court, Fay E. Hudson +1 more

We present radio-frequency measurements on a single-Cooper-pair-transistor in which individual quasiparticle poisoning events were observed with microsecond temporal resolution. Thermal activation of the quasiparticle dynamics is investigated, and consequently, we are able to determine energetics of the poisoning and unpoisoning processes. In particular, we are able to assign an effective quasiparticle temperature to parametrize the poisoning rate.

arXiv:cond-mat/0604403Physical Review LettersSep 7, 200660 citations

Towards Single Shot Measurements of a Cooper Pair Box Qubit Using an RF Single Electron Transistor

Carlos Alberto Sánchez

I report an investigation into using an rf Single Electron Transistor (rf-SET) to implement single shot readout of the quantum state of a Cooper pair box. I fabricated Al/AlOx/Al SETs and Cooper pair boxes using e-beam lithography and double-angle evaporation. The devices had typical charging energies of about 1 K and were measured at 30 mK in a dilution refrigerator. I built rf-SETs with an LC resonant circuit integrated on the chip and operating frequencies of 650 MHz and 1.5 GHz. The rf-SETs had a typical bandwidth of 30 MHz and the best achieved charge sensitivity was 8X10-6 erms/(Hz)1/2 at 1.5 GHz. The rf-SETs were used to measure the Coulomb staircase on the Cooper pair box. Measurements of the staircase show features due to quasiparticle poisoning that are dependent on temperature and SET bias. The data was compared to a theory I developed to extend existing theories of quasiparticle poisoning. My model is based on the calculation of tunneling rates to explain observations of parity effects in the Cooper pair box over a wide temperature range. Quantum state control of the Cooper pair box was demonstrated by using fast voltage pulses applied to the gate. Temporal oscillations of charge states and spectroscopy measurements reveal a dephasing time T2=200 ps at 30 mK, for a drive frequency of 30 GHz. The energy relaxation time T1=140 ns was determined from time domain measurements of the decay of the Cooper pair box charge. The fidelity of single shot measurements and the feasibility of this quantum readout scheme based on the results are discussed.

Dec 5, 20050 citations