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arXiv:cond-mat/0601199 · Physical Review B

Ground-state characterization of Nb charge-phase Josephson qubits

H. Zangerle, Jens Könemann, Brigitte Mackrodt, R. Dolata, S. V. Lotkhov, Sergey A. Bogoslovsky, Martin Götz, Alexander B Zorin

Published Jun 28, 2006• 9 citations
arXivHTMLPDF

Abstract

We present investigations of Josephson charge-phase qubits of superconducting quantum interference device (SQUID) configuration, inductively coupled to a radio-frequency-driven tank circuit, enabling the readout of the states by measuring the Josephson inductance of the qubit. The circuits, including junctions with linear dimensions of $60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ifmmode\times\else\texttimes\fi{}60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ifmmode\times\else\texttimes\fi{}80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, are fabricated from $\mathrm{Nb}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Nb}$ trilayer and allow the determination of relevant sample parameters at liquid helium temperature. The observed partial suppression of the circulating supercurrent in the qubit loop at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is explained within the framework of a quantum-statistical model. We have probed the ground-state properties of qubit structures with different ratios of the Josephson coupling to Coulomb charging energy at $20\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$, demonstrating both the magnetic control of phase and the electrostatic control of charge of the qubit.

DOI
10.1103/physrevb.73.224527

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We present investigations of Josephson charge-phase qubits of superconducting quantum interference device (SQUID) configuration, inductively coupled to a radio-frequency-driven tank circuit, enabling the readout of the states by measuring the Josephson inductance of the qubit. The circuits, including junctions with linear dimensions of 60nm\ifmmode×\else\texttimes\fi60nm60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ifmmode\times\else\texttimes\fi{}60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}60nm\ifmmode×\else\texttimes\fi60nm and 80nm\ifmmode×\else\texttimes\fi80nm80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ifmmode\times\else\texttimes\fi{}80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}80nm\ifmmode×\else\texttimes\fi80nm, are fabricated from Nb∕AlOx∕Nb\mathrm{Nb}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Nb}Nb∕AlOx​∕Nb trilayer and allow the determination of relevant sample parameters at liquid helium temperature. The observed partial suppression of the circulating supercurrent in the qubit loop at 4.2K4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}4.2K is explained within the framework of a quantum-statistical model. We have probed the ground-state properties of qubit structures with different ratios of the Josephson coupling to Coulomb charging energy at 20mK20\phantom{\rule{0.3em}{0ex}}\mathrm{mK}20mK, demonstrating both the magnetic control of phase and the electrostatic control of charge of the qubit.
The 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 qubit operation with minimum decoherence are given.
We investigated transitions in an Al charge-phase qubit of superconducting quantum interference device configuration which was inductively coupled to an rf tank circuit that made it possible to read out the state by measuring the Josephson inductance of the qubit. Depending on the flux and charge bias and on 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 quasiparticle tunneling onto and off the qubit island, involving an exchange of energy with the qubit. Within the scope of this model, a selection rule for quasiparticle-induced transitions in the qubit is discussed.
We implemented experimentally an interferometer-type charge qubit consisting of a single-Cooper-pair transistor closed by a superconducting loop that is in flip-chip configuration inductively coupled to a radio-frequency tank circuit. The tank permits us to readout the qubit state, acting as an inductance measuring apparatus. By applying continuous microwave power to the quantum device, we observed inductance alterations caused by redistributions of the energy-level populations. From the measured data we extracted the energy gap between ground and upper levels as a function of the transistor quasicharge as well as the Josephson phase across both junctions.
We present the design, fabrication, and characterization of a barrier-tunable superconducting quantum interference device (SQUID) qubit for the study of Maxwell’s demon experiment. In this work, a compound Josephson junction (CJJ) radio-frequency (RF)-SQUID qubit with an overdamped resistively shunted direct-current (DC)-SQUID magnetometer is used to continuously monitor the state of the qubit. The circuit is successfully fabricated with the standard Nb/Al-AlO x /Nb trilayer process of our laboratory and characterized in a low noise measurement system, which is capable of measuring coherent dynamics of superconducting qubits, in an Oxford dilution refrigerator. All circuit parameters are determined accurately by fitting experimental data to theoretical analysis and simulation, which allows us to make a quantitative comparison between the results of the experiment and theory.
The research work discussed in this thesis deals with the study of superconducting Josephson qubits. Superconducting qubits are solid-state artificial atoms which are based on lithographically defined Josephson tunnel junctions properties. When sufficiently cooled, these superconducting devices exhibit quantized states of charge, flux or junction phase depending on their design parameters. This allows to observe coherent evolutions of their states. The results presented can be divided into two parts. In a first part we investigate operations of superconducting qubits based on the quantum coherence in superconducting quantum interference devices (SQUID). We explain experimental data which has been observed in a SQUID subjected to fast, large-amplitude modifications of its effective potential shape. The motivations for this work come from the fact that in the past few years there have been attempts to interpret the supposed quantum behavior of physical systems, such as Josephson devices, within a classical framework. Moreover, we analyze the possibility of generating GHZ states, namely maximally entangled states, in a quantum system made out of three Josephson qubits. In particular, we investigate the possible limitations of the GHZ state generation due to coupling to bosonic baths. In the second part of the thesis we address a particular cause of decoherence of flux qubits which has been disregarded until now: thermal gradients, which can arise due to accidental non equilibrium quasiparticle distributions. The reason for these detrimental effects is that heat currents flowing through Josephson tunnel junctions in response to a temperature gradient are periodic functions of the phase difference between the electrodes. The phase dependence of the heat current comes from Andreev reflection, namely an interplay between the quasiparticles which carry heat and the superconducting condensate which is sensitive to the superconducting phase difference. Generally speaking, the flux qubit states are characterized by different values of the phase difference through their Josephson junctions. Consequently, the phase-dependent thermal current through a device subject a temperature gradient is related to the phase-dependent qubit states. We study how the thermal currents change according to the state of the qubits hence yielding a measurement of the qubit state. This in turn leads to an impact of temperature gradient on the dynamics of the system. We show that flux qubits in the Delft qubit design can have limitations of the decoherence time to the order of microseconds as a result of this newly discovered source of decoherence. In contrast, the fluxonium qubit is found to be well protected due to its superinductance.
We have designed and experimentally implemented a circuit of inductively coupled superconducting charge qubits, where a Josephson junction is used as an inductance, and the coupling between the qubits is controlled by an applied magnetic flux. Spectroscopic measurements on the circuit are in good agreement with theoretical calculations. We observed anticrossings which originate from the coupling between the qubit and the plasma mode of the Josephson junction. Moreover, the size of the anticrossing depends on the external magnetic flux, which demonstrates the controllability of the coupling.