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Journal of Physics Conference Series

Observation of 65 GHz Coherent Oscillation in a Superconducting Flux Qubit Manipulated by Pulses

F. Chiarello, P. Carelli, Maria Gabriella Castellano, C. Cosmelli, G. Torrioli

Published May 12, 2014• 0 citations
Publisher

Abstract

A superconducting flux qubit can be manipulated by a fast modification of its potential, with a rapid transition from a double well to a single well shape, and coming back to the initial condition. This mechanism is based on a non-trivial quantum phenomenon, involving "partial" Landau-Zener transitions, coherent evolution in an harmonic potential and quantum interference. The study of this system enables a deep insight in decoherence mechanisms typical of superconducting qubits. Moreover, this procedure allows quantum operations with extreme high speeds, not possible with other standard manipulations. We present the experimental observation of coherent oscillations showing tunable frequencies with a 65 GHz top value. © Published under licence by IOP Publishing Ltd.

DOI
10.1088/1742-6596/507/4/042006

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A superconducting flux qubit can be manipulated by a fast modification of its potential, with a rapid transition from a double well to a single well shape, and coming back to the initial condition. This mechanism is based on a non-trivial quantum phenomenon, involving "partial" Landau-Zener transitions, coherent evolution in an harmonic potential and quantum interference. The study of this system enables a deep insight in decoherence mechanisms typical of superconducting qubits. Moreover, this procedure allows quantum operations with extreme high speeds, not possible with other standard manipulations. We present the experimental observation of coherent oscillations showing tunable frequencies with a 65 GHz top value. © Published under licence by IOP Publishing Ltd.
A particular superconducting quantum interference device (SQUID) qubit, namely the double SQUID qubit, can be manipulated by rapidly modifying its potential with the application of fast flux pulses.In this system we observe coherent oscillations exhibiting non-exponential decay, indicating an unconventional decoherence mechanism.Moreover, via tuning the qubit in different conditions (different oscillation frequencies) by changing the pulse height, we observe a crossover between two distinct decoherence regimes and the existence of an 'optimal' point where the qubit is only weakly sensitive to intrinsic noise.We find that this behavior is in agreement with a model considering the decoherence caused essentially by low-frequency noise contributions, and we discuss the experimental results and possible issues.
Superconducting devices based on the Josephson effect are effectively used for the implementation of qubits and quantum gates. The manipulation of superconducting qubits is generally performed by using microwave pulses with frequencies from 5 to 15 GHz, obtaining a typical operating frequency from 100 MHz to 1 GHz. A manipulation based on simple pulses in the absence of microwaves is also possible. In our system, a magnetic flux pulse modifies the potential of a double SQUID qubit from a symmetric double well to a single deep-well condition. By using this scheme with a Nb/AlO x /Nb system, we obtained coherent oscillations with sub-nanosecond period (tunable from 50 to 200 ps), very fast with respect to other manipulating procedures, and with a coherence time up to 10 ns, of the order of that obtained with similar devices and technologies but using microwave manipulation. We introduce ultrafast manipulation, presenting experimental results, new issues related to this approach (such as the use of a compensation procedure for canceling the effect of 'slow' fluctuations) and open perspectives, such as the possible use of RSFQ logic for qubit control.
A double SQUID qubit (Superconducting Quantum Interference Device) can be handled by applying microwave trains, but also by using fast flux pulses. In this second case the manipulation is based on the fast and radical modification of the qubit potential shape that induces non-adiabatic transitions between the computational states (the two lowest energy eigenstates), still avoiding transitions to upper levels. This modality is interesting because it allows faster operations with respect to other techniques, but also because it gives access to interesting nontrivial physical features, concerning in particular decoherence and adiabaticity. About decoherence, we observed experimentally the existence of an “optimal” bias region and the transition between two distinct decoherence regimes. These results can be explained by considering the effect of first and second order slow fluctuations which dominate on high frequency noise contributions. This allows a deep insight in the qubit decoherence mechanisms.
We experimentally demonstrate coherent oscillations of a tunable superconducting flux qubit by manipulating its energy potential with a nanosecond-long pulse of magnetic flux. The occupation probabilities of two persistent current states oscillate at a frequency ranging from 6 GHz to 21 GHz, tunable by changing the amplitude of the flux pulse. The demonstrated operation mode could allow quantum gates to be realized in less than 100 ps, which is much shorter than gate times attainable in other superconducting qubits. Another advantage of this type of qubit is its immunity to both thermal and magnetic field fluctuations.
We report an experimental measurement of Landau-Zener transitions on an individual flux qubit within a multiqubit superconducting chip. The method used isolates a single qubit, tunes its tunneling amplitude $\ensuremath{\Delta}$ into the limit where $\ensuremath{\Delta}$ is much less than both the temperature TTT and the decoherence-induced energy level broadening, and forces it to undergo a Landau-Zener transition. We find that the behavior of the qubit agrees to a high degree of accuracy with theoretical predictions for Landau-Zener transition probabilities for a double-well quantum system coupled to a nonMarkovian 1/f1/f1/f magnetic flux noise.
Landau-Zener-Stuckelberg interferometry has been extensively investigated in quantum two-level systems, with particular interests on artificial system such as superconducting flux qubits. With increasing the driving field amplitude, more energy levels will be involved into the quantum evolution, which results in population inversion and many interesting interference patterns. These interference patterns can be used to obtain the parameters characterizing the system and probe dephasing mechanisms of the qubit. Most recently, experiments have been extended to the regime with higher-frequency and larger-amplitude driving field, in which the interference pattern exhibited more complicated characteristics. In this article, we give a universal description of the characteristics observed in both low-frequency and high-frequency regimes. Besides explaining the already observed experimental results, our theoretical model predicted many interesting phenomenon, which can be demonstrated by future experiments.