Decoherence in quantum bit circuits is presently a major limitation to their
use for quantum computing purposes. We present experiments, inspired from NMR,
that characterise decoherence in a particular superconducting quantum bit
circuit, the quantronium. We introduce a general framework for the analysis of
decoherence, based on the spectral densities of the noise sources coupled to
the qubit. Analysis of our measurements within this framework indicates a
simple model for the noise sources acting on the qubit. We discuss various
methods to fight decoherence.Comment: Long paper. 65 pages, 18 Figure
We have measured the rate of thermally induced escape from the zero-voltage state in long Josephson junctions of both overlap and in-line geometry as a function of applied magnetic field. The statistical distribution of switching currents is used to evaluate the escape rate and derive an activation energy ⌬U for the process. Because long junctions correspond to the continuum limit of multidimensional systems, ⌬U is in principle the difference in energy between stationary states in an infinite-dimensional potential. We obtain good agreement between calculated and measured activation energies for junctions with lengths a few times the Josephson penetration depth J . ͓S0163-1829͑96͒01145-9͔
We experimentally demonstrate the 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 via the amplitude of the flux pulse. The demonstrated operation mode allows to realize quantum gates which take less than 100 ps time and are thus much faster compared to other superconducting qubits. An other advantage of this type of qubit is its insensitivity to both thermal and magnetic field fluctuations.PACS numbers: 03.67. Lx, 85.25.Dq Superconducting qubits stand between the most promising systems for the realization of quantum computation. Coherent quantum evolution and manipulation have been demonstrated and extensively studied for single [1,2,3,4,5] and coupled superconducting qubits [6,7,8,9,10,11]. In most cases, the state of superconducting qubits are manipulated by means of microwave pulses, with a technique similar to the NMR manipulation of atoms. An alternative way to manipulate qubits is based on modifying their energy potential without applying any microwave signals [1,5]. The latter approach requires a much simpler experimental technique and offers the possibility of using classical logic signals to control a quantum processor in situ, which is advantageous for the large scale implementation of a quantum circuits.In this Letter, we report the observation of tunable coherent oscillations in a SQUID-based flux qubit. These oscillations are obtained by manipulating the qubit with nanosecond-long pulses of magnetic flux rather than microwaves. By this technique, we could increase the oscillation frequency up to 21 GHz, which allows to perform very fast logical quantum gates. Since the relevant quality factor of a qubit is the number of gate operations which can be performed during its coherence time, this result is of particular interest towards the realization of a solid-state quantum computer.The investigated circuit, shown in Fig. 1(a), is a double SQUID consisting of a superconducting loop of inductance L = 85 pH, interrupted by a small dc SQUID of loop inductance l = 6 pH. This dc SQUID is operated as a single Josephson junction (JJ) whose critical current is tunable by an external magnetic field. Each of the two JJs embedded in the dc SQUID has a critical current I 0 = 8µA and capacitance C = 0.4 pF. The qubit is manipulated by changing two magnetic fluxes Φ x and Φ c , applied to the large and small loops by means of two coils of mutual inductance M x = 2.6 pH and M c = 6.3 pH, respectively. The readout of the qubit flux is performed by measuring the switching current of an unshunted dc SQUID, which is inductively coupled to the qubit [12]. The circuit was manufactured by Hypres [13] using standard Nb/AlO x /Nb technology in a 100 A/cm 2 critical current density process. The dielectric material used for junction isolation is SiO 2 . The whole circuit is designed gradiometrically in order...
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