116 research outputs found

    Crossover of phase qubit dynamics in presence of negative-result weak measurement

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    Coherent dynamics of a superconducting phase qubit is considered in the presence of both unitary evolution due to microwave driving and continuous non-unitary collapse due to negative-result measurement. In the case of a relatively weak driving, the qubit dynamics is dominated by the non-unitary evolution, and the qubit state tends to an asymptotically stable point on the Bloch sphere. This dynamics can be clearly distinguished from conventional decoherence by tracking the state purity and the measurement invariant (``murity''). When the microwave driving strength exceeds certain critical value, the dynamics changes to non-decaying oscillations: any initial state returns exactly to itself periodically in spite of non-unitary dynamics. The predictions can be verified using a modification of a recent experiment.Comment: 5 pages, 4 eps figure

    Continuous quantum feedback of coherent oscillations in a solid-state qubit

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    We have analyzed theoretically the operation of the Bayesian quantum feedback of a solid-state qubit, designed to maintain perfect coherent oscillations in the qubit for arbitrarily long time. In particular, we have studied the feedback efficiency in presence of dephasing environment and detector nonideality. Also, we have analyzed the effect of qubit parameter deviations and studied the quantum feedback control of an energy-asymmetric qubit.Comment: 11 page

    Quantum feedback control of a solid-state qubit

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    We have studied theoretically the basic operation of a quantum feedback loop designed to maintain a desired phase of quantum coherent oscillations in a single solid-state qubit. The degree of oscillations synchronization with external harmonic signal is calculated as a function of feedback strength, taking into account available bandwidth and coupling to environment. The feedback can efficiently suppress the dephasing of oscillations if the qubit coupling to the detector is stronger than coupling to environment.Comment: Extended version of cond-mat/0107280 (5 pages, 5 figures); to be published in PRB (RC

    Speaker Authorization for Air Traffic Control Security

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    The number of incidents in which unauthorized persons break into frequencies used by Air Traffic Controllers (ATCOs) and give false instructions to pilots, or transmit fake emergency calls, is a permanent and apparently growļæ½ing threat. One of the measures against such attacks could be to use automatic speaker recognition on the voice radio channel to disclose the potential unauļæ½thorized speaker. This work describes the solution for a speaker authorization system in the Security of Air Transport Infrastructures of Europe (SATIE) project, presents the architecture of the system, gives details on training and testing proceļæ½dures, analyses the influence of the number of authorized persons on the systemā€™s performance and describes how the system was adapted to work on the radio channel

    Spectrum of qubit oscillations from Bloch equations

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    We have developed a formalism suitable for calculation of the output spectrum of a detector continuously measuring quantum coherent oscillations in a solid-state qubit, starting from microscopic Bloch equations. The results coincide with that obtained using Bayesian and master equation approaches. The previous results are generalized to the cases of arbitrary detector response and finite detector temperature.Comment: 8 page

    Quantum Zeno stabilization in weak continuous measurement of two qubits

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    We have studied quantum coherent oscillations of two qubits under continuous measurement by a symmetrically coupled mesoscopic detector. The analysis is based on a Bayesian formalism that is applicable to individual quantum systems. Measurement continuously collapses the two-qubit system to one of the sub-spaces of the Bell basis. For a detector with linear response this corresponds to measurement of the total spin of the qubits. In the other extreme of purely quadratic response the operator \sigma_y^1 \sigma_y^2 + \sigma_z^1 \sigma_z^2 is measured. In both cases, collapse naturally leads to spontaneous entanglement which can be identified by measurement of the power spectrum and/or the average current of the detector. Asymmetry between the two qubits results in evolution between the different measurement subspaces. However, when the qubits are even weakly coupled to the detector, a kind of quantum Zeno effect cancels the gradual evolution and replaces it with rare, abrupt switching events. We obtain the asymptotic switching rates for these events and confirm them with numerical simulations. We show how such switching affects the observable power spectrum on different time scales.Comment: 18 pages, 8 eps figures, reference adde

    Entanglement of solid-state qubits by measurement

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    We show that two identical solid-state qubits can be made fully entangled (starting from completely mixed state) with probability 1/4 just measuring them by a detector, equally coupled to the qubits. This happens in the case of repeated strong (projective) measurements as well as in a more realistic case of weak continuous measurement. In the latter case the entangled state can be identified by a flat spectrum of the detector shot noise, while the non-entangled state (probability 3/4) leads to a spectral peak at the Rabi frequency with the maximum peak-to-pedestal ratio of 32/3.Comment: 5 pages, 2 figure

    High-intensity interval training improves VO2peak, maximal lactate accumulation, time trial and competition performance in 9ā€“11-year-old swimmers

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    Training volume in swimming is usually very high when compared to the relatively short competition time. High-intensity interval training (HIIT) has been demonstrated to improve performance in a relatively short training period. The main purpose of the present study was to examine the effects of a 5-week HIIT versus high-volume training (HVT) in 9ā€“11-year-old swimmers on competition performance, 100 and 2,000Ā m time (T100Ā m and T2,000Ā m), VO2peak and rate of maximal lactate accumulation (Lacmax). In a 5-week crossover study, 26 competitive swimmers with a mean (SD) age of 11.5Ā Ā±Ā 1.4Ā years performed a training period of HIIT and HVT. Competition (PĀ <Ā 0.01; effect sizeĀ =Ā 0.48) and T2,000Ā m (PĀ =Ā 0.04; effect sizeĀ =Ā 0.21) performance increased following HIIT. No changes were found in T100Ā m (PĀ =Ā 0.20). Lacmax increased following HIIT (PĀ <Ā 0.01; effect sizeĀ =Ā 0.43) and decreased after HVT (PĀ <Ā 0.01; effect sizeĀ =Ā 0.51). VO2peak increased following both interventions (PĀ <Ā 0.05; effect sizesĀ =Ā 0.46ā€“0.57). The increases in competition performance, T2,000Ā m, Lacmax and VO2peak following HIIT were achieved in significantly less training time (~2Ā h/week)
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