R. G. Herb Condensed Matter Seminars |
Events During the Week of March 6th through March 13th, 2016
Monday, March 7th, 2016
- No events scheduled
Tuesday, March 8th, 2016
- No events scheduled
Wednesday, March 9th, 2016
- No events scheduled
Thursday, March 10th, 2016
- Phonon-mediated decay of singlet-triplet qubits in double quantum dots in GaAs/AlGaAs and Si/SiGe heterostructures
- Time: 10:00 am - 11:00 am
- Place: Chamberlin 5310
- Speaker: Viktoriia Kornich, University of Basel
- Abstract: We study theoretically the phonon-induced relaxation and decoherence times of singlet-triplet qubits in lateral double quantum dots (DQDs) in GaAs/AlGaAs and Si/SiGe heterostructures. We show that the two-phonon processes can dominate over the one-phonon processes even at rather low temperature, e.g. for the decoherence time at 50 mK for biased GaAs/AlGaAs DQDs. We find that when the DQD is unbiased the qubit lifetimes are orders of magnitude longer than for the case of biased DQD. The lifetimes of the qubits in Si/SiGe DQDs are remarkably longer than the ones in GaAs/AlGaAs DQDs, and following interest in S-T_- qubits we consider the phonon-induced decoherence and relaxation particularly in the region of the singlet-triplet anticrossing. We show that the hybridization of the singlet state plays a crucial role. Among other parameters of the system, we study the effect of micromagnet-induced magnetic field gradient on the qubit, which reveals an non-trivial behavior of the relaxation and decoherence times as a result of interplay between one-phonon and two-phonon processes. We suggest how our theory can be tested experimentally.
- Host: Vavilov
Friday, March 11th, 2016
- Microwave response and photon emission of a voltage baised Josephson junction
- Time: 10:00 am - 11:00 am
- Place: 5310 Chamberlin Hall
- Speaker: Salha Jebari, CEA Grenoble
- Abstract: The readout of superconducting qubits and other quantum devices operating at microwave frequencies requires amplifiers combining noise close to the quantum limit[1 ], high gain, large bandwidth, and sufficient dynamic range[2 ]. Josephson parametric amplifiers using Josephson junctions in the 0-voltage state, driven by a large microwave signals, begin to perform sufficiently well in all 4 of these aspects to be of practical use, but remain difficult to optimize and use. Recent experiments with superconducting circuits consisting of a DC voltage-biased Josephson junction in series with a resonator, showed that a tunneling Cooper pair can emit one or several photons with a total energy of 2e times the applied voltage[3 ]. We present microwave reflection measurements on this device in [3], indicating that amplification is possible with a simple DC voltage-biased Josephson junction.<br>
We compare these measurements with the noise power emitted by the junction and show that, for low Josephson energy, transmission and noise emission can be explained within the framework of P(E) theory of inelastic Cooper pair tunneling. Combined with a theoretical model, our results indicate that voltage-biased Josephson junctions might be useful for amplification near the quantum limit, offering simpler design and a different trade-off between gain, bandwidth and dynamic range, which could be advantageous in some situations. - Host: Robert McDermott