Events at Physics |
Events on Thursday, May 15th, 2008
- R. G. Herb Condensed Matter Seminar
- Mach-Zehnder Interferometry and Microwave-Induced Cooling in Persistent-Current Qubits
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Will Oliver, MIT/Lincoln Labs
- Abstract: Superconducting persistent-current qubits are quantum-coherent artificial atoms with multiple energy levels. In the presence of large-amplitude harmonic excitation, the qubit state can be driven through one or more of the energy-level avoided crossings. The resulting Landau-Zener transitions mediate a rich array of quantum-coherent phenomena as a function of the driving amplitude and frequency.
In this talk, we present three such demonstrations of quantum coherence in a strongly-driven niobium persistent-current qubit. The first is Mach-Zehnder-type interferometry [1], for which we observe quantum interference fringes for 1-50 photon transitions. The second is a new operating regime exhibiting coherent quasi-classical dynamics [2], for which the MZ quantum interference persists even for driving frequencies smaller than the resonance linewidth. The third is microwave-induced cooling [3], for which we achieve effective qubit temperatures < 3 mK, a factor 10x-100x lower than the dilution refrigerator ambient temperature.
These experiments exhibit a remarkable agreement with theory, and are extensible to other solid-state qubit modalities. In addition to our interest in these techniques for fundamental studies of quantum coherence in strongly-driven solid-state systems, we anticipate they will find application to nonadiabatic qubit control and state-preparation methods for quantum information science and technology.
[1] W.D. Oliver, Y. Yu, J.C. Lee, et al., Science 310, 1653 (2005).
[2] D.M. Berns, W.D. Oliver, S.O. Valenzuela et al., PRL 97, 150502 (2006).
[3] S.O. Valenzuela, W.D. Oliver, D.M. Berns, et al., Science (2006).
The work at Lincoln Laboratory was sponsored by the Air Force under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author(s) and are not necessarily endorsed by the United States Government.
- Host: McDermott
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Probing the TeV Scale with Parity-Violating Electron Scattering
- Time: 4:00 pm
- Place: 5280 Chamberlin
- Speaker: Krishna Kumar, UMass
- Abstract: The technique of parity-violating electron scattering, involving measurements of the asymmetry in the scattering of longitudinally polarized electrons off fixed targets, has become increasingly precise and broad in its scope over the past two decades. Such asymmetries are sensitive to weak neutral current interactions (mediated by the Z boson) between electrons and quarks, or between two electrons, and are being used to investigate the strangeness content of the nucleon, the neutron distribution in heavy nuclei and to probe for the limits of the validity of the electroweak theory in a manner complementary to direct searches for new physics at high energy scales at colliders.
In this talk, we focus on the last of the abovementioned topics. We begin by motivating and describing the results of the E158 experiment at the Stanford Linear Accelerator Center. We then discuss an ongoing project and possible future complementary measurements that can be carried out at Jefferson Laboratory. The first experiment will be carried out with a 1 GeV electron beam. With the completion of the 12 GeV upgrade, further precise measurements become feasible, in parity-violating deep inelastic scattering and in electron-electron scattering. In particular, the latter measurement could potentially lead to a measurement of the mixing angle with precision equal to or better than the two best collider measurements. In deep inelastic scattering, apart from testing the electroweak theory, the measurements would provide new precision probes the valence quark structure of the nucleon. Some aspects of the experimental challenges in carrying out this ambitious program will also be discussed.