R. G. Herb Condensed Matter Seminars |
Events During the Week of March 3rd through March 10th, 2013
Monday, March 4th, 2013
- No events scheduled
Tuesday, March 5th, 2013
- Analysis of high-fidelity gate design and error thresholds for fault-tolerant superconducting quantum computing architectures
- Time: 10:00 am
- Place: 5280 Chamberlin Hall
- Speaker: Joydip Ghosh, University of Georgia
- Abstract: Quantum computing with superconducting elements promises scalability and is widely regarded as a viable approach to develop a fault-tolerant architecture of a candidate quantum computer. In this talk, I first discuss our recent proposal to design high-fidelity controlled-σz (CZ) operations using only DC bias control and then explore the performance of various existing superconducting surface code based architectures under a realistic multi-parameter error model. Assuming phase or transmon qubits and using only low frequency qubit-bias control, our CZ operation exhibits threshold fidelity (intrinsic) with a realistic two-parameter pulse profile. In addition we have an analytic model that estimates the fidelities of CZ gates as a function of various pulse parameters as well as quantifies the error due to any perturbation over an optimal pulse shape. Next we consider a realistic, multi-parameter error model and investigate the performance of the surface code for three possible fault-tolerant superconducting architectures. We map amplitude and phase damping to an asymmetric depolarization channel via the Pauli twirl approximation, and obtain the logical error rate as a function of the qubit coherence time, intrinsic state preparation and gate and readout errors. A numerical Monte Carlo simulation is performed to obtain the logical error rates and a leading order analytic model is constructed to estimate their scaling behavior below threshold. Our results suggest that large-scale fault-tolerant quantum computation should be possible with existing superconducting devices.
- Host: Friesen & Coppersmith
Wednesday, March 6th, 2013
- No events scheduled
Thursday, March 7th, 2013
- The two sides of a semiclassical spin
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Feifei Li, Northwestern University
- Abstract: A spinning electron and a spinning gyroscope represent the two ultra limits that spin can behave. One is purely quantum, while the other is purely classical. In this talk, I would like to discuss what happens for a semiclassical spin with intermediate magnitude of angular momentum. My talk consists two parts. In the first half, I would like to tell a story about the single-molecular-magnet Fe8. Fe8 is a molecule made of about a hundred atoms, yet it behaves like a single giant spin of J = 10 at low temperatures. Quantum interference causes the tunneling gap of this molecule to oscillate with applied magnetic field and to vanish at certain magnitude and direction of the magnetic fields, known as diabolical points. My story is about how these diabolical points were discovered, missed and rediscovered. The second half of my talk will focus on the quantum-classical correspondence for spin. The quantum-classical correspondence for a particle has been formulated by Moyal, who in a seminal paper, showed that quantum mechanics can be expressed as a quasi-statistical theory in the phase space of coordinate and momentum. Moyal's formalism unified Weyl ordering and Wigner quasi-distribution function, providing an invertible map between dynamical variables on the classical phase space and operators on the quantum mechanical Hilbert space. Moyal has also shown that the commutator of two operators is the Poisson bracket to leading order of $hbar$. All this was done for position and momentum. Here I present a Moyal treatment for spin, and show that, in the classical limit, the Weyl symbol for a spin commutator is i times the Poisson bracket of the corresponding Weyl symbols.
References
[1] Feifei Li and Anupam Garg, Numerical search for diabolical points in the energy spectrum of the single-molecule magnet Fe8, Phys. Rev. B 83, 132401 (2011).
[2] José E. Moyal, Quantum Mechanics as a Statistical Theory, Proc. Cambridge Philos. Soc. 45, 99 (1949).
[3] Feifei Li, Carol Braun, and Anupam Garg, The Weyl-Wigner-Moyal Formalism for Spin, arXiv:1210.4075v2 (2012).
- Host: Friesen & Coppersmith
Friday, March 8th, 2013
- No events scheduled