Southwest Quantum Information and Technology

Tenth Annual Meeting, February 14-17, 2008
University of New Mexico, Santa Fe, New Mexico

Full Program | Thursday Tutorials | Friday | Saturday | Sunday

SESSION 4: Quantum Control
Session Chair: Todd Brun
16:00-16:30	Brad Chase, University of New Mexico
	Efficient feedback controllers for continuous-time quantum error correction
	Abstract. We present an efficient approach to continuous-time quantum error correction that extends the low-dimensional quantum filtering methodology developed by van Handel and Mabuchi [quant-ph/0511221 (2005)] to include error recovery operations in the form of real-time quantum feedback. We expect this paradigm to be useful for systems in which error recovery operations cannot be applied instantaneously. While we could not find an exact low-dimensional filter that combined both continuous syndrome measurement and a feedback Hamiltonian appropriate for error recovery, we developed an approximate reduced-dimensional model to do so. Simulations of the five-qubit code subjected to the symmetric depolarizing channel suggests that error correction based on our approximate filter performs essentially identically to correction based on an exact quantum dynamical model.
17:00-17:30	Seth Merkel, University of New Mexico
	Optimal Control of Large Spin-Atomic Systems with Coherent Electromagnetic Fields
	Abstract. Cold atomic systems provide an excellent testing ground for quantum control protocols due to the isolation of these systems from their environment and the availability of high precision fields from the “quantum optics toolbox”. In this talk, we look at a variety of way to control large spins confined to the ground state hyperfine manifold of 133Cs. In particular, we present a scheme for controlling spins coherently using microwaves and rf-magnetic fields and compare this some previous experiments that utilized quasi-static magnetic fields and a nonlinear AC-Stark shift. We look at the requirements for controllability and find state preparations protocols, fields that map a fiducial state to an arbitrary target state, through a simple stochastic search algorithm. Additionally, we show that in this system the ability to easily find state preparation protocols translates into the ability to easily find arbitrary unitary maps.
17:30-18:00	Souma Chaudhury, University of Arizona
	A Quantum Kicked Top with Cold Atomic Spins
	Abstract. Complexity in classical as well as quantum physics arises through the coupling of multiple degrees of freedom. Recent theoretical studies have shown a connection between the dynamical rate of entanglement generation in a bipartite quantum system and the presence of chaos in the corresponding classical dynamics. In order to explore this and similar questions that lie at the boundary between quantum information science and quantum chaos we have developed a version of the quantum kicked top based on laser cooled atomic spins driven by a pulsed magnetic field and a rank 2 tensor light shift. Among the advantages offered by our system are the ability to prepare arbitrary initial spin states, the ability to precisely implement the desired non-linear dynamics, and the ability to accurately measure the entire spin density matrix and thus obtain accurate snapshots of the evolving quantum state. We will present results from an experiment that implemented a quantum kicked top for the F=3 hyperfine ground state of Cs. Initial spin states were chosen to overlap with regular or chaotic areas of the classical phase space map, and the resulting spin Husimi distribution measured after each step in a series of 50 kicks. The spin dynamics seen in the experiment agrees closely with the predictions of theory, including dynamical tunneling between regular islands, rapid spreading of states throughout the chaotic sea, and surprisingly robust signatures of classical phase space structures even after many kicks and significant decoherence. As expected, the entanglement generated between electronic and nuclear spin is larger when the corresponding classical dynamics is chaotic, though the difference "while clear" is modest due to the small size of the total spin. Future versions of the experiment may circumvent this limitation by driving the electronic and nuclear spins independently, or by working with the collective spin of an ensemble of atoms.