Title: Quantum state preparation and readout with Landau-Zener sweeps in circuit QED
M. Wubs (1), K. Saito (2,4), D. Zueco (3), S. Kohler (3), P. Hanggi (3), Y. Kayanuma (5)

Universities of (1) Copenhagen, (2) Tokyo, and (3) Augsburg; (4) CREST (JST) Saitama, and (5) Osaka Prefecture University

The Landau-Zener (LZ) problem is an exactly
solvable textbook example in time-dependent quantum mechanics dating back to 1932. It describes a quantum two-state system ('qubit') whose energies cross with constant speed. In recent years artificial atoms have been fabricated with parameters that are highly tunable, for example in superconducting circuits. Their tunability can be exploited to prepare their quantum states with LZ sweeps.

In this theoretical talk we consider LZ sweeps in a qubit which has wanted or unwanted coupling to another system. In circuit QED, LZ sweeps can be used to create single-photon states, or Bell states in the analogy of two-cavity QED [1]. Interestingly, when starting in the ground state, transition probabilities can still be found exactly for these multilevel generalizations of the LZ problem.

Generalizing further, we present the exact Landau-Zener transition probabilities for a qubit with arbitrary coupling to a dissipative environment at zero temperature [2]. Applications in circuit QED will be discussed. Recent master-equation calculations agree with the exact transition probabilities in the zero-temperature limit, and show that their temperature dependence can often be neglected [3].

[1] K. Saito et al., EPL 76, 22 (2006); M. Wubs et al., Physica E 40, 187 (2007).
[2] M. Wubs et al., PRL 97, 200404 (2006); K. Saito et al., PRB 75, 214308 (2007).
[3] D. Zueco et al., arXiv:quant-ph/0807.1748.