Recent results:
Decoherence and Decay Manipulation: Quantum Zeno and Anti-Zeno Effects
Noam Erez, Goren Gordon, Mathias Nest and Gershon Kurizki, Nature 452, 724-727(2008).
Heat flow between a large thermal 'bath' and a smaller system brings them progressively closer to thermal equilibrium while increasing their entropy1. Fluctuations involving a small fraction of a statistical ensemble of systems interacting with the bath result in deviations from this trend. In this respect, quantum and classical thermodynamics are in agreement. Here we predict a different trend in a purely quantum mechanical setting: disturbances of thermal equilibrium between two-level systems (TLSs) and a bath6, caused by frequent, brief quantum non-demolition measurements of the TLS energy states. By making the measurements increasingly frequent, we encounter first the anti-Zeno regime and then the Zeno regime (namely where the TLSs' relaxation respectively speeds up and slows down). The corresponding entropy and temperature of both the system and the bath are then found to either decrease or increase depending only on the rate of observation, contrary to the standard thermodynamical rules that hold for memory-less (Markov) baths. From a practical viewpoint, these anomalies may offer the possibility of very fast control of heat and entropy in quantum systems, allowing cooling and state purification over an interval much shorter than the time needed for thermal equilibration or for a feedback control loop. Top of pageG. Gordon, G. Kurizki, S. Mancini, D. Vitali and P. Tombesi, J. Phys. B-AMO 40, S61(2007).
We present and compare stochastic open-loop techniques aimed at controlling quantum coherence in dissipative environments. One approach describes the evolution time as a random non-Gaussian variable. The other implements dynamical control on non-Markovian time-scales via stochastic modulation of the system-bath coupling.I.E. Mazets and G. Kurizki, J. Phys. B-AMO 40, F105(2007).
We investigate conditions under which multiatom absorption of a single photon leads to cooperative decay. Our analysis reveals the symmetry properties of the multiatom Dicke states underlying the cooperative decay dynamics and their spatio-temporal manifestations, particularly, the forward-directed spontaneous emission investigated by Scully et al ( 2006 Phys. Rev. Lett. 96 010501).G. Gordon, N. Erez and G. Kurizki, J. Phys. B-AMO 40, S75(2007).
In this paper, we develop, step by step, the framework for universal dynamical control of two-level systems (TLS) or qubits experiencing amplitude or phase noise (AN or PN) due to coupling to a thermal bath. A comprehensive arsenal of modulation schemes is introduced and applied to either AN or PN, resulting in completely analogous formulae for the decoherence rates, thus underscoring the unified nature of this universal formalism. We then address the extension of this formalism to multipartite decoherence control, where symmetries are exploited to overcome decoherence.G. Gordon and G. Kurizki, Phys. Rev. A 76, (2007).
Dephasing is a ubiquitous phenomenon that leads to the loss of coherence in quantum systems and the corruption of quantum information. We present a universal dynamical control approach to combat dephasing during all stages of quantum computation, namely, storage and single- and two-qubit operators. We show that (a) tailoring multifrequency gate pulses to the dephasing dynamics can increase fidelity; (b) cross-dephasing, introduced by entanglement, can be eliminated by appropriate control fields; (c) counterintuitively and contrary to previous schemes, one can increase the gate duration, while simultaneously increasing the total gate fidelity.G. Gordon and G. Kurizki, Phys Rev Lett 97, 110503(2006).
An entangled multipartite system coupled to a zero-temperature bath undergoes rapid disentanglement in many realistic scenarios due to local, symmetry-breaking differences in the particle-bath couplings. We show that locally controlled perturbations, addressing each particle individually, can impose a symmetry allowing the existence of decoherence-free multipartite entangled systems.G. Gordon and G. Kurizki, Phys. Rev. Lett. 97, 110503 (2006)
An entangled multipartite system coupled to a zero-temperature bath undergoes rapid disentanglement in many realistic scenarios due to local, symmetry-breaking differences in the particle-bath couplings. We show that locally controlled perturbations addressing each particle individually, can impose a symmetry allowing the existence of decoherence-free multipartite entangled systems.G. Gordon, G. Kurizki and A. G. KofmanOpt. Comm. 264, 398 (2006).
A unified theory is presented of dynamically modified decay and decoherence in driven multilevel quantum systems that are weakly coupled to arbitrary zero-temperature reservoirs. Examples of different phase and amplitude modulations are given for two-level systems (qubits). Analysis of modulations on multilevel systems is detailed with a numerical example using quasiperiodic impulsive phase jumps. The merits and disadvantages of the different modulation types are discussed.