A scalable multatom entangled system, capable of high-performance quantum computations, can be realized by resonant dipole-dipole interacting dopants in a solid state host. In one realization, the qubits are represented by ground and subradiant states of effective dimers formed by pairs of closely spaced two-level systems (TLS). Such qubits are highly robust against radiative decay. The two-qubit entanglement in this scheme relies on coherent excitation exchange between the dimers by external laser fields. This scheme is challenging because of the nanosize control and addressability it requires. Another realization involves dipole-dipole interacting TLS whose resonance frequency lies in a photonic band gap of a dielectric photonic crystal. A sequence of abrupt changes of the resonance frequency can produce controlled entanglement (logic gates) with improved protection from radiation decay and decoherence.
Relaxation of a two-level system (TLS) into a resonant
infinite-temperature reservoir with a Lorentzian spectrum is studied.
The reservoir is described by a complex Gaussian-Markovian field
coupled to the nondiagonal elements of the TLS Hamiltonian.
The theory can be relevant for electromagnetic interactions in
microwave high-Q cavities and muon spin depolarization.
Analytical results are obtained for the strong-coupling regime,
0>ν, where Ω0 is the rms coupling amplitude (Rabi frequency) and ν is the width of the reservoir spectrum. In this regime, the population difference and half of the initial coherence decay with two characteristic rates: the most part of the decay occurs at t < (Ω0ν)-1/3 the relaxation becomes reversible for t ~ Ω0-1/3, whereas for t > (Ω02ν)-1/3 the relaxation becomes irreversible and is practically over. The other half of the coherence decays with the rate on the order of ν, which may be slower by orders of magnitude than the time scale of the population relaxation. The above features are explained by the fact that at t < ν-1 the reservoir temporal fluctuations are effectively one-dimensional (adiabatic). Moreover, we identify the pointer basis, in which the reduction of the state vector occurs.
The pointer states are correlated with the reservoir, being dependent on the reservoir phase.
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