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Physical Review A

Quantum control of entanglement by phase manipulation of time-delayed pulse sequences. I

Vladimir S. Malinovsky, Ignacio R. Solá

Published Oct 8, 2004• 34 citations
Publisher

Abstract

We investigate coherent population transfer in double-$\ensuremath{\Lambda}$ and diamond-type four-level systems driven by two pairs of time-delayed laser pulses. We show that the relative phase between the pulses induces interference so that the adiabatic passage dynamics is only possible for a particular phase. Both pulse area and phase are control knobs that can be used to prepare specific coherent superpositions using counterintuitive and intuitive pulse sequences. Since the diamond-type four-level system maps the Hamiltonian of two interacting two-level systems, the schemes can be used to manipulate entanglement. In particular we propose robust schemes to prepare specific entangled states by phase control with time-delayed pulse sequences.

DOI
10.1103/physreva.70.042304

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A method of entangled states preparation of two-qubit systems is proposed. The method combines the techniques of coherent control by manipulation of the relative phase between the pulses and adiabatic control by using time-delayed pulse sequences. Simple analytic expressions are given for the wave function of the system as a function of phase and pulse area which show that complete control of entanglement is possible with resonant pulses. The connection between the phase of the entanglement and the control parameters is clarified and the effects of different pulse sequences are analyzed to design the optimal schemes. An off-resonant scheme is proposed that simplifies the preparation of certain entangled states.
The manipulation of population transfer and the creation of an arbitrary superposition between two atomic states in the framework of a closed inverse Y-type four-level system driven by a series of pulse pairs and an external control field is reported. It is shown that when the laser fields are tuned to resonance with respective transitions, the non-adiabatic coupling between two degenerate adiabatic states can be reduced tremendously and complete population transfer from the initial state to the target state can be achieved in a piecewise manner. Moreover, an arbitrary superposition between the initial state and target state can be created by suitably tuning the Rabi frequency of the control laser field, inter-pair time delay, pulse number or intra-pair time delay between the pump and Stokes pulses. This scheme combines stimulated Raman adiabatic passage and temporal coherent control, and may be useful for quantum information processing and control of chemical reactions.
We propose a selective, efficient, and robust way to realize control of coherent population transfer in a $\ensuremath{\Lambda}$-type four-level system with a closely spaced doublet in the final state with time-separated chirped pump pulse and Stokes pulse pair by merging stimulated Raman adiabatic passage, temporal coherent control, and chirped adiabatic passage techniques. Moreover, an arbitrary coherent superposition between the final doublet, or between the intermediate state and either of the doublets can be created. This method holds the ability to ``control with control'' and has potential applications in coherent control of chemical reactions and quantum information processing.
We investigate the effect of the spontaneous decay-induced coherence on the control of coherent population transfer in a closed double $\ensuremath{\Lambda}$-type four-level system with the energy separation of the excited doublet comparable to their decay rates by using the stimulated Raman adiabatic passage technique. It is shown that when the two pulse laser fields are tuned to the particular frequency where the condition for quantum interference is satisfied, remarkable enhancement or suppression of population transfer can be realized through the spontaneous emission constructive or destructive quantum interference even with a finite pulse area, which is very favorable to coherent population transfer in quantum systems with weak oscillator strengths.
We propose a scheme for coherent control of population transfer and arbitrary superpositions of quantum states by a single-chirped laser pulse in a Λ-type excited-doublet four-level system. It is shown that selective, efficient, and robust population transfer from the initial state to the target states (excited or ground) can be achieved via chirped adiabatic passage when the pulse bandwidth is smaller than about 1/10 of the energy separations between the excited-doublet levels and between the ground states. Moreover, arbitrary coherent superpositions between the excited-doublet states, or between the higher ground state and either of the excited-doublet states, can be created by appropriately tuning the chirp rate and detuning the laser field. The proposed flexible method of coherent control of population transfer and arbitrary superpositions between the initially unpopulated states requires only a single pulse, and is robust against variations in the laser parameters, which has potential applications in quantum information processing.