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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum processing through a manifold of dark states

Santosh Kumar, Deepak Kumar

Published Oct 15, 2012• 0 citations
Publisher

Abstract

We propose a scalable network, in which all quantum operations can be executed through external controls. Nodes of this network are high-finesse electromagnetic cavities, each coupled to a single three-level atom. The nodes are connected by optical fibers. Each atom is addressed by a control laser, which along with the cavity field drives atomic transitions. The network can be in the form of arrays of N-cavities connected by NB fibers in one to three dimensions. We find that under certain conditions, the system possesses two kinds of degenerate dark states. The first kind are N states corresponding to atomic excitations at each node and these are our logical states for quantum processing. The second kind are NB states on pairs of sites connected by a fibre. By manipulating intensities and phases of control lasers on the cavities, one can pass adiabatically among these dark states due to their degeneracy. This network operates as a N-level quantum system in which one can generate computationally useful states by protocols of external controls. We obtain numerical results for small chains and square lattices to demonstrate some quantum operations like the transport of states across the array, generation of superposed states and phase-flipping in a network. We also discuss effects of dissipation and limitations of the model.

DOI
10.1117/12.929552

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We propose a scalable network, in which all quantum operations can be executed through external controls. Nodes of this network are high-finesse electromagnetic cavities, each coupled to a single three-level atom. The nodes are connected by optical fibers. Each atom is addressed by a control laser, which along with the cavity field drives atomic transitions. The network can be in the form of arrays of N-cavities connected by NB fibers in one to three dimensions. We find that under certain conditions, the system possesses two kinds of degenerate dark states. The first kind are N states corresponding to atomic excitations at each node and these are our logical states for quantum processing. The second kind are NB states on pairs of sites connected by a fibre. By manipulating intensities and phases of control lasers on the cavities, one can pass adiabatically among these dark states due to their degeneracy. This network operates as a N-level quantum system in which one can generate computationally useful states by protocols of external controls. We obtain numerical results for small chains and square lattices to demonstrate some quantum operations like the transport of states across the array, generation of superposed states and phase-flipping in a network. We also discuss effects of dissipation and limitations of the model.
We consider a network whose nodes are electromagnetic cavities, each coupled to a single three-level atom. The nodes are connected by optical fibers. Each atom is addressed by a control laser, which along with the cavity field drives atomic transitions. The network can be in the form of chain or two- and three-dimensional arrays of NNN cavities connected by NB{N}_{B}NB​ fibers. Following the work on a two-cavity system by Pellizzari, we find that under certain conditions the system possesses two kinds of dark states. The first kind are NNN states corresponding to atomic excitations at each node and these are our logical states for quantum processing. The second kind are NB{N}_{B}NB​ degenerate dark states on pairs of sites connected by a fiber. By manipulating intensities and phases of control lasers on the cavities, one can pass adiabatically among these dark states due to their degeneracy. This network operates as an NNN-level quantum system in which one can generate computationally useful states by protocols of external controls. We obtain numerical results for small chains and lattices to demonstrate some quantum operations such as the transport of states across the array, generation of WWW states, and Fourier-like states. We also discuss effects of dissipation and limitations of the model.
We show that the coherent coupling of atomic qubits at distant nodes of a quantum network, composed of several cavities linked by optical fibers, can be arbitrarily controlled via the selective pairing of Raman transitions. The adiabatic elimination of the atomic excited states and photonic states leads to selective qubit-qubit interactions, which would have important applications in quantum-information processing. Quantum gates between any pair of distant qubits and parallel two-qubit operations on selected qubit pairs can be implemented through suitable choices of the parameters of the external fields. Selective pairing of Raman transitions also allows the generation of spin chains and cluster states without the requirement that the cavity-fiber coupling be smaller than the detunings of the Raman transitions.
A scheme is presented to realize transfer of a three-dimensional quantum state between two atoms trapped in distant cavities connected by an optical fiber, whose mode is resonant with the polarization cavity modes. Performing an adiabatic passage along dark states, the fiber mode remains in the vacuum state and the population of the cavities being excited can be negligible under certain conditions. In addition, atomic transitions in our scheme are largely detuned from cavity modes. As a consequence, the atomic spontaneous emission and the fiber decay can be effectively eliminated. Furthermore, we give some discussions about the fidelity of our scheme.
The paper reports on the experimental demonstration of elementary quantum-network links and nodes with narrowband and transform-limited single-photon pulses that cannot be distinguished from one another. The photon source is realized in form of a strongly coupled atom-cavity system and it employs a controlled energy exchange between the atom and the quantized light field of the cavity. This is accomplished by laser pulses adiabatically driving one branch of a Raman transition in a /spl Lambda/-type three-level atom, which is located in a high-finesse optical cavity resonant with the other branch of the transition. The transition goes hand-in-hand with a change of the photon number in the cavity.
We consider the dynamics of an open three-level quantum degenerate system. One of the levels in this system is degenerate. The system interacts with three reservoirs (quantum fields) and a classical external field. We show that nondecaying so-called dark states are generated in this system. Since the interactions of the degenerate level with two different reservoirs are different (correspond to different spaces of dark states), we can describe excitation and manipulations for this kind of states (in particular, observation in spectroscopical experiments). Possible applications of this model in quantum optics, quantum computations, and quantum photosynthesis are discussed.