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Progress In Electromagnetics Research M

POLARIZED PHOTON GENERATION FOR THE TRANSPORT OF QUANTUM STATES: A CLOSED-SYSTEM SIMULATION APPROACH

Md. Mijanur Rahman, P. K. Choudhury

Published Jan 1, 2009• 9 citations
Publisher

Abstract

A novel approach for logic state dependent generation of polarized photon is proposed, where the logic states '0' and '1' are represented by two sub-spaces in the Hilbert space of the hyperflne states of rubidium atom ( 87 Rb). Each subspace consists of a ground state, an intermediate state and an excited state. The atom is placed at the center of a two-mode cavity, and the cavity modes correspond to frequencies of the generated photon. Photon generation process involves raising the atom to the excited state within the corresponding subspace and letting it decay back to the initial (ground) state, emitting thereby a photon of logic state dependent polarization. In order to keep the driving laser frequencies far ofi from the cavity mode frequencies, the atom is raised to the excited state in two steps | flrst from the ground state to the intermediate state and then from the intermediate state to the excited state. Polarization states of the photon represent the logic states, and can be used to transport logic from one node to another of the quantum network.

DOI
10.2528/pierm09081603

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A novel approach for logic state dependent generation of polarized photon is proposed, where the logic states '0' and '1' are represented by two sub-spaces in the Hilbert space of the hyperflne states of rubidium atom ( 87 Rb). Each subspace consists of a ground state, an intermediate state and an excited state. The atom is placed at the center of a two-mode cavity, and the cavity modes correspond to frequencies of the generated photon. Photon generation process involves raising the atom to the excited state within the corresponding subspace and letting it decay back to the initial (ground) state, emitting thereby a photon of logic state dependent polarization. In order to keep the driving laser frequencies far ofi from the cavity mode frequencies, the atom is raised to the excited state in two steps | flrst from the ground state to the intermediate state and then from the intermediate state to the excited state. Polarization states of the photon represent the logic states, and can be used to transport logic from one node to another of the quantum network.
An enhanced approach for transferring quantum state between quantum nodes is proposed wherein photons serve as the information carrier. Each node consists of a Rubidium (87Rb) atom trapped inside a two-mode optical cavity. The approach is based on cavity quantum electrodynamics (QED) wherein a system of lasers is applied on the atom in order to generate photon through Raman transition. Logic states ‘0’ and ‘1’ are represented by two subspaces of the hyperfine energy levels with magnetic sub-levels of 87Rb atom. A static magnetic field is applied upon the atoms so that the hyperfine states of 87Rb atom are split into the magnetic sub-levels (due to Zeeman effect). Depending on the logic state of the transmit node, a right- or left-circularly polarized photon with designated frequency is produced through a cavity assisted Raman process. When the photon is received at the receive node via an optical fiber, the logic state of the transmit node is restored (through a cavity QED process) into the receive node. A desirable feature of the approach is that, during the transmission of logic state, the transmit node itself should not significantly change its quantum state; this is successfully validated through simulations.
A scheme to generate the entanglement between two distant atoms by the interference of polarized photons is proposed. The model consists of two identical /spl Lambda/-type three-level atoms trapped in two distant optical cavities respectively. Each atom has a excited state |e> and two degenerate ground states |g/sub L/> and |g/sub R/>. The scheme is based on the interference effect where the coherent superposition of atomic states is generated in photon emission process. The scheme gives several advantages: (i) both atoms are excited simultaneously; (ii) the driving condition does not influence the fidelity of the final entangled states; and (iii) the influence of atomic recoil on the entanglement fidelity could be eliminated.
A single atom strongly coupled to a high-finesse optical-cavity is a versatile tool for quantum information processing. Utilized as a single-photon source, it allows one to generate single photons very efficiently in a well defined spatio-temporal mode. In a first experiment, polarization-control over the photons is shown. A time-resolved two-photon interference experiment proves the indistinguishability of these photons - required in various quantum information processing schemes. Moreover, in a second experiment, entanglement between the polarization of the emitted photon and the population of the atomic Zeeman levels is created. Subsequent state mapping of the atomic state onto another photon results in a pair of polarization-entangled photons emitted one after the other from the cavity. Although these schemes are in principle possible in free space, the cavity boosts the efficiency by several orders of magnitude.
We describe a cavity-QED scheme to deterministically generate polarization entangled photon pairs by using a single atom successively coupled to two single longitudinal mode cavities presenting polarization degeneracy. The cavities are initially prepared either in the vacuum state or in a single photon Fock state for each orthogonal polarization. Sharing the same basic elements, the source can operate on different physical processes. For a V -type three-level atom initially prepared in the ground state two implementations of the source are possible using either: i) two truncated Rabi Oscillations, or ii) a counterintuitive Stimulated Raman Adiabatic Passage process. Although slower than the former implementation, this second one is very efficient and robust under fluctuations of the experimental parameters and, particularly interesting, almost insensitive to atomic decay. For a four-level atom in a diamond configuration initially prepared in the upper state, the source can produce entangled photon pairs even in the bad cavity limit via an adiabatic passage process. We have performed Monte Carlo wave function simulations to characterize these sources by means of: i) the success probability P of producing the desired entangled state, ii) the fidelity F in the reduced space of two emitted cavity photons, and iii) the S parameter of the Clauser-Horne-Shimony-Holt (CHSH) inequality to quantify the entanglement capability.
The cause of decoherence in a quantum system can be traced back to the interaction with the environment. As it has been pointed out by Dicke, in a system of N two-level atoms, where each of the atoms is individually dipole coupled to the environment, there are collective, subradiant states, that have no dipole coupling to photon modes, and therefore, they are expected to decay slower. This property also implies that these type of states decohere slower. We propose a scheme which will create such states. First, the two-level atoms are placed in a strongly detuned cavity and one of the atoms, called the control atom is excited. The time evolution of the coupled atom-cavity system leads to an appropriately entangled state of the atoms. By the aid of a suitable single-atom phase transformation, it is possible to drive the atomic state into the subradiant, i.e., decoherence-free subspace. Up to a certain average number of photons, the result is independent of the state of the cavity. The analysis of the conditions shows that this scheme is feasible with present day techniques achieved in atom-cavity interaction experiments.
Single-photons of well-defined polarization that are deterministically generated in a single spatio-temporal field mode are the key to the creation of multi-partite entangled states in photonic networks. Here, we present a novel scheme to produce such photons from a single atom in an optical cavity, by means of vacuum-stimulated Raman transitions between the Zeeman substates of a single hyperfine state. Upon each transition, a photon is emitted into the cavity, with a polarization that depends on the direction of the Raman process.