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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Generation of photonic time-bin qubit in dense atomic media

Narek Sisakyan, Yu. P. Malakyan

Published Jun 1, 2007• 0 citations
Publisher

Abstract

In this paper, we report a scheme for generation of entangled single-photon states coherently delocalized in two well-separated temporal modes. The proposed scheme employs strong nonclassical correlations between the Stokes and anti-Stokes photons generated and represents a remotely source for Stokes time-bin. We analyze the entangled nature and nonlocality in Stokes qubit by employing the Bell's inequality formulated for two-mode Wigner function.

DOI
10.1109/cleoe-iqec.2007.4386798

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In this paper, we report a scheme for generation of entangled single-photon states coherently delocalized in two well-separated temporal modes. The proposed scheme employs strong nonclassical correlations between the Stokes and anti-Stokes photons generated and represents a remotely source for Stokes time-bin. We analyze the entangled nature and nonlocality in Stokes qubit by employing the Bell's inequality formulated for two-mode Wigner function.
Time-bin entangled single-photons are highly demanded for long distance quantum communication. We propose a heralded source of tunable narrowband single photons entangled in well-separated multiple temporal modes (time bins) with controllable amplitudes. The detection of a single Stokes photon generated in a cold atomic ensemble via Raman scattering of a weak write pulse heralds the preparation of one spin excitation stored within the atomic medium. A train of read laser pulses deterministically converts the atomic excitation into a single anti-Stokes photon delocalized in multi-time-bins. The waveforms of bins are well-controlled by the read pulse parameters. A scheme to measure the phase coherence across all time bins is suggested.
A method of preparing a single photon in temporally delocalized entangled modes is proposed and analyzed. We show that two single-photon pulses propagating in a driven nonabsorbing medium with different group velocities are temporally split under parametric interaction into well-separated pulses. As a consequence, the single-photon ``time-bin-entangled'' states are generated with a programmable entanglement, which is easily controlled by driving field intensity. The experimental study of nonclassical features and nonlocality in generated states by means of balanced homodyne tomography is discussed.
A new method of preparing a single photon in temporally delocalized entangled modes is proposed and analyzed. We show that in a medium of four-level atoms with tripod-level configuration a strong parametric interaction between two one-photon pulses emerges, if the medium was initially prepared in a coherent superposition of the two ground states, while a strong classical field drives the neighbor transition from the third ground state. The analytical solution of Maxwell equations reveals that under the electromagnetically induced transparency (EIT), the quantum fields propagate with significantly small absorption and with different group velocities that leads to their temporal split into well-separated pulses, the amplitudes of which are well controlled by the driving field. The proposed scheme can serve as a robust source of narrow-band one-photon qubits with an entanglement between two temporal modes, not sensitive to losses in atmosphere and in telecommunication waveguides. We show also that in our scheme a controllable degree of temporal entanglement can be achieved.
Entanglement between light and matter combines the advantage of long distance transmission of photonic qubits with the storage and processing capabilities of atomic qubits. To distribute photonic states efficiently over long distances several schemes to encode qubits have been investigated-time-bin encoding being particularly promising due to its robustness against decoherence in optical fibers. Here, we demonstrate the generation of entanglement between a photonic time-bin qubit and a single collective atomic spin excitation (spin wave) in a cold atomic ensemble, followed by the mapping of the atomic qubit onto another photonic qubit. A magnetic field that induces a periodic dephasing and rephasing of the atomic excitation ensures the temporal distinguishability of the two time bins and plays a central role in the entanglement generation. To analyze the generated quantum state, we use largely imbalanced Mach-Zehnder interferometers to perform projective measurements in different qubit bases and verify the entanglement by violating a Clauser-Horne-Shimony-Holt Bell inequality.
We present a scheme to realize a deterministic solid state source of time-bin entangled photon pairs using cavity-assisted stimulated Raman adiabatic passage (STIRAP) in a single quantum dot. The quantum dot is embedded inside a semiconductor cavity, and the interaction of a coherent superposition of two temporally separated input pulses and the cavity mode leads to a two-photon Raman transition, which produces a time-bin entangled photon pair through the biexciton-exciton cascade. We show that the entanglement of the generated state can be measured using triple coincidence detection, and the degree of entanglement is quantified as the visibility of the interference. We also discuss the effect of pure dephasing on entanglement of the generated photon pair. Pronounced interference visibility values of greater than 1/21/\sqrt{2}1/2​ are demonstrated in triple coincidence measurement using experimentally achievable parameters, thus demonstrating that the generated photons are suitable for applications with Bell's inequality violation and quantum cryptography.
We present the quantum theory for creation of collective atomic memory and generation of nonclassically correlated photon pairs from an ensemble via the protocol of Duan et al. [Nature (London) 414, 413 (2001)]. The temporal evolution of photon numbers, photon statistics, and cross-correlation between the Stokes and anti-Stokes fields is found by solving the equation of motion for atomic spin-wave excitations. We consider a low-finesse cavity model with collectively enhanced signal-to-noise ratio, which remains still considerably large in the free-space limit. Our results describe analytically the dependence of quantum correlations on spin decoherence time and time delay between the write and read lasers and reproduce the observed data very well including the generated pulse shapes, strong violation of Cauchy-Schwarz inequality and conditional generation of anti-Stokes single-photon pulse. The approach we developed may be used also for quantum description of storage and retrieval of quantum information, especially when the statistical properties of nonclassical pulses are studied.