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Quantum Electronics and Laser Science Conference

Many-spin quantum entanglement induced by femtosecond pulses

Gregory A. Garrett, Alka Ingale, Roberto D. Merlin, T.E. Stevens, M. Kudas, J.B. Michean-Cunningham, I. Miotkowski, A. K. Ramdas

Published Jan 20, 2003• 0 citations
Publisher

Abstract

Summary form only given. Femtosecond pulses were used to generate long-lived (/spl tau/>0.1 ns) quantum-entangled states involving the spins of many individual Mn/sup 2+/ ions in Cd/sub 1-x/Mn/sub x/Te. The experiments were performed on a single crystal of Cd/sub 0.98/Mn/sub 0.02/Te with a standard pump-probe setup in the Voigt reflection geometry. We used a mode-locked Ti:sapphire laser which provided /spl sim/ 95 fs pulses at a repetition rate of 85 MHz an average-power of 80 mW and focused to a 80-/spl mu/m-diameter spot. The intensity of the reflected probe pulse measures changes in the optical constants that depend on the expectation value of the total spin for the particular entangled state /spl Psi/ created by the pump. The signature for an entanglement between states |j) and |l) is the observation of oscillations of frequency n/spl Omega//sub 0/ where n=(n/sub j/-n/sub l/). We notice that the highest harmonic of /spl Omega//sub 0/ provides a lower bound for the number of magnetic ions N that are entangled. The entanglement process, relying on coupling to localized excitons, holds promise for quantum-gate operations.

DOI
10.1109/qels.1999.807668

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Quantum Electronics and Laser Science Conference•Jan 20, 2003•1 citation
Summary form only given. Femtosecond pulses were used to generate long-lived (/spl tau/>0.1 ns) quantum-entangled states involving the spins of many individual Mn/sup 2+/ ions in Cd/sub 1-x/Mn/sub x/Te. The experiments were performed on a single crystal of Cd/sub 0.98/Mn/sub 0.02/Te with a standard pump-probe setup in the Voigt reflection geometry. We used a mode-locked Ti:sapphire laser which provided /spl sim/ 95 fs pulses at a repetition rate of 85 MHz an average-power of 80 mW and focused to a 80-/spl mu/m-diameter spot. The intensity of the reflected probe pulse measures changes in the optical constants that depend on the expectation value of the total spin for the particular entangled state /spl Psi/ created by the pump. The signature for an entanglement between states |j) and |l) is the observation of oscillations of frequency n/spl Omega//sub 0/ where n=(n/sub j/-n/sub l/). We notice that the highest harmonic of /spl Omega//sub 0/ provides a lower bound for the number of magnetic ions N that are entangled. The entanglement process, relying on coupling to localized excitons, holds promise for quantum-gate operations.
A quantum-mechanical many-particle system may exhibit nonlocal behavior in that measurements performed on one of the particles can affect a second one that is far apart. These so-called entangled states are crucial for the implementation of quantum information protocols and gates for quantum computation. Here, we use ultrafast optical pulses and coherent pump-probe techniques to create and control spin entangled states in an ensemble of up to three non-interacting electrons bound to donors in a ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$Te quantum well. Our method, relying on the exchange interaction between optically excited excitons and the paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of electrons. A microscopic theory of impulsive stimulated Raman scattering and a model for multi-spin entanglement are presented. The signature of entanglement is the observation of overtones of donor spin-flips in the differential reflectivity of the probe pulse. Results are shown for resonant excitation of localized excitons below the gap, and above the gap where the signatures of entanglement are significantly enhanced. Data is also presented on the generation of coherent excitations of antiferromagnetically coupled manganese pairs, folded acoustic phonons, exciton Zeeman beats and entanglement involving two Mn+2{\mathrm{Mn}}^{+2}Mn+2 ions.
We present an experimental setup for generation of entangled-photon pairs via spontaneous parametric down-conversion, based on the femtosecond-pulsed laser. Our entangled-photon source utilizes a 76-MHz-repetition-rate, 100-fs-pulsewidth, mode-locked, ultrafast femtosecond laser, which can produce, on average, more photon pairs than a cw laser of an equal pump power. The output infrared pump photons (λ = 810 nm) are first up-converted to blue light (λ = 405 nm) and, subsequently, down-converted in a 1.5-mm-thick, type-II BBO crystal via spontaneous down-conversion. The resulting entangled pairs are counted by a pair of high-quantum-efficiency, single-photon, silicon avalanche photodiodes. The total down-conversion efficiency of our system, corresponding criterion of the pump power for real entangled coincident events, has been calculated to be 0.86 × 10-9. Our apparatus is intended as an efficient source/receiver system for the quantum communications and quantum cryptography applications.
A quantum-mechanical many-particle system may exhibit non-local behavior in that measurements performed on one of the particles can affect a second one that is far apart. These so-called entangled states are crucial for the implementation of quantum information protocols and gates for quantum computation. Here, we use ultrafast optical pulses and coherent techniques to create and control spin entangled states in an ensemble of up to three non-interacting electrons bound to donors in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of spins.
Summary form only given. We present experimental results that demonstrate a high degree of control over the spin state of an atom. The experiment is carried out on a sample of about 10/sup 7/ laser-cooled /sup 133/Cs atoms in their F=4 ground state hyperfine level released from a magneto-optical trap. Our method is to apply a static magnetic field B/sub 0/, a time-dependent transverse magnetic field B/sub x/(t), and a linearly polarized off-resonance laser field to the atoms. The Hamiltonian is H=-/spl mu//spl middot/B(t)+ M/sup 2//spl omega//sub E/|M><M|, where B(t)=B/sub 0/z+B/sub x/(t)x. B/sub 0/ induces a precession of the spins at the Larmor frequency /spl omega//sub L/=/spl mu//sub B/B/4/spl planck/. The second term in H arises from the laser field light shift; together with the static field B/sub 0/ it yields a finite anharmonic ladder of quantum states, labeled by the spin projection quantum number M=-F,...,F. Because of the anharmonicity, B/sub x/(t) induces motions of the spin that are not simply rotations of the initial state. Rather, through the application of quantum control theory, it is possible to find a pulse B/sub x/(t) that produces time evolution connecting arbitrary spin states.
We demonstrate quantum entanglement of two trapped atomic ion qubits using a sequence of ultrafast laser pulses. Unlike previous demonstrations of entanglement mediated by the Coulomb interaction, this scheme does not require confinement to the Lamb-Dicke regime and can be less sensitive to ambient noise due to its speed. To elucidate the physics of an ultrafast phase gate, we generate a high entanglement rate using just ten pulses, each of ∼20 ps duration, and demonstrate an entangled Bell state with (76±1)% fidelity. These results pave the way for entanglement operations within a large collection of qubits by exciting only local modes of motion.
Summary form only given. We have observed a stimulated Raman resonance between free and bound molecular states of atoms in a Bose-condensed gas. In our experiment, we first produce a Bose-condensed cloud containing about 5/spl times/10/sup 5/ /sup 87/Rb 5/sup 2/S/sub 1/2/ (f=1,m/sub f/=-1) atoms in a magnetic TOP trap, with a typical condensate fraction of about 20%. We then illuminate the atoms with a /spl tau/=10 ms duration pulse of light from two laser beams 1 and 2 of frequency /spl nu//sub 1/ and /spl nu//sub 2/, and intensities I/sub 1/=120 W/cm/sup 2/ and I/sub 2/=2.2 W/cm/sup 2/, respectively. We measure the fraction of atoms N(/spl tau/)/N/sub 0/ remaining after the 10 ms pulse with laser-induced fluorescence.