Many-spin quantum entanglement induced by femtosecond pulses

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.