Ideas for Experimental Realization of Neutral Atom Quantum Computing 演 講 者：蔡 錦 俊 成功大學物理系 年 10 月 18 日

Outline Motivation Trapping and manipulation of Single or Few Atoms Entanglement of two Macroscopic Objects

Motivation Using neutral atoms to realize quantum computing Advantages: Atoms, photons, and fields are involved Weak interactions with external fields Many internal states Long-lived coherence time Disadvantages: Exponential decrease of preparing efficiency Noise and imperfections in setup

Entanglement of two Macroscopic Objects / Nature 413, 400 (2001), Aarhur, Denmark. Experimental set-up and the sequence of optical pulses.

Entanglement of two Macroscopic Objects / Nature 413, 400 (2001), Aarhur, Denmark. Internal state of neutral Cs atoms and optical pumping 6s 2 S 1/2 F=4, m F -4, -3, -2, -1, 0, 1, 2, 3, 4 6p 2 P 3/2 F=3, m F -4, -3, -2, -1, 0, 1, 2, 3, 4 ++ Cs 6s 2 S 1/2 n=6 l=0 2S+1, S=1/2 J=1/2 Nuclear spin, I=7/2 F = J+I

Entanglement of two Macroscopic Objects / Nature 413, 400 (2001), Aarhur, Denmark. Sample: Two 3x3 cm paraffin coated cells place in a highly homogenous B field 0f 0.9 G. Coherence time of spin-state 5~30 msec Optical pumping: Cell 1 : |F=4, m F =4>; Cell 2 : |F=4, m F =-4> Optical pulses: 0.45msec, 0.5 mW at 852 nm with 700 MHz of blue detuned. Entangling pulse and verifying pulse are separated by 0.5 msec, no entanglement at 0.8 msec.

Entanglement of two Macroscopic Objects / Nature 413, 400 (2001), Aarhur, Denmark. Measurement Cos(  t) and Sin(  t) Special variance:  = (S ycos (  )) 2 + (S ysin (  )) 2 out

Entanglement of two Macroscopic Objects / Nature 413, 400 (2001), Aarhur, Denmark. Normalized special variance  EPR /  (J x ) vs. J x Below unity level for entangled State of the two atomic samples Maximum possible entanglement (dotted line) Shot noise of verifying pulse (dashed line) Degree of entanglement  = (35+7)%

Trapping and manipulation of Single or Few Atoms Single atom trap/ Science 293, 278 (2001), Bonn, Germany Normal MOT device Dipole Trap : Nd:YAG laser, =1064nm, counter propagated, Beam waist  0 ~ 30  m Dipole potential, U(z, t) = U 0 cos[  (  t-2z/ )]  controlled with two acousto-optic modulation (AOM) Detection: position sensitive LIF at Cs F=4  F’=5 and Repumping at F=3  F’=4 Advantages for dipole trapping: Trap all spin states Very long spin relaxation time ~ 30 sec High Modulation speed

Trapping and manipulation of Single or Few Atoms Single atom trap/ Science 293, 278 (2001), Bonn, Germany Experimental set-up

Trapping and manipulation of Single or Few Atoms Single atom trap/ Science 293, 278 (2001), Bonn, Germany Few atoms detection

Good for extracting definite number of neutral atoms from reservoir, Bose-Einstein condensation. Quantum Tweezer for Atoms Deterministic loading of single atom/PRL 89,70401(2002),Austin,USA Loading atom from a Condensate and dot potential The probability of extracting a single atom vs. dot speed. Using 1D BEC harmonic trap, N=10 5 and square dot well.

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