Ideas for Experimental Realization of Neutral Atom Quantum Computing

演 講 者：蔡 錦 俊 成功大學物理系chintsai@mail.ncku.edu.tw http://www.phys.ncku.edu.tw/~cctsai

20022002年年 1010月月 1818日日

OutlineOutline

Motivation

Trapping and manipulation of Single or Few Atoms

Entanglement of two Macroscopic Objects

MotivationMotivationUsing neutral atoms to realize quantum computing

Advantages: Atoms, photons, and fields are involvedWeak interactions with external fieldsMany internal statesLong-lived coherence time

Disadvantages: Exponential decrease of preparing efficiencyNoise 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 2S1/2

F=4, mF

-4, -3, -2, -1, 0, 1, 2, 3, 4

6p 2P3/2

F=3, mF

-4, -3, -2, -1, 0, 1, 2, 3, 4

+Cs 6s 2S1/2

n=6

l=0

2S+1, S=1/2

J=1/2Nuclear spin, I=7/2F = J+I

Entanglement of two Macroscopic Objects/ Nature 413, 400 (2001), Aarhur, Denmark.

Sample: Two 3x3 cm paraffin coated cells place in a highlyhomogenous B field 0f 0.9 G.Coherence time of spin-state 5~30 msecOptical pumping:Cell1: |F=4, mF=4>; Cell2: |F=4, mF=-4>

Optical pulses: 0.45msec, 0.5 mW at 852 nm with 700 MHz ofblue 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:

= (Sycos())2 + (Sysin())2

out out

Entanglement of two Macroscopic Objects/ Nature 413, 400 (2001), Aarhur, Denmark.

Normalized special varianceEPR/(Jx) vs. Jx

Below unity level for entangledState 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 AtomsSingle atom trap/ Science 293, 278 (2001), Bonn, Germany

Normal MOT deviceDipole Trap : Nd:YAG laser, =1064nm, counter propagated,Beam waist 0 ~ 30mDipole potential, U(z, t) = U0cos[(t-2z/)] controlled with two acousto-optic modulation (AOM)Detection: position sensitive LIF at Cs F=4 F’=5 and Repumping at F=3F’=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 AtomsSingle atom trap/ Science 293, 278 (2001), Bonn, Germany

Experimental set-up

Trapping and manipulation of Single or Few AtomsSingle 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 AtomsDeterministic loading of single atom/PRL 89,70401(2002),Austin,USA

Loading atom from a Condensateand dot potential

The probability of extractinga single atom vs. dot speed. Using1D BEC harmonic trap, N=105 and square dot well.

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