Quantum Memory with cold atoms and it’s Applications

18th, July 2009, Shanghai

Outline

• What’s quantum memory• Atomic ensemble work as quantum memory• Generation of atom-photon entanglement and

applications in quantum communication• Lifetime extension to 1 millisecond• “NOON” state generation and phase super-

resolution• Summary

| ⟩| ⟩ orClassical Physics:“bit”

| ⟩| ⟩ +Quantum Physics:“qubit”

| ⟩| ⟩ | ⟩| ⟩ +

Entanglement:

Quantum foundations: Bell’s inequality, quantum nonlocality…Quantum information processing: quantum communication, Quantum computation, high precision measurement etc …

Quantum Superposition and entanglement

• A quantum state of a two level system

• A quantum memory is a system that can keep both the amplitude and phase of a quantum state over a period of time.

ia e bφψ α β= +

1|||| 22 =+ βα

Three level atoms: medium ofquantum memory-DLCZ

|aÚ|bÚ

iatom

a e bφψ α β= +

Optically dense Atomic Ensemble:N atoms with Lambda System

Step 1:State

preparation

Step 2:Anti-Stokes

Photon

Step 3:Stokes Photon

Storage time t

[L.Duan et al., Nature 414, 413 (2001)]

∑

∏

=

=

iNib

iib

abaN

a

......11

0

1

Non-classical photon pair Generation

,

:

as

s

as s

pp

Quantum mechanics

p

χχχ

===

,(2),

1 1, ( 1)·

as sas s

as s

pg

p pχ

χ= =

Cross-correlation is used to show quantum correlation between the single-photon pair

,

: ·as s as s

Classical statistip

cp

sp=

, 00 11 22AS Sψ χ χ= + +

Quantum Memory System

Quantum Memory with cold atoms

write

Read

[S. Chen et al., Phys. Rev. Lett. 97 173004 (2006)]

t~13 µs

Atom-photon entanglement

[Shuai Chen et. al, Phys. Rev. Lett. 99, 180505 (2007)]

Momentum conservation

Entangled state

Entanglement verify

Phase lock of the entanglement source

MOT on MOT offMOT

Lock beam

Time sequence for the phase lock

Entanglemant signal to noise ratio, 15:1@excitation rate of 3â10-3

Short term fluctuation: <p/30Long term drift: cancelled

Phase stability after lock

stabilize of the phase f1+f2

Visibility of the entanglement

Character of the novel atom-photon entanglement source

entanglement storage~4 km for the light to transmit in fiber

Teleport a photonic qubit to atomic qubit

Memory-built-in Teleportation, Fidelities and Storage feature

[Yu-Ao Chen et. al, Nature Physics 4, 103 (2008)]

Entanglement swapping between remote atomic ensembles

[Z.-S. Yuan et. al, Nature 454, 1038 (2008)]

Result of Swapping

Violation of CHSH-type Bell’s inequality in 500 ns

S=2.26±0.07

Excitation rate: 32 10−×

Alice and Bob are connected with 300 m fiber

Mechanism of decoherence

• External stray magnetic field – 10 mG ~ 10ms life time

• Movement of the atoms – Time for defuse: ~ 1ms for 100mK atoms

• Gravity effect – fall down time: ~ 3ms

Extend the lifetime to miliseconds

• clock states:write

ReadA weak B field is applied to polarize the atoms

Dephase of the spin wave: theoretical model

Wave vector of spin wave

Position of “j”th atom

Effect of the atomic motion

collective excitation

retrieve efficiency

Life time of the spin wave is limited by the motion of atoms

Life time

Finally,

0θ =

Life time

The life time of the quantum memory is limited by the free expansion of theatomic ensemble

[ B. Zhao et. al, Nature Physics 5, 95 (2009)]

Further extend the life time

• Lower temperature• Load the atoms in optical lattices (R.Zhao

et.al. Nature Physics 5, 101 (2009))

• Blue dipole trap• Mott insulator• Spin echo• …

Entanglement assisted spin-wave interferometer

Wave vector of the spin wave

tvkt c⋅Δ=Δ )(φ

The centre of mass motion of the atomic ensemble will give a “phase shift ” to the spin wave

Photon:

Atoms:

Interference of single excitations

Pump power, A: 6mW, B: 4.5mW, C: 3mW,D: 1.5mW, E: 0.75mW, F: 0

The coincidence evolution after retrieve:

NOON state preparation

Once the ensemble have a collective motion

Post selected “NOON”state generation

If “N” detector fired, the wave vector of the spin wave becomes

phase super-resolution

N=2 N=1

[ Y. -A. Chen et. al, arXiv: 0904.3617, submit to Nature Physics]

Detail presentation, please see: Xiao-Hui Bao, 12:00-12:20, July 20th, in Hall A

Summary

• Atomic ensembles can be used as the medium of quantum memory

• Atom-Photon entanglement source is generated– Memory-built-in quantum teleportation– Entangle two distant atomic qubit via entanglement

swapping• Long life time of quantum memory up to 1ms is

reached in cold atoms• NOON state is generated and show the ability

of the “phase super-resolution”

Future Plan: Further Study of quantum memory

• Load the atoms into the optical trap or optical lattices to get rid of the effect caused by motion and gravity, further increase the lifetime of the memory

• With help of CQED, increase the coupling strength between the atomic ensemble and the signal mode of the photon, increase the read out efficiency

• Guide the atoms into a photonic crystal fiber, for the smart devise.

Thanks!