quantum memory with cold atoms and it’s...
TRANSCRIPT
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 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
Memory-built-in Teleportation, Fidelities and Storage feature
[Yu-Ao Chen et. al, Nature Physics 4, 103 (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
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.