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Future Challenges in Long-Distance Quantum
Communication
Jian-Wei Pan
Hefei National Laboratory for Physical Sciences at Microscale, USTC and
Physikalisches Institut der Universität Heidelberg
December 15, 2005
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Classical 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
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When information is encoded in quantum states one may outperform classical communication systems in terms of
• absolute security• efficiency• channel capacity
Because quantum information systems allow encoding information by means of
coherent superposition of quantum states.
Why Quantum Communication?
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Qubits: Polarization of Single Photons
One bit of information per photon(encoded in polarization)
"1|"|
"0|"|
V
H
Qubit: VH |||
Non-cloning theorem:
An unknown quantum state can not be copied precisely!
1|||| 22 2||
2||
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Bell states – maximally entangled states:
212112
212112
||||2
1|
||||2
1|
HVVH
VVHH
Polarization Entangled Photon Pair 1-2
)|||(|2
1
)|||(|2
1|
2121
212112
HVVH
HVVH
)|(|2
1|
)|(|2
1|
VHV
VHH
Singlet:
where
45-degree polarization
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Quantum Cryptographic Key Distribution
•Single-particle-based secret key distribution:
•Entanglement-based secret key distribution:[A. Ekert, Phys. Rev. Lett. 67, 661 (1991). ]
[C. H. Bennett & G. Brassard, BB84 protocol (1984) ]
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Quantum Teleportation
111 ||| VH
Initial state
The shared entangled pair
323223 ||||2
1| VVHH
,|||
|||
|||
|||
|||
3312
3312
3312
3312
231123
HV
HV
VH
VH
212112
212112
||||2
1|
||||2
1|
HVVH
VVHH
where
[C.H. Bennett et al., Phys. Rev. Lett. 73, 3801 (1993)]
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Entanglement Swapping
[M. Zukowski et al., Phys. Rev. Lett. 71, 4287 (1993)]
34123412
34123412
23141234
||||
||||
|||
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achieved distance: 100km fiber-based (Toshiba Research Europe) 23km free-space (TU Munich)
Key Distribution with Single Photons
[C. Kurtsiefer et al., Nature 419, 450 (2002)]
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Generation of Photonic Entanglement
[P. G. Kwiat et al., Phys. Rev. Lett. 75, 4337 (1995).]
212112
212112
||||2
1|
||||2
1|
HVVH
VVHH
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Key Distribution with Entangled Photons
achieved distance: 1km for both fibre-based and free-space
Fibre: [T. Jennewein et al., Phys. Rev. Lett. 84, 4729 (2000).] [D. S. Naik, et al., Phys. Rev. Lett. 84, 4733 (2000).] [W. Tittel et al., Phys. Rev. Lett. 84, 4737 (2000).]Free-space: [M. Aspelmeyer et al., Science 301, 621 (2003).]
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Experimental Quantum Teleportation
Teleportation: [D. Bouwmeester & J.-W. Pan et al., Nature 390, 575 (1997)]
The setup
Entanglement Swapping: [J.-W. Pan et al., Phys. Rev. Lett. 80, 3891 (1998)]
The result
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Our dream:
achieving long-distance quantum communication!
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However, due to the noisy quantum channel
photon loss(1) absorption
(2) decoherence degrading entanglement quality
Difficulties in Long-Distance Quantum Communication
Free-Space Distribution of Entangled Photons
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Free-Space Distribution of Entangled Photons over 13km
[C.-Z. Peng et al., Phys. Rev. Lett. 94, 150501 (2005)]
Free-space entanglement distribution - we are working on 20km and 500km scale…
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Entanglement swapping: solution to photon loss: [N. Gisin et al., Rev. Mod. Phys. 74, 145 (2002)]
Entanglement purification: solution to decoherence[C. H. Bennett et al., Phys. Rev. Lett. 76, 722 (1996)][D. Deutsch et al., Phys. Rev. Lett. 77, 2818 (1996)]
Another Solution to Photon Loss and Decoherence
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Generating Entangled States over Long-Distance
Quantum repeaters:
[H. Briegel et al., Phys. Rev. Lett. 81, 5932(1998)]
Require
• entanglement swapping with high precision• entanglement purification with high precision• quantum memory
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Experimental Entanglement Purification and Swapping
0.0
0.1
0.2
0.3
0.4
0.5
|V>|H>
|V>|V>
|H>|V>
|H>|H>
Fraction
0.0
0.1
0.2
0.3
0.4
0.5
|V>|H>
|V>|V>
|H>|V>
|H>|H>
Fraction
0.0
0.1
0.2
0.3
0.4
0.5
|-45o>|+45o>
|-45o>|-45o>
|+45o>|-45o>
|+45o>|+45o>
Fraction
0.0
0.1
0.2
0.3
0.4
0.5
|-45o>|+45o>
|-45o>|-45o>
|+45o>|-45o>
|+45o>|+45o>
Fraction
Before purification, F=3/4
After purification, F=13/14
[J.-W. Pan et al., Nature 423, 417 (2003)]
[J.-W. Pan et al., Nature 410, 1067 (2001)]
[J.-W. Pan et al., Nature 421, 721 (2003)]
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Drawback in Former Experiments
• Probabilistic entangled photon source• Probabilistic entanglement purification• Bad weather
Quantum memory
!P
• In N -stage realization, the cost of resource is proportional to NPN/
• With the help of quantum memory, the total cost is then PN/
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Storage of single-photon states in atomic ensembles
[C. Liu et al., Nature 409, 490 (2001)][D. F. Phillips et al., Phys. Rev. Lett. 86, 783 (2001)]
Storage of light in atomic ensembles
motivate
[L.-M. Duan et al., Nature 414, 413 (2001)]
Solution with Atomic Ensembles
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Entanglement Generation
Maximally entangled in
the number basis!
RLppLRaa00
LRaaRL hh 00
LRaaLRaa 0,11,0
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Entanglement Connection
Steps :
1. Apply a reverse read laser pulse to transfer
atomic excitation to optical exc.
2.Succeeds if D1 or D2 registers one photon
3.Fails otherwise, and repeat every step from entanglement generation
0000' RIIL hhhh 00 RLLR
hh
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The most recent experiment results
Observation of Stokes and anti-Stokes photon • Harvard: M. D. Lukin… [C. H. Van der Wal et al., Science 301, 196 (2003)]• Caltech: H. J. Kimble… [A. Kuzmich et al., Nature 423, 731 (2003)]• Gatech: A. Kuzmich… [D. N. Matsukevich et al., Science 306, 663 (2004)]• Heidelberg: J.-W. Pan … long-life time quantum memory [S. Chen et al., in preparation for Phys. Rev. Lett.] working on a phase insensitive scheme…Synchronization of two independent lasers• USTC: J.-W. Pan, J. Zhang and Z.-Y. Wei … [T. Yang et al., submitted to Phys. Rev. Lett. (2005)]
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|Photons>
|Atoms> +
Powerful Quantum Superposition
Promising Long-Distance Quantum Communication