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

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?

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||

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

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) ]

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)]

Entanglement Swapping

[M. Zukowski et al., Phys. Rev. Lett. 71, 4287 (1993)]

34123412

34123412

23141234

||||

||||

|||

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)]

Generation of Photonic Entanglement

[P. G. Kwiat et al., Phys. Rev. Lett. 75, 4337 (1995).]

212112

212112

||||2

1|

||||2

1|

HVVH

VVHH

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).]

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

Our dream:

achieving long-distance quantum communication!

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

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…

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

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

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)]

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/

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

Entanglement Generation

Maximally entangled in

the number basis!

RLppLRaa00

LRaaRL hh 00

LRaaLRaa 0,11,0

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

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)]

|Photons>

|Atoms> +

Powerful Quantum Superposition

Promising Long-Distance Quantum Communication