Recent develpoment in Quantum Communication

周志隆

physics dept. CYCUchoucl@cycu.edu.tw

Why do people study QIS

computations effectively performed simultaneously (quantum parallelism)

classically intractable problems may become feasible quantum-mechanically

unbreakable shared codes teleportation

QIS

ComputationComputation

CommunicationCommunication

Modern Modern electronicselectronics

Quantum V.S. Classical (1)

In principle, classical states can be faithfully distinguished from each other.

Quantum V.S. Classical (2)

Quantum states are non-orthogonal in general.

Quantum states cannot be identified faithfully.

0

Quantum V.S. Classical (3)

Quantum states can be superposed states.

qubit = a | 0 > + b | 1 >

The principle of superposition

Quantum V.S. Classical (4)

Quantum states can be non-local.

可能狀

態古典的 | 1 1 >| 1 0 >

| 0 1 >| 0 0 >

Entangled states ：)11|00(|

21

)10|01(|2

1

Quantum no-cloning theorem unknown quantum states cannot

be copied due to linearity of QM.S US U

Not possible

Do we need Quantum Communication?

(1) Quantum computer can break some of the best public key cryptosystems.

(2) Quantum key distribution provides provably secure distribution of private information.

(3) Quantum cryptography is technically viable and affordable.

However, QC..

does not provide a complete solution for all cryptographic purposes.

authentication

rapid delivery of keys

robustness

distance and location independence

resistance to traffic analysisQuantum communication network?

Quantum Communcation

Protocols

BB84 protocol

利用光的 2 個偏振方向，代表 “ 0” 與“ 1”

竊聽者

Bennett & Brassard

BB84

Alice 隨機選擇光的極化

Bob 隨機選擇測量方向Bob 公告測量方向Alice 告訴 Bob 哪些測量方向選對了。這些方向當作共同的加解密金匙！

Bit value “1”

BB84--example

Alice

Bob

Bob

Alice

Bob

Key 1 0 0 0

Bit survival rates: 50%Key-breaking prob.

34N

B92 protocol

Alice prepares a random classical bit “a”, and sends Bob the following quantum state depending on the “a” value.

0 if a 0012

if a 1

Bennett, PRL 68,3121(1992)

Bit values “0”

B92 protocol

Bob also prepares a random classical bit a’, and measureshis quantum bit with one of the following bases accordingto the value of a’.

(Z basis) 1 , 0

(X basis)1 0

2

a’=0

a’=1

“0” bits

B92 protocol—example

Alice(a)

0 0 1 0 1 1 1 0

Bob(a’)

1 0 1 1 0 1 0 1

Bit-result

s

1 0 0 0 1 0 1 0

key 01

10

10

bases

Bob anoucesit publicly

for Alice

for Bob

Bit survival rates: 25%

Key-breaking prob.

34N

Quantum nature of signals

Signals are non-orthogonal states.

Eavesdropper cannot clone signals Eavesdropping will inevitably incur

disturbance to the signal.qubits.

cannot be distinguished with 100% confidence

Ekert scheme (EPR protocol)

EkertPRL 67, 661(1991)

Entangled pairs of qubits are prepared as EPR states

01102

Alice

Bob

EPR protocol

Randomly select a subset of EPR pairs

Test violation of Bell’s inequality

The fidelity of the remaining EPR pairs is then inferred from the test.

Alice and Bob obtain correlated classical bit strings------ secret keys

Other protocols Variations of BB84 Two-state protocols Six-state protocol

Respect the symmetry of the qubit state space

Reduce Eve’s info gains

QC experiments…

Photon source

Photon counter

Quantum channel

Optical fiberFree space

Single-mode fibers

Free-space links

Telecommunication optical fiber

~ 1300, 1550 nm

~ 800 nmcommercial photon counterlow absorption

Silicon-base APD

Quantum channels

< 0.3 dB/km

InGaAs/InP APD

Free space links

Transmission in free space

low-loss window

line-of-sight communication

~ 770 nm

~ < 10 km

beam-pointingis difficult for movingtargets

Free space links High transmission window at ~ 770 nm compatible w. commercial silicon APD photon-

counter

Atmosphere is weakly dispersive and nonbirefirngent at the wavelengths. plain polarization coding is possible.

Energy transmitted spread out in space- higher loss during transmission.

Background lights can couple into the receiver - higher error rate.

It depends on weather conditions.

QC via optical fiber

~ 100 km

Cambridge research laboratory of Toshiba Research Europe

Swiss communication group & idQuantique in Europe

MagiQ technology in USA

Photon sources

Faint laser pulses

Entangled photon pairs by parametric down conversion or Franson’s method

Single-photon Fock states are difficult to realizedexperimently.

coherent stateswith an untra-lowmean photon number

semiconductor laser & attenuators

Producing entangled photons

Parametric down conversion.

Franson’s method.(using unbalanced Mach-Zender interferometer)

Parametric down conversion PDC is a process that a pump photon in a

nonlinear crystal has a small probability of splitting into two photons of lower frequency.

p 1 2 conserv. of energy

kp k1 k2 conserv. of momentum

Type II: two converted photons have orthogonal polarizations

PDCbeta-BaB2O4

Faint laser pulses

Pn, n

neProb. of finding n photons

Most pulses are empty Pn 0 1

decrease in bit rate

Pn 1n 0

2Pulse contains more than 1 photon

mean photon number ~ 0.1

Photon pairs by PDC Very inefficient ----- it takes 1010 pump

photons to create a pair in a given mode. The system can be made compact and handy.

40 cm × 45 cm ×15 cm

Single-photon detectors Avalanche photodiodes, photomultipliers,

Josephson junctions, quantum dots, MODFET

Most experiments used APD’s Silicon APD’s ~ 800 nm Ge APD’s ~ 1300 nm InGaAs/InP APD’s ~ 1500 nm Some group design photon-detecting

device for QC experiment. Toshiba team at Cambridge use MODFET

Exp. with faint laser pulses Polarization coding

Phase coding

Polarization coding

Bennett, Bessette, etc.1992

30 cm

BB84 four-state protocol

the pulseattenuated bythe filters

emits classical pulse

Beamsplitter:base choice

Polarization coding

(1) Polarization transformation induced by long optical fibers is unstable. require active alignment of bases.

(2) No polarization-maintaining fibers actually maintain polarization.

Drawbacks:

Polarization coding

Swisscom used optical fibers for QCexperiments between Geneva and Nyon, 1996

Nyon

Geneva

Transmission distance 23km

Phase coding

LD

PM AAlicePM B

APD’s

time

cou

nt

sDouble Mach-Zender implementation

Bob

~ 100 km

long+short

Phase coding(BB84)

1 eiAB2 Cos2A B2Interference

Bit value(Alice)

Bit value(Bob)

0 0 0 0 00 0 /2 3/2 ?1 0 11 /2 /2 ?0 /2 0 /2 ?0 /2 /2 0 01 3/2 0 3/2 ?1 3/2 /2 1

A B A B

Destructive interfere

Exp with photon pairs Polarization entanglement

Energy-time entanglement

dis/advantage w. photon pairs

advantage disadvantage Prevent unintended info leakage

Prevent multiphoton splitting attacks

Analyzers are simple and efficient. (polarization entanglement)

(polarization -based)

Decoherence is more serious than faint laser pulses (energy-time entanglement) Chromatic dispersion will destroy strong time correlation not adequate for QC over long optical fibers

Low key generation rate

Polarization entanglement

BBO pumped by argon laser

analyzer: simple and efficient

Both BB84 & Ekert’sscheme were realizedwith distances less than 1 km.

Energy-time-entangled photon pairs yr 2001

KNbO3 crystal

BB84 protocol, yr 2001.

Active groups worldwideJapan National Institute of

Informatics Australia Innsbruck

USA Caltech, Columbia UniversityHarvard UniversityIBM, Los Alamos,MagiQ TechnologiesMIT, Stanford, Berkeley,…

EuropeVienna - Quantum Experiments and the Foundations of Physics

Genève – GAP OptiqueUniversity of Padova, ItalyENS - LKB ,FranceIPT Russian Academy of Sciences - Quantum Computer Physics LaboratoryMax-Planck-Institut für QuantenoptikQuantum Information in Braunschweigid Quantique

UK CQC – CambridgeCQC - OxfordBangor UniversityImperial CollegeLoughborough University

Asia Quantum Lah in SingaporeNCKU, Taiwan

Quantum cryptography in Taiwan?

We can do it, since we have

(1) physicists who know quantum information theory well.

(2) experienced researchers who have good knowledge on quantum optics.

(3) well-established laboratories of quantum optics.

and most of the exp. components are commercial and easy to get.

Possible first QC lab in Taiwan

中央大學：徐子民教授，欒丕綱、陳彥宏助理教授 , 中原大學：周志隆助理教授 NCTS: 徐立義博士中華電信：蔡一鳴博士

以及

願意投入 QC 研究的學者、學生… ..

The End

Key distillation

Classical error correction

Privacy amplification

raw key distilled key

Producing entangled photons

Parametric down conversion.

Franson’s method.(using unbalanced Mach-Zender interferometer)

Parametric down conversion PDC is a process that a pump photon in a

nonlinear crystal has a small probability of splitting into two photons of lower frequency.

p 1 2 conserv. of energy

kp k1 k2 conserv. of momentum

Type II: two converted photons have orthogonal polarizations

PDCbeta-BaB2O4

Franson’s method

1 2D1

D2

D4

D3

12

time correlatedphoton source

s1, s2 ei121, 2

Mach-Zenderinterferometer

By looking in coincidence, we get the entangled state

Single-mode fibers well suited to carry single quanta Transmission loss. geometric phase Even small birefringence will remain a

concern in quantum communication.(both polarization-based & phase-based systems)

Polarization mode dispersion. Polarization-dependent loss. Chromatic dispersion (phase-and time coding).

Q-dot as single-photon detector Conventionally single photons are detected by

multiplying a photo-generated electron using an avalanche process. The Toshiba researchers, in collaboration with Cambridge University, developed a device for detection of single photons based on a GaAs/AlGaAs modulation doped field effect transistor (MODFET) which does not rely on avalanche processes.