Differential Phase Shift Quantum Key Distribution

Kyo Inoue

Osaka University

Quantum Key Distribution (QKD)

Alice Bob

QKD System

secret key secret key

- QKD provides a secret key for cryptography to legitimate parties.

- The security of the key is guaranteed by quantum mechanics.

Quantum mechanics utilized in QKD

traditional

Non-cloning theory:

A quantum state cannot be perfectly copied without changed.

Uncertainty principle:

copier

Two nonorthogonal physical quantity cannot be precisely measured at

the same time.

PBS

「？」

lin. pol. cir. pol.

PBS

0

1

(H,V) (R,L)

basis

bit

in

fo.

meas.

(H, V)

meas.

(R, L)

BB84 protocol traditional

Polarization coding scheme

Phase coding scheme

Alice Bob

0

1

(0, p)

basis

bit

in

fo.

meas.

(0, p)

meas.

(-p/2, p/2)

Alice Bob

2/π

0

p

-p/2

p/2

q

Contents

(1) Physics utilized in DPS-QKD

(2) Setup & Protocol

(3) Experiments

(4) Security issue

(5) Modified versions

Differential phase shift (DPS) QKD

- A unique QKD protocol different from BB84 -

this presentation

featuring simplicity, practicality, high efficiency, , , .

B

A

B|A|| ba

Wave & particle properties of light

physics utilized in DPS-QKD

(observing which slit)

A||

observation

or B||

B

A

B|A|| ba

attenuation attenuation

normal

eavesdropped

no interference

interference

state change due to observation

Eavesdropping is revealed from no or incorrect interference.

QKD based on particle & wave properties

T

. . . . att.

< 1 photon/pulse Alice

{0, p}

DET-1

DET-2

Bob

coherent pulse source

phase mod.

T

Setup

. . . .

. . . .

Dq = 0 DET-1

Dq = p DET-2

time

A photon is detected occasionally and randomly in time.

Protocol

(1) Signal transmission

(2) Bob Alice: photon detection time

(3) Alice knows which detector clicked at Bob.

(4) Key bits are created according to

DET-1 = “0” DET-2 = “1”

Bob

Eavesdropping

measurement

.. .. .. .. Eve 0.2 photon/pls.

Eve cannot fully measure the phase differences.

security

Alice

Eve

. . . .

no interference

0.2 photon/pls.

The state collapses when measured.

Bob

meas.

Intercept-resend attack

Beam splitting attack

trans.

lossless Alice

Features

- Simple configuration

- Efficient usage of the time domain

- No photon discarded

- Robustness against photon number splitting attack

att.

Alice

{0, p}

{-p/2, p/2}

Bob

light source p/2

0

intensity mod.

phase mod.

(ref.) Phase encoding BB84 using a laser

att.

pulse pattern generator

(rep. rate: 10 GHz)

{0, p}

data generator

time interval analyzer

waveguide

interferometer

Superconducting

Single Photon

Detector

[Takesue et al., Nat. Photon., 1, 343 (2007)]

Long distance experiment

Configuration

17 kbit/s over 100 km.

12 bit/s over 200 km.

Experimental effort

Result

intensity mod.

phase mod. cw-laser

Other experiments

performance detector year organization note

0.33Mbps@15km APD 2007 NTT/Nihon U.

1.3Mbps@10km up-conv. 2009 NTT/Stanford U.

24kbps@100km APD 2011 NTT/Nihon U.

2.1kbps@90km SSPD 2011 NTT/NICT field experiment

(Tokyo QKD-NW)

1.85bps@260km SSPD 2012 U. Sic. Tech. China

Bob

Security issue

Security analysis is challenging for DPS-QKD.

Alice

. . . . . .

DPS

Many key bits are embedded in one quantum state.

(one coherent sequence)

BB84

Bob

One key bit is carried by one quantum state.

Alice

Sequential attack was proposed (2007).

General individual attack was analyzed (2006).

Bob

Alice

. . . . . .

Eve individually attacks each photon in one sequence.

meas. ‥ ‥

Alice trans. lossless

Eve resends a signal when conclusive results are sequentially obtained.

Eve

Bob

Unconditional security was proved for single-photon DPS (2009).

Unconditional security is discussed for block-wise DPS (2012-).

Alice

One photon is super-positioned over a number of pulses.

Bob

Bob

A pulse sequence is assumed to be composed of individual uncorrelated blocks.

Alice

. . . . . .

Security issue in practical system

Bright illumination attack was proposed (2011).

Bob ‥ ‥

Alice

Eve manipulates Bob’s detectors

by sending bright light.

meas.

Eve

trans.

threshold detection

A counter measure was proposed (2013).

interferometer

Bob

Bright light is detected

by using 4 detectors.

Laser light is not a pure coherent state (2013-).

Theoretical analyses assumes ideal coherent states.

However, laser light has the finite spectral linewidth, which is not pure coherent state.

0 40 80 120 k

ey r

ate

(per

puls

e) 10-1

10-2

10-3

10-4

10-5

10-6

distance (km)

ideal 10 kHz 100 kHz 500 kHz 1 MHz

Bob meas. ‥ ‥

Alice trans. lossless

Eve

Umambiguous

State

Discrimination

phase tracking

(Sequential attack)

Differential quadrature phase shift scheme

att.

Alice

{0, p}

{-p/2, p/2}

Bob

p/2

0

phase mod.

coherent light source

. . . .

Slow phase modulation scheme

att.

Alice

{0, p} Bob

phase mod.

coherent light source

. . . . phase mod.

fmod << fdata

Modified DPS protocol

A combination of DPS & BB84

phase randomization

Segmented sequence scheme

att.

Alice

{0, p}

Bob phase mod.

coherent light source

intensity mod.

Blanks are randomly inserted.

Eve cannot distinguish between signal and blank pulses.

Temporal photon distribution is changed when eavesdropped.

Summary

(4) Modified protocol

Efforts to improve the system performance

Differential-phase-shift (DPS) QKD is presented.

(1) Setup & Protocol featuring simplicity, practicality, high key efficiency

(3) Security issues

(2) Experiments A long distance QKD has been achieved.

Security analysis is challenging because the system structure is much

different .