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 .