POINT-TO-POINT LINK
INTRODUCTION
MULTI-USER
Quantum key distribution can also be used in a multi-user system,
such as a QKD system with more than one receiver (multiple Bobs).
The multiple receivers can be linked to the transmitter via a 1N
splitter. When each photon reaches the splitter, because it
constitutes an indivisible particle, the outcome at any one of the
ports will be random. This is essential to make possible that each
Bob is provided with a unique and verifiably secure key. We have
demonstrated
a
gigahertz-clocked
multi-user
application
comprising a 132 splitter and standard telecom fibre designed for
single-mode operation at  ~ 1.3/1.55m.
Quantum key distribution (QKD) allows two parties to share a
verifiably secure encryption key, guaranteed by the laws of
quantum mechanics.
In these experiments we have used an implementation of the B92
protocol. The binary values ‘0’ and ‘1’ are encoded using two nonorthogonal linear polarisation states at a separation angle of 45º.
In this case the Heisenberg’s Uncertainty Principle states that the
measurement of one property necessarily affects the other. Hence,
if the communication channel is tapped by an eavesdropper,
changes in the photon states will create an increase in the error
that the transmitter (Alice) and receiver (Bob) can quantify when
they perform a comparison of a sub-set of the transmitted bits.
~850nm
fibre
~850nm
fibre
~1/3/1.55m
fibre
Fusion splice
Bob1
1
Bob2
The transmitter and receiver (Alice and Bob) use single mode fibre
at  ~ 850nm, which allows them to take full advantage of the
mature technology of silicon single photon avalanche diodes
(SPAD’s), in conjunction with standard telecommunication fibre.
This system is capable of significantly higher key-exchange rates
than systems using InGaAs/InP single-photon detectors at e.g.  ~
1550nm.
DFB: Distributed feedback laser SPC: Fibre based polarisation controller APD: Avalanche photodiode
VCSEL: Vertical-cavity surface-emitting laser
PBS: Polarising Beam Splitter
“?”: Ambiguous measurement
1,000,000
Bob32
30
1GHz [0.5ns]
QBER (%)
100,000
10,000
1,000
8
25
20
15
10
100MHz [5ns]
100
 = 1550nm
5
1GHz [0.5ns]
100
10
10
2
4
6
8
10
12
14
16
0
2
4
8
10
12
14
16
Multi-user
6
Point-to-point
4
3
2
18
1
Distance (km)
Distance (km)
1
6
7
5
0
0
0
0
20
40
60
80
100
5
IEEE Journal of Quantum Electronics - July 2004
Two vertical-cavity surface-emitting lasers (VCSEL’s) are used at Alice,
as sources of the two encoded states. Both outputs are then
attenuated to achieve an average of ~ 0.1 photons per pulse. This
ensures that the probability of two photons appearing in the same bit
period is less than 0.5%, thus reducing the probability of a photon
splitting eavesdropper attack.
10
15
Distance (km)
Fibre distance (km)
DEVELOPMENT OF 2GHZ SYSTEM
Currently we are investigating methods of improving the performance of the
point-to point QKD system at a clock frequency of 2 GHz. Perkin Elmer
commercially available SPAD’s were used for the 100 MHz and 1 GHz results
shown above. However these SPAD’s have a jitter of ~ 350 ps which is
insufficient to resolve a data stream at a clock rate of 2 GHz.
QBER (%)
25
20
15
10
Shallow Junction SPAD
Perkin Elmer Commercial Device
5
0
0
2
4
6
8
10
Distance (km)
12
14
16
18
Shallow-junction SPAD’s developed by Professor Sergio Cova’s group at the
Politecnico di Milano in Italy are currently being investigated as detectors in this
system. The initial results shown opposite exhibit a significant improvement in
QBER when the Shallow-junction SPAD is implemented in the QKD system. The
shallow junction SPAD has a temporal response of ~ 35 ps, an order of
magnitude less than the Perkin Elmer commercially available device.
25
20
QBER (%)
30
The  ~ 850 nm single-mode fibre is fusion spliced to the transmission
medium, which is standard telecommunications  ~ 1.3/1.55m optical
fibre.
Two polarising beam splitters (PBS’s) and two SPAD’s are used to filter
and detect the two non-orthogonal polarisation states.
In the point-to-point application, we have achieved net bit rates greater
than 100,000 bits-1 for a 4.2 km transmission range and a
corresponding quantum bit error rate (QBER) of 2.1%, which, to the
best of our knowledge, are the highest key exchange rates
demonstrated in an optical fiber system (IEEE Journal of Quantum
Electronics - July 2004).
Bob4
35
QBER (%)
10000
0
Bob3
100MHz [5ns]
Net bit rate (s-1)
100000
Raw Bit rate (Hz)
Splitter
1x32
Point-to-Point Link at 100MHz and 1 GHz Clock Frequencies
 = 850nm
1000
WDM: Wavelength division multiplexor
Fusion splice
Comparison of Bit Rates of Heriot-Watt’s
 = 850 nm and  = 1550 nm QKD Systems
1000000
SPAD: Single photon avalanche diode
Alice
As the clock frequency of the QKD
system is increased the QBER
becomes
higher
as
a
direct
consequence of the temporal response
of the single photon detector. This
graph illustrates this at a fixed fibre
length of 4km for both the Perkin Elmer
device and the developmental shallowjunction SPAD.
Perkin Elmer Commercial Device
Shallow Junction SPAD
15
10
5
0
250
500
750
1000
1250
1500
Clock Frequency (MHz)
1750
2000