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. 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). 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 non- orthogonal 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 Fibre distance (km) Raw Bit rate (Hz) = 850nm = 1550nm Comparison of Bit Rates of Heriot-Watt’s = 850 nm and = 1550 nm QKD Systems 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 Distance (km) QBER (%) 100MHz [5ns] 1GHz [0.5ns] ,000 10, ,000 1,000, Distance (km) 100MHz [5ns] 1GHz [0.5ns] Net bit rate (s -1 ) DFB: Distributed feedback laserSPC: Fibre based polarisation controllerAPD: Avalanche photodiode VCSEL: Vertical-cavity surface-emitting laser PBS: Polarising Beam SplitterSPAD: Single photon avalanche diodeWDM: Wavelength division multiplexor “?”: Ambiguous measurement Point-to-Point Link at 100MHz and 1 GHz Clock Frequencies IEEE Journal of Quantum Electronics - July 2004 POINT-TO-POINT LINK 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 Distance (km) QBER (%) Multi-user Point-to-point MULTI-USER INTRODUCTION 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. 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. Splitter 1x32 Alice 1 Bob 2 Bob 4 ~850nm fibre Fusion splice Bob 32 Bob 1 Bob 3 ~850nm fibre ~1/3/1.55  m fibre DEVELOPMENT OF 2GHZ SYSTEM 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 shallow- junction SPAD.