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@fibresystemsmag | www.fibre-systems.com


FEATURE NETWORK SECURITY


vibrations, which were seen as errors.’ Toshiba similarly uses attenuated lasers in its


approach, the T12 protocol, based on DV-QKD. Marco Lucamarini, senior research scientist at Toshiba Research Europe Ltd in Cambridge, UK, emphasises that attenuated lasers are cheaper than single photon sources. Tey are also more efficient for QKD as they emit more pulses per second. T12, like BB84, has two options for encoding 0s and 1s, both involving shiſting the phase difference between two coherent light pulses. One option records 0 when the phase is shiſted 0° and 1 when it is shiſted 180°, and the other counts 0 at 90° and 1 at 270°. Te T12 protocol also modulates intensity, with


The quantum cryptography labs at the Australian National University, whose research QuintessenceLabs is founded on


random bits, and from this you can make a key for cryptographic applications,’ explains ID Quantique’s Bruno Huttner. And, unlike classical optical communication, where information can be stolen from the physical layer by splitting light out, nobody can know what this key is, Huttner underlines. Tat’s because quantum information encoded on a single photon cannot be split. Te only choices are to take it all or leave it all, meaning stolen data completely disappears. Alice and Bob will know there’s an eavesdropper if the error rate gets too high. Rather than the asymmetric RSA algorithm,


QKD systems therefore use symmetric encryption schemes, with ID Quantique’s Cerberis systems employing the Advanced Encryption Standard with a 256-bit key (AES-256). ID Quantique’s initial instruments used photon polarisation, but the company found this property hard to control in optical fibres, Huttner reveals.


Coherent story ID Quantique now uses a protocol devised in collaboration with the University of Geneva called coherent one-way (COW). In COW, Alice sends attenuated coherent laser pulses containing either no photons or that probably have one photon, with very low probability of having more than one. 0 is encoded by a photon-containing pulse followed by an empty pulse, and 1 is encoded by an empty pulse followed by a photon-containing pulse. Interspersed among these are decoy pulses with two consecutive photon-containing pulses. Bob can use the decoy pulses to measure coherence, from which he can determine an error


rate. Separately, Alice can tell Bob which parts of the bit string to use as a key without needing to hear back from Bob. ‘You only need an active optical transmission


component on Alice’s side, which makes the whole system easier to implement,’ Huttner explained. ‘From the error rate you derive the maximum amount of information that could have leaked to an eavesdropper. Ten you use “privacy amplification”, which allows you to reduce this information down to about zero.’ ID Quantique has not yet discovered any eavesdroppers in the real world, he adds. ‘In our deployments, sometimes the number of errors would increase dramatically for a few seconds. In a few cases this was just a nearby train making


Figure 1: A true random number generator


Alice randomly choosing between three possible average photon numbers. Te intensity where 40 per cent of the optical pulses contain photons is the true signal, and intensities where only 4 per cent or 0.1 per cent of pulses contain photons are decoy signals. As in COW, the decoy signals help the recipient estimate the fraction of photons actually detected on the receiver’s side. ‘In the T12 protocol, these two parts are optimised to guarantee the highest possible key rate,’ Lucamarini says. NTT Basic Research Laboratories in


Kanagawa, Japan, has developed two DV-QKD protocols where Alice again exploits phase shiſts to send a bit sequence, explains senior research scientist Hiroki Takesue. ‘In differential phase-shiſt QKD, we measure only the phase difference between adjacent pulses in a continuous pulse train,’ Takesue explained. ‘In round-robin DPS-QKD, we implement a measurement setup by which we can measure the phase differences of any two pulses at random from a packet containing many pulses.’ Te former is much easier to implement than the


QuintessenceLabs uses quantum fluctuations that create differences between the two halves of a split beam of light in its qStream random number generator


Issue 14 • Winter 2017 FIBRE SYSTEMS 21


ANU/Tim Wetherill


Quintessence Labs


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