Christoffer Wittmann

How well does a quantum channel preserve the quantum properties of the transmitted quantum states? We investigate this question in the context of a continuous-variable quantum communication system using the framework of effective entanglement. This framework allows for a quantification of the transmitted entanglement using only coherent states and the well-established double homodyne detection. Experimentally, we investigated fiber channels up to a length of 40 km for a wide range of coherent state amplitudes. Additionally, we induced phase noise to study the quantum-classical transition within the framework. From the measured parameters we are able to identify the optimal point of operation for each quantum channel with respect to the rate of transmitted entanglement. We note that the benchmarking procedure is independent of the physical implementation of the quantum channel and would therefore be a promising candidate for benchmarking of future quantum technologies.

We propose and experimentally demonstrate the tools to implement a Trojan-horse attack to break the security of the commercial quantum cryptosystem ‘Clavis2’ from ID Quantique while it is operating the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocol. Eve launches a bright optical pulse into Bob and analyses the back-reflections that arise from different components and interfaces in Bob. By homodyning the weak coherent state in a suitable back-reflected pulse with an appropriately delayed local oscillator, Eve can get information about Bob’s basis choice with high accuracy. Since the basis choice is essentially the raw secret key in SARG04 protocol, Eve could in principle know the whole key. However, a problem that needs to be circumvented is that of afterpulsing caused by the bright Trojan-horse pulses impinging on the avalanche photodiode based single-photon detectors in Bob. This increases the dark counts or false clicks, thus directly translating into a higher quantum bit error rate (QBER) that could expose Eve. Nonetheless, we show that there exist attack regimes that allow Eve to get a partial information of the key without being discovered, thus breaching the security of the QKD system.