Christian S. Jacobsen

High-rate measurement-device-independent quantum cryptography

Stefano Pirandola, Carlo Ottaviani, Gaetana Spedalieri, Christian Weedbrook, Samuel L. Braunstein, Seth Lloyd, Tobias Gehring, Christian S. Jacobsen, and Ulrik L. Andersen Department of Computer Science, University of York, York YO10 5GH, United Kingdom Department of Physics, University of Toronto, Toronto M5S 3G4, Canada and QKD Corp., 112 College St., Toronto M5G 1L6, Canada MIT – Department of Mechanical Engineering and Research Laboratory of Electronics, Cambridge MA 02139, USA Department of Physics, Technical University of Denmark, Fysikvej, 2800 Kongens Lyngby, Denmark

Reply to 'Discrete and continuous variables for measurement-device-independent quantum cryptography'

Stefano Pirandola, Carlo Ottaviani, Gaetana Spedalieri, Christian Weedbrook, Samuel L. Braunstein, Seth Lloyd, Tobias Gehring, Christian S. Jacobsen, and Ulrik L. Andersen Computer Science and York Centre for Quantum Technologies, University of York, York YO10 5GH, United Kingdom Department of Physics, University of Toronto, Toronto M5S 3G4, Canada Department of Mechanical Engineering and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge MA 02139, USA Department of Physics, Technical University of Denmark, Fysikvej, 2800 Kongens Lyngby, Denmark

Continuous-variable quantum computing on encrypted data

The ability to perform computations on encrypted data is a powerful tool for protecting a clients privacy, especially in todays era of cloud and distributed computing. In terms of privacy, the best solutions that classical techniques can achieve are unfortunately not unconditionally secure in the sense that they are dependent on a hackers computational power. Here we theoretically investigate, and experimentally demonstrate with Gaussian displacement and squeezing operations, a quantum solution that achieves the security of a users privacy using the practical technology of continuous variables. We demonstrate losses of up to 10 km both ways between the client and the server and show that security can still be achieved. Our approach offers a number of practical benefits (from a quantum perspective) that could one day allow the potential widespread adoption of this quantum technology in future cloud-based computing networks.

Continuous Variable Quantum Key Distribution with a Noisy Laser

Existing experimental implementations of continuous-variable quantum key distribution require shot-noise limited operation, achieved with shot-noise limited lasers. However, loosening this requirement on the laser source would allow for cheaper, potentially integrated systems. Here, we implement a theoretically proposed prepare-and-measure continuous-variable protocol and experimentally demonstrate the robustness of it against preparation noise stemming for instance from technical laser noise. Provided that direct reconciliation techniques are used in the post-processing we show that for small distances large amounts of preparation noise can be tolerated in contrast to reverse reconciliation where the key rate quickly drops to zero. Our experiment thereby demonstrates that quantum key distribution with non-shot-noise limited laser diodes might be feasible.

Complete elimination of information leakage in continuous-variable quantum communication channels

In all lossy communication channels realized to date, information is inevitably leaked to a potential eavesdropper. Here we present a communication protocol that does not allow for any information leakage to a potential eavesdropper in a purely lossy channel. By encoding information into a restricted Gaussian alphabet of squeezed states we show, both theoretically and experimentally, that the Holevo information between the eavesdropper and the intended recipient can be exactly zero in a purely lossy channel while minimized in a noisy channel. This result is of fundamental interest, but might also have practical implications in extending the distance of secure quantum key distribution.Quantum communication: no information for the eavesdroppersA new study demonstrates that information leakage from purely lossy quantum channels can be prevented without the need for data post-processing. Christian Jacobsen and colleagues from the Technical University of Denmark and the Palacky University of Czech Republic have shown that communicating quantum information encoded into the squeezed states of a restricted Gaussian alphabet decouples any potential eavesdropper from the communication. The authors experimentally demonstrate complete elimination of information leakage for photonic continuous-variable quantum-key distribution using squeezed states of light and homodyne detection. Through this scheme, security is achieved without the need of burdensome data post-processing such as privacy amplification – common to standard quantum-key distribution protocols. In the presence of noise, information leakage cannot be entirely eliminated, but can still be efficiently minimised. These findings will be important to future developments in secure quantum communications.

Quantum cryptography with an ideal local relay

We consider two remote parties connected to a relay by two quantum channels. To generate a secret key, they transmit coherent states to the relay, where the states are subject to a continuous-variable (CV) Bell detection. We study the ideal case where Alices channel is lossless, i.e., the relay is locally in her lab and the Bell detection is perfomed with unit efficiency. This configuration allows us to explore the optimal performances achievable by CV measurement-device-independent quantum key distribution. This corresponds to the limit of a trusted local relay, where the detection loss can be re-scaled. Our theoretical analysis is confirmed by an experimental simulation where 10-4 secret bits per use can potentially be distributed at 170km assuming ideal reconciliation.

Correction: Jacobsen, C.S., et al. Continuous Variable Quantum Key Distribution with a Noisy Laser. Entropy 2015, 17, 4654-4663

This errata contains the mentioned plots where the revised expressions have been applied, such that the replacement for Figure 2 in [1] is shown in Figure 1, and Figure 4 in [1] is shown in Figure 2. We keep the corresponding (b) panels for comparison. We note that the corrections only reinforce the conclusions of our paper, which are that reverse reconciliation is vulnerable to preparation noise, while direct reconciliation is not.