Daniel Oblak

Quantum Key Distribution (QKD) enables two parties to establish common secret keys by transmitting bits encoded in quantum systems (qubits), which provides unconditional security. QKD introduces errors during quantum communication, which need to be corrected post-key exchange. Typical error-correcting codes in the context of QKD include Cascade, Low-density parity check (LDPC) codes, and polar codes. In our work, we use polar codes, which are state-of-the-art error-correcting codes meeting the requirements of a QKD system. We provide an implementation of an encoder for polar codes based on reliability sequence, which is computationally efficient and can be implemented in QKD postprocessing. Our work on improving the efficiency of QKD postprocessing is highly relevant for the commercialization of QKD.

We have performed Bell state measurements with time-bin qubits encoded into attenuated laser pulses using SNSPDs. Their short recovery time allows not only projections onto |?-> but also onto |?+> Bell states. Together with their high quantum efficiency, around 80% this yields an 60-fold improvement in the efficiency of BSMs with time-bin qubits compared to standard InGaAs detectors. Our results have an impact on quantum repeaters and measurement device independent QKD.

If two parties were to exploit today’s quantum key distribution (QKD) systems, they would be limited to being at most ~100 km apart [1]. It is possible to overcome this limit with a quantum repeater that exploits quantum memories for qubit synchronization [1]. Among other criteria desired for quantum memories, simultaneous storage of multiple qubits (multiplexing) and recall of any desired qubit on-demand is required for a quantum repeater [1,2]. These properties are generally associated with the ability to trigger the re-emission of any previously stored qubit at a desired time [3]. We will argue that this view is too restricted, and that it is possible to build a quantum repeater using quantum memories that allow storage of frequency multiplexed qubits supplemented with frequency-selective read-out on demand. Furthermore we report on measurements exploiting the atomic frequency comb protocol in a Ti:Tm:LiNbO3 waveguide cooled to 3 K [4,5] that shows the required on-demand readout with average fidelities of 0.95 ± 0.03 thereby significantly violating the maximum fidelity of 0.67 possible using a classical memory. Our demonstration constitutes an important step towards the development of a quantum repeater. [1] N. Sangouard et al., Reviews of Modern Physics 83, 33 (2011). [2] A. I. Lvovsky, W. Tittel, and B.C. Sanders, Nature Photonics 3, 706 (2009). [3] C. Simon et al., Phys. Rev. Lett. 98, 190503 (2007). [4] M. Afzelius et al., Phys. Rev. A 79, 052329 (2009). [5] E. Saglamyurek et al., Nature 469, 512 (2011).