Hanqing Liu

Quantum Dots can generate on-demand, highly indistinguishable photons for quantum information purposes. We make use of a high performance quantum dot with dynamic polarization modulation in order to experimentally implement the BB84 protocol. State preparation is achieved with a custom built pulse-pattern generator and a 10^5-bit random sequence controlling an electro-optical modulator and an 80 MHz repetition rate. These are then detected in a 4 state polarization analyzer with which we record a QBER of 2.9%. In order to gauge the performance of the protocol, we make use of advanced numerical techniques to compute lower bounds on secure key rates. These techniques allow us to demonstrate the security and performance of this protocol while considering device imperfections such as the source on Alice's side and unequal detection efficiencies on Bob's side. Our detailed analysis of the protocol’s performance including device imperfections is an important step towards practical implementations of QKD.

In quantum cryptography, fundamental laws of quantum physics are exploited to enhance the security of cryptographic tasks. Quantum key distribution (QKD) is by far the most studied protocol to date, enabling the establishment of a secret key between trusted parties. Many practical use-cases in communication networks, however, involve parties who do not know or trust each other. The most fundamental quantum cryptographic building block in such a distrustful setting is quantum coin flipping, which, in its original version has been proposed in the seminal work by C.H. Bennett and G. Brassard in 1984. Interestingly, few experimental studies of quantum coin flipping have been reported to date using weak coherent pulses (WCPs), sources based on spontaneous parametric down conversion (SPDC) exploiting entanglement, or heralded single-photon states. Here, we experimentally implement a quantum strong coin flipping (QSCF) protocol using single-photon states and demonstrate an advantage compared to both classical realizations and implementations using faint laser pulses. We achieve this by employing a state-of-the-art deterministic single-photon source based on the Purcell-enhanced emission of a semiconductor quantum dot in combination with fast polarization-state encoding with sufficiently low quantum bit error ratio. The reduced multi-photon emission of the single-photon source yields a smaller bias of the coin flipping protocol compared to an attenuated laser implementation, both in simulations and in the experiment. By demonstrating a single-photon quantum advantage in a cryptographic primitive beyond QKD, our work represents an important advance towards the implementation of complex cryptographic tasks in a future quantum internet.