Several vulnerabilities of single-photon detectors have recently been exploited to compromise the security of quantum-key-distribution (QKD) systems. In this Letter, we report the first proof-of-principle implementation of a new quantum-key-distribution protocol that is immune to any such attack. More precisely, we demonstrated this new approach to QKD in the laboratory over more than 80 km of spooled fiber, as well as across different locations within the city of Calgary. The robustness of our fiber-based implementation, together with the enhanced level of security offered by the protocol, confirms QKD as a realistic technology for safeguarding secrets in transmission. Furthermore, our demonstration establishes the feasibility of controlled two-photon interference in a real-world environment and thereby removes a remaining obstacle to realizing future applications of quantum communication, such as quantum repeaters and, more generally, quantum networks.

We present a detailed description of a widely applicable mathematical model for quantum key distribution (QKD) systems implementing the measurement-device-independent (MDI) protocol. The model is tested by comparing its predictions with data taken using a proof-of-principle, time-bin qubit-based QKD system in a secure laboratory environment (i.e. in a setting in which eavesdropping can be excluded). The good agreement between the predictions and the experimental data allows the model to be used to optimize mean photon numbers per attenuated laser pulse, which are used to encode quantum bits. This in turn allows optimization of secret key rates of existing MDI-QKD systems, identification of rate-limiting components, and projection of future performance. In addition, we also performed measurements over deployed fiber, showing that our system's performance is not affected by environment-induced perturbations.

We assess the overall performance of our quantum key distribution (QKD) system implementing the measurement-device-independent (MDI) protocol using components with varying capabilities such as different single-photon detectors and qubit preparation hardware. We experimentally show that superconducting nanowire single-photon detectors allow QKD over a channel featuring 60 dB loss, and QKD with more than 600 bits of secret key per second (not considering finite key effects) over a 16 dB loss channel. This corresponds to 300 and 80 km of standard telecommunication fiber, respectively. We also demonstrate that the integration of our QKD system into FPGAbased hardware (instead of state-of-the-art arbitrary waveform generators) does not impact on its performance. Our investigation allows us to acquire an improved understanding of the trade-offs between complexity, cost and system performance, which is required for future customization of MDI-QKD. Given that our system can be operated outside the laboratory over deployed fiber, we conclude that MDI-QKD is a promising approach to information-theoretic secure key distribution.

Increasing control of single photons enables new applications of photonic quantum-enhanced technology and further experimental exploration of fundamental quantum phenomena. Here, we demonstrate quantum logic using narrow linewidth photons that are produced under nearly perfect quantum control from a single 87 Rb atom strongly coupled to a high-finesse cavity. We use a controlled-NOT gate integrated into a photonic chip to entangle these photons, and we observe non-classical correlations between events separated by periods exceeding the travel time across the chip by three orders of magnitude. This enables quantum technology that will use the properties of both narrowband single photon sources and integrated quantum photonics, such as networked quantum computing, narrow linewidth quantum enhanced sensing and atomic memories.New applications of single photons will continue to emerge from increased control of both their emission and their subsequent processing with photonic components. Today, intrinsically probabilistic photon sources, such as spontaneous parametric down conversion, are widely used for proof-of-principle photonic quantum technologies. This is because of control over properties such as entanglement [1] and spectrum [2], and increasingly because of the demonstrated compatibility with integrated quantum photonics [3]. But probabilistic sources can only generate high numbers of photons with an overhead of fast switching and optical delays [4]. Deterministic single photon emitters circumvent this overhead whilst providing valuable capabilities such as mediating entangling operations and acting as quantum memories. Here we demonstrate that it is also possible to operate integrated quantum logic with ultra-narrowband photons emitted on-demand from single 87 Rb atoms.Integrated optics is a viable approach to control photons after they have been generated, with increasingly complex, miniature, and programable quantum circuits [3,5,6]. Single photon emitters are being used with photonic quantum logic with the aim of increasing capability. For instance, sequentially emitted photons from single quantum dots have been used to measure the logical truth table of an on-chip controlled-NOT gate (CNOT) [7] and entangled using a bulk-optical CNOT [8]; photons emitted from diamond colour centres have been manipulated with an on-chip interferometer [9]. These emitters can be regarded as artificial atomic systems. In contrast to these, ultra-narrowband indistinguishable photons can be readily obtained on-demand from real single atoms in strong coupling to high-finesse cavities [10][11][12]. These systems emit mutually coherent photons [13], they have been used to generate photonatom entanglement [14] and distant atom-atom entanglement [15], they can be used for quantum memories [16] and they can be used to individually tailor the phase and coherence envelope of each emitted single photon [17,18]. We seek the benefits of both integrated quantum photonic circuits and atom-cavity photon sources.Our demonstration operates i...

Abstract:We experimentally demonstrate a high-efficiency Bell state measurement for time-bin qubits that employs two superconducting nanowire single-photon detectors with short dead-times, allowing projections onto two Bell states, |ψ − and |ψ + . Compared to previous implementations for time-bin qubits, this yields an increase in the efficiency of Bell state analysis by a factor of thirty.