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arXiv:1306.3425 · New Journal of Physics

A complete characterization of the heralded noiseless amplification of photons

Natalia Bruno, Valerio Pini, Anthony Martin, Rob Thew

Published Sep 2, 2013• 18 citations
arXivHTMLPDF

Abstract

Heralded noiseless amplification of photons has recently been shown to provide a means to overcome losses in complex quantum communication tasks. In particular, to overcome transmission losses that could allow for the violation of a Bell inequality free from the detection loophole, for device independent quantum key distribution (DI-QKD). Several implementations of a heralded photon amplifier have been proposed and the first proof of principle experiments realized. Here we present the first full characterization of such a device to test its functional limits and potential for DI-QKD. This device is tested at telecom wavelengths and is shown to be capable of overcoming losses corresponding to a transmission through 20 km of single mode telecom fibre. We demonstrate heralded photon amplifier with a gain >100 and a heralding probability >83%, required by DI-QKD protocols that use the Clauser–Horne–Shimony–Holt inequality. The heralded photon amplifier clearly represents a key technology for the realization of DI-QKD in the real world and over typical network distances.

DOI
10.1088/1367-2630/15/9/093002

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Heralded noiseless amplification of photons has recently been shown to provide a means to overcome losses in complex quantum communication tasks. In particular, to overcome transmission losses that could allow for the violation of a Bell inequality free from the detection loophole, for device independent quantum key distribution (DI-QKD). Several implementations of a heralded photon amplifier have been proposed and the first proof of principle experiments realized. Here we present the first full characterization of such a device to test its functional limits and potential for DI-QKD. This device is tested at telecom wavelengths and is shown to be capable of overcoming losses corresponding to a transmission through 20 km of single mode telecom fibre. We demonstrate heralded photon amplifier with a gain >100 and a heralding probability >83%, required by DI-QKD protocols that use the Clauser–Horne–Shimony–Holt inequality. The heralded photon amplifier clearly represents a key technology for the realization of DI-QKD in the real world and over typical network distances.
Abstract Device-independent quantum key distribution (DI-QKD) represents one of the most fascinating challenges in quantum communication, exploiting concepts of fundamental physics, namely Bell tests of nonlocality, to ensure the security of a communication link. This requires the loophole-free violation of a Bell inequality, which is intrinsically difficult due to losses in fibre optic transmission channels. Heralded photon amplification (HPA) is a teleportation-based protocol that has been proposed as a means to overcome transmission loss for DI-QKD. Here we demonstrate HPA for path entangled states and characterise the entanglement before and after loss by exploiting a recently developed displacement-based detection scheme. We demonstrate that by exploiting HPA we are able to reliably maintain high fidelity entangled states over loss-equivalent distances of more than 50 km.
In device-independent quantum key distribution (DIQKD), the violation of a Bell inequality is exploited to establish a shared key that is secure independently of the internal workings of the QKD devices. An experimental implementation of DIQKD, however, is still awaited, since hitherto all optical Bell tests are subject to the detection loophole, making the protocol unsecured. In particular, photon losses in the quantum channel represent a fundamental limitation for DIQKD. Here we introduce a heralded qubit amplifier based on single-photon sources and linear optics that provides a realistic solution to overcome the problem of channel losses in Bell tests.
We demonstrate postselection free heralded qubit amplification for Time-Bin qubits and single photon states in an all-fibre, telecom-wavelength, scheme that highlights the simplicity, stability and potential for fully integrated photonic solutions. Exploiting high-efficiency superconducting detectors, the gain, fidelity and the performance of the amplifier are studied as a function of loss. We also demonstrate the first heralded single photon amplifier with independent sources. This provides a significant advance towards demonstrating device-independent quantum key distribution as well as fundamental tests of quantum mechanics over extended distances.
The concept of the Device-Independent Quantum Key Distribution (DI-QKD) constitutes the minimalist paradigm for quantum cryptography, in which the security of the distributed secret key is fully assured by the statistical properties of the data being shared between the parties that perform a quantum-based protocol. In particular, the secrecy of the distributed key is ultimately guaranteed not only thanks to the quantum nature of the underlying scheme, but also without making any assumptions about the operation of the devices being employed. Such a conservative approach is possible thanks to the non-local correlations exhibited within the shared data, i.e., the correlations of genuine quantum origin that, due to violation of a particular Bell inequality, cannot be explained by any form of common randomness pre-available to the parties. Such violation, however, must be revealed without performing any post-selection on the data, what would then open the so-called detection loophole and jeopardize the security of the protocol. In spite of the tremendous advances recently made to achieve higher detection efficiencies in Bell-violation experiments, DI-QKD remains a very experimentally difficult task due to the exponential increase of loss in the channel, e.g., implemented with optical fibres, with the distance separating the parties involved. Here, we describe a new and plausible solution to overcome the crucial problem of channel loss in the frame of DI-QKD optical implementations. In particular, we propose a novel protocol inspired by the entanglement swapping schemes, which by the usage of the state-of-the-art (e.g., quantum-dot-based or heralded) single-photon sources has potential, for the first time, to be implementable with current photonic and linear optics technologies. While allowing for any transmission losses that only decrease the rate of the key distribution without creating vulnerability, it tolerates overall detection efficiency at the 90% level even when requiring strict device-independence. We compare our scheme against protocols that involve sources based on spontaneous parametric down-conversion (SPDC), in order to explicitly show and explain why such SPDC-based proposals—even when enhanced by the entanglement swapping or qubit amplification techniques—are then largely outperformed when physical imperfections are rigorously taken into account.
Heralded noiseless amplification based on single-photon sources and linear optics is ideally suited for long-distance quantum communication tasks based on discrete variables. We experimentally demonstrate such an amplifier, operating at telecommunication wavelengths. Coherent amplification is performed with a gain of $G=1.98\ifmmode\pm\else\textpm\fi{}0.20$ for a state with a maximum expected gain G=2G=2G=2. We also demonstrate that there is no need for a stable phase reference between the initial signal state and the local auxiliary photons used by the amplifier. We discuss these results in the context of experimental device-independent quantum key distribution based on heralded qubit amplification, and we highlight several key challenges for its realization.