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Nature Communications

All-photonic intercity quantum key distribution

Koji Azuma, Kiyoshi Tamaki, William J. Munro

Published Dec 16, 2015• 74 citations
Publisher

Abstract

Recent field demonstrations of quantum key distribution (QKD) networks hold promise for unconditionally secure communication. However, owing to loss in optical fibres, the length of point-to-point links is limited to a hundred kilometers, restricting the QKD networks to intracity. A natural way to expand the QKD network in a secure manner is to connect it to another one in a different city with quantum repeaters. But, this solution is overengineered unless such a backbone connection is intercontinental. Here we present a QKD protocol that could supersede even quantum repeaters for connecting QKD networks in different cities below 800 km distant. Nonetheless, in contrast to quantum repeaters, this protocol uses only a single intermediate node with optical devices, requiring neither quantum memories nor quantum error correction. Our all-photonic 'intercity' QKD protocol bridges large gaps between the conventional intracity QKD networks and the future intercontinental quantum repeaters, conceptually and technologically.

DOI
10.1038/ncomms10171

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Recent field demonstrations of quantum key distribution (QKD) networks hold promise for unconditionally secure communication. However, owing to loss in optical fibres, the length of point-to-point links is limited to a hundred kilometers, restricting the QKD networks to intracity. A natural way to expand the QKD network in a secure manner is to connect it to another one in a different city with quantum repeaters. But, this solution is overengineered unless such a backbone connection is intercontinental. Here we present a QKD protocol that could supersede even quantum repeaters for connecting QKD networks in different cities below 800 km distant. Nonetheless, in contrast to quantum repeaters, this protocol uses only a single intermediate node with optical devices, requiring neither quantum memories nor quantum error correction. Our all-photonic 'intercity' QKD protocol bridges large gaps between the conventional intracity QKD networks and the future intercontinental quantum repeaters, conceptually and technologically.
It has long been known that quantum networks will enable a whole new range of communication tasks to be undertaken. The simplest is quantum key distribution (QKD) and are commercially available but currently only operate securely over distances around 100 km. A significant advance has been the development of mdiQKD, a scheme where Alice and Bob send one photon at a time to an intermediate node where a Bell measurement is performed. This Bell measurement can only succeed when both Alice and Bob photons arrive at the same time and so the key rate is limited by the exponential losses in both fibres. It limits the practical distance keys can be generated to less than 400km. Spatial or temporal multiplexing is a natural solution to this where one stores the photons that independently arrive from Alice and Bob. Only when the immediate node has both does it perform the Bell measurement. This means we are effectively only limited by fibres losses in one half of the channel. It however means one requires quantum memories at this immediate node, a technically challenging feat and one that changes the general resources used in QKD schemes. In our spatial multiplexed approach, we propose the use of an “all photonic non-destructive measurement (QND)” to herald whether the photon has arrived successfully from either Alice or Bob. Optical switches can them be used to route these photons to the Bell measurement, meaning that we are only limited by the channel loss between either Alice and the immediate node or Bob and the intermediate node, but not both. Further this can achieved without the use of quantum memories at all. Only optical switches, single-photon sources, photon detectors, and passive feed-forward techniques are required. Our approach can be applied naturally to entanglement distribution and so has applications beyond QKD.
In this paper we present our work demonstrating the transfer of quantum cryptographic information over a 78 km optical fiber, as a base for future long distance public quantum links, using a commercial quantum key distribution (QKD) system based on the BBM92 protocol. The system communicates with pairs of polarization-entangled photons generated by the nonlinear optical process of spontaneous parametric down-conversion. The system was put to the test in the laboratory using optical fiber of different lengths up to 112 km and with different link losses up to 29 dB. The QKD system was implemented on single mode standard optical fiber link of 78 km length deployed between the two cities of Braunschweig and Hannover. Characterizations and optimizations of the QKD system and the fiber link have been carried out in order to improve the synchronization, to reduce the quantum bit error rate, and to successfully establish the intercity quantum communication.
Abstract Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 × 10−5 with an average quantum bit error ratio of ~ 0.65% are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.
Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 × 10−5 with an average quantum bit error ratio of ~ 0.65% are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.
Entanglement-based quantum links are the backbone of future quantum internet networks, enabling secure communication between distant cities. Realizing such networks requires addressing multiple practical challenges in long-distance quantum key distribution : time synchronisation, interferometer stabilisation and automation. Here, we report several advances. First, we maintained an operational QKD link continuously for 325hours over 50km between two remotes locations, demonstrating the feasibility of long-duration key generation. We further extended secure key distribution up to a 100km operational link connecting the University of Nice to a ground-based optical station, a setup compatible with future quantum satellite connections. Finally, by employing wavelength demultiplexing to separate photons of entangled pairs, we performed QKD across multiple ITU channels, achieving secure key exchange via the BBM92 protocol and time-energy observables.