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arXiv:1508.01471

Ultrabroadband Quantum-Secured Communication

Quntao Zhuang, Zheshen Zhang, Justin Dove, Franco N. C. Wong, Jeffrey H. Shapiro

Published Aug 6, 2015• 5 citations
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Abstract

We propose a two-way secure-communication protocol in which Alice uses an amplified spontaneous emission source while Bob employs binary phase-shift keying and an optical amplifier. Against an eavesdropper who captures all the light lost in fibers linking Alice and Bob, this protocol is capable of 3.5 Gbps quantum-secured direct communication at 50 km range. If Alice augments her terminal with a spontaneous parametric downconverter and both Alice and Bob add channel monitors, they can realize 2 Gbps quantum key distribution at that range against an eavesdropper who injects her own light into Bob's terminal. Compared with prevailing quantum key distribution methods, this protocol has the potential to significantly increase secure key rates at long distances by employing many ultrabroadband photons per key bit to mitigate channel loss.

DOI
10.48550/arxiv.1508.01471

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We propose a two-way secure-communication protocol in which Alice uses an amplified spontaneous emission source while Bob employs binary phase-shift keying and an optical amplifier. Against an eavesdropper who captures all the light lost in fibers linking Alice and Bob, this protocol is capable of 3.5 Gbps quantum-secured direct communication at 50 km range. If Alice augments her terminal with a spontaneous parametric downconverter and both Alice and Bob add channel monitors, they can realize 2 Gbps quantum key distribution at that range against an eavesdropper who injects her own light into Bob's terminal. Compared with prevailing quantum key distribution methods, this protocol has the potential to significantly increase secure key rates at long distances by employing many ultrabroadband photons per key bit to mitigate channel loss.
A two-way protocol for defeating passive eavesdropping is proposed. For each information bit, Alice sends Bob TTT sec of signal-beam output from a spontaneous parametric down-converter over a pure-loss channel while retaining the idler beam with which it is maximally entangled. Bob imposes a single information bit on the light he receives from Alice via binary phase-shift keying. He then amplifies the modulated beam and sends the resulting light back to Alice over the same pure-loss channel. Even though the loss and amplifier noise destroy any entanglement between the light that Alice receives from Bob and the idler she has retained, she can decode Bob's bit with an error probability that can be orders of magnitude lower than what is achieved by a passive eavesdropper who receives all the photons that are lost en route from Alice to Bob and from Bob to Alice. In particular, Alice and Bob can communicate at 50 Mbit/s over 50 km of low-loss fiber with an error probability of less than ${10}^{\ensuremath{-}6}$ while the passive eavesdropper's error probability must exceed 0.28.
A new paradigm for secure communication, based on quantum illumination, is proposed. Alice uses spontaneous parametric down-conversion to send Bob a set of signal modes over a pure-loss channel while retaining the set of idler modes with which they are maximally entangled. Bob imposes a single information bit on the modes he receives from Alice via binary phase-shift keying. He then adds classical Gaussian noise and sends the noisy modulated modes back to Alice over the same pure-loss channel. Even though the loss and noise destroy any entanglement between the modes that Alice receives from Bob and the idler modes she has retained, she can decode Bob's bit with an error probability that can be orders of magnitude lower than what is achieved by a passive eavesdropper who receives all the photons that are lost en route from Alice to Bob and from Bob to Alice.
This paper presents an investigation of practical quantum secure communication using multi-photon tolerant protocols. Multi-photon tolerant protocols loosen the limit on the number of photons imposed by currently used quantum key distribution protocols. The multi-photon tolerant protocols investigated in this paper are multi-stage protocols that do not require any prior agreement between a sender Alice and a receiver Bob. The security of such protocols stems from the fact that the optimal detection strategies between the legitimate users and the eavesdropper are asymmetrical, allowing Bob to obtain measurement results deterministically while imposing unavoidable quantum noise to the eavesdropper Eve's measurement. Multi-photon tolerant protocols are based on the use of transformations known only to the communicating party applying them i.e. either Alice or Bob. In this paper multi-photon tolerant protocols are used in order to share a key or a message between a sender Alice and a receiver Bob. Thus such protocols can be either used as quantum key distribution (QKD) protocols or quantum communication protocols. In addition, multi-stage protocols can be used to share a key between Alice and Bob, followed by the shared key used as a seed key to a single-stage protocol, called the braiding concept. This paper presents a practical study of multi-photon tolerant multi-stage protocols. Security aspects as well as challenges to the practical implementation are discussed. In addition, secret raw key generation rates are calculated with respect to both losses and distances over a fiber optical channel. It is well-known that raw key generation rates decreases with the increase in channel losses and distances. In this paper, coherent non-decoying quantum states are used to transfer the encoded bits from Alice to Bob. Raw key generation rates are calculated for different average photon numbers µ and compared with the case of µ=0.1, which is the average number of photons used in most single-photon based QKD protocols. Furthermore, an optimal average number of photons to be used within the secure region of the multi-photon tolerant protocols is calculated. It is worth noting that, with the increased key generation rates and distances of communication offered by the multi-photon tolerant protocols, quantum secure communication need not be restricted to quantum key distribution; it can be elevated to attain direct quantum secure communication.
We suggest here a two-point eavesdropping strategy aimed at a two nonorthogonal states protocol of quantum key distribution over a fiber-optic channel. When the single-photon sources and detectors of Alice, Bob and the two Eve are all ideal, the two-point attack can break the two nonorthogonal states protocol if the distance between Alice and Bob is longer than 30 km. However, Bennett's original multi-photon protocol is secure against both two-point and beam splitting attacks, though the protocol is realized with a weak pulsed source.
Quantum key distribution can provide unconditionally secure key exchange for remote users in theory. In practice, however, in most quantum key distribution systems, quantum hackers might steal the secure keys by observing the side channels in the emitted photons, such as the photon frequency spectrum, emission time, propagation direction, spatial angular momentum, and so on. It is hard to prevent such kinds of attacks because side channels may exist in many dimensions of the emitted photons. Here we report an experimental realization of a side-channel-secure quantum key distribution protocol which is not only measurement-device independent, but also immune to all side-channel attacks to the photons emitted from Alice's and Bob's labs. We achieve a secure key rate of 1.73×10^{-6} per pulse through 50 km fiber spools.
We suggest here a two-point eavesdropping strategy to two nonorthogonal states protocol of quantum key distribution over a fiber-optic channel. Suppose that the single-photon sources and detectors of Alice, Bob and Eves are ideal ones, the two-point attack can break the two nonorthogonal states protocol if the distance between Alice and Bob is sufficiently long. For single-photon type B92 protocol, the upper bound of distance is about 30 kilometers. It is interesting that Bennett's original multi-photon protocol is secure against both two-point attack and beam splitting attack, even if the protocol is realized with weak pulse source.