Practical quantum key distribution protocol without monitoring signal disturbance

Abstract

Quantum cryptography exploits the fundamental laws of quantum mechanics to provide a secure way to exchange private information. Such an exchange requires a common random bit sequence, called a key, to be shared secretly between the sender and the receiver. The basic idea behind quantum key distribution (QKD) has widely been understood as the property that any attempt to distinguish encoded quantum states causes a disturbance in the signal. As a result, implementation of a QKD protocol involves an estimation of the experimental parameters influenced by the eavesdropper's intervention, which is achieved by randomly sampling the signal. If the estimation of many parameters with high precision is required, the portion of the signal that is sacrificed increases, thus decreasing the efficiency of the protocol. Here we propose a QKD protocol based on an entirely different principle. The sender encodes a bit sequence onto non-orthogonal quantum states and the receiver randomly dictates how a single bit should be calculated from the sequence. The eavesdropper, who is unable to learn the whole of the sequence, cannot guess the bit value correctly. An achievable rate of secure key distribution is calculated by considering complementary choices between quantum measurements of two conjugate observables. We found that a practical implementation using a laser pulse train achieves a key rate comparable to a decoy-state QKD protocol, an often-used technique for lasers. It also has a better tolerance of bit errors and of finite-sized-key effects. We anticipate that this finding will give new insight into how the probabilistic nature of quantum mechanics can be related to secure communication, and will facilitate the simple and efficient use of conventional lasers for QKD.

Detector-decoy quantum key distribution without monitoring signal disturbance

“…Intuitively, Eve seems to have some control over the generation of index i, though it was shown by a virtual measurement scheme proposed in Ref. [10] that her control is in fact rather limited. Therefore, it is possible to ignore the signal disturbance and analyze errors, namely the privacy amplification, based only on the outcomes of Alice and Bob.…”

“…In standard calculations, it holds that H ER = h(e b ) and H PA = h(e ph ), where h(x) is the Shannon entropy h(x) = −x log 2 x − (1 − x) log 2 (1 − x), e b and e ph are the bit error rate and phase error rate. In our computations, we inspect specifically the key rate per pulse, with its formula written as [10] …”

“…Meanwhile, in general QKD protocols, the length of secure key K 1 is obtained after subtracting the bits used for error reconciliation and privacy amplification [10], written as…”

“…Therefore, for qubit-based quantum communication protocols [1][2][3][4][5][6][18][19][20][21][22], the decoy state method [4,5] has solved the multiphoton problems at the source with great enhancements, while squash models [23][24][25][26] are proposed to solve problems at the detector. In the RRDPS-QKD protocol [10], a practical vulnerability lies in that Bob's measurement device requires experimentally challenging detectors that are able to discriminate between single photons from two or more photons, i.e., photon number resolving detectors. Laboratories are presently equipped with conventional threshold photon detectors, and while actual PNR detectors are slowly entering commercial use, they are still highly temperature sensitive and can only resolve a limited number of photons received [27].…”

“…Recently, Sasaki, Yamamoto, and Koashi proposed a ground-breaking approach, a qudit-based protocol, i.e., the round-robin differential phase-shift QKD (RRDPS-QKD) protocol that does not require disturbance monitoring since the limit on leaked information, namely, the portion of the sifted key subjected to privacy amplification, can be acquired in advance and maintains a high tolerance of noise [10]. Since the RRDPS-QKD was proposed, it has been studied both theoretically [11][12][13] and experimentally [11,[14][15][16].…”