Based on the fundamental concept of quantum counterfactuality, we propose a protocol to achieve quantum private database queries, which is a theoretical study of how counterfactuality can be employed beyond counterfactual quantum key distribution (QKD). By adding crucial detecting apparatus to the device of QKD, the privacy of both the distrustful user and the database owner can be guaranteed. Furthermore, the proposed private-database-query protocol makes full use of the low efficiency in the counterfactual QKD, and by adjusting the relevant parameters, the protocol obtains excellent flexibility and extensibility.
Through introducing discrete-time quantum walks on the infinite line and on circles, we present a kind of two-particle interacting quantum walk which has two kinds of interactions. We investigate the characteristics of this kind of quantum walk and the time evolution of the two particles. Then we put forward a kind of quantum Hash scheme based on two-particle interacting quantum walks and discuss their feasibility and security. The security of this kind of quantum Hash scheme relies on the infinite possibilities of the initial state rather than the algorithmic complexity of hard problems, which will greatly enhance the security of the Hash schemes.
The way to compare the efficiencies of different detect strategies (DSs) in the "ping-pong" protocol is studied. The trade-off between information gain and disturbance is calculated and compared for different DSs. The comparison result primely tallies with our intuitional analysis. It is shown that the analysis of this trade-off is a feasible way to compare the performances of different DSs in theory.quantum key distribution, quantum cryptography, quantum secure direct communicationThe task of cryptography is to make secret messages intelligible only for the two legitimate parties of the secret communication, Alice and Bob, and unreadable for other unauthorized users such as Eve. To this end, Alice and Bob have to encrypt their secret messages using a suitable encryption scheme. In 1926, the one-time pad (OTP) cipher was invented by American AT&T Engineer Vernam [1] . It was later shown, by Shannon [2] , that as long as the key is truly random, has the same length as the message, and is never reused, then OTP is perfectly secure. However, there is a problem, called key distribution. Although the public key cryptography accomplishes this task, it is based on computational security. That is, if and when mathematicians or computer scientists come up with fast and clever procedures for resolving mathematical difficult problems such as factoring large integers, the whole privacy of public-key cryptosystems could vanish overnight. Quantum cryptography, which is based on fundamental physical principles, has been proved to be an effective technique for secure key distribution [3][4][5][6] . It overcomes the drawbacks possessed by conventional cryptography and the public key cryptography, and has the vast developing prospect.Boström and Felbinger [7] presented a "ping-pong" communication protocol which can be used both to distribute a secure key (i.e. quantum dey distribution (QKD)) and to transfer information in a deterministic secure manner (i.e., quantum secure direct communication, or QSDC for short).
Cryptanalysis is an important branch in the study of cryptography, including
both the classical cryptography and the quantum one. In this paper we analyze
the security of two three-party quantum key distribution protocols (QKDPs)
proposed recently, and point out that they are susceptible to a simple and
effective attack, i.e. the dense-coding attack. It is shown that the
eavesdropper Eve can totally obtain the session key by sending entangled qubits
as the fake signal to Alice and performing collective measurements after
Alice's encoding. The attack process is just like a dense-coding communication
between Eve and Alice, where a special measurement basis is employed.
Furthermore, this attack does not introduce any errors to the transmitted
information and consequently will not be discovered by Alice and Bob. The
attack strategy is described in detail and a proof for its correctness is
given. At last, the root of this insecurity and a possible way to improve these
protocols are discussed.Comment: 6 pages, 3 figure
A quantum key distribution protocol based on entanglement swapping is proposed. Through choosing particles by twos from the sequence and performing Bell measurements, two communicators can detect eavesdropping and obtain the secure key. Because the two particles measured together are selected out randomly, we need neither alternative measurements nor rotations of the Bell states to obtain security.
The length of signal pulses is finite in practical quantum key distribution. The finite-key analysis of an unconditional quantum key distribution is a burning problem, and the efficient quantum key distribution protocol suitable for practical implementation, measurement-device-independent quantum key distribution (MDI QKD), was proposed very recently. We give the finite-key analysis of MDI QKD, which removes all detector side channels and generates many orders of key rate higher than that of full-device-independent quantum key distribution. The secure bound of the ultimate key rate is obtained under the statistical fluctuations of relative frequency, which can be applied directly to practical threshold detectors with low detection efficiency and highly lossy channels. The bound is evaluated for reasonable values of the observed parameters. The simulation shows that the secure distance is around 10 km when the number of sifted data is 10 10. Moreover the secure distance would be much longer in practice because of some simplified treatments used in our paper.
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