We propose a scheme for quantum key distribution using macroscopic nonclassical pulses of light having of the order ${10}^{6}$ photons per pulse. Sub-shot-noise quantum correlation between the two polarization modes in a pulse gives the necessary sensitivity to eavesdropping that ensures the security of the protocol. We consider pulses of two-mode squeezed light generated by a type-II seeded parametric amplification process. We analyze the security of the system in terms of the effect of an eavesdropper on the bit error rates for the legitimate parties in the key distribution system. We also consider the effects of imperfect detectors and lossy channels on the security of the scheme.
We propose a scheme for quantum key distribution using macroscopic nonclassical pulses of light having of the order 106 photons per pulse. Sub-shot-noise quantum correlation between the two polarization modes in a pulse gives the necessary sensitivity to eavesdropping that ensures the security of the protocol. We consider pulses of two-mode squeezed light generated by a type-II seeded parametric amplification process. We analyze the security of the system in terms of the effect of an eavesdropper on the bit error rates for the legitimate parties in the key distribution system. We also consider the effects of imperfect detectors and lossy channels on the security of the scheme.
We address the effect of classical correlations, introduced to a quantum resource used for the continuous-variable quantum key distribution. The set-up is based on an entangled source with two trusted parties performing homodyne measurements on their modes, thus corresponding to the preparation of squeezed states, while one of the modes is traveling to the remote party through lossy and noisy channel. The security of the scheme is considered against individual and collective eavesdropping attacks. It is shown that the classical correlations added to the entangled source increase the performance of the scheme both quantatively in terms of the secure key rate and qualitatively in terms of the security region with respect to the tolerable excess noise for both types of attacks and the improvement is essentially significant for sources possessing low degree of nonclassicality.
Quantum key distribution (QKD) protocols allow two trusted parties to distribute a cryptographic key which they can further use for the unconditionally secure classical communication. During the last decades the field has grown mature with the commercial prototypes being available. They mostly rely on the faint laser pulses implementing the protocols on the basis of single qubits and qubit pairs. Alternatively, the continuous-variable (CV) protocols based on the multi-photon states of light were developed recently. They are typically built using the quadrature modulation of the coherent or squeezed states and subsequent homodyne detection. In the case of Gaussian modulation the security of the protocols is based on the extremality of Gaussian states, which allows estimating bounds on the leaked information. The Gaussian CV QKD protocols were shown secure for any degree of channel attenuation, but are restricted by the channel noise. Moreover, the applicability of Gaussian CV QKD protocols is limited by the effectiveness of classical post-processing algorithms, used by the trusted parties to process the measured data and distill the cryptographic key. On the other hand, the squeezedstate CV QKD protocols were previously considered ineffective under limited degrees of squeezing, while strong squeezing remains experimentally challenging. In the present work we distinguish between the classical and quantum resources in the Gaussian CV QKD and address the role of nonclassicality in the Gaussian protocols. We demonstrate, that by properly combining squeezed resource and coherent modulation, trusted parties are able to decouple eavesdropper from the channel, thus being able to establish the secure key from any amount of the classical mutual information. Our result shows a very promising path towards the long-distance CV QKD.
Abstract Ideal quantum key distribution (QKD) protocols require perfect single photon sources, detectors, and lossless channels. However, the present technology cannot ensure all of the requirements. Among the variants of practical QKD schemes, decoy state QKD in its present form approaches nearly the theoretical security. In any decoy state method, a compromise is made between the security of the key distribution and the rate of key generation by optimizing the two major protocol parameters: the number of decoy states and their relative intensities, and the length of the data string as well. Use of more number of decoy states and higher range of intensities though improve security, their optimization becomes too complex. Therefore, from practical consideration, some restrictions are to be imposed on the choice of decoy states and their intensities. In addition, the treatment of correlations among the laser pulses in security analysis is not fully understood. Normally, lack of correlations or randomness among the signal states is considered to gain security, but, in a different approach, described in the proposed protocol, additional correlation is introduced to improve key generation rate without compromising security. To do that, a one-way function is shared among the legitimate users to provide a semi-random choice of bases depending on the outcome of the receiver’s detector. The initial communication is made through a set of bits with predefined bases and intensity distribution. Then, with the help of the positive outcome of the receiver’s detector, the basis set as well as the length of the bit string for subsequent communications are computed. This results in correlation among the bases and the relative intensities of the signal states and provides a self-checking mechanism to identify eavesdropping. The security analysis of the protocol provides a low error rate and a relatively high key generation rate.
Conventional quantum key distribution (QKD) protocols include a basis selection process for providing a secure secret key. In contrast, this paper proposes an entanglement-based QKD with no basis selection procedure. Entangled-photon pulse trains with an average photon number less than one per pulse are sent to two legitimate parties, from which a secret key is created utilizing the entanglement nature. Eavesdropping on a transmission line is prevented by a condition of less than one photon per pulse, and sending classically correlated coherent pulses instead of quantum correlated ones is revealed by monitoring coincident count rates.
A new family of two-parameter quantum key distribution protocols is discussed where eavesdropping is detected by using two parameters: bit error rate Q and photon count rate q in control time slots. When a single-photon source is used and mutually orthogonal states are prepared in each basis, the protocol’s maximum tolerable error rate for secure key distribution is the highest, reaching a theoretical upper limit of 50%. When the signal states emitted by the source of attenuated laser light include multiphoton coherent states, the protocol also guarantees secure key distribution over the longest distance as compared to other quantum cryptography systems, up to the channel length for which the channel losses are sufficiently high that all five-photon pulses can be blocked by an eavesdropper.
Quantum correlations or entanglement is a basic ingredient for many applications of quantum information theory.One important application using quantum entanglement exploits the correlation nature of entangled photon states is quantum key distribution, which is proven unbreakable in principle and provides the highest possible security that is impossible in classical information theory. However, generating entangled photon pairs is not a simple task -- only approximately one out of a million pump photons decay into a signal and idler photon pair. This low rate of entangled photon pairs is further reduced by the overhead required in order for the rectification of the inevitable errors due to channel imperfections or caused by potential eavesdroppers. As a consequence, quantum key distribution suffers from a low bit rate, which is in the order of hundreds to thousands bits per second or below. On the other hand, the classical public key distribution does not impose a tight limit on the transmission rate. However, it is subject to the risks of eavesdroppers sitting in the middle of the insecure channel. In this paper, we propose a hybrid key distribution method which uses public key distribution method to generate a raw key, and then uses entanglement assisted communication to modify the raw key by inserting a number of quantum bits in the raw key. Building upon the foundation of the unconditional security of quantum key distribution, we use the privacy amplification to make the affection of inserted bits expand to a whole key. Our quantum entanglement assisted key distribution scheme greatly improves the efficiency of key distribution while without compromising the level of security achievable by quantum cryptography.