Practical issues in quantum-key-distribution postprocessing

Quantum key distribution (QKD) is a secure key generation method between two distant parties by wisely exploiting properties of quantum mechanics. In QKD, experimental measurement outcomes on quantum states are transformed by the two parties to a secret key. This transformation is composed of many logical steps (as guided by security proofs), which together will ultimately determine the length of the final secret key and its security. We detail the procedure for performing such classical postprocessing taking into account practical concerns (including the finite-size effect and authentication and encryption for classical communications). This procedure is directly applicable to realistic QKD experiments and thus serves as a recipe that specifies what postprocessing operations are needed and what the security level is for certain lengths of the keys. Our result is applicable to the BB84 protocol with a single or entangled photon source. © 2010 The American Physical Society.published_or_final_versio

Universal squash model for optical communications using linear optics and threshold detectors

Transmission of photons through open-air or optical fibers is an important primitive in quantum-information processing. Theoretical descriptions of this process often consider single photons as information carriers and thus fail to accurately describe experimental implementations where any number of photons may enter a detector. It has been a great challenge to bridge this big gap between theory and experiments. One powerful method for achieving this goal is by conceptually squashing the received multiphoton states to single-photon states. However, until now, only a few protocols admit a squash model; furthermore, a recently proven no-go theorem appears to rule out the existence of a universal squash model. Here we show that a necessary condition presumed by all existing squash models is in fact too stringent. By relaxing this condition, we find that, rather surprisingly, a universal squash model actually exists for many protocols, including quantum key distribution, quantum state tomography, Bell's inequality testing, and entanglement verification. © 2011 American Physical Society.published_or_final_versio

Unconditional security proof of a deterministic quantum key distribution with a two-way quantum channel

In a deterministic quantum key distribution (DQKD) protocol with a two-way quantum channel, Bob sends a qubit to Alice who then encodes a key bit onto the qubit and sends it back to Bob. After measuring the returned qubit, Bob can obtain Alice's key bit immediately, without basis reconciliation. Since an eavesdropper may attack the qubits traveling on either the Bob-Alice channel or the Alice-Bob channel, the security analysis of DQKD protocol with a two-way quantum channel is complicated and its unconditional security has been controversial. This paper presents a security proof of a single-photon four-state DQKD protocol against general attacks. © 2011 American Physical Society.published_or_final_versio

Solution to Time-energy Costs of Quantum Channels

We derive a formula for the time-energy costs of general quantum channels proposed in [Phys. Rev. A 88, 012307 (2013)]. This formula allows us to numerically find the time-energy cost of any quantum channel using positive semidefinite programming. We also derive a lower bound to the time-energy cost for any channels and the exact the time-energy cost for a class of channels which includes the qudit depolarizing channels and projector channels as special cases.postprin

Universally composable and customizable post-processing for practical quantum key distribution

In quantum key distribution (QKD), a secret key is generated between two distant parties by transmitting quantum states. Experimental measurements on the quantum states are then transformed to a secret key by classical post-processing. Here, we propose a construction framework in which QKD classical post-processing can be custom made. Though seemingly obvious, the concept of concatenating classical blocks to form a whole procedure does not automatically apply to the formation of a quantum cryptographic procedure since the security of the entire QKD procedure rests on the laws of quantum mechanics and classical blocks are originally designed and characterized without regard to any properties of these laws. Nevertheless, we justify such concept of concatenating classical blocks in constructing QKD classical post-processing procedures, along with a relation to the universal-composability-security parameter. Consequently, effects arising from an actual QKD experiment, such as those due to the finiteness of the number of signals used, can be dealt with by employing suitable post-processing blocks. Lastly, we use our proposed customizable framework to build a comprehensive generic recipe for classical post-processing that one can follow to derive a secret key from the measurement outcomes in an actual experiment. © 2010 Elsevier Ltd. All rights reserved.postprin

Genotype analyses using SNP (using MALDI-TOF Mass spectrometry) and STR (microsatellite) markers in the determination of zygosity status of Chinese Twins

It has been argued that single nucleotide polymorphisms (SNPs) is a very useful tool complementary to microsatellite analyses in the near future due to its relative ease of automation and interpretation. However, because allele frequencies can vary greatly among populations, it is important to validate and identify the informative SNPs that are useful for sample identification in biomedical and epidemiological studies. We have genotyped 768 individuals (384 pairs of twins) with a panel of SNPs based mostly on the SNPs chosen by an earlier work conducted by Lee et al., 2005 for Koreans, which we hypothesized to be useful for our Chinese subjects. The MALDI-TOF mass spectrometry based iPLEX Gold assay on the MassARRAY® Platform method using Sequenom was used for determination of the zygosity status of the twins. The ABI microsatellite kit (AmpFISTR identifiler PCR amplifcation kit) for measuring 16 loci was used to confirm the zygosity status. We found that some of the SNPs (2/22) used in Lee’s paper were not suitable for our Hong Kong Chinese samples. The zygosity status as determined by the mass spectrometry method can be validated with microsatellite method using 76 samples. In summary, we have studied a panel of 25 SNPs on their effectiveness in distinguishing identical and non-identical twins (768 individuals) for the Hong Kong Chinese population using the iPLEX method of Sequenom. It is important to optimize SNP panels for genotyping depending on the population and the platform used for the studies