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W. -Y. Hwang

Publications and source records attributed to W. -Y. Hwang.

11 recordsLinked to original sources
arXiv2008

Picture Invariance in Quantum Optics

We clarify the controversy over the coherent-state (CS) versus the number-state (NS) pictures in quantum optics. The NS picture is equivalent to the CS picture, as long as the phases $ϕ$ in the laser fields are randomly distributed, as Mølmer argues [\pra {\bf 55}, 3195 (1997)]. However, the claim by Rudolph and Sanders [Phys. Rev. Lett. {\bf 87}, 077903 (2001)] has a few gaps. First, they make an assumption that is not necessarily true in the calculation of a density operator involved with a two-mode squeezed state. We show that there exists entanglement in the density operator without defying the assumption that phases are randomly distributed. Moreover, using a concept of picture-invariance, we argue that it is not that criteria for quantum teleportation are not satisfied. We discuss an analogy between the controversy on the CS versus NS pictures to that on the heliocentric versus geocentric pictures.

quant-ph↗
arXiv2008

No-clicking Event in Quantum Key Distribution

We discuss the `no-clicking' event, which is harmful for legitimate users in Bennett-Brassard 1984 quantum key distribution: We describe an attack where no-clicking events are utilized in the same way as double-clicking events are utilized in the quantum Trojan-pony attack. We discuss how to deal with the no-clicking events. We discuss how to estimate the security of the protocol against the proposed attack by using a formula involved with the fraction of adversarial removals of events.

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arXiv2005

Bell Measurement and Local Measurement in the Modified Lo-Chau Quantum Key Distribution Protocol

We clarify the argument on the how (nonlocal) degenerate Bell measurement can be replaced by local measurements in the modified Lo-Chau quantum key distribution protocol. Discussing security criterion for users, we describe how eavesdropper's refined information on the final state is not helpful. We argue that current discussions on the equivalence of the Bell and the local measurements are not clear. We show how the problem of equivalence can be resolved using the fact that eavesdropper's refined information is not helpful for her.

quant-ph↗
arXiv2005

Bell's Inequality, Random Sequence, and Quantum Key Distribution

The Ekert 91 quantum key distribution (QKD) protocol appears to be secure whatever devices legitimate users adopt for the protocol, as long as the devices give a result that violates Bell's inequality. However, this is not the case if they ignore non-detection events because Eve can make use of the detection-loophole, as Larrson showed. We show that even when legitimate users take into account non-detection events Eve can successfully eavesdrop if the QKD system has been appropriately designed by the manufacturer. A loophole utilized here is that of `free-choice' (or `real randomness'). Local QKD devices with pseudo-random sequence generator installed in them can apparently violate Bell's inequality.

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arXiv2005

Helstrom Theorem by No-Signaling Condition

We prove a special case of Helstrom theorem by using no-signaling condition in the special theory of relativity that faster-than-light communication is impossible.

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arXiv2003

Quantum Key Distribution with High Loss: Toward Global Secure Communication

We propose a decoy-state method to overcome the photon-number-splitting attack for Bennett-Brassard 1984 quantum key distribution protocol in the presence of high loss: A legitimate user intentionally and randomly replaces signal pulses by multi-photon pulses (decoy-states). Then they check the loss of the decoy-states. If the loss of the decoy-states is abnormally less than that of signal pulses, the whole protocol is aborted. Otherwise, to continue the protocol, they estimate loss of signal multi-photon pulses based on that of decoy-states. This estimation can be done with an assumption that the two losses have similar values, that we justify.

quant-ph↗
arXiv2003

Irrversibility of Entanglement Manipulations: Vagueness of the Entanglement of Cost and Entanglement of Distillation

We show that the entanglement of cost and entanglement of distillation can be vague when we consider a more general form of entanglement manipulation in which we collectively deal with not only states of our concern but also other states. We introduce the most general entanglement manipulation in which the formation and distillation can be simultaneously performed. We show that in a certain case entanglement manipulations are reversible with respect to the most general entanglement manipulation. This broadens our scope of vision on the irreversibility of entanglement manipulations.

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arXiv2002

Shor-Preskill Type Security-Proof for the Quantum Key Distribution without Public Announcement of Bases

We give a Shor-Preskill type security-proof to the quantum key distribution without public announcement of bases [W.Y. Hwang et al., Phys. Lett. A 244, 489 (1998)]. First, we modify the Lo-Chau protocol once more so that it finally reduces to the quantum key distribution without public announcement of bases. Then we show how we can estimate the error rate in the code bits based on that in the checked bits in the proposed protocol, that is the central point of the proof. We discuss the problem of imperfect sources and that of large deviation in the error rate distributions. We discuss when the bases sequence must be discarded.

quant-ph↗
arXiv2002

Quantum Gambling Using Three Nonorthogonal States

We provide a quantum gambling protocol using three (symmetric) nonorthogonal states. The bias of the proposed protocol is less than that of previous ones, making it more practical. We show that the proposed scheme is secure against non-entanglement attacks. The security of the proposed scheme against entanglement attacks is shown heuristically.

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