arXiv subjects
Publications and source records attributed to Wang Xiang-bin.
We propose a very simple scheme to test the quantum enatanglement swapping in a priori. In the scheme, we only assume the photon detector can distinguish the vacuum and non-vacuum Fock state rather than the exact Fock number states.
We propose a scheme to produce the maximally two photon polarization entangled state(EPR state) with single photon sources and the linear optics devices. In particular, our scheme requires the photon detectors only to distinguish the vacuum and non-vacuum Fock number states. A sophisticated photon detector distinguishing one or two photon states is unnecessary.
Two remote parties, Alice and Bob initially share some non-maximally entangled states. Through the entanglement concentration by local operation and classical communication(LOCC), they may obtain an outcome of maximally entangled states in the price of decreasing the number of pairs shared by them. Recently, entanglement concentration have been experimently demonstrated by post-selection (T. Yamamoto et al, Nature, 421, 343(2003) and Z. Zhao et al, quant-ph/0301118). Here we give a modified scheme which can be used for the entanglement concentration without any post selection by using only practically existing linear optical devices. In particular, a sophisticated photon detector to distinguish one photon or two photons is not required. Our scheme can be used to really produce the event-ready maximally entangled pairs through LOCC provided that the requested raw pairs are supplied deterministically. A detailed experimental plan with spontaneous parametric down conversion(SPDC) is shown.
An explicit formula is given for the quantity of entanglement in the output state of a beam splitter, given the squeezed vacuum states input in each mode.
We consider a strategic problem of the Evesdropping to quantum key distribution. Evesdropper hopes to obtain the maxium information given the disturbance to the qubits is often For this strategy, the optimized individual attack have been extensively constructed under various conditions. However, it seems a difficult task in the case of coherent attack, i.e., Eve may treat a number of intercepted qubits collectively, including the collective unitary transformations and the measurements. It was conjectured by Cirac and Gisin that no coherent attack can be more powerful for this strategy for BB84 protocol. In this paper we give a general conclusion on the role of coherent attacks for the strategy of maxmizing the information given the disturbance. Suppose in a quantum key distribution(QKD) protocol, all the transmitted bits from Alice are independent and only the individual disturbances to each qubits are examined by Alice and Bob. For this type of protocols(so far almost all QKD protocols belong to this type), in principle no coherent attack is more powerful than the product of optimized individual attack to each individual qubits. All coherent attacks to the above QKD protocols can be disregarded for the strategy above.
So far all the proven unconditionally secure prepare and measure protocols for the quantum key distribution(QKD) must solve the very complex problem of decoding the classical CSS code. In the decoding stage, Bob has to compare his string with an exponentially large number of all the strings in certain code space to find out the closest one. Here we have spotted that, in an entanglement purification protocol(EPP), the random basis in the state preparation stage is only necessary to those check qubits, but uncessary to the code qubits. In our modified two way communication EPP(2-EPP) protocol, Alice and Bob may first take all the parity checks on $Z$ basis to reduce the bit flip error to strictly zero with a high probability, e.g., $1-2^{-30}$, and then use the CSS code to obtain the final key. We show that, this type of 2-EPP protocol can be reduced to an equivalent prepare and measure protocol. In our protocol, the huge complexity of decoding the classical CSS code is totally removed.
We show that the existing argument on the non-classicality of the continuous variable quantum teleportation (CVQT) experiment by the average fidelity criterion is incomplete therefore so far it is still unclear whether the CVQT experiment(Furusawa et al, Science,282, 706(1998)) is really non-classical.
We show that the Innsbruck quantum teleportation experiment(Bouwmeester D et al, Nature 390, 575(1997)) can be modified by using the polarized beam splitter. In this modified scheme, Bob does not have to do any meassurement and an unknown state from Victor can be verified to be teleported to Bob from Alice by the currently exisiting technology, i.e., by using the normal photon detectors which do not distinguish one photon and two photons.
We propose a simple scheme to produce the polarization entangled photon pairs without the type II phase match. The same scheme can also be used to produce the macroscopic entangled photon states in both photon number space and the polarization space(A. Lamas-Linares, J.C. Howell and D. Bouwmeester, Nature, 412887(2001)). Advantages and applications of our scheme in quantum key distribution are discussed.
It is conjectured that the an entanglement output states from a beam splitter requires the nonclassicality in the input state(M.S. Kim, W. Son, V. Buzek and P. L. Knight, Phys. Rev. A, 65, 032323(2002)). Here we give a proof for this conjecture.
Almost all novel observable phenomena in quantum optics are related to the quantum coherence. The coherence here is determined by the relative phase inside a state. Unfortunately, so far all the relevant experimental results in quantum optics are insensitive to the phase information of the coherent state. Lack of phase information may cause serious consequences in many problems in quantum optics. For example, an ensemble of two mode squeezed states is a classical ensemble if the phase in each state is totally random; but it is a non-classical ensemble if the phase in each state is fixed. As a timly application, verification of this type of phase information in an ensemble of two mode squeezed states from the conventional laser is crucial to the validity of the continuous variavable quantum teleportation(CVQT) experiment. Here we give a simple scheme to distinguish two different ensemble of states: the Rudolph-Sanders ensemble, by which each squeezed states emitted has a uniform distribution from $0-2π$ on the phase value; and the van Enk-Fuchs ensemble, which emmits identical states with a fixed(but unknown) phase for every state. We believe our proposal can help to give a clear picture on whether the existing two mode squeezed states so far are indeed non-classical states which can be used as the entanglement resource.
The paper is withdrawn.
The paper has been withdrawn by the author since the protocol is not new. It is just the oldest version of BB84.