Yu-Bo Sheng

Efficient polarization-entanglement purification based on parametric down-conversion sources with cross-Kerr nonlinearity

We present a way for entanglement purification based on two parametric down-conversion (PDC) sources with cross-Kerr nonlinearities. It is comprised of two processes. The first one is a primary entanglement purification protocol for PDC sources with nondestructive quantum nondemolition (QND) detectors by transferring the spatial entanglement of photon pairs to their polarization. In this time, the QND detectors act as the role of controlled-NOT (CNOT) gates. Also they can distinguish the photon number of the spatial modes, which provides a good way for the next process to purify the entanglement of the photon pairs kept more. In the second process for entanglement purification, new QND detectors are designed to act as the role of CNOT gates. This protocol has the advantage of high yield and it requires neither CNOT gates based on linear optical elements nor sophisticated single-photon detectors, which makes it more convenient in practical applications.

Nonlocal entanglement concentration scheme for partially entangled multipartite systems with nonlinear optics (7 pages)

We present a nonlocal entanglement concentration scheme for reconstructing some maximally entangled multipartite states from partially entangled ones by exploiting cross-Kerr nonlinearities to distinguish the parity of two polarization photons. Compared with the entanglement concentration schemes based on two-particle collective unitary evolution, this scheme does not require the parties to know accurately information about the partially entangled states--i.e., their coefficients. Moreover, it does not require the parties to possess sophisticated single-photon detectors, which makes this protocol feasible with present techniques. By iteration of entanglement concentration processes, this scheme has a higher efficiency and yield than those with linear optical elements. All these advantages make this scheme more efficient and more convenient than others in practical applications.

One-step deterministic polarization-entanglement purification using spatial entanglement

We present a one-step deterministic entanglement purification protocol with linear optics and postselection. Compared with the Simon-Pan protocol [C. Simon and J. W. Pan, Phys. Rev. Lett. 89, 257901 (2002)], this one-step protocol has some advantages. First, it can obtain a maximally entangled pair from each photon pair with only one step, instead of improving the fidelity of less-entangled photon pairs by performing the entanglement purification process repeatedly in other protocols. Second, it works in a deterministic way, not a probabilistic one, which greatly reduces the number of entanglement resources needed. Third, it does not require the polarization state be entangled; only spatial entanglement is needed. Moreover, it is feasible with current techniques [J. W. Pan, S. Gasparonl, R. Ursin, G. Weihs, and A. Zellinger, Nature (London) 423, 417 (2003)]. All these advantages make this one-step protocol more convenient than others in quantum-communication applications.

Deterministic entanglement purification and complete nonlocal Bell-state analysis with hyperentanglement

Entanglement purification is a very important element for long-distance quantum communication. Different from all the existing entanglement purification protocols (EPPs) in which two parties can only obtain some quantum systems in a mixed entangled state with a higher fidelity probabilistically by consuming quantum resources exponentially, here we present a deterministic EPP with hyperentanglement. Using this protocol, the two parties can, in principle, obtain deterministically maximally entangled pure states in polarization without destroying any less-entangled photon pair, which will improve the efficiency of long-distance quantum communication exponentially. Meanwhile, it will be shown that this EPP can be used to complete nonlocal Bell-state analysis perfectly. We also discuss this EPP in a practical transmission.

Fault tolerant quantum key distribution based on quantum dense coding with collective noise

We present two robust quantum key distribution protocols against two kinds of collective noise, following some ideas in quantum dense coding. Three-qubit entangled states are used as quantum information carriers, two of which form the logical qubit, which is invariant with a special type of collective noise. The information is encoded on logical qubits with four unitary operations, which can be read out faithfully with Bell-state analysis on two physical qubits and a single-photon measurement on the other physical qubit, not three-photon joint measurements. Two bits of information are exchanged faithfully and securely by transmitting two physical qubits through a noisy channel. When the losses in the noisy channel is low, these protocols can be used to transmit a secret message directly in principle.

Efficient and economic five-party quantum state sharing of an arbitrary m-qubit state

Abstract.We present an efficient and economic scheme for five-party quantum state sharing of an arbitrary m-qubit state with 2m three-particle Greenberger-Horne-Zeilinger (GHZ) states and three-particle GHZ-state measurements. It is more convenient than other schemes as it only resorts to three-particle GHZ states and three-particle joint measurement, not five-particle entanglements and five-particle joint measurements. Moreover, this symmetric scheme is in principle secure even though the number of the dishonest agents is more than one. Its total efficiency approaches the maximal value.

Efficient polarization entanglement concentration for electrons with charge detection

We present an entanglement concentration protocol for electrons based on their spins and their charges. The combination of an electronic polarizing beam splitter and a charge detector functions as a parity check device for two electrons, with which the parties can reconstruct maximally entangled electron pairs from those in a less-entanglement state nonlocally. This protocol has a higher efficiency than those based on linear optics and it does not require the parties to know accurately the information about the less-entanglement state, which makes it more convenient in a practical application of solid quantum computation and communication.

Multipartite entanglement purification with quantum nondemolition detectors

AbstractWe present a scheme for multipartite entanglement purification of quantum systems in a Greenberger-Horne-Zeilinger state with quantum nondemolition detectors (QNDs). This scheme does not require the controlled-not gates which cannot be implemented perfectly with linear optical elements at present, but QNDs based on cross-Kerr nonlinearities. It works with two steps, i.e., the bit-flipping error correction and the phase-flipping error correction. These two steps can be iterated perfectly with parity checks and simple single-photon measurements. This scheme does not require the parties to possess sophisticated single photon detectors. These features maybe make this scheme more efficient and feasible than others in practical applications.

Multipartite electronic entanglement purification with charge detection

Abstract We present a multipartite entanglement purification scheme in a Greenberger–Horne–Zeilinger state for electrons based on their spins and their charges. This scheme works for purification with two steps, i.e., bit-flip error correction and phase-flip error correction. By repeating these two steps, the parties in quantum communication can get some high-fidelity multipartite entangled electronic systems.

Efficient faithful qubit transmission with frequency degree of freedom

We propose an efficient faithful polarization-state transmission scheme by utilizing frequency degree of freedom besides polarization and an additional qubit prepared in a fixed polarization. An arbitrary single-photon polarization state is protected against the collective noise probabilistically. With the help of frequency beam splitter and frequency shifter, the success probability of our faithful qubit transmission scheme with frequency degree of freedom can be 1/2 in principle.