Guo-Yong Xiang

Remote preparation of mixed states via noisy entanglement

We present a practical and general scheme of remote preparation for pure and mixed states, which is proposed with an auxiliary qubit and controlled-NOT gate. We discuss the remote state preparation (RSP) in two important types of decoherent channel (depolarizing and dephasing). We realize RSP in the dephasing channel in our experiment by using spontaneous parametric down-conversion, linear optical elements, and single photon detector. Our experimental results match the theoretical prediction well.

Heralded noiseless linear amplification and distillation of entanglement

A noiseless linear amplifier for quantum states of an optical field is demonstrated. The amplifier is also used to enhance entanglement through a technique known as distillation. Such amplification and distillation may be useful for quantum cloning, metrology and communications.

Entanglement-enhanced measurement of a completely unknown optical phase

We demonstrate a method for achieving phase measurements with accuracy beyond the standard quantum limit using entangled states. A sophisticated feedback scheme means that no initial estimate of the phase is required.

Quantum Storage of Orbital Angular Momentum Entanglement in an Atomic Ensemble

Constructing a quantum memory for a photonic entanglement is vital for realizing quantum communication and network. Because of the inherent infinite dimension of orbital angular momentum (OAM), the photons OAM has the potential for encoding a photon in a high-dimensional space, enabling the realization of high channel capacity communication. Photons entangled in orthogonal polarizations or optical paths had been stored in a different system, but there have been no reports on the storage of a photon pair entangled in OAM space. Here, we report the first experimental realization of storing an entangled OAM state through the Raman protocol in a cold atomic ensemble. We reconstruct the density matrix of an OAM entangled state with a fidelity of 90.3%±0.8% and obtain the Clauser-Horne-Shimony-Holt inequality parameter S of 2.41±0.06 after a programed storage time. All results clearly show the preservation of entanglement during the storage.

Quantum State Tomography via Linear Regression Estimation

A simple yet efficient state reconstruction algorithm of linear regression estimation (LRE) is presented for quantum state tomography. In this method, quantum state reconstruction is converted into a parameter estimation problem of a linear regression model and the least-squares method is employed to estimate the unknown parameters. An asymptotic mean squared error (MSE) upper bound for all possible states to be estimated is given analytically, which depends explicitly upon the involved measurement bases. This analytical MSE upper bound can guide one to choose optimal measurement sets. The computational complexity of LRE is O(d4) where d is the dimension of the quantum state. Numerical examples show that LRE is much faster than maximum-likelihood estimation for quantum state tomography.

Teleporting a rotation on remote photons

Quamtum remote rotation allows implement local quantum operation on remote systems with shared entanglement. Here we report an exper- imental demonstration of remote rotation on single photons using linear optical element. And the local dephase is also teleported during the pro- cess. The scheme can be generalized to any controlled rotaion commutes withz. PACS: 03.67.Lx, 03.67.Hk, 03.65.Od, 42.50.-P The development of quantum information in the last two decades promises the powerful applications in information manipulation(1). Using quantum op- erations, some missions, for example large number factoring, can be solved by quantum algorithm effectively, which is impossible for classical computer. Ar- bitrary rotation gates on single qubit and controlled-NOT(CNOT) gate on two qubits are sufficient and necessary, for universal quantum computation(2). These two kinds of operations have been realized locally in laboratory on physical sys- tems such as trapped ion, neutral atom, and so on(1). If the qubits involved in the process are separated in distance(e.g. distributed computation(3)(4)), opera- tions must be implemented nonlocally via sharing entanglement, local operation and classical communication, i.e. remote operation(5)(6)(7)(8)(9)(10). Suppose only two parties, Alice and Bob, are involved in the process(9). The former has the operation gate, while the later has the state to be operated on. The trivial realisation of remote operation can be finished by the bi-directional quantum state teleportation. The target state is teleported from Bob to Alice. Then Alice applies the operation on the state and sends the resulting state back to Bob via another state teleportation. The total resources for this trivial protocol are two maximally-entangled states (2 ebits) shared and two bits of classical communication (2 cbits) in each direction. And theres no restriction for either the target state or the operation. If the gate is restricted in the unitary operation set Ucom ∪ Uanti, where Ucom (Uanti) is the set of single qubit operation commuting (anti-commuting) withz, the resource can be reduced to two ebits shared, two cbits from Bob to Alice and one cbit reversely in the optimal nontrivial scheme. Moreover, if the set is either Ucom or Uanti, the scheme can be simplified by using only one ebit shared and one cbit in each direction(9). Specially, if the operation at Alices site is a collective one with an auxiliary qubit, the two-qubit gate can be performed nonlocally(5)(10). Here we present an experimental demonstration of a remote rotation on sin- gle photons. Unitary rotation commuting withz is implemented remotely on

Demonstration of temporal distinguishability in a four-photon state and a six-photon state.

An experiment is performed to demonstrate the temporal distinguishability of a four-photon state and a six-photon state, both from parametric down-conversion. The experiment is based on a multiphoton interference scheme in a recently discovered projection measurement of a maximally entangled N-photon state. By measuring the visibility of the interference dip, we can distinguish the various scenarios in the temporal distribution of the pairs and, thus, quantitatively determine the degree of temporal distinguishability of a multiphoton state.

Remote implementation of quantum operations

Shared entanglement allows, under certain conditions, the remote implementation of quantum operations. We revise and extend recent theoretical results on the remote control of quantum systems as well as experimental results on the remote manipulation of photonic qubits via linear optical elements.

Scheme for preparation of the W-state by using linear optical elements

We present an experimental scheme of preparing the three-photon W-state in this paper. This scheme is based on a linear optical element, four polarization-entangled photons produced in spontaneous parametric down-conversion and single-photon detection. Effects of the imperfection of optical elements on nonlocality are also discussed.

Robust high-fidelity teleportation of an atomic state through the detection of cavity decay

We propose a scheme for the quantum teleportation of an atomic state based on the detection of cavity decay. The internal state of an atom trapped in a cavity can be disembodiedly transferred to another atom trapped in a distant cavity by measuring interference of polarized photons through single-photon detectors. In comparison with the original proposal by Bose, Knight, Plenio, and Vedral [Phys. Rev. Lett. 83, 5158 (1999)], our protocol of teleportation has a high fidelity of almost unity, and inherent robustness, such as the insensitivity of fidelity to randomness in the atoms position, and to detection inefficiency. All these favorable features make the scheme feasible with the current experimental technology.