Yan-Xiao Gong

Experimental demonstration of phase measurement precision beating standard quantum limit by projection measurement

We propose and demonstrate experimentally a projection scheme to measure the quantum phase with a precision beating the standard quantum limit. The initial input state is a twin Fock state |N,N proposed by Holland and Burnett (Phys. Rev. Lett., 71 (1993) 1355) but the phase information is extracted by a quantum state projection measurement. The phase precision is about 1.4/N for large photon number N, which approaches the Heisenberg limit of 1/N. Experimentally, we employ a four-photon state from type-II parametric down-conversion and achieve a phase uncertainty of 0.291±0.001 beating the standard quantum limit of for four photons.

Methods for a linear optical quantum Fredkin gate

We consider the realization of a quantum Fredkin gate with only linear optics and single photons. First we construct a heralded Fredkin gate using four heralded controlled-NOT (CNOT) gates. Then we simplify this method to a post-selected one utilizing only two CNOT gates. We also give a possible realization of this method which is feasible with current experimental technology. Another post-selected scheme requires time entanglement of the input photons but needs no ancillary photons.

Observable estimation of entanglement for arbitrary finite-dimensional mixed states

We present observable upper bounds of squared concurrence, which are the dual inequalities of the observable lower bounds introduced in [F. Mintert and A. Buchleitner, Phys. Rev. Lett. 98, 140505 (2007)] and [L. Aolita, A. Buchleitner and F. Mintert, Phys. Rev. A 78, 022308 (2008)]. These bounds can be used to estimate entanglement for arbitrary experimental unknown finite-dimensional states by few experimental measurements on a twofold copy

Four-photon interference with asymmetric beam splitters

\rho\otimes\rho

Dependence of the decoherence of polarization states in phase-damping channels on the frequency spectrum envelope of photons

of the mixed states. Furthermore, the degree of mixing for a mixed state and some properties of the linear entropy also have certain relations with its upper and lower bounds of squared concurrence.

Demonstration of controllable temporal distinguishability in a three-photon state

Two experiments of four-photon interference are performed with two pairs of photons from parametric downconversion with the help of asymmetric beam splitters. The first experiment is a generalization of the Hong-Ou-Mandel interference effect to two pairs of photons while the second one utilizes this effect to demonstrate a four-photon de Broglie wavelength of lambda/4 by projection measurement.

Observation of a generalized bunching effect of six photons

We consider the decoherence of photons suffering in phase-damping channels. By exploring the evolutions of single-photon polarization states and two-photon polarization-entangled states, we find that different frequency spectrum envelopes of photons induce different decoherence processes. A white frequency spectrum can lead the decoherence to an ideal Markovian process. Some color frequency spectrums can induce asymptotical decoherence, while, some other color frequency spectrums can make coherence vanish periodically with variable revival amplitudes. These behaviors result from the non-Markovian effects on the decoherence process, which may give rise to a revival of coherence after complete decoherence.

A simple scheme for expanding a polarization-entangled W state by adding one photon

By using two processes of parametric down-conversion, we are able to generate three photons with controllable degree of temporal distinguishability. This is confirmed by the visibility in a three-photon interference experiment, which can be generalized to provide a practical technique for characterizing the temporal distinguishability of a multi-photon state. This method of quantitative description of multi-photon indistinguishability will have practical implications in the implementation of quantum information protocols.

Linear optical quantum computation with imperfect entangled photon-pair sources and inefficient non-photon-number-resolving detectors

When photons are indistinguishably in the same temporal mode, their detection probability is greatly enhanced due to constructive multiphoton interference, as compared to the case when they are distinguishable. We observed for what is believed to be the first time such a photon bunching effect for six photons. The observed enhancement factor in six-photon coincidence measurement is 17+/-2, which is close to a factor of 20 for an ideal case. Our result confirms that the six photons that we obtain have a high degree of indistinguishability.

Experimental demonstration of quantum contextuality with nonentangled photons

We propose a simple scheme for expanding a polarization-entangled W state. By mixing a single photon and one of the photons in an n-photon W state at a polarization-dependent beam splitter (PDBS), we can obtain an (n + 1)-photon W state after post-selection. Our scheme also opens the door to generating n-photon W states using single photons and linear optics.