609 research outputs found
Experimental detection of quantum coherent evolution through the violation of Leggett-Garg-type inequalities
We discuss the use of inequalities of the Leggett-Garg type (LGtI) to witness
quantum coherence and present the first experimental violation of this type of
inequalities using a light-matter interfaced system. By separately benchmarking
the Markovian character of the evolution and the translational invariance of
the conditional probabilities, the observed violation of a LGtI is attributed
to the quantum coherent character of the process. These results provide a
general method to benchmark `quantumness' when the absence of memory effects
can be independently certified and confirm the persistence of quantum coherent
features within systems of increasing complexity.Comment: published version, including supplementary materia
Revisiting Bohr's principle of complementarity using a quantum device
Bohr's principle of complementarity lies at the central place of quantum
mechanics, according to which the light is chosen to behave as a wave or
particles, depending on some exclusive detecting devices. Later, intermediate
cases are found, but the total information of the wave-like and particle-like
behaviors are limited by some inequalities. One of them is Englert-Greenberger
(EG) duality relation. This relation has been demonstrated by many experiments
with the classical detecting devices. Here by introducing a quantum detecting
device into the experiment, we find the limit of the duality relation is
exceeded due to the interference between the photon's wave and particle
properties. However, our further results show that this experiment still obey a
generalized EG duality relation. The introducing of the quantum device causes
the new phenomenon, provides an generalization of the complementarity
principle, and opens new insights into our understanding of quantum mechanics.Comment: 5 pages, 4 figure
Limitations of practical multi-photon decoherence-free states
It is shown in this paper that decoherence-free subspace (DFS) of practical
multi-photon polarization can not avoid the exponential decoherence even in the
same extra-environment if the photons are frequency-anticorrelated. The reason
lies in that the condition of collective decoherence is not satisfied in this
case. As an example, the evolution of biphoton's decoherence-free state is
given. Possible solution for feasible multi-photon's DFS state is also given.Comment: 6 pages, no figur
Probabilistic teleportation and entanglement matching
Teleportation may be taken as sending and extracting quantum information
through quantum channels. In this report, it is shown that to get the maximal
probability of exact teleportation through partially entangled quantum
channels, the sender (Alice) need only to operate a measurement which satisfy
an ``entanglement matching'' to this channel. An optimal strategy is also
provided for the receiver (Bob) to extract the quantum information by adopting
general evolutions.Comment: 3.5 pages, No figure
Measurement Induced Quantum Coherence Recovery
We show that measurement can recover the quantum coherence of a qubit in a
non-Markovian environment. The experimental demonstration in an optical system
is provided by comparing the visibilities (and fidelities) of the final states
with and without measurement. This method can be extended to other two-level
quantum systems and entangled states in a non-Markovian evolution environment.
It may also be used to implement other quantum information processing.Comment: 9 pages, 5 figure
Optical realization of universal quantum cloning
Beyond the no-cloning theorem, the universal symmetric quantum cloning
machine was first addressed by Buzek and Hillery. Here, we realized the
one-to-two qubits Buzek-Hillery cloning machine with linear optical devices.
This method relies on the representation of several qubits by a single photon.
We showed that, the fidelities between the two output qubits and the original
qubit are both 5/6 (which proved to be the optimal fidelity of one-to-two
qubits universal cloner) for arbitrary input pure states.Comment: 5 Pages, 2 Figure
Efficient quantum key distribution scheme with nonmaximally entangled states
We propose an efficient quantum key distribution scheme based on
entanglement. The sender chooses pairs of photons in one of the two equivalent
nonmaximally entangled states randomly, and sends a sequence of photons from
each pair to the receiver. They choose from the various bases independently but
with substantially different probabilities, thus reducing the fraction of
discarded data, and a significant gain in efficiency is achieved. We then show
that such a refined data analysis guarantees the security of our scheme against
a biased eavesdropping strategy.Comment: 5 Pages, No Figur
Experimental preparation of Werner state via spontaneous parametric down-conversion
We present an experiment of preparing Werner state via spontaneous parametric
down-conversion and controlled decoherence of photons in this paper. In this
experiment two independent BBO (beta-barium borate) crystals are used to
produce down-conversion light beams, which are mixed to prepare Werner state.Comment: 6 pages, 4 figures and 2 table
Simple scheme for expanding a polarization-entangled W state by adding one photon
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 for generating
n-photon W states using single photons and linear optics.Comment: 3 pages, 2 figure
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