321 research outputs found
Delayed-choice gedanken experiments and their realizations
The wave-particle duality dates back to Einstein's explanation of the
photoelectric effect through quanta of light and de Broglie's hypothesis of
matter waves. Quantum mechanics uses an abstract description for the behavior
of physical systems such as photons, electrons, or atoms. Whether quantum
predictions for single systems in an interferometric experiment allow an
intuitive understanding in terms of the particle or wave picture, depends on
the specific configuration which is being used. In principle, this leaves open
the possibility that quantum systems always either behave definitely as a
particle or definitely as a wave in every experimental run by a priori adapting
to the specific experimental situation. This is precisely what is tried to be
excluded by delayed-choice experiments, in which the observer chooses to reveal
the particle or wave character -- or even a continuous transformation between
the two -- of a quantum system at a late stage of the experiment. We review the
history of delayed-choice gedanken experiments, which can be traced back to the
early days of quantum mechanics. Then we discuss their experimental
realizations, in particular Wheeler's delayed choice in interferometric setups
as well as delayed-choice quantum erasure and entanglement swapping. The latter
is particularly interesting, because it elevates the wave-particle duality of a
single quantum system to an entanglement-separability duality of multiple
systems
Entangling quantum and classical states of light
Entanglement between quantum and classical objects is of special interest in
the context of fundamental studies of quantum mechanics and potential
applications to quantum information processing. In quantum optics, single
photons are treated as light quanta while coherent states are considered the
most classical among all pure states. Recently, entanglement between a single
photon and a coherent state in a free-traveling field was identified to be a
useful resource for optical quantum information processing. However, it was
pointed out to be extremely difficult to generate such states since it requires
a clean cross-Kerr nonlinear interaction. Here, we devise and experimentally
demonstrate a scheme to generate such hybrid entanglement by implementing a
coherent superposition of two distinct quantum operations. The generated states
clearly show entanglement between the two different types of states. Our work
opens a way to generate hybrid entanglement of a larger size and to develop
efficient quantum information processing using such a new type of qubits.Comment: 9 pages, 4 figure
Linear optics quantum Toffoli and Fredkin gates
We design linear optics multiqubit quantum logic gates. We assume the
traditional encoding of a qubit onto state of a single photon in two modes
(e.g. spatial or polarization). We suggest schemes allowing direct
probabilistic realization of the fundamental Toffoli and Fredkin gates without
resorting to a sequence of single- and two-qubit gates. This yields more
compact schemes and potentially reduces the number of ancilla photons. The
proposed setups involve passive linear optics, sources of auxiliary single
photons or maximally entangled pairs of photons, and single-photon detectors.
In particular, we propose an interferometric implementation of the Toffoli gate
in the coincidence basis, which does not require any ancilla photons and is
experimentally feasible with current technology.Comment: 8 pages, 4 figures, RevTeX
Experimental delayed-choice entanglement swapping
Motivated by the question, which kind of physical interactions and processes
are needed for the production of quantum entanglement, Peres has put forward
the radical idea of delayed-choice entanglement swapping. There, entanglement
can be "produced a posteriori, after the entangled particles have been measured
and may no longer exist". In this work we report the first realization of
Peres' gedanken experiment. Using four photons, we can actively delay the
choice of measurement-implemented via a high-speed tunable bipartite state
analyzer and a quantum random number generator-on two of the photons into the
time-like future of the registration of the other two photons. This effectively
projects the two already registered photons onto one definite of two mutually
exclusive quantum states in which either the photons are entangled (quantum
correlations) or separable (classical correlations). This can also be viewed as
"quantum steering into the past"
Multi-photon entanglement and interferometry
Multi-photon interference reveals strictly non-classical phenomena. Its
applications range from fundamental tests of quantum mechanics to photonic
quantum information processing, where a significant fraction of key experiments
achieved so far comes from multi-photon state manipulation. We review the
progress, both theoretical and experimental, of this rapidly advancing
research. The emphasis is given to the creation of photonic entanglement of
various forms, tests of the completeness of quantum mechanics (in particular,
violations of local realism), quantum information protocols for quantum
communication (e.g., quantum teleportation, entanglement purification and
quantum repeater), and quantum computation with linear optics. We shall limit
the scope of our review to "few photon" phenomena involving measurements of
discrete observables.Comment: 71 pages, 38 figures; updated version accepted by Rev. Mod. Phy
Manipulating Transverse Modes of Photons for Quantum Cryptography
Several schemes have been proposed to extend Quantum Key Distribution
protocols aiming at improving their security or at providing new physical
substrates for qubit implementation. We present a toolbox to jointly create,
manipulate and measure qubits stored in polarization and transverse-modes
degrees of freedom of single photons. The toolbox includes local operations on
single qubits, controlled operations between the two qubits and projective
measurements over a wide variety of non-local bases in the four dimensional
space of states. We describe how to implement the toolbox to perform an
extended version of the BB84 protocol for this Hilbert space (ideally
transmitting two key bits per photon). We present the experimental
implementation of the measurement scheme both in the regimes of intense light
beams and with single photons. Thus, we show the feasibility of implementing
the protocol providing an interesting example of a new method for quantum
information processing using the polarization and transverse modes of light as
qubits.Comment: 9 pages, 7 figures, 5 table
Protocol for Counterfactually Transporting an Unknown Qubit
Quantum teleportation circumvents the uncertainty principle using dual
channels: a quantum one consisting of previously-shared entanglement, and a
classical one, together allowing the disembodied transport of an unknown
quantum state over distance. It has recently been shown that a classical bit
can be counterfactually communicated between two parties in empty space,
"Alice" and "Bob". Here, by using our "dual" version of the chained quantum
Zeno effect to achieve a counterfactual CNOT gate, we propose the first
protocol for transporting an unknown qubit counterfactually, that is without
any physical particles travelling between Alice and Bob - no classical channel
and no previously-shared entanglement.Comment: Minor improvement
Quantum Teleportation and Bell's Inequality Using Single-Particle Entanglement
A single-particle entangled state can be generated by illuminating a beam
splitter with a single photon. Quantum teleportation utilizing such a
single-particle entangled state can be successfully achieved with a simple
setup consisting only of linear optical devices such as beam splitters and
phase shifters. Application of the locality assumption to a single-particle
entangled state leads to Bell's inequality, a violation of which signifies the
nonlocal nature of a single particle.Comment: 7 pages including 3 figures in eps-forma
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