235 research outputs found
Interference contrast in multi-source few photon optics
Many recent experiments employ several parametric down conversion (PDC)
sources to get multiphoton interference. Such interference has applications in
quantum information. We study here how effects due to photon statistics,
misalignment, and partial distinguishability of the PDC pairs originating from
different sources may lower the interference contrast in the multiphoton
experiments.Comment: 23 pages, 9 figures, journal versio
A posteriori teleportation
The article by Bouwmeester et al. on experimental quantum teleportation
constitutes an important advance in the burgeoning field of quantum
information. The experiment was motivated by the proposal of Bennett et al. in
which an unknown quantum state is `teleported' by Alice to Bob. As illustrated
in Fig. 1, in the implementation of this procedure, by Bouwmeester et al., an
input quantum state is `disembodied' into quantum and classical components, as
in the original protocol. However, in contrast to the original scheme,
Bouwmeester et al.'s procedure necessarily destroys the state at Bob's
receiving terminal, so a `teleported' state can never emerge as a freely
propagating state for subsequent examination or exploitation. In fact,
teleportation is achieved only as a postdiction.Comment: 1 page LaTeX including 1 figure. Scientific Correspondence about:
"Experimental quantum teleportation" Nature 390, 575 (1997
Loss Tolerant Optical Qubits
We present a linear optics quantum computation scheme that employs a new
encoding approach that incrementally adds qubits and is tolerant to photon loss
errors. The scheme employs a circuit model but uses techniques from cluster
state computation and achieves comparable resource usage. To illustrate our
techniques we describe a quantum memory which is fault tolerant to photon loss
Probabilistic quantum multimeters
We propose quantum devices that can realize probabilistically different
projective measurements on a qubit. The desired measurement basis is selected
by the quantum state of a program register. First we analyze the
phase-covariant multimeters for a large class of program states, then the
universal multimeters for a special choice of program. In both cases we start
with deterministic but erroneous devices and then proceed to devices that never
make a mistake but from time to time they give an inconclusive result. These
multimeters are optimized (for a given type of a program) with respect to the
minimum probability of inconclusive result. This concept is further generalized
to the multimeters that minimize the error rate for a given probability of an
inconclusive result (or vice versa). Finally, we propose a generalization for
qudits.Comment: 12 pages, 3 figure
Quantum teleportation and entanglement swapping with linear optics logic gates
We report on the usage of a linear optics phase gate for distinguishing all
four Bell states simultaneously in a quantum teleportation and entanglement
swapping protocol. This is demonstrated by full state tomography of the one and
two qubit output states of the two protocols, yielding average state fidelities
of about 0.83 and 0.77, respectively. In addition, the performance of the
teleportation channel is characterised by quantum process tomography. The non
classical properties of the entanglement swapping output states are further
confirmed by the violation of a CHSH-type Bell inequality of 2.14 on average.Comment: 11 pages, 3 figure
High-Fidelity Teleportation of Independent Qubits
Quantum teleportation is one of the essential primitives of quantum
communication. We suggest that any quantum teleportation scheme can be
characterized by its efficiency, i.e. how often it succeeds to teleport, its
fidelity, i.e. how well the input state is reproduced at the output, and by its
insensitivity to cross talk, i.e. how well it rejects an input state that is
not intended to teleport. We discuss these criteria for the two teleportation
experiments of independent qubits which have been performed thus far. In the
first experiment (Nature {\bf 390},575 (1997)) where the qubit states were
various different polarization states of photons, the fidelity of teleportation
was as high as 0.80 0.05 thus clearly surpassing the limit of 2/3 which
can, in principle, be obtained by a direct measurement on the qubit and
classical communication. This high fidelity is confirmed in our second
experiment (Phys. Rev. Lett. {\bf 80}, 3891 (1998)), demonstrating entanglement
swapping, that is, realizing the teleportation of a qubit which itself is still
entangled to another one. This experiment is the only one up to date that
demonstrates the teleportation of a genuine unknown quantum state.Comment: 13 pages, Latex, 5 figures(eps), to appear in Journal of Modern
Optic
Aligning Sub-national Climate Actions for the new post-Paris Climate Regime
The rise of sub-national actors in global climate governance underscores the
need for clear alignment between these efforts and their national
counterparts. As these sub-national climate actions are filling gaps in
mitigation, adaptation, and financing, among other functions, a critical
question is how these efforts complement or overlap with national climate
pledges. This consideration is particularly important in the context of the
Paris Agreement’s mandate for fiveyear review cycles, where national
governments will be asked to demonstrate progress towards climate mitigation
goals and increase their ambition. In this paper, we argue that alignment –
both vertically between multiple jurisdictions and horizontally with external
networks and actors – is critical to clarifying climate actions between
multiple levels of actors and to maximizing mitigation potential. We use nine
case studies to demonstrate the varying degrees and modes of vertical
integration between subnational and national climate actors. We find that the
case studies embody different styles of vertical alignment, and exhibit
significant variation in the degree and direction of vertical alignment within
each of these modes. We also find that many case studies rely on horizontally-
aligned international networks and coalitions to fill gaps in financial
resources or technical support. As a proof of concept, we demonstrate that an
additional 1 gigaton carbon dioxide equivalent (CO2e) in 2020 can be achieved
in these nine case studies through stronger alignment that makes it possible
to scale sub-national climate actions to the national level. These findings
suggest there may be a missed opportunity to realize greater mitigation
potential by fostering stronger vertical alignment, and enhancing coordination
between horizontal networks of climate action and national governments
Optical Bell-state analysis in the coincidence basis
Many quantum information protocols require a Bell-state measurement of
entangled systems. Most optical Bell-state measurements utilize two-photon
interference at a beam splitter. By creating polarization-entangled photons
with spontaneous parametric down-conversion using a first-order
Hermite-Gaussian pump beam, we invert the usual interference behavior and
perform an incomplete Bell-state measurement in the coincidence basis. We
discuss the possibility of a complete Bell-state measurement in the coincidence
basis using hyperentangled states [Phys. Rev. A, \textbf{58}, R2623 (1998)].Comment: 5 pages, 5 figure
Solving the liar detection problem using the four-qubit singlet state
A method for solving the Byzantine agreement problem [M. Fitzi, N. Gisin, and
U. Maurer, Phys. Rev. Lett. 87, 217901 (2001)] and the liar detection problem
[A. Cabello, Phys. Rev. Lett. 89, 100402 (2002)] is introduced. The main
advantages of this protocol are that it is simpler and is based on a four-qubit
singlet state already prepared in the laboratory.Comment: REVTeX4, 4 page
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