586 research outputs found
Hybrid Qubit gates in circuit QED: A scheme for quantum bit encoding and information processing
Solid state superconducting devices coupled to coplanar transmission lines
offer an exquisite architecture for quantum optical phenomena probing as well
as for quantum computation implementation, being the object of intense
theoretical and experimental investigation lately. In appropriate conditions
the transmission line radiation modes can get strongly coupled to a
superconducting device with only two levels -for that reason called artificial
atom or qubit. Employing this system we propose a hybrid two-quantum bit gate
encoding involving quantum electromagnetic field qubit states prepared in a
coplanar transmission line capacitively coupled to a single charge qubit. Since
dissipative effects are more drastic in the solid state qubit than in the field
one, it can be employed for storage of information, whose efficiency against
the action of an ohmic bath show that this encoding can be readily implemented
with present day technology. We extend the investigation to generate
entanglement between several solid state qubits and the field qubit through the
action of external classical magnetic pulses.Comment: 9 pages, 10 figure
Non-Gaussian states for continuous variable quantum computation via Gaussian maps
We investigate non-Gaussian states of light as ancillary inputs for
generating nonlinear transformations required for quantum computing with
continuous variables. We consider a recent proposal for preparing a cubic phase
state, find the exact form of the prepared state and perform a detailed
comparison to the ideal cubic phase state. We thereby identify the main
challenges to preparing an ideal cubic phase state and describe the gates
implemented with the non-ideal prepared state. We also find the general form of
operations that can be implemented with ancilla Fock states, together with
Gaussian input states, linear optics and squeezing transformations, and
homodyne detection with feed forward, and discuss the feasibility of continuous
variable quantum computing using ancilla Fock states.Comment: 8 pages, 6 figure
The Higgs boson in the MSSM in light of the LHC
We investigate the expectations for the light Higgs signal in the MSSM in
different search channels at the LHC. After taking into account dark matter and
flavor constraints in the MSSM with eleven free parameters, we show that the
light Higgs signal in the channel is expected to be at most at
the level of the SM Higgs, while the from W fusion
and/or the can be enhanced. For the main discovery
mode, we show that a strong suppression of the signal occurs in two different
cases: low or large invisible width. A more modest suppression is
associated with the effect of light supersymmetric particles. Looking for such
modification of the Higgs properties and searching for supersymmetric partners
and pseudoscalar Higgs offer two complementary probes of supersymmetry.Comment: 19 pages, 8 figure
Entanglement degradation of a two-mode squeezed vacuum in absorbing and amplifying optical fibers
Applying the recently developed formalism of quantum-state transformation at
absorbing dielectric four-port devices [L.~Kn\"oll, S.~Scheel, E.~Schmidt,
D.-G.~Welsch, and A.V.~Chizhov, Phys. Rev. A {\bf 59}, 4716 (1999)], we
calculate the quantum state of the outgoing modes of a two-mode squeezed vacuum
transmitted through optical fibers of given extinction coefficients. Using the
Peres--Horodecki separability criterion for continuous variable systems
[R.~Simon, Phys. Rev. Lett. {\bf 84}, 2726 (2000)], we compute the maximal
length of transmission of a two-mode squeezed vacuum through an absorbing
system for which the transmitted state is still inseparable. Further, we
calculate the maximal gain for which inseparability can be observed in an
amplifying setup. Finally, we estimate an upper bound of the entanglement
preserved after transmission through an absorbing system. The results show that
the characteristic length of entanglement degradation drastically decreases
with increasing strength of squeezing.Comment: Paper presented at the International Conference on Quantum Optics and
VIII Seminar on Quantum Optics, Raubichi, Belarus, May 28-31, 2000, 11 pages,
LaTeX2e, 4 eps figure
Continuous Variable Quantum Cryptography using Two-Way Quantum Communication
Quantum cryptography has been recently extended to continuous variable
systems, e.g., the bosonic modes of the electromagnetic field. In particular,
several cryptographic protocols have been proposed and experimentally
implemented using bosonic modes with Gaussian statistics. Such protocols have
shown the possibility of reaching very high secret-key rates, even in the
presence of strong losses in the quantum communication channel. Despite this
robustness to loss, their security can be affected by more general attacks
where extra Gaussian noise is introduced by the eavesdropper. In this general
scenario we show a "hardware solution" for enhancing the security thresholds of
these protocols. This is possible by extending them to a two-way quantum
communication where subsequent uses of the quantum channel are suitably
combined. In the resulting two-way schemes, one of the honest parties assists
the secret encoding of the other with the chance of a non-trivial superadditive
enhancement of the security thresholds. Such results enable the extension of
quantum cryptography to more complex quantum communications.Comment: 12 pages, 7 figures, REVTe
No extension of quantum theory can have improved predictive power
According to quantum theory, measurements generate random outcomes, in stark
contrast with classical mechanics. This raises the question of whether there
could exist an extension of the theory which removes this indeterminism, as
suspected by Einstein, Podolsky and Rosen (EPR). Although this has been shown
to be impossible, existing results do not imply that the current theory is
maximally informative. Here we ask the more general question of whether any
improved predictions can be achieved by any extension of quantum theory. Under
the assumption that measurements can be chosen freely, we answer this question
in the negative: no extension of quantum theory can give more information about
the outcomes of future measurements than quantum theory itself. Our result has
significance for the foundations of quantum mechanics, as well as applications
to tasks that exploit the inherent randomness in quantum theory, such as
quantum cryptography.Comment: 6 pages plus 7 of supplementary material, 3 figures. Title changed.
Added discussion on Bell's notion of locality. FAQ answered at
http://perimeterinstitute.ca/personal/rcolbeck/FAQ.htm
General framework for estimating the ultimate precision limit in noisy quantum-enhanced metrology
The estimation of parameters characterizing dynamical processes is central to
science and technology. The estimation error changes with the number N of
resources employed in the experiment (which could quantify, for instance, the
number of probes or the probing energy). Typically, it scales as 1/N^(1/2).
Quantum strategies may improve the precision, for noiseless processes, by an
extra factor 1/N^(1/2). For noisy processes, it is not known in general if and
when this improvement can be achieved. Here we propose a general framework for
obtaining attainable and useful lower bounds for the ultimate limit of
precision in noisy systems. We apply this bound to lossy optical interferometry
and atomic spectroscopy in the presence of dephasing, showing that it captures
the main features of the transition from the 1/N to the 1/N^(1/2) behaviour as
N increases, independently of the initial state of the probes, and even with
use of adaptive feedback.Comment: Published in Nature Physics. This is the revised submitted version.
The supplementary material can be found at
http://www.nature.com/nphys/journal/v7/n5/extref/nphys1958-s1.pd
Quantum optical coherence can survive photon losses: a continuous-variable quantum erasure correcting code
A fundamental requirement for enabling fault-tolerant quantum information
processing is an efficient quantum error-correcting code (QECC) that robustly
protects the involved fragile quantum states from their environment. Just as
classical error-correcting codes are indispensible in today's information
technologies, it is believed that QECC will play a similarly crucial role in
tomorrow's quantum information systems. Here, we report on the first
experimental demonstration of a quantum erasure-correcting code that overcomes
the devastating effect of photon losses. Whereas {\it errors} translate, in an
information theoretic language, the noise affecting a transmission line, {\it
erasures} correspond to the in-line probabilistic loss of photons. Our quantum
code protects a four-mode entangled mesoscopic state of light against erasures,
and its associated encoding and decoding operations only require linear optics
and Gaussian resources. Since in-line attenuation is generally the strongest
limitation to quantum communication, much more than noise, such an
erasure-correcting code provides a new tool for establishing quantum optical
coherence over longer distances. We investigate two approaches for
circumventing in-line losses using this code, and demonstrate that both
approaches exhibit transmission fidelities beyond what is possible by classical
means.Comment: 5 pages, 4 figure
Classical communication and non-classical fidelity of quantum teleportation
In quantum teleportation, the role of entanglement has been much discussed.
It is known that entanglement is necessary for achieving non-classical
teleportation fidelity. Here we focus on the amount of classical communication
that is necessary to obtain non-classical fidelity in teleportation. We
quantify the amount of classical communication that is sufficient for achieving
non-classical fidelity for two independent 1-bit and single 2-bits noisy
classical channels. It is shown that on average 0.208 bits of classical
communication is sufficient to get non-classical fidelity. We also find the
necessary amount of classical communication in case of isotropic
transformation. Finally we study how the amount of sufficient classical
communication increases with weakening of entanglement used in the
teleportation process.Comment: Accepted in Quantum Info. Proces
Free randomness can be amplified
Are there fundamentally random processes in nature? Theoretical predictions,
confirmed experimentally, such as the violation of Bell inequalities, point to
an affirmative answer. However, these results are based on the assumption that
measurement settings can be chosen freely at random, so assume the existence of
perfectly free random processes from the outset. Here we consider a scenario in
which this assumption is weakened and show that partially free random bits can
be amplified to make arbitrarily free ones. More precisely, given a source of
random bits whose correlation with other variables is below a certain
threshold, we propose a procedure for generating fresh random bits that are
virtually uncorrelated with all other variables. We also conjecture that such
procedures exist for any non-trivial threshold. Our result is based solely on
the no-signalling principle, which is necessary for the existence of free
randomness.Comment: 5+7 pages, 2 figures. Updated to match published versio
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