2,915 research outputs found
Optimized pulse sequences for suppressing unwanted transitions in quantum systems
We investigate the nature of the pulse sequence so that unwanted transitions
in quantum systems can be inhibited optimally. For this purpose we show that
the sequence of pulses proposed by Uhrig [Phys. Rev. Lett. \textbf{98}, 100504
(2007)] in the context of inhibition of environmental dephasing effects is
optimal. We derive exact results for inhibiting the transitions and confirm the
results numerically. We posit a very significant improvement by usage of the
Uhrig sequence over an equidistant sequence in decoupling a quantum system from
unwanted transitions. The physics of inhibition is the destructive interference
between transition amplitudes before and after each pulse.Comment: 5 figure
A rate equation approach to cavity mediated laser cooling
The cooling rate for cavity mediated laser cooling scales as the Lamb-Dicke
parameter eta squared. A proper analysis of the cooling process hence needs to
take terms up to eta^2 in the system dynamics into account. In this paper, we
present such an analysis for a standard scenario of cavity mediated laser
cooling with eta << 1. Our results confirm that there are many similarities
between ordinary and cavity mediated laser cooling. However, for a weakly
confined particle inside a strongly coupled cavity, which is the most
interesting case for the cooling of molecules, numerical results indicate that
even more detailed calculations are needed to model the cooling process
accurately.Comment: 15 pages, 10 figures, minor corrections, PRA (in press
Loss-Induced Limits to Phase Measurement Precision with Maximally Entangled States
The presence of loss limits the precision of an approach to phase measurement
using maximally entangled states, also referred to as NOON states. A
calculation using a simple beam-splitter model of loss shows that, for all
nonzero values L of the loss, phase measurement precision degrades with
increasing number N of entangled photons for N sufficiently large. For L above
a critical value of approximately 0.785, phase measurement precision degrades
with increasing N for all values of N. For L near zero, phase measurement
precision improves with increasing N down to a limiting precision of
approximately 1.018 L radians, attained at N approximately equal to 2.218/L,
and degrades as N increases beyond this value. Phase measurement precision with
multiple measurements and a fixed total number of photons N_T is also examined.
For L above a critical value of approximately 0.586, the ratio of phase
measurement precision attainable with NOON states to that attainable by
conventional methods using unentangled coherent states degrades with increasing
N, the number of entangled photons employed in a single measurement, for all
values of N. For L near zero this ratio is optimized by using approximately
N=1.279/L entangled photons in each measurement, yielding a precision of
approximately 1.340 sqrt(L/N_T) radians.Comment: Additional references include
Vacuum state truncation via the quantum Zeno effect
In the context of quantum state engineering we analyze the effect of
observation on nonlinear optical -photon Fock state generation. We show that
it is possible to truncate the vacuum component from an arbitrary photon number
superposition without modifying its remaining parts. In the course of the full
dynamical analysis of the effect of observation, it is also found that the Zeno
and the anti-Zeno effects repeat periodically. We discuss the close
relationship between vacuum state truncation and so-called "interaction-free"
measurement.Comment: 4 pages, 2 figures, LaTeX; TeX errors fixe
Concentration and purification of entanglement for qubit systems with ancillary cavity fields
We propose schemes for entanglement concentration and purification for qubit
systems encoded in flying atomic pairs. We use a cavity-quantum electrodynamics
setting as the paradigmatic scenario within which our proposals can be
implemented. Maximally entangled pure states of qubits can be produced as a
result of our protocols. In particular, the concentration protocol yields Bell
states with the largest achievable theoretical probability while the
purification scheme produces arbitrarily pure Bell states. The requirements for
the implementation of these protocols are modest, within the state of the art,
and we address all necessary steps in two specific set-ups based on
experimentally mature microwave technology.Comment: 10 pages, 6 figure
Beam splitting and Hong-Ou-Mandel interference for stored light
Storing and release of a quantum light pulse in a medium of atoms in the
tripod configuration are studied. Two complementary sets of control fields are
defined, which lead to independent and complete photon release at two stages.
The system constitutes a new kind of a flexible beam splitter in which the
input and output ports concern photons of the same direction but well separated
in time. A new version of Hong-Ou-Mandel interference is discussed.Comment: 8 pages, 3 figure
Generalized Toffoli gates using qudit catalysis
We present quantum networks for a n-qubit controlled gate C^{n-1}(U) which
use a higher dimensional (qudit) ancilla as a catalyser. In its simplest form
the network has only n two-particle gates (qubit-qudit) -- this is the minimum
number of two-body interactions needed to couple all n+1 subsystems (n qubits
plus one ancilla). This class of controlled gates includes the generalised
Toffoli gate C^{n-1}(X) on n qubits, which plays an important role in several
quantum algorithms and error correction. A particular example implementing this
model is given by the dispersive limit of a generalised Jaynes-Cummings
Hamiltonian of an effective spin-s interacting with a cavity mode.Comment: 5 pages, 3 fig
Generating entanglement of photon-number states with coherent light via cross-Kerr nonlinearity
We propose a scheme for generating entangled states of light fields. This
scheme only requires the cross-Kerr nonlinear interaction between coherent
light-beams, followed by a homodyne detection. Therefore, this scheme is within
the reach of current technology. We study in detail the generation of the
entangled states between two modes, and that among three modes. In addition to
the Bell states between two modes and the W states among three modes, we find
plentiful new kinds of entangled states. Finally, the scheme can be extend to
generate the entangled states among more than three modes.Comment: 2 figure
Quantum Cryptography Approaching the Classical Limit
We consider the security of continuous-variable quantum cryptography as we
approach the classical-limit, i.e., when the unknown preparation noise at the
sender's station becomes significantly noisy or thermal (even by as much as
10,000 times the variance of the vacuum mode). We show that, provided the
channel transmission losses do not exceed 50%, the security of quantum
cryptography is not dependent on the channel transmission, and is therefore,
incredibly robust against significant amounts of excess preparation noise. We
extend these results to consider for the first time quantum cryptography at
wavelengths considerably longer than optical and find that regions of security
still exist all the way down to the microwave.Comment: Letter (4 pages) followed by appendix (4 pages). Updated from
published version with some minor correction
Highly-functionalised difluorinated cyclohexane polyols via the Diels–Alder reaction : regiochemical control via the phenylsulfonyl group
A difluorinated dienophile underwent cycloaddition reactions with a range of furans to afford cycloadducts whichcould be processed regio- and stereoselectively via episulfonium ions, generated by the reaction between their alkenyl groups and phenylsulfenyl chloride. The oxabicyclic products were oxidised to the phenylsulfonyl level and ring opened via E1CB or reductive desulfonative pathways to afford, ultimately, difluorinated cyclohexene or cyclohexane polyols
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