121 research outputs found
Optical diode based on the chirality of guided photons
Photons are nonchiral particles: their handedness can be both left and right.
However, when light is transversely confined, it can locally exhibit a
transverse spin whose orientation is fixed by the propagation direction of the
photons. Confined photons thus have chiral character. Here, we employ this to
demonstrate nonreciprocal transmission of light at the single-photon level
through a silica nanofibre in two experimental schemes. We either use an
ensemble of spin-polarised atoms that is weakly coupled to the nanofibre-guided
mode or a single spin-polarised atom strongly coupled to the nanofibre via a
whispering-gallery-mode resonator. We simultaneously achieve high optical
isolation and high forward transmission. Both are controlled by the internal
atomic state. The resulting optical diode is the first example of a new class
of nonreciprocal nanophotonic devices which exploit the chirality of confined
photons and which are, in principle, suitable for quantum information
processing and future quantum optical networks
Design and Stability of Discrete-Time Quantum Filters with Measurement Imperfections
This work considers the theory underlying a discrete-time quantum filter
recently used in a quantum feedback experiment. It proves that this filter
taking into account decoherence and measurement errors is optimal and stable.
We present the general framework underlying this filter and show that it
corresponds to a recursive expression of the least-square optimal estimation of
the density operator in the presence of measurement imperfections. By
measurement imperfections, we mean in a very general sense unread measurement
performed by the environment (decoherence) and active measurement performed by
non-ideal detectors. However, we assume to know precisely all the Kraus
operators and also the detection error rates. Such recursive expressions
combine well known methods from quantum filtering theory and classical
probability theory (Bayes' law). We then demonstrate that such a recursive
filter is always stable with respect to its initial condition: the fidelity
between the optimal filter state (when the initial filter state coincides with
the real quantum state) and the filter state (when the initial filter state is
arbitrary) is a sub-martingale.Comment: Submitted to the American Control Conference 201
Phase space tweezers for tailoring cavity fields by quantum Zeno dynamics
We discuss an implementation of Quantum Zeno Dynamics in a Cavity Quantum
Electrodynamics experiment. By performing repeated unitary operations on atoms
coupled to the field, we restrict the field evolution in chosen subspaces of
the total Hilbert space. This procedure leads to promising methods for
tailoring non-classical states. We propose to realize `tweezers' picking a
coherent field at a point in phase space and moving it towards an arbitrary
final position without affecting other non-overlapping coherent components.
These effects could be observed with a state-of-the-art apparatus
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