10,346 research outputs found
The Langevin Equation for a Quantum Heat Bath
We compute the quantum Langevin equation (or quantum stochastic differential
equation) representing the action of a quantum heat bath at thermal equilibrium
on a simple quantum system. These equations are obtained by taking the
continuous limit of the Hamiltonian description for repeated quantum
interactions with a sequence of photons at a given density matrix state. In
particular we specialise these equations to the case of thermal equilibrium
states. In the process, new quantum noises are appearing: thermal quantum
noises. We discuss the mathematical properties of these thermal quantum noises.
We compute the Lindblad generator associated with the action of the heat bath
on the small system. We exhibit the typical Lindblad generator that provides
thermalization of a given quantum system.Comment: To appear in J.F.
Extreme faint flux imaging with an EMCCD
An EMCCD camera, designed from the ground up for extreme faint flux imaging,
is presented. CCCP, the CCD Controller for Counting Photons, has been
integrated with a CCD97 EMCCD from e2v technologies into a scientific camera at
the Laboratoire d'Astrophysique Experimentale (LAE), Universite de Montreal.
This new camera achieves sub-electron read-out noise and very low Clock Induced
Charge (CIC) levels, which are mandatory for extreme faint flux imaging. It has
been characterized in laboratory and used on the Observatoire du Mont Megantic
1.6-m telescope. The performance of the camera is discussed and experimental
data with the first scientific data are presented.Comment: 33 pages, 17 figures, accepted for publication in PAS
Quantum control and measurement of atomic spins in polarization spectroscopy
Quantum control and measurement are two sides of the same coin. To affect a
dynamical map, well-designed time-dependent control fields must be applied to
the system of interest. To read out the quantum state, information about the
system must be transferred to a probe field. We study a particular example of
this dual action in the context of quantum control and measurement of atomic
spins through the light-shift interaction with an off-resonant optical probe.
By introducing an irreducible tensor decomposition, we identify the coupling of
the Stokes vector of the light field with moments of the atomic spin state.
This shows how polarization spectroscopy can be used for continuous weak
measurement of atomic observables that evolve as a function of time.
Simultaneously, the state-dependent light shift induced by the probe field can
drive nonlinear dynamics of the spin, and can be used to generate arbitrary
unitary transformations on the atoms. We revisit the derivation of the master
equation in order to give a unified description of spin dynamics in the
presence of both nonlinear dynamics and photon scattering. Based on this
formalism, we review applications to quantum control, including the design of
state-to-state mappings, and quantum-state reconstruction via continuous weak
measurement on a dynamically controlled ensemble
X-ray imaging of spin currents and magnetisation dynamics at the nanoscale
Understanding how spins move in time and space is the aim of both fundamental
and applied research in modern magnetism. Over the past three decades, research
in this field has led to technological advances that have had a major impact on
our society, while improving the understanding of the fundamentals of spin
physics. However, important questions still remain unanswered, because it is
experimentally challenging to directly observe spins and their motion with a
combined high spatial and temporal resolution. In this article, we present an
overview of the recent advances in X-ray microscopy that allow researchers to
directly watch spins move in time and space at the microscopically relevant
scales. We discuss scanning X-ray transmission microscopy (STXM) at resonant
soft X-ray edges, which is available at most modern synchrotron light sources.
This technique measures magnetic contrast through the X-ray magnetic circular
dichroism (XMCD) effect at the resonant absorption edges, while focusing the
X-ray radiation at the nanometre scale, and using the intrinsic pulsed
structure of synchrotron-generated X-rays to create time-resolved images of
magnetism at the nanoscale. In particular, we discuss how the presence of spin
currents can be detected by imaging spin accumulation, and how the
magnetisation dynamics in thin ferromagnetic films can be directly imaged. We
discuss how a direct look at the phenomena allows for a deeper understanding of
the the physics at play, that is not accessible to other, more indirect
techniques. Finally, we present an overview of the exciting opportunities that
lie ahead to further understand the fundamentals of novel spin physics,
opportunities offered by the appearance of diffraction limited storage rings
and free electron lasers.Comment: 21 pages, 10 figure
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