29 research outputs found
Determining the Quantum Expectation Value by Measuring a Single Photon
Quantum mechanics, one of the keystones of modern physics, exhibits several
peculiar properties, differentiating it from classical mechanics. One of the
most intriguing is that variables might not have definite values. A complete
quantum description provides only probabilities for obtaining various
eigenvalues of a quantum variable. These and corresponding probabilities
specify the expectation value of a physical observable, which is known to be a
statistical property of an ensemble of quantum systems. In contrast to this
paradigm, we demonstrate a unique method allowing to measure the expectation
value of a physical variable on a single particle, namely, the polarisation of
a single protected photon. This is the first realisation of quantum protective
measurements.Comment: Nature Physics, in press (this version corresponds to the one
initially submitted to Nature Physics
Le Monde islamique, du Maghreb Ă l'Iran
Dossiers des Images Economiques du Monde [DIEM n° 26
Territoires et politiques dans les périphéries des grandes villes du Maghreb
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Controlling quantum flux through measurement: An idealised example
Classically, no transfer occurs between two equally filled reservoirs no
matter how one looks at them, but the situation can be different quantum
mechanically. This paradoxically surprising phenomenon rests on the distinctive
property of the quantum world that one cannot stare at a system without
disturbing it. It was recently discovered that this seemingly annoying feature
could be harnessed to control small quantum systems using weak measurements.
Here we present one of the simplest models -- an idealised double quantum dot
-- where by toying with the dot measurement strength, i.e. the intensity of the
look, it is possible to create a particle flux in an otherwise completely
symmetric system. The basic property underlying this phenomena is that
measurement disturbances are very different on a system evolving unitarily and
a system evolving dissipatively. This effect shows that adaptive measurements
can have dramatic effects enabling transport control but possibly inducing
biases in the measurement of macroscopic quantities if not handled with care.Comment: Published version. Principal: 8 pages + references + 6 figures;
Supplementary: 6 pages + references + 2 figure