Nonclassical light

Nonclassical light is any state of light that cannot be described using classical electromagnetism; its characteristics are described by the quantised electromagnetic field and quantum mechanics. Nonclassical light has nonclassical noise properties called quantum noise, which can be understood on the basis of quantum optics.

Common described forms of nonclassical light are the following:

Squeezed light exhibits reduced noise in one quadrature component. The most familiar kinds of squeezed light have either reduced amplitude noise or reduced phase noise, with increased noise of the other kind.
Fock states (also called photon number states) have a well defined number of photons (stored e.g. in a cavity), while the phase is totally undefined.

The quantum mechanical version of a classical light wave is a coherent state of the quantum electromagnetic field.

1 Glauber-Sudarshan P-representation
2 Non-classicality of measurements
3 External links
4 References

Glauber-Sudarshan P-representation

Main article: Glauber-Sudarshan P-representation

It has been shown that the density matrix for any state of light can be written as:

$\widehat{\rho} = \int \varphi(\alpha) |{\alpha}\rangle \langle {\alpha}| \rm{d}^2 \alpha,$

where $\scriptstyle|\alpha\rangle$ is a coherent state. A classical state of light is one in which $\scriptstyle\varphi(\alpha) \,$ is a probability distribution. If it is not, the state is said to be nonclassical.

Aspects of $\scriptstyle \varphi(\alpha) \,$ that would make it nonclassical are:

a negative value at any point;
being more singular than a Dirac delta function.

Non-classicality of measurements

A recent work^[1] gives a meaning to the non-classicality of a measurement in the framework of the usual interpretation of quantum physics, which is predictive about measurement results and retrodictive about state preparations.

Thus, the non-classicality of a measurement corresponds in fact to the non-classicality of its pre-measurement state, for which such a notion is well-defined by different signatures, such as the negativity in the Wigner representation for instance. The pre-measurement state retrodicted from a measurement is simply linked to the POVM element describing this measurement. It allows us to make predictions about state preparations leading to the result. We can therefore determine in which kind of states the measured system was prepared for leading to such a result. The non-classicality of measurements is a necessary condition for preparing more exotic non-classical states in protocols based on measurements.

External links

Nonclassical light at Encyclopedia of Laser Physics and Technology

References

^ Taoufik Amri, Quantum behavior of measurement apparatus, arXiv:1001.3032 (2010).

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