Alladi Ramakrishnan Hall

Nonclassicality depth for quantum-optical channels: a noisy approach

Krishnakumar Sabapathy

Fısica Teorica: Informacio i Fenomens Quantics, Universitat Autonoma de Barcelona, Spain

The quantum-optical notion of classical/nonclassical divide among
states of light is naturally described in terms of the Glauber-Sudarshan
diagonal `weight' function. Analogously, a demarcation exits between
quantum-optical processes that output only classical states and those do
not, i.e., the nonclassicality breaking/non-nonclassicality breaking divide
for channels. We exploit these divisions to quantify the `nonclassicality'
of a multimode quantum-optical channel in terms of the following: what is
the `minimal' additive classical noise channel, characterised by a noise
matrix $Y_{\rm min}$, that when composed with a general channel results in
a nonclassicality breaking channel? We then extract a suitable scalar from
$Y_{\rm min}$ which we define as the nonclassicality depth of a
quantum-optical channel, as a measure of nonclassicality of the channel.
The approach also provides an operational interpretation for the
nonclassicality depth of general multimode states. We compute the
nonclassicality depth of a few practically relevant channels which include
amplifiers and attenuators.