By Cillian Harney and Stefano Pirandola
Submitted to arXiv on 23 October 2020.
The characterisation of Quantum Channel Discrimination (QCD) offers critical performative insight for future quantum technologies in quantum metrology, sensing and communications. The task of multi-channel discrimination creates a scenario in which the discrimination of multiple quantum channels can be equated to the idea of pattern recognition, highly relevant to the tasks of quantum reading, illumination and more. Whilst the optimal quantum strategy for many scenarios has been shown to consist of a tensor product of entangled idler-assisted input states per channel, the extension to a multi-ary setting invites the exploration of discrimination strategies based on multipartite probe states. In this work, we expand the space of possible quantum enhanced protocols by formulating general classes of multipartite entangled probe states which are not assisted by idler modes. We perform this investigation in the Continuous Variable (CV) setting, studying prominent Gaussian channel discrimination problems for real world applications. Our findings uncover the existence of strongly quantum advantageous, idler-free protocols for the discrimination of bosonic loss and environmental noise. This circumvents the necessity for idler assistance to achieve quantum advantage in some of the most relevant discrimination settings, significantly loosening practical requirements for prominent quantum sensing applications.