BELOW THE STANDARD QUANTUM LIMIT
F. E. Becerra, J. Fan, G. Baumgartner, Goldhar, J. T. Kosloski, and A. Migdall
Quantum state discrimination is a central task for quantum information and is fundamental in quantum mechanics. Nonorthogonal states, such as coherent states, cannot be discriminated with total certainty because of their intrinsic overlap. This nonorthogonality is at the heart for quantum key distribution for ensuring absolute secure communications between a transmitter and a receiver, and can enable many quantum information protocols based in coherent states. At the same time, while coherent states are used for communication because of their robustness to loss and simplicity of generation and detection, their nonorthogonality inherently produces errors in the process of decoding the information. The minimum error in the discrimination of nonorthogonal coherent states measured by ideal lossless and noiseless conventional receivers is given by the standard quantum limit (SQL). This limit sets strict bounds in the ultimate performance of coherent communications and many coherent-state quantum information protocols.
We demonstrate the first quantum receiver that can discriminate multiple nonorthogonal coherent states below the SQL for a wide range of input powers, and with errors up to 4 times lower than this ideal SQL. This receiver implements a discrimination strategy based on optical displacement, photon counting and fast feedback, and can increase the information transmitted in the communication channel above what is possible with ideal conventional receivers.