Preparation of non-local superpositions of quasi-classical light states

Abstract

Quantum superpositions of coherent light waves offer several advantages for quantum-information processing compared to single photons. A novel scheme for generating these so-called Schrodinger-cat states circumvents problems arising from their fragility, which has been a key obstacle towards applications. ‘Schrodinger cat’ states of light1, defined as quantum superpositions of quasi-classical coherent states, have recently emerged as an alternative to single-photon qubits for quantum-information processing2,3,4,5,6. Their richer structure provides significant advantages for quantum teleportation, universal quantum computation, high-precision measurements and fundamental tests of quantum physics7,8,9,10,11,12,13. Local superpositions of free-propagating coherent states have been realized experimentally, but their applications were so far limited by their extreme sensitivity to losses, and by the lack of quantum gates for coherent qubit rotations. Here, we demonstrate a simple approach to generating strongly entangled non-local superpositions of coherent states, using a very lossy quantum channel. Such superpositions should be useful for implementing coherent qubit-rotation gates, and for teleporting these qubits over long distances. The generation scheme may be extended to creating entangled coherent superpositions with arbitrarily large amplitudes.