Phase modulation at the few-photon level for weak-nonlinearity-based quantum computing

The ability of a few-photon light field to impart an appreciable phase shift on another light field is critical for many quantum information applications(1,2). A recently proposed paradigm(3) for quantum computation utilizes weak nonlinearities, where a strong field mediates such cross-phase shifts between single photons. Such a protocol promises to be feasible in terms of scalability to many qubits if a cross-phase shift of 10(-5) to 10(-2) radians per photon can be achieved. A promising platform to achieve such cross-phase shifts is the hollow-core photonic bandgap fibre(4), which can highly confine atomic vapours and light over distances much greater than the diffraction length(5,6). Here, we produce large cross-phase shifts of 0.3 mrad per photon with a fast response time (<5 ns) using rubidium atoms confined to a hollow-core photonic bandgap fibre, which represents, to our knowledge, the largest such nonlinear phase shift induced in a single pass through a room-temperature medium.