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 applications1,2. A recently proposed paradigm3 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 fibre4, which can highly confine atomic vapours and light over distances much greater than the diffraction length5,6. 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.

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Figure 1: XPM using a three-level scheme in rubidium vapour confined to a hollow-core PBGF.
Figure 2: Non-demolition measurement of signal power.
Figure 3: Measurement of system response time.
Figure 4: Large XPM at the few-photon level.


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This work was supported by the National Science Foundation (NSF, grant no. PHY-0969996). The authors thank P. Londero for useful discussions during the conception of the experiment, and A. R. Bhagwat and A. D. Slepkov for sharing their expertise regarding design of the experimental chamber.

Author information

V.V. conceived and designed the experiment in consultation with A.L.G., and V.V. and K.S. performed the experiment. V.V. analysed the data and carried out the theoretical modelling. V.V. wrote the paper in discussion with all authors. A.L.G. is the principal investigator on the project.

Correspondence to Alexander L. Gaeta.

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Venkataraman, V., Saha, K. & Gaeta, A. Phase modulation at the few-photon level for weak-nonlinearity-based quantum computing. Nature Photon 7, 138–141 (2013) doi:10.1038/nphoton.2012.283

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