Session H4: Invited Session: Quantum Manipulation of Photons

Linear Optical Quantum Computing in a Single Spatial Mode

We present a scheme for linear optical quantum computing using time-bin encoded qubits in a single spatial mode. This scheme allows arbitrary numbers of qubits to be encoded in the same mode, circumventing the requirement for many spatial modes that challenges the scalability of other schemes, and exploiting the inherent stability and robustness of time-frequency optical modes. This approach leverages the architecture of modern telecommunications systems, and opens a door to very high dimensional Hilbert spaces while maintaining compact device designs. Further, temporal encodings benefit from intrinsic robustness to inhomogeneities in transmission mediums. These advantages have been recognized in works exploring the preparation of time-frequency entangled states both for tests of fundamental quantum phenomena, and for quantum communications technologies including key distribution and teleportation. Here we extend this idea to computation. In particular, we present methods for single-qubit operations and heralded controlled phase (CPhase) gates, providing a sufficient set of operations for universal quantum computing with the Knill-Laflamme-Milburn scheme. As a test of our scheme, we demonstrate the first entirely single spatial mode implementation of a two-qubit quantum gate and show its operation with an average fidelity of 0.84 /pm 0.07. An analysis of the performance of current technologies suggests that our scheme offers a promising route for the construction of quantum circuits beyond the few-qubit level. In addition, we foresee that our investigation may motivate further development of the approaches presented into a regime in which time bins are temporally overlapped and frequency based manipulations become necessary, opening up encodings of even higher densities.   

Coupling, controlling, and processing non-transversal photons with a single atom

I will report on recent experimental investigations of the interaction between single rubidium atoms and light that is confined by continuous total internal reflection in a whispering-gallery-mode (WGM) bottle microresonator. These resonators offer the advantage of very long photon lifetimes in conjunction with near lossless in- and out-coupling of light via tapered fiber couplers. We discovered that the non-transversal polarization of WGMs fundamentally alters the physics of light-matter interaction [1]. Taking advantage of this effect, we recently demonstrated switching of signals between two distinct optical fibers controlled by a single atom [2]. Owing to the excellent optical properties of our bottle microresonator, the scheme yields high switching fidelities and low losses. Furthermore, we exploited the strong birefringence of the bottle microresonator and implemented a single-atom-controlled polarization flip of the light that is guided through the coupling fiber [3]. And finally, we made use of the strong nonlinear response of the atom-resonator system and experimentally realized an optical Kerr nonlinearity at the level of single photon [3]. Analyzing the transmitted light, we observe a nonlinear phase shift of $\pi$ between the cases of one and of two photons passing the resonator. This phase shift leads to entanglement between previously independent fiber-guided photons, which we verify by performing a full quantum state tomography of the transmitted two-photon state. \\[4pt] [1] C. Junge et al., Phys. Rev. Lett. \textbf{110}, 213604 (2013).\\[0pt] [2] D. O'Shea et al., Phys. Rev. Lett. \textbf{111}, 193601 (2013).\\[0pt] [3] J. Volz et al., submitted (2013).   

Dynamics of Superradiant Lasers

A superradiant laser has been shown to operate with less than one photon on average inside of the optical cavity. In this regime, almost all of the phase information of the laser is stored in the atoms rather than the cavity field. As a result, the laser's phase is highly insensitive to both technical and fundamental thermal cavity mirror vibrations. This vibration noise presently limits the coherence of the best lasers as well as the precision of the optical lattice clocks that these lasers interrogate. We have explored the physics of superradiant lasers utilizing Raman transitions between hyperfine states in rubidium to mimic narrow optical transitions. In this talk, we will discuss the amplitude stability of our superradiant Raman laser, and the dynamics of phase synchronization in our system. We will also consider the prospects for future superradiant lasers that would lase on the same highly-forbidden transitions used in optical lattice clocks.   

Controlled photon-photon interactions using Rydberg polaritons

By coupling a strong optical transition to a highly-excited Rydberg state [1] it is possible to realise giant optical non-linearities [2] and hence strong photon-photon interactions [3-5]. A remaining challenge is to implement an interaction that does not distort the photon mode and hence to realise high-fidelity photonic quantum gates. In this talk we will discuss how to control the photon-photon interact using microwave fields [6] and how this could be used to implement deterministic non-linear optical computation [7].\\[4pt] [1] A. Mohapatra et al. Phys. Rev. Lett. {\bf 98}, 113003 (2007).\\[0pt] [2] J. Pritchard et al. Phys. Rev. Lett. {\bf 105}, 193603 (2010).\\[0pt] [3] Y. Dudin and A. Kuzmich, Science {\bf 336}, 887 (2012).\\[0pt] [4] T. Peyronel et al. Nature {\bf 488}, 57 (2012).\\[0pt] [5] D. Maxwell et al. Phys. Rev. Lett. {\bf 110}, 103001 (2013).\\[0pt] [6] D. Maxwell et al. arXiv:1308.1425\\[0pt] [7] D. Paredes-Barato and C. S. Adams, arXiv:1309.7933