Strong nonlinearities mediated by atomic vapours have the potential to achieve optically controlled switching at room temperature, opening the way to scalable quantum computing. A strong candidate for engineering such interactions is the combination of optical waveguides and cavities with warm alkali vapours. Here we report the implementation of a doubly resonant cavity that translates Kerr nonlinearity induced by an optical control field into a switch for a signal field. We investigate losses and consider optimised designs. We discuss this model in the context of building practical switching devices with applications to logic gate operations at room temperature.
Strong nonlinearities mediated by atomic vapours have the potential to achieve optically controlled switching at room temperature, opening the way to scalable quantum computing. A strong candidate for engineering such interactions is the combination of optical waveguides and cavities with warm alkali vapours. Here we report the implementation of a doubly resonant cavity that translates Kerr nonlinearity induced by an optical control field into a switch for a signal field. We investigate losses and consider optimised designs. We discuss this model in the context of building practical switching devices with applications to logic gate operations at room temperature.
The ability to dynamically route and manage optical signals at high speeds with low-loss is a substantial challenge in quantum optics. Existing optical switches, such as electro-optic phase modulators either achieve high-speed or low insertion losses, but rarely both. However, optically-controlled phase modulation in atomic vapour promises a pathway for efficient, high-bandwidth switching [1]. These characteristics are critical for single photon multiplexing in quantum technologies, implementing loop memory schemes and addressing scalability challenges in photonic quantum computation [2]. We present a cavity based system that can deliver high-speed all-optical switching of a signal field with minimal loss by accessing the strong two photon absorption in warm rubidium vapour via a counter-propagating control field.
Optical switching remains a key outstanding challenge for scalable photonic quantum computing due to the trade-off between speed, bandwidth, and loss. Quantum photonics demands all three to enable high computational clock rates and resource-efficient scaling to large systems. We present a cavity-based optical switch that overcomes this limitation, demonstrating a rise time of 22 ns, an insertion loss of 2.4 dB, and an extinction ratio of 17.5, which bridges the gap between speed and efficiency in photonic switching. All-optical control is achieved via phase modulation of a signal field detuned from the near-degenerate two-photon absorption ladder in warm rubidium vapor. The ultimate performance of our switch will find applications in active multiplexing, loop-based quantum memory, and feed-forward for quantum error-correction protocols.
We consider the dynamics of single photons in a nonlinear optical cavity. When the Kerr nonlinearities of atomic dark resonances are utilized, the cavity mode is well described by a spin- 1/2 Hamiltonian. We show that it is possible to achieve coherent control of the cavity-mode wave function using $\ensuremath{\pi}$ pulses for single photons that switch the state of the cavity with very high accuracy. The underlying physics is best understood as the nonlinearity induced anticorrelation between single-photon injection/emission events, which we refer to as photon blockade. We also propose a method which uses these strong dispersive interactions to realize a single-photon turnstile device.
A fast, low-loss optical switch would be a powerful tool for photonic quantum technologies, allowing the implementation of, for example, rapid quantum logic gates, loop memories, and improved heralded single photon sources. Integration into optical fibre promises an avenue to future scalability, but existing optical switches cannot achieve both high speed and high efficiency. Here I present two routes to fibre-integrated, low -loss and fast optical switching. The first uses the acousto-optic effect in a tapered fibre. We demonstrate that light in the tapered region of the fibre can be coupled between optical modes by an acoustic wave, which introduces a phase-shift. The second route is by a two-photon transition in rubidium vapour. A strong control field modulates the mode of a weak signal field. We demonstrate this effect with low loss (1dB) and on short timescales (100MHz) in bulk optics, and work towards a fibre-integrated vapour cell.
The ability to control light with light at ultralow powers has been a major avenue of research in photonics with applications to optical communications, computation, and signal processing. Such light-by-light scattering is achieved in a medium with a strong light-matter interaction, and for the development of quantum information networks it is important to demonstrate such effects near the single-photon level. Alkali-metal vapors such as rubidium (Rb) enable strong light-matter interactions due to the large cross section per atom and well-defined energy level structure, while the use of optical fibers offers the advantage of possible integration with modern optical communication systems. Hollow-core photonic band-gap fibers (PBGFs) can combine both these technologies such that both the atoms and the optical fields are transversely confined to a region that is a few wavelengths in size, which offers the prospect of exploring few-photon nonlinear interactions. We generate large optical depths in such a Rb-PBGF system, and the tight light confinement, high vapor density and long interaction length allow us to perform nonlinear optics at ultralow power. We demonstrate large signal amplification (>100) and frequency conversion using a four-wave mixing process with only microwatts of pump power. This is, to our knowledge, the largest gain observed at such low power. We perturb the coherence of this four-wave mixing to demonstrate all-optical modulation at unprecedented bandwidths (~300 MHz) for an atomic-vapor system, with an energy density of only tens of pho- tons per atomic cross-section, comparable to that achieved in more elaborate setups based on cold-atomic clouds. We then demonstrate an enhancement of several orders of magnitude in degenerate two-photon absorption in our RbPBGF system over that achieved in bulk vapor cells in a focused beam geometry. This allows us to directly measure two-photon absorption from a beam by detecting its intensity on a photodiode. Further, employing a near-resonant, non-degenerate two-photon transition in Rb, we demonstrate all-optical intensity modulation with just a few photons (<20), or only a few attojoules of energy, at relatively large bandwidths (~50 MHz) for such a sensitive scheme. This result takes us to within an order of magnitude of single-photon switching, and improves upon previous experiments for freely propagating optical fields, including those in cold-atoms. Finally, we produce relatively large cross-phase shifts of a few milliradians on a meter beam with <20 signal photons by tuning slightly away from resonance on the same non-degenerate two-photon transition. This corresponds to a phase shift of 0.3 milliradian per photon, with a fast response time of <5 ns. This represents, to our knowledge, the largest such nonlinear phase shift induced in a single-pass through a room temperature medium. Our Rb-PBGF system can thus potentially be employed to realize weak-nonlinearity based quantum computation and quantum non-demolition measurement of photon number. Through these experiments, we show the potential of a Rb-PBGF system for exploring quantum nonlinear optics at ultralow powers. Moreover, our system is simpler and easier to control and manipulate than setups based on cold atomic clouds and/or high-finesse cavities, and holds promise for integration with fiber-optic communication networks.
Strong interactions between light and atoms at the single-quantum level are an important ingredient for quantum technologies, as well as for studies of fundamental effects in quantum optics. This thesis describes the development of a novel experimental platform that allows for trapping a single rubidium atom in the evanescent mode of a nano-fabricated optical cavity with sub-wavelength dimensions. By virtue of their small size, these cavities provide extremely large atom-photon coupling strengths and good prospects for scalability and integration into complex quantum optical circuits. Positioning the atom near the nano-structure is accomplished using a scanning optical tweezer dipole trap. As a first application, we have demonstrated a coherent optical switch, where a single gate photon controls the propagation of many subsequent signal photons, with the interaction mediated by the atom and cavity. We have also shown that the optical response of the combined atom-cavity system is nonlinear at the level of one or two photons.