Mark Sadgrove

Cavity quantum electrodynamics on a nanofiber using a composite photonic crystal cavity.

We demonstrate cavity QED conditions in the Purcell regime for single quantum emitters on the surface of an optical nanofiber. The cavity is formed by combining an optical nanofiber and a nanofabricated grating to create a composite photonic crystal cavity. By using this technique, significant enhancement of the spontaneous emission rate into the nanofiber guided modes is observed for single quantum dots. Our results pave the way for enhanced on-fiber light-matter interfaces with clear applications to quantum networks.

Rectified momentum transport for a kicked Bose-Einstein condensate.

We report the experimental observation of rectified momentum transport for a Bose-Einstein condensate kicked at the Talbot time (quantum resonance) by an optical standing wave. Atoms are initially prepared in a superposition of the 0 and -2hkl momentum states using an optical pi/2 pulse. By changing the relative phase of the superposed states, a momentum current in either direction along the standing wave may be produced. We offer an interpretation based on matter-wave interference, showing that the observed effect is uniquely quantum.

A Pseudoclassical Method for the Atom-Optics Kicked Rotor: from Theory to Experiment and Back

Abstract We review the concept and applications of a semiclassical ( ϵ -classical or pseudoclassical) approximation to the resonant dynamics of an atom “kicked” by a pulsed, periodic potential. This powerful method allows us to derive analytical results in the deep quantum limit of the kicked rotor. Additionally, classical phase space portraits may be used to represent the dynamics even though the system is fundamentally quantum mechanical. The technique has been successfully adapted for systems including noise and decoherence, as well as systems for which the initial state is a trivial quantum superposition (leading to directed transport at quantum resonance). For almost a decade, theoretical investigations and experimental investigations have been proceeding hand-in-hand in this field, which has been stimulated regularly by experimental progress in controlling driven dynamical systems. Here, we review both theoretical and experimental advances, which in turn may inspire future applications of the presented pseudoclassical method.

Photonic crystal nanofiber using an external grating.

We implemented a photonic crystal nanofiber device by reversibly combining an optical nanofiber and a nanofabricated grating. Using the finite-difference time-domain method, we designed the system for minimal optical loss while tailoring the resonant wavelength and bandwidth of the device. Experimentally, we demonstrated that the combined system shows a strong photonic stop band in good agreement with numerical predictions. The resulting device may be used to realize strong light-matter coupling near the nanofiber surface.

Diameter measurement of optical nanofibers using a composite photonic crystal cavity

We demonstrate a method for making precise measurements of the diameter of a tapered optical fiber with a sub-wavelength diameter waist (an optical nanofiber). The essence of the method is to create a composite photonic crystal cavity by mounting a defect-mode grating on an optical nanofiber. The resultant cavity has a resonance wavelength that is sensitive to the nanofibers diameter, allowing the diameter to be inferred from optical measurements. This method offers a precise, nondestructive, and in situ way to characterize the nanofiber diameter.

Spin-echo-based magnetometry with spinor Bose-Einstein condensates

We demonstrate detection of a weak alternate-current magnetic field by application of the spin echo technique to F = 2 Bose-Einstein condensates. A magnetic field sensitivity of 12 pT/Hz^1/2 is attained with the atom number of 5*10^3 at spatial resolution of 99 \mu m^2. Our observations indicate magnetic field fluctuations synchronous with the power supply line frequency. We show that this noise is greatly suppressed by application of a reverse phase magnetic field. Our technique is useful in order to create a stable magnetic field environment, which is an important requirement for atomic experiments which require a weak bias magnetic field.

Scaling law and stability for a noisy quantum system.

We show that a scaling law exists for the near-resonant dynamics of cold kicked atoms in the presence of a randomly fluctuating pulse amplitude. Analysis of a quasiclassical phase-space representation of the quantum system with noise allows a new scaling law to be deduced. The scaling law and associated stability are confirmed by comparison with quantum simulations and experimental data.

Ramsey interferometry using the Zeeman sublevels in a spin-2 Bose gas

We perform atom interferometry using the Zeeman sublevels of a spin-2 Bose–Einstein condensate of 87Rb. The observed fringes are strongly peaked, and fringe repetition rates higher than the fundamental Ramsey frequency are found in agreement with a simple theory based on spin rotations. With a suitable choice of initial states, the interferometer could function as a useful tool for magnetometry and studies of spinor dynamics in general.We perform atom interferometry using the Zeeman sublevels of a spin-2 Bose–Einstein condensate of 87Rb. The observed fringes are strongly peaked, and fringe repetition rates higher than the fundamental Ramsey frequency are found in agreement with a simple theory based on spin rotations. With a suitable choice of initial states, the interferometer could function as a useful tool for magnetometry and studies of spinor dynamics in general.

Fast, externally triggered, digital phase controller for an optical lattice

We present a method to control the phase of an optical lattice according to an external trigger signal. The method has a latency of less than 30 μs. Two phase locked digital synthesizers provide the driving signal for two acousto-optic modulators which control the frequency and phase of the counter-propagating beams which form a standing wave (optical lattice). A micro-controller with an external interrupt function is connected to the desired external signal, and updates the phase register of one of the synthesizers when the external signal changes. The standing wave (period λ/2 = 390 nm) can be moved by units of 49 nm with a mean jitter of 28 nm. The phase change is well known due to the digital nature of the synthesizer, and does not need calibration. The uses of the scheme include coherent control of atomic matter-wave dynamics.

Polarization response and scaling law of chirality for a nanofibre optical interface

Two port optical devices couple light to either port dependent on the input photon state. An important class of two-port devices is that of evanescently-coupled interfaces where chirality of photon coupling can lead to important technological applications. Here, we perform a fundamental characterization of such an interface, reconstructing the two-port polarization response over the surface of the Poincaré sphere for an optical nanofibre. From this result, we derive a chirality measure which is universal, obeying a one parameter scaling law independent of the exact parameters of the nanofibre and wavelength of light. Additionally, we note that the polarization response differs qualitatively for single and multiple coupled emitters, with possible implications for sensing and the characterization of waveguide coupled spins.