K. J. Weatherill

Cooperative atom-light interaction in a blockaded Rydberg ensemble

By coupling a probe transition to a Rydberg state using electromagnetically induced transparency (EIT) we map the strong dipole-dipole interactions onto an optical field. We characterize the resulting cooperative optical nonlinearity as a function of probe strength and density. We demonstrate good quantitative agreement between the experiment and an N-atom cooperative model for N=3 atoms per blockade sphere and the n=60 Rydberg state. The measured linewidth of the EIT resonance places an upper limit on the dephasing rate of the blockade spheres of <110 kHz.We demonstrate a cooperative optical non-linearity caused by dipolar interactions between Rydberg atoms in an ultra-cold atomic ensemble. By coupling a probe transition to the Rydberg state we map the strong dipoledipole interactions between Rydberg pairs onto the optical field. We characterize the non-linearity as a function of electric field and density, and demonstrate the enhancement of the optical non-linearity due to cooperativity.

Storage and control of optical photons using Rydberg polaritons.

We use a microwave field to control the quantum state of optical photons stored in a cold atomic cloud. The photons are stored in highly excited collective states (Rydberg polaritons) enabling both fast qubit rotations and control of photon-photon interactions. Through the collective read-out of these pseudospin rotations it is shown that the microwave field modifies the long-range interactions between polaritons. This technique provides a powerful interface between the microwave and optical domains, with applications in quantum simulations of spin liquids, quantum metrology and quantum networks.

A giant electro-optic effect using polarizable dark states

The electro-optic effect, where the refractive index of a medium is modified by an electric field, is of central importance in nonlinear optics, laser technology, quantum optics and optical communications. In general, electro-optic coefficients are very weak and a medium with a giant electro-optic coefficient could have profound implications for precision electrometry and nonlinear optics at the single-photon level. Here we propose and demonstrate a giant d.c. electro-optic effect on the basis of polarizable (Rydberg) dark states. When a medium is prepared in a dark state consisting of a superposition of ground and Rydberg energy levels, it becomes transparent and acquires a refractive index that is dependent on the energy of the highly polarizable Rydberg state. We demonstrate phase modulation of the light field in the Rydberg-dark-state medium and measure an electro-optic coefficient that is more than six orders of magnitude larger than in usual Kerr media. Coupling of the Rydberg states of an ensemble of rubidium atoms gives rise to a d.c. Kerr effect that is six orders of magnitude greater than in conventional Kerr media. Such phenomena could enable the development of high-precision electric field sensors and other nonlinear optical devices.

Laser frequency stabilization to excited state transitions using electromagnetically induced transparency in a cascade system

We demonstrate laser frequency stabilization to excited state transitions using cascade electromagnetically induced transparency. Using a room temperature Rb vapor cell as a reference, we stabilize a first diode laser to the D2 transition and a second laser to a transition from the intermediate 5P3/2 state to a highly excited state with principal quantum number n=19–70. A combined laser linewidth of 280±50 kHz over a 100 μs time period is achieved. This method may be applied generally to any cascade system and allows laser stabilization to an atomic reference in the absence of a direct absorption signal.

Nonequilibrium phase transition in a dilute Rydberg ensemble.

We demonstrate a nonequilibrium phase transition in a dilute thermal atomic gas. The phase transition, between states of low and high Rydberg occupancy, is induced by resonant dipole-dipole interactions between Rydberg atoms. The gas can be considered as dilute as the atoms are separated by distances much greater than the wavelength of the optical transitions used to excite them. In the frequency domain, we observe a mean-field shift of the Rydberg state which results in intrinsic optical bistability above a critical Rydberg number density. In the time domain, we observe critical slowing down where the recovery time to system perturbations diverges with critical exponent α=-0.53±0.10. The atomic emission spectrum of the phase with high Rydberg occupancy provides evidence for a superradiant cascade.

Narrow absorptive resonances in a four-level atomic system

We study the effect of a control beam on a Λ electromagnetically induced transparency (EIT) system in 87Rb. The control beam couples one ground state to another excited state forming a four-level -system. Phase coherent laser beams to drive the -system are produced using a double injection locking scheme. We show that the control beam can be used to Stark shift or split the EIT resonance. Finally, we show that, when the control beam is on resonance, one observes a Doppler-free and sub-natural absorptive resonance with a width of order 100 kHz. Crucially, this narrow absorptive resonance only occurs when atoms with a range of velocities are present, as is the case in a room-temperature vapour.

Real-time near-field terahertz imaging with atomic optical fluorescence

A time-averaged intensity distribution of terahertz waves is imaged by converting terahertz waves to optical fluorescence. The conversion becomes possible by exciting Cs atoms to a Rydberg state. The image acquisition time is 40 ms. Terahertz (THz) near-field imaging is a flourishing discipline1,2, with applications from fundamental studies of beam propagation3 to the characterization of metamaterials4,5 and waveguides6,7. Beating the diffraction limit typically involves rastering structures or detectors with length scale shorter than the radiation wavelength; in the THz domain this has been achieved using a number of techniques including scattering tips8,9 and apertures10. Alternatively, mapping THz fields onto an optical wavelength and imaging the visible light removes the requirement for scanning a local probe, speeding up image collection times11,12. Here, we report THz-to-optical conversion using a gas of highly excited Rydberg atoms. By collecting THz-induced optical fluorescence we demonstrate a real-time image of a THz standing wave and use well-known atomic properties to calibrate the THz field strength.

Nonlinear optics using cold Rydberg atoms

The implementation of electromagnetically induced transparency (EIT) in a cold Rydberg gas provides an attractive route towards strong photon--photon interactions and fully deterministic all-optical quantum information processing. In this brief review we discuss the underlying principles of how large single photon non-linearities are achieved in this system and describe experimental progress to date.

Electromagnetically induced transparency of an interacting cold Rydberg ensemble

We study electromagnetically induced transparency (EIT) of a weakly interacting cold Rydberg gas. We show that for Rydberg states with principal quantum numbers in the range n = 19–26, the onset of interactions is manifest as a depopulation of the Rydberg state. In the limit of a weak probe where the depopulation effect is negligible, we observe no evidence of interaction-induced decoherence and obtain a narrow Rydberg dark resonance with a linewidth of <600 kHz.

Polarization spectroscopy of an excited state transition

We demonstrate polarization spectroscopy of an excited state transition in room-temperature cesium vapor. An anisotropy induced by a circularly polarized pump beam on the D2 transition is observed using a weak probe on the 6P(3/2)→7S(1/2) transition. At high pump power, a subfeature due to Autler-Townes splitting is observed that theoretical modeling shows is enhanced by Doppler averaging. Polarization spectroscopy provides a simple modulation-free signal suitable for laser frequency stabilization to excited state transitions.