Strong Interactions and Rydberg Excitation in Thermal Vapours
Rubidium: Kate Whittaker, Robert Bettles, James Keaveney, Ifan Hughes & Charles Adams
Caesium: Christopher Wade, Nikola Sibalic, James Keaveney, Kevin Weatherill & Charles Adams

Cooperative atom-light interactions occur when the properties an atomic ensemble cannot be described the sum of the optical response from each individual atom. This non-collective process results from strong dipole-dipole interactions between the atoms. The atomic and optical response of the system is non-linear and gives rise to such phenomena as the dipole blockade effect and non-classical states of light.

Project Aim: Exploit the dipole blockade effect to produce and manipulate non-classical states of light in a thermal vapour.

Three Photon Rydberg Excitation in a Thermal Vapour

Thermal vapours in thin cells allow access to strong dipole-dipole atomic interactions and highly non-linear phenomena. They also offer significant advantages over cold atom experiments:

  • Ability to work at high densities without being optically thick by changing cell thickness.
  • Number density can be controlled by changing the temperature of the cell.
  • Simple, inexpensive setup for continuous measurements.
  • Scalable system for quantum information processing applications.

Rydberg atoms are highly excited atomic states with exagerated properties which result from the large orbital radius of the outer electron. Some of their interesting properties as a function of principal quantum number n include:

  • Electron orbital radius ~ n2.
  • Atomic state lifetime ~ n3.
  • Electric field polarisability ~ n7.
  • Van der Waals atom-atom interaction ~ n11.

We perform a novel three-photon excitation scheme to Rydberg states in Caesium. Unlike two-photon excitation which requires a frequency-doubled high power blue laser, the three-photon scheme uses only inexpensive diode lasers. The ground state 62S1/2 is coupled to the excited state 62P3/2 on the D2 line using the 852nm probe laser. The intermediate coupling laser at 1470nm excites on the strong transition to the 72S1/2 state. Finally, the upper coupling laser at 785nm couples to the 322P3/2 Rydberg state. As this is a common wavelength used for Rubidium spectroscopy, high power lasers (in excess of 1W) are available to drive this transition. The three laser beams are focussed into the thin cell co-linearly, with a residual three-photon Doppler shift:

Δ3ph/v = (k852 – k1470 – k785).

Rydberg Electromagnetically Induced Transparency

By coupling a probe transition to a Rydberg state using Electromangetically Induced Transparency (EIT), it is possible to map the strong dipole-dipole atomic interactions onto an optical field. EIT results from quantum inteference between the excitation pathways in an atomic system. Under specific conditions, the system evolves into a dark state where the probe is no longer absorbed. As the dark state is a coherent superposition of the ground and Rydberg state, the transparency lineshape is highly sensitive to the coherence of the atomic ensemble.

In our four-level system, a weak 852nm probe beam couples the ground and first excited state and strong coupling beams at 1470nm and 785nm couple the first excited, second excited and Rydberg states. The transmission lineshape of three-photon EIT is shown in (a) for a scanning 852nm probe and (b) for a scanning 785nm coupling beam. In (a), the Doppler-broadened absorption of the probe is apparent - here both coupling beams are locked on resonance. In (b), the probe and first coupling laser are locked on resonance.

(a) (b)

We find that it is possible to produce Doppler-free bright and dark state resonances by matching the lower and upper coupling Rabi frequencies. In (a-c) the Rabi frequency ratio Ω7851470 is varied. The upper panel shows the absorption of the probe beam as a function of Rydberg laser detuning and atomic velocity. The lower panel shows both experimental and theoretical spectra obtained by summing over the velocity classes that are present in the thermal ensemble. At the optimum ratio (b), a velocity insensitive absorption resonance is obtained - the three-photon Doppler shift is balanced by the light shifts produced by the strong coupling lasers.

Dipole-Dipole Interactions and Non-Classical Light
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Content © Christopher Carr, Durham University 2011/2012. Last updated 20/03/2012.