Photon entanglement via Rydberg blockade
Jonathan Pritchard, Daniel Maxwell, Alex Gauguet, Matt Jones and Prof. Charles Adams
Funding: EPSRC EP/F040253/1
Latest Results:
In our current experiment (see arXiv:0911.3523v2 ) optical pumping is used to prepare 87Rb atoms in the 5s 2s1/2 state. Counter-propagating probe and coupling lasers are then turned on simultaneously to perform EIT spectroscopy as shown in the figure on the left. The probe beam absorption is detected using a single photon counter. The coupling laser is frequency stabilized on resonance to the 5p 2P3/2 &rarr ns 2S1/2 transition. The probe laser drives the 5s 2S1/2 to 5p 2P3/2 transition and is scanned across the resonance.

The figures below show the effect of dressing the Rydberg state with a microwave field coupled to the 45p1/2 state. The figure on the left shows the probe beam absorption after 1 shot i.e. one scan through resonance. The figure on the right shows the result after 100 shots.

We are builing a new experiment to entangle single photons using Rydberg atom dipole blockade. Rydberg atoms have enhanced dipole-dipole interactions which detune the doubly excited state off-resonance. This leads to a blockade which permits only a single excitation within a given volume known as a blockade sphere. Using this single excitation it is possible to manipulate light at the single photon level, such as creating a single photon source or creating non-classical optical fields. The ultimate goal is to realise a two-photon photonic phase gate for use in quantum information processing (QIP).
Rydberg atoms and dipole blockade:
Rydberg atoms have a dipole moment ~n2 which leads to very strong dipole-dipole interactions, with the long range van der Waals interaction scaling as Δ~n11/R6. These strong interactions make Rydberg atoms ideal for use in QIP as it allows very fast gate operations. For closely spaced atoms in highly excited Rydberg states this interaction suppresses excitation to the Rydberg state due to the dipole blockade. If we consider a pair of atoms initially in the groundstate |g> which are resonantly coupled to the Rydberg state |r> then either atom can be excited to the Rydberg state. However, if both atoms are excited to the Rydberg state then they will experience a large force due to the dipole interaction and this causes the doubly excited state |rr> to shift by an amount Δ. If this shift is larger than the laser linewidth, then the laser is no longer resonant and only a single Rydberg excitation is allowed within a sphere of radius Rb, the blockade radius. As an example the shift for a pair of Rubidium atoms at n=80 is 250 MHz for a 5 μm separation.
From dipole blockade to photon entanglement:
A blockaded medium offers an ideal tool with which to manipulate single photons. This can be done by storing single excitations in the Rydberg state, which can be read out at some later time. Initial experiments will explore using the single excitation as a single photon source, before developing entanglement operations between target and control photons propagating through the medium. To see how this can lead to entanglement, consider the case of two identical photons propagating through a blockaded medium. Since it can only support a single excitation, only one photon will cause a Rydberg excitation. However since they are identical, the outgoing state is an entangled state with equal probabilty of each photon causing the excitation. It may also be possible to read out the collective excitation of a Rydberg state to give single photon emission with a high degree of directionality.
In order to use the dipole-blockade to develop a two-photon quantum gate it is necessary to confine all atoms within the blockade sphere Rb such that only a single Rydberg excitation is supported. To do this a pair of diffraction limited lenses with NA=0.63 will be used to confine ultracold Rubidium atoms in a dipole trap with a 1 μm focus. We use a two-photon excitation to the Rybderg state, using light at 780nm to drive the 5S1/2 to 5P3/2 and a second laser at 480nm to excite from 5P3/2 to nS or nD Rydberg states. A new frequency doubled 480nm laser has recently been installed in the lab and stabilised to the atomic transition. For our experiments using Rydberg atom the blockade regime requires a very small relative linewidth for the two photon transition. To narrow the linewidth, a frequency modulation technique is used to generate an error signal from electromagnetically induced transparency (EIT) in a thermal cell. This scheme is detailed in Appl. Phys. Lett. 94, 071107 (2009).
Progress so far:
Our vacuum chamber has now been fully assembled. To allow good optical access to the centre of the chamber we are using custom-made viewports which are sealed in place using an indium solder (see Rev. Sci. Instrum. 80, 026105 (2009)), which means a much larger clear aperture can be achieved than for standard CF fittings. High numerical aperture lenses are mounted in the centre of the chamber. We are currently setting up the MOT and dipole trap optics around the chamber.
Our Rydberg activities are part of the EU COHERENCE network involving 12 partners across Europe. See Link

If you are interested in Post-doctoral or Ph.D. positions please contact Dr. M. P. A. Jones or Prof. C. S. Adams.