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Initial Rydberg dressing experiments were performed by shining the coupling laser onto a single frequency red MOT for a range of coupling beam detunings and dressing times, shown in Figure 5.1. The first observation, even before considering the shape of the MOT, is that the trap continues to operate. After 5 ms of dressing we still observe atoms when the coupling beam is red- and blue-detuned. The 5sns3_S

1 Rydberg series exhibits

repulsive interactions, and we might expect a blue-detuned coupling laser to excite pairs of Rydberg atoms, leading to heavy depletion, but at initial densities of 1011 cm−3, we don’t see this.

When the coupling beam is red-detuned from resonance we observe a dimple protruding into the MOT, shown in the top two lines of Figure 5.1. Atoms within this dimple are not trapped and fall under gravity. When blue-detuned (bottom two rows of Figure 5.1), we see a dimple outside of the MOT, into which atoms flow. On resonance we see heavy loss due to Rydberg excitation, shown in the third row of Figure 5.1.

The cause of this dimple is illustrated in Figure 5.2. The red-detuned coupling beam results in an AC Stark shift that reduces the MOT beam detuning from the dressed

Position/µm Position/µm Position/µm

Figure 5.1: Dressing a MOT with 5s37s 3S1 Rydberg character, using Ω = 2.5 MHz,

δMOT = −400 kHz and PMOT = 150 µW for different dressing times (left to right) and

coupling beam detunings (top to bottom). The scale shows cloud optical depth, averaged over two repeats. The single frequency red MOT is held for 50 ms for cooling to complete before the coupling laser is turned on.

state. MOT beam resonance with the dressed state then occurs for a smaller Zeeman shift, closer to the quadrupole field centre. The reverse is true for blue-detuned coupling - MOT beam resonance with the dressed state occurs further from the quadrupole field centre, creating a dimple into which atoms can flow.

The effect observed in Figure 5.1 is spatially asymmetric, suggesting the coupling beam is not perfectly aligned onto the cloud centre. As the cloud is larger than the coupling beam the AC Stark shift only affects part of the cloud.

A two-stage dressed MOT

To better understand the effect that the AC Stark shift has we have developed a two- stage MOT detuning scan, shown in Figure 5.3. We form a single frequency red MOT at δMOT = −350 kHz and PMOT= 90 µW, which we then dress with 5s37s 3S1 Rydberg

|gi |ei |ri δMOT δC < 0 |r0i |e0_i δAC MOT light δMOT− δAC Coupling Bare Dressed (a) (b) (c)

Figure 5.2: (a) When the coupling beam δC is red-detuned it reduces the energy of the

dressed state |e0i below the bare state |ei, effectively reducing the MOT beam detuning magnitude, illustrated on the left. The smaller effective MOT beam detuning results in resonance occuring for a smaller Zeeman shift, closer to the quadrupole field centre. (b) shows the dependence on the coupling beam detuning δC - when the coupling beam is

blue-detuned the dressed state |e0i is shifted higher in energy than the bare state |ei, and resonance occurs for a larger Zeeman shift. The colour of the dressed state indicates the fraction of |ei (blue) and |ri (red) in the dressed state and the black dotted line indicates the MOT beam detuning. (c) shows the effective MOT beam detuning δMOT− δAC from

the dressed state |e0i.

protrude into the cloud. When the coupling laser is turned on the MOT beam detuning δMOT is changed. This was repeated with and without the coupling laser, for 10 ms of

dressing.

In the bare state case (Figure 5.3(a-e)), as the MOT beam frequency moves further from resonance the cloud gets larger and moves lower, retaining the ellipse outline set by the quadrupole field as we expect. When the coupling beam is on (Figure 5.3(f-j)) we see very different MOT shapes - as the MOT beam detuning increases the cloud gets larger but also splits into two clouds that form either side of the dimple due to the coupling beam. We also see a fraction of the cloud that is not trapped and falls under gravity, seen in Figure 5.3(f).

Position/µm Position/µm Position/µm Position/µm Position/µm Figure 5.3: Stepping the MOT detuning to a new value (row a-e), we see the MOT form in a new position. Adding the red-detuned coupling laser to this (row f-j) modifies the cloud shape - the cloud flows under gravity to either side of the coupling beam dimple. Taken with δC = −6 MHz, Ω = 2.5 MHz. Predicted resonance curves described in Section

5.1.2 are overlaid.

we can map out the shape of the dimple that protrudes into the MOT. This is shown in Figure 5.4, and offers a very striking representation of the effect of the coupling beam. Rather than atoms simply sagging to the lowest point of a resonance ellipse set by the quadrupole field, the resonance condition is met either side of the coupling beam, and above the coupling beam. Atoms sag under gravity to the lowest positions on the reso- nance curve, either side of the coupling beam. As the coupling beam is slightly off-centre, the atoms preferentially fall to the right of the coupling beam. The effect does not appear to be significantly blurred by the 30◦ angle between the coupling beam and the imaging axis.