Nonlinear photon-atom coupling with 4Pi microscopy

Abstract

Implementing nonlinear interactions between single photons and single atoms is at the forefront of optical physics. Motivated by the prospects of deterministic all-optical quantum logic, many efforts are currently underway to find suitable experimental techniques. Focusing the incident photons onto the atom with a lens yielded promising results, but is limited by diffraction to moderate interaction strengths. However, techniques to exceed the diffraction limit are known from high-resolution imaging. Here we adapt a super-resolution imaging technique, 4Pi microscopy, to efficiently couple light to a single atom. We observe 36.6(3)% extinction of the incident field, and a modified photon statistics of the transmitted field–indicating nonlinear interaction at the single-photon level. Our results pave the way to few-photon nonlinear optics with individual atoms in free space.

Transmission spectroscopy of a single atom in the presence of tensor light shifts

“…We find an excellent agreement between the observed spectrum T(Δ) and the model for ò 0 =24.7%, a value consistent with our previous experiment [21] (figures 3(b), (d)). We further test our model by comparing the resonant extinction at various trap depths, but a fixed magnetic field strength of 1.44 mT ( figure 4).…”

“…As the magnetic field strength is reduced, the spectrum shows a lower peak extinction and multiple, broad resonances. This is in stark contrast to the strong extinction(≈22%) we observed in our previous experiments with the same optical setup with a circularly polarized FORT [21]. We then repeat the experiment, but this time, after polarization gradient cooling, we lower the trap depth to 0.88 mK.…”

“…To determine the impact of the light shift on the optical coupling, we perform transmission spectroscopy on a single 87 Rb atom in a tightly focused red-detuned FORT [5]. The atom is held between two high numerical aperture lenses (NA=0.75, focal length f=5.95 mm) with a 2.24 mK-deep FORT operating at a wavelength 851 nm [21,27]. The trapping beam is linearly polarized (with a polarization extinction ratio ≈34 dB [28]) and focused to a waistw 0 ≈1.4 μm.…”

“…strong extinction, are compatible with a long qubit coherence time. In contrast to our earlier experiments [20][21][22][23][24], we use a linearly polarized dipole trap, which strongly reduces atomic motion-induced qubit dephasing, but affects the light-atom coupling through a tensor light shift. We perform transmission spectroscopy to investigate the impact of the tensor light shift on the optical coupling in detail.…”

Maltese cross coupling to individual cold atoms in free space

“…Initial experiments used a single high-NA lens or objective [1], and soon thereafter developed co-linear pairs of high-NA lenses for bi-directional access [19,20]. Already the use of lens pairs offers an important advantage in coupling strength and allows the observation of nonlinearities at the single atom [21]. Achieving a still greater coupling is a major challenge, and has motivated exotic optical techniques [18].…”