6.6 Predicted microdisk cavity QED parameters
6.6.3 Practical limitations
The above analysis assumes that the cavity Q is limited by either material or radiation loss. In practice, effects such as surface roughness and surface state absorption can become the dominant microcavity loss channels [16]. Although experimental evidence in Sec. 6.2.1 indicates that this was not the case for the microdisks studied there, surface effects be- come increasingly pronounced as the microdisk diameter shrinks and the field becomes less confined. Similarly, the TM-like mode is a comparatively better sensor of the microdisk surface than is the TE-like mode, and is more sensitive to surface related loss mechanisms. Further systematic experimental studies ofQvs. microdisk diameter and mode polarization are needed to better understand the impact of these effects.
0 400 800 1200 0 5 10 15 20 25 2 3 4 5 6 7 ge 2 / κ γsp Microdisk diameter [μm] TE, p = 1 TM, p = 1 3.6 4.4 5.2 Microdisk diameter [μm] 2.8 [GHz ] 0.01 100 10 1 0.1 [GHz ] Microdisk diameter [μm] κ ge γsp 3 5 2 4 6 Microdisk diameter [μm] ge / max[ κ , γsp ] TE, p = 1 TM, p = 1 (a) (b) (d) (c) 0.01 100 10 1 0.1 3 5 2 4 6 TE, p = 1 TM, p = 1 λ = 637 nm 30 κ ge γsp
Figure 6.11: Cavity QED parameters for an diamond NV center interacting with the mi- crodisk near field, as a function of microdisk diameter. The NV center is taken to be at the field maximum outside of the microdisk. The microdisk thickness is h = 250 nm, and
λ∼637 nm. In calculatingκ,Q= min4×106, Qrad
. (a,b) Interaction and decoherence rates for the fundamental (a) TE mode, (b) TM mode. (c) Strong coupling parameter. (d) Bad cavity parameter.
Wavelength dependent material absorption was also ignored in the analysis of the mi- crodisk modes at 637 nm. A difference in the intrinsic material optical loss rate at 637 nm compared with that at 852 nm would modify the maximum obtainable Q for devices in this wavelength range. However, based on existing literature [119, 118], we expect the optical attenuation coefficient of SiNx to fall within the same order of magnitude at both wavelengths.
6.7
Conclusion
In this chapter, we have shown that microdisk optical cavities fabricated from SiNx have sufficiently low optical loss rates and sufficiently large single photon peak field strengths for cavity QED experiments with Cs atoms operating within the strong-coupling regime. These cavities should allow GHz atom-photon coupling rates, which are higher than any other high-Q microcavity operating at λ= 852 nm demonstrated to date. Because of the low optical loss of SiNx in the visible wavelength range, these cavities should also be useful for experiments studying a wide class of solid state quantum emitters, such as diamond NV centers. Ultimately, by taking advantage of the planar, CMOS compatible nature of the SiNx material system, fully integrated photonic chips for visible wavelengths, consisting of many cavities connected through on-chip waveguides, can be designed and fabricated. In the next chapter, we will show how these devices can be integrated with atom chips, eventually promising fully “on-chip” cavity QED and quantum information processing with neutral atoms.
Chapter 7
An atom-cavity chip
Atom chips [152, 50, 51, 153] have rapidly evolved over the last decade as a valuable tool in experiments involving the cooling, trapping, and transport of ultra-cold neutral atom clouds. Fabricated using standard semiconductor processing techniques, atom chips are formed by conducting microwires lithographically patterned on a planar insulating substrate. For modest microwire currents, extremely high magnetic field gradients can be formed close to the atom chip surface [154], and by combining appropriate microwire configurations with externally generated magnetic bias fields, magnetic traps for cold atoms can be realized [155, 156, 157]. Crucially, the position of the magnetic trap, and hence the atoms, can be moved dynamically by varying the current through the microwire configuration.
Examples of experiments that leverage the planar, scalable, micron-sized features of atom chips include studies of Bose-Einstein condensates [158, 159] and degenerate Fermi gases [160] “on-chip”, “portable” Bose-Einstein condensates [161], atom waveguides [162] and conveyer belts [163], and atom interferometers [164, 165, 166, 167]. The field of cavity QED [6, 5, 4] and, in particular, cavity QED with neutral atoms and micropho- tonic devices [168, 169, 170, 171, 172, 173, 131, 29] is poised to significantly benefit from atom chips. Integration of atomic and microphotonic chips [170, 173, 171, 174, 175] of- fers several advancements to the current state-of-the-art Fabry-P´erot cavity QED systems [176, 34, 177, 178, 179], most notably a scalable platform for locally controlling multiple quantum bits. Ultimately, the atom chip can be used to deliver and possibly trap single atoms within the near field of a microcavity.
In this chapter, we describe and demonstrate a technique for integrating the fiber coupled microcavities studied in previous chapters with atom chips developed by Benjamin Lev [180]. Integrated “atom-cavity” chips fabricated using this technique can be installed in atom
trapping vacuum systems while maintaining an efficient fiber input and output channel between the microcavity and the outside world. By taking advantage of the small size of the microcavities, we show that they can be directly integrated with the metal layer used in optically and magnetically trapping atoms near the surface the atom chip, and hence, the cavity. In Sec. 7.1, we describe the fiber mounting technique, and in Sec. 7.2 use a fiber coupled device installed in the atom trapping UHV system to study the sensitivity of a SiNx microdisk to a dilute cesium (Cs) vapor. In Sec. 7.3, we show how Cs atoms can be trapped directly above an array of microdisks integrated with a mirrored surface attached to an atom chip. Much of the work presented in Sec. 7.1 and Sec. 7.2 first appeared in Ref. [17].