A Platform for Evanescently Trapping Rb-87 Using Silicon Nitride Strip Waveguides Buried in Silica

TL;DR

This work tackles the challenge of miniaturizing quantum sensors by presenting a PIC-based platform for evanescent trapping of 87Rb near buried SiN waveguides in SiO$_2$. The approach combines grating MOT loading, magnetic-wire transfer, evaporative cooling to a BEC, and chip-scale trapping using red- and blue-detuned evanescent fields in a buried SiN strip, enabling three-dimensional confinement through higher-order modes and lattice potentials. Key contributions include detailed trap design with vertical, lateral, and lattice confinement, quantitative trap properties (depths, lifetimes, scattering rates), and a thorough analysis of failure modes (stray charges and chiral effects) alongside practical considerations for photonic components and mode multiplexing. The platform offers a path toward compact, CMOS-compatible quantum sensors and clocks, with potential for inertial sensing and multi-axis interferometry in chip-scale architectures, while highlighting fabrication and integration steps required to realize a fully integrated device.

Abstract

Cold-atom systems have emerged as a highly promising avenue for quantum-enhanced position, navigation, and timing applications. However, their wider adoption is currently hampered in part by the large footprint of the systems. In leveraging the miniaturisation possible through photonic integrated circuits, cold-atom sensors would be able to reach much wider commercial adoption. In this paper, we introduce a platform for evanescently trapping 87Rb using strip silicon nitride waveguides buried in silica using red- and blue-detuned fundamental and higher-order modes, providing a three-dimensional adjustable trap for BEC-based, chip-scale work in quantum science and technologies.

Figures (8)

• Figure 1: Illustration of the integrated gMOT-PIC system. Atoms are initially trapped using a downwards-incident beam which diffracts to form a MOT cloud. These can then be trapped in the magnetic field generated by current carrying wires (crossed thick wires below the chip), evaporated to BEC, and controllably lowered to the PIC region of the chip (located in the centre of the grating), where the atoms can be trapped in the evanescent fields above the waveguides.
• Figure 2: (a) Desired trap parameters $\Delta T$ and $\omega_\textrm{trap}$ against trap height $z_\textrm{trap}$. (b) Required current $I$ and $B_\textrm{ext}$ to produce the 2D trap with the desired trap parameters against trap height $z_\textrm{trap}$.
• Figure 3: (Top) Red TE0, blue TE0, and red TE1 spatial potentials from a SiN waveguide in buried silica, expressed in effective temperature $T=\frac{2}{3}\frac{U}{k_B}$ which is proportional to $|E_y|^2$. (Bottom) Schematic of the combination of red, blue, lateral, and lattice modes to produce copropagation in a single snaking waveguide in the atom trapping region.
• Figure 4: (a) Simulation of the potential of the atoms expressed in µK along the central $y=0$ axis. A clear peak and trough is formed for atom trapping 205 nm above the waveguide surface. (b) 2D simulation of the trap including a lateral trapping TE1 mode for two-dimensional confinement ($y=0$ line from (a) overlaid).
• Figure 5: Effect of sweeping blue and red laser power on the trap/barrier's potential and position along the $z$ axis. Regions are marked where $\Delta T_\textrm{trap}\approx\{7,30\}\, E_\textrm{rec}$. The minimum viable trap with the $7E_\textrm{rec}$ criterion used predominantly in this paper is at $P_{r}$ = 1.65 mW and $P_{b}$ = 9 mW. The rightmost white dot represents the trap parameters plotted in Figure \ref{['fig:PIC_trap_profile']} and the leftmost dot is the potential at saddle points in the optical lattice formed (see Section \ref{['sec:standing_wave']}).