Quantum nanophotonics with single erbium ions and silicon nanophotonic cavities (@Thompson Lab):

Single atoms and atom-like defects in solids are actively pursued for realizing single photon sources and long-lived quantum memories, which are essential ingredients for the development of long-range quantum networks based on spin-photon entanglement. A primary challenge to achieving this is overcoming attenuation in fiber networks used for transmitting light over large distances. Minimal losses occur for light propagating in the “telecom band” wavelength of 1.5 µm, whereas most atomic transitions are situated in the ultraviolet-NIR regions with wavelengths shorter than 1 µm, where fiber transmission losses are significantly higher. A notable exception is Er3+ ion, which has an optical transition at 1.5 µm that has been thoroughly exploited by modern fiber-based telecommunication networks. But isolating and addressing single Er3+ ions via an optical interface have been elusive so far because of the poor emission rate (<100 Hz) from single Er3+ ions due to its electric dipole-forbidden nature of the intra-4f optical transition. 

We have demonstrated fluorescence from single Er3+ ions embedded in a solid-state host for the first time. This has been achieved by integrating the Er3+ ions in a low loss, small mode-volume silicon nanophotonic cavity and enhancing emission from the ions by a factor of more than 650. We have addressed dozens of spectrally distinct ions within the inhomogeneous distribution of ions coupled to the same cavity. These results are a significant step towards realizing long-distance quantum networks by utilizing multiplexed quantum repeater protocols and deterministic quantum logic for photons based on a scalable silicon nanophotonics architecture.

Selected other works:

Momentum switch construction for multi-particle quantum walk [paper, arXiv] 

It is possible to realize universal quantum computation by multi-particle quantum walk, making it a possible candidate for building the architecture of a scalable quantum computer with time-independent control. Certain continuous-time quantum walks can be viewed as scattering processes. These processes can perform quantum computations, but it is challenging to design graphs with desired scattering behavior. We studied and constructed momentum switches, graphs that route particles depending on their momenta. We also give an example where there is no exact momentum switch, although we construct an arbitrarily good approximation. 

A holey waveguide with an adiabatically-introduced defect

We know that defects in photonic crystals (PhC) entertain the presence of localized modes having frequencies inside the band gaps such that they will decay exponentially once they enter the crystal. We introduce defects in a PhC, which consists of a pseudo one-dimensional dielectric slab with an array of air-holes along one direction as the periodic dielectric arrangement, by reducing the size of one or more air-holes. This effectively increases the volume of the higher dielectric constant region, which enables in pulling down a set of discrete modes into the gap from the upper bands. If we introduce just a single defect in the structure by creating an air-hole with a smaller radius, we can obtain a resonant cavity mode – localized along the direction of the periodic dielectric arrangement (because of the band gap) and index-guided in the transverse direction. But since the gap is an incomplete one, there will be radiating modes (the modes within the light cone existing at all frequencies) causing the mode to slowly leak away into the surrounding medium (air) and hence a low Q factor. We show that in lieu of abruptly decreasing the radius of one single air-hole to create a defect, if we adiabatically decrease the radius of a series of air-holes, we get a much better Electric field localization, and hence a far-improved Q factor.

Multi-photon interferometry

This project is relevant to the experimental implementation of the Boson Sampling model of linear optics quantum computing. We theoretically compute coincidence detection probabilities of photons originating from heralded SPDC photon sources in a Hong-Ou-Mandel experiment while including effects of higher order multi-photon terms. The results are expressed in terms of Permanents and Immanants of the corresponding transfer matrices. The absolute visibility of coincidence detection is shown to be strongly dependent on the multi-photon input terms. 

Topological Superconductivity in a two-dimensional system

We numerically investigate topological superconductivity in a two-dimensional electron-gas system, formed at the LaAlO3/SrTiO3 interface. The out-of-plane, critical magnetic field is studied as a function of electronic correlation parameter, chemical potential, and spin-orbit coupling strength.