arXiv subjects

Rahul Shrestha

Publications and source records attributed to Rahul Shrestha.

5 recordsLinked to original sources

arXiv2025

Enabling atom-clad waveguide operation in a microfabricated alkali vapor-photonic integrated circuit

Integrating alkali atomic vapors with nanophotonic devices offers a scalable route to quantum technologies that leverage strong atom-photon interactions. While there have been many approaches to such integration, the general reliance on traditional glass vapor cells, distilled alkali metals, and epoxy sealing limits reproducibility and scalability. Moreover, mitigating adverse Rb-photonics interactions is essential, particularly as devices become more compact and the alkali source lies in close proximity to the photonic elements. Here, we demonstrate the successful operation of compact and fully integrated devices that combine silicon nitride photonic integrated circuits (PICs) with microfabricated borosilicate vapor cells and pill-type rubidium (Rb) dispensers through hermetic seals via anodic bonding. We show how successful operation hinges on optically activating the dispenser in a low-power pulsed mode, releasing controlled amounts of Rb vapor on demand while mitigating photonic degradation. Simultaneously, a counter-propagating desorption laser completely suppresses Rb-induced losses and enables waveguide-based atomic vapor spectroscopy. Using this approach, we demonstrate repeatable control of vapor density by tuning activation pulse length, duty cycle, and device temperature. These results establish a compact, manufacturable, and scalable vapor-PIC device, and set the stage for future demonstrations in cavity quantum electrodynamics, quantum nonlinear optics, and chip-scale atomic sensors.

physics.optics↗

arXiv2024

Strong interactions between integrated microresonators and alkali atomic vapors: towards single-atom, single-photon operation

Cavity quantum electrodynamics (cQED), the interaction of a two-level system with a high quality factor (Q) cavity, is a foundational building block in different architectures for quantum computation, communication, and metrology. The strong interaction between the atom and the cavity enables single photon operation which is required for quantum gates and sources. Cold atoms, quantum dots, and color centers in crystals are amongst the systems that have shown single photon operations, but they require significant physical infrastructure. Atomic vapors, on the other hand, require limited experimental infrastructure and are hence much easier to deploy outside a laboratory, but they produce an ensemble of moving atoms that results in short interaction times involving multiple atoms, which can hamper quantum operations. A solution to this issue can be found in nanophotonic cavities, where light-matter interaction is enhanced and the volume of operation is small, so that fast single-atom, single-photon operations are enabled. In this work, we study the interaction of an atomically-clad microring resonator (ACMRR) with different-sized ensembles of Rb atoms. We demonstrate strong coupling between an ensemble of ~50 atoms interacting with a high-quality factor (Q > 4 x 10^5) ACMRR, yielding a many-atom cooperativity C ~ 5.5. We continue to observe signatures of atom-photon interaction for a few (< 3) atoms, for which we observe saturation at the level of one intracavity photon. Further development of our platform, which includes integrated thermo-optic heaters to enable cavity tuning and stabilization, should enable the observation of interactions between single photons and single atoms.

quant-ph↗

arXiv2024

Broadband Visible Wavelength Microcomb Generation In Silicon Nitride Microrings Through Air-Clad Dispersion Engineering

The development of broadband microresonator frequency combs at visible wavelengths is pivotal for the advancement of compact and fieldable optical atomic clocks and spectroscopy systems. Yet, their realization necessitates resonators with anomalous dispersion, an arduous task due to the prevailing normal dispersion regime of materials within the visible spectrum. In this work, we evince that silicon nitride microring resonators with air cladding on top and sides -- a deviation from the frequently employed silica-embedded resonators -- allows for the direct generation of broadband microcombs in the visible range. We experimentally demonstrate combs pumped at 1060~nm (283~THz) that reach wavelengths as short as 680~nm (440 THz), and combs pumped at 780~nm (384 THz) that reach wavelengths as short as 630 nm (475 THz). We further show through simulations that microcombs extending to wavelengths as low as 461 nm (650 THz) should be accessible in this platform.

physics.optics↗

arXiv2023

Energy-preserving Indirect-feedback for Wireless Power Transfer

Recognising the limitations of various existing channel-estimation schemes for energy beamforming, we propose an energy-preserving indirect feedback-based approach for finding the optimal beamforming vector. Upon elaborating on the key ideas behind the proposed approach -- dynamics of the harvest-then-transmit protocol and the latency associated with the charging process -- we present an algorithm and its hardware architecture to concretise the proposed approach. The algorithm and the hardware architecture are supplemented by mathematical analysis, numerical simulation and hardware utilisation details, ASIC synthesis and post-layout simulation details, respectively. We firmly believe this paper, due to its unified algorithm-hardware design, will open up new avenues for research in radio frequency (RF) wireless power transfer.

eess.SP↗

arXiv2023

Kerr-Induced Synchronization of a Cavity Soliton to an Optical Reference

The phase-coherent frequency division of a stabilized optical reference laser to the microwave domain is made possible by optical frequency combs (OFCs). Fundamentally, OFC-based clockworks rely on the ability to lock one comb tooth to this reference laser, which probes a stable atomic transition. The active feedback process associated with locking the comb tooth to the reference laser introduces complexity, bandwidth, and power requirements that, in the context of chip-scale technologies, complicate the push to fully integrate OFC photonics and electronics for fieldable clock applications. Here, we demonstrate passive, electronics-free synchronization of a microresonator-based dissipative Kerr soliton (DKS) OFC to a reference laser. We show that the Kerr nonlinearity within the same resonator in which the DKS is generated enables phase locking of the DKS to the externally injected reference. We present a theoretical model to explain this Kerr-induced synchronization (KIS), and find that its predictions for the conditions under which synchronization occur closely match experiments based on a chip-integrated, silicon nitride microring resonator. Once synchronized, the reference laser is effectively an OFC tooth, which we show, theoretically and experimentally, enables through its frequency tuning the direct external control of the OFC repetition rate. Finally, we examine the short- and long-term stability of the DKS repetition rate and show that the repetition rate stability is consistent with the frequency division of the expected optical clockwork system.

physics.optics↗