2026 Theses Doctoral

On-Chip Frequency Conversion for Quantum Networks

Raghunathan, Sidarth

The past several decades have seen the emergence of a wide variety of potential candidates for quantum memories and single-photon sources in many different material platforms, all operating at unique optical frequencies. These include semiconductor quantum dots, color centers in diamond, alkali vapors, rare-earth-ion-doped solids, and trapped ion systems. Each of these platforms has its own set of advantages and drawbacks, rendering them desirable in different applications. To create a versatile hybrid quantum network that can interface between all of these wavelength-incompatible nodes using optical fiber links requires robust quantum frequency conversion (QFC). An ideal QFC device performs unitary conversion of a single photon input with high efficiency and minimal added noise.

In this dissertation, we develop devices for on-chip QFC using the process of Bragg-scattering four-wave mixing (BS-FWM). Different applications have their own unique set of requirements for a QFC device, which are explored in each chapter, however all the devices in this dissertation are united by the fact that they utilize the process of BS-FWM and are performed in an integrated, CMOS-compatible silicon nitride platform.

In the first part of this dissertation, we demonstrate a ring-based microresonator device that can perform QFC between the Sr optical clock transition at 698 nm, the Rb D2 transition at 780 nm, and the telecom O-band (1260-1360 nm). Such a telecom-visible frequency converter connects promising transitions in two distinct atomic species with the low transmission loss window of optical fibers. We characterize sources of noise photons in SiN, describe ways of experimentally distinguishing them, and perform conversion of heralded photon pairs.

In the second part, we describe some limitations imposed by microring resonator-based QFC devices on the tunability and conversion bandwidth when dealing with broadband single-photon inputs. We demonstrate a single-pass waveguide that overcomes these limitations, converting photons near the Rb D1 transition with >50% internal efficiency and low added noise.

In the third part, we introduce distributed Bragg reflector (DBR) cavities as an alternative platform for QFC. This platform resolves the bandwidth limitation of the microring resonator while reducing the device footprint and pump power requirements of the single-pass waveguide, by enabling simultaneous single-pass and multi-pass operation in different wavelength regimes. We show intraband conversion in a DBR cavity that allows for broadband inputs while still providing significant resonant enhancement. We also describe other experiments in which the flexibility introduced by the DBR cavity design allows accessing regimes challenging to achieve with conventional microring resonators.

In the last part, we use integrated thermometry to perform fast and reliable measurement of the thermo-optic coefficient of SiN at 780 nm. Characterization of the thermo-optic properties of SiN at near-visible wavelengths is useful for understanding spectral shifts of cavity resonances caused by optical heating in dual-pump schemes such as that described in part one.