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Optical and Quantum Sensing and Precision Metrology

Interfacing quantum dots with laser-cooled atomic ensembles

Divya Bharadwaj, Paul Anderson, Sreesh Venuturumilli, Rubayet Al Maruf, Jiawei Qiu, Taehyun Yoon, Behrooz Semnani, Yujia Yuan, Jinjin Du, Mohd Zeeshan, Philip J. Poole, Dan Dalacu, Michael E. Reimer, Michal Bajcsy

Published Mar 8, 2021• 1 citation
Publisher

Abstract

We describe the experimental progress and the challenges of integrating a single photon source based on quantum dots embedded in semiconductor nanowires with a cold-atom experiment in which laser-cooled caesium atoms are loaded and confined inside a hollow-core micro-structured optical fiber. We focus in particular on wavelength conversion of the photons between 895nm and wavelengths suitable for satellite links (~794nm).

DOI
10.1117/12.2586902

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We describe the development and applications of a single-photon source based on a quantum dot embedded in a semiconductor nanowire, which can be precision-tuned to emit ∼1ns long photons at wavelengths that match the transitions of caesium D1 line. We discuss interfacing such single-photon source with atomic ensembles and present our experimental results demonstrating a new method of tuning the emission of the quantum dot by condensing inert gas (N2) on the nanowire. Next, we describe how these single photons at ∼895nm can be efficiently converted to wavelength suitable for satellite QKD links (∼794 nm) and optical fiber links (∼1469 nm) using a laser-cooled atomic ensemble that is loaded and confined inside a hollow-core optical fiber. Lastly, we inroduce our proposal of integrating the semiconductor nanowire with a lensed fiber to create a compact single-photon source with improved photon-collection efficiency compared to conventional setups.

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Integrated Waveguide Interfaces for Quantum Optics Applications

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This dissertation explores the integration and interfacing of a variety of photonic devices with optical waveguides, aiming to minimize the optical losses and physical footprint of such systems. The specific investigations include interfacing solid-core and hollow-core optical fibers, development of a monolithic single-photon source based on a quantum dot embedded in a semiconductor nanowire aligned with a single-mode fiber, a proposal for a waveguide-integrated power limiter that can protect detectors in a quantum key distribution (QKD) network, and integrating superconducting-nanowire single photon detectors (SNSPD) with laser-written waveguides. \n \nHollow-core photonic-crystal fibers (HCPCFs), which can allow simultaneous tight confinement of both photons and atoms in their hollow core, offer a platform for enhancing interactions between light and atomic ensembles. However, interfacing HCPCFs with conventional solid core (SC) fibers presents a unique set of challenges including significant losses in the joint region due to partial melting and deformation of the photonic crystal during the conventional arc-fusion splicing process. We address this issue with a lithographically defined, vacuum-compatible on-chip structure acting as a mechanical splicer that allows efficient injection of light from a SC fiber to a HCPCF and vice versa, with maximum observed light injection efficiency from a single-mode SC fiber into a HCPCF of 90%. \n \nSingle photon sources (SPS) are often required in photonic quantum information applications. Among various candidates, quantum dots (QD) embedded in tapered semiconductor nanowires have demonstrated excellent progress so far to realize a SPS. However, the overall collection efficiency of the emitted photon into a single mode fiber remains as one of the major challenges, mainly due to loss in the intermediate optical components. We design a complete scheme for high efficiency light coupling from such quantum dot to a single mode fiber. We optimize the geometry of the tapered nanowire to achieve a low numerical aperture Gaussian output mode profile from the wire tip and use a lithographically defined structure to align the SMF collecting the emitted photons. A graded-index multimode fiber acting as a lens is spliced on the SMF end for high efficiency coupling of light from the nanowire into the fiber. \n \nExtremely sensitive photonic devices are often used in quantum optics applications for detection and manipulation of signals with very low power. To prevent malfunction or even damage to these sensitive devices by excess incident power, we propose an optical power limiter based on a pair of cavities integrated with optical waveguides. Such power limiter can be used to protect sensitive electro-optical devices and prevent certain types of attacks on QKD systems. \n \nLastly, a 2D photonic lattice formed by laser-written waveguides in bulk glass can be used as an analogue quantum simulator. We explore the integration of SNSPDs with laser-written waveguides with the goal of making this platform fully on-chip.

•Sep 30, 2020•1 citation
We describe the experimental progress and the challenges of integrating a single photon source based on quantum dots embedded in semiconductor nanowires with a cold-atom experiment in which laser-cooled caesium atoms are loaded and confined inside a hollow-core micro-structured optical fiber. We focus in particular on wavelength conversion of the photons between 895nm and wavelengths suitable for satellite links (~794nm).
We describe the development and applications of a single-photon source based on a quantum dot embedded in a semiconductor nanowire, which can be precision-tuned to emit ∼1ns long photons at wavelengths that match the transitions of caesium D1 line. We discuss interfacing such single-photon source with atomic ensembles and present our experimental results demonstrating a new method of tuning the emission of the quantum dot by condensing inert gas (N2) on the nanowire. Next, we describe how these single photons at ∼895nm can be efficiently converted to wavelength suitable for satellite QKD links (∼794 nm) and optical fiber links (∼1469 nm) using a laser-cooled atomic ensemble that is loaded and confined inside a hollow-core optical fiber. Lastly, we inroduce our proposal of integrating the semiconductor nanowire with a lensed fiber to create a compact single-photon source with improved photon-collection efficiency compared to conventional setups.
This thesis explores hollow-core fibres as a platform for quantum optics experiments with laser-cooled atomic ensembles. The non-diffracting, tightly-confined guided modes of these fibers grant us a ~µm-wide one-dimensional space to study atom-light interactions. In order to describe on-going experiments, simulations are carried out to understand atomic motion into the hollow fibers. Following which, a preliminary case study of a quantum optics experiment to convert wavelengths of single photons with Cs atomic ensembles inside the hollow fiber is presented. Lastly, basic optical properties of photonic crystal membranes are briefly explored. These can form novel cavities when appended to hollow fibers.
We describe the loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber. Inside the fiber, the atoms are confined by a far-detuned optical trap and probed by a weak resonant beam. We describe different loading methods and compare their trade-offs in terms of implementation complexity and atom-loading efficiency. The most efficient procedure results in loading of  30~30 30,000 rubidium atoms, which creates a medium with an optical depth of  180~180 180 inside the fiber. Compared to our earlier study [1] this represents a sixfold increase in the maximum achieved optical depth in this system.
This dissertation explores the integration and interfacing of a variety of photonic devices with optical waveguides, aiming to minimize the optical losses and physical footprint of such systems. The specific investigations include interfacing solid-core and hollow-core optical fibers, development of a monolithic single-photon source based on a quantum dot embedded in a semiconductor nanowire aligned with a single-mode fiber, a proposal for a waveguide-integrated power limiter that can protect detectors in a quantum key distribution (QKD) network, and integrating superconducting-nanowire single photon detectors (SNSPD) with laser-written waveguides. \n \nHollow-core photonic-crystal fibers (HCPCFs), which can allow simultaneous tight confinement of both photons and atoms in their hollow core, offer a platform for enhancing interactions between light and atomic ensembles. However, interfacing HCPCFs with conventional solid core (SC) fibers presents a unique set of challenges including significant losses in the joint region due to partial melting and deformation of the photonic crystal during the conventional arc-fusion splicing process. We address this issue with a lithographically defined, vacuum-compatible on-chip structure acting as a mechanical splicer that allows efficient injection of light from a SC fiber to a HCPCF and vice versa, with maximum observed light injection efficiency from a single-mode SC fiber into a HCPCF of 90%. \n \nSingle photon sources (SPS) are often required in photonic quantum information applications. Among various candidates, quantum dots (QD) embedded in tapered semiconductor nanowires have demonstrated excellent progress so far to realize a SPS. However, the overall collection efficiency of the emitted photon into a single mode fiber remains as one of the major challenges, mainly due to loss in the intermediate optical components. We design a complete scheme for high efficiency light coupling from such quantum dot to a single mode fiber. We optimize the geometry of the tapered nanowire to achieve a low numerical aperture Gaussian output mode profile from the wire tip and use a lithographically defined structure to align the SMF collecting the emitted photons. A graded-index multimode fiber acting as a lens is spliced on the SMF end for high efficiency coupling of light from the nanowire into the fiber. \n \nExtremely sensitive photonic devices are often used in quantum optics applications for detection and manipulation of signals with very low power. To prevent malfunction or even damage to these sensitive devices by excess incident power, we propose an optical power limiter based on a pair of cavities integrated with optical waveguides. Such power limiter can be used to protect sensitive electro-optical devices and prevent certain types of attacks on QKD systems. \n \nLastly, a 2D photonic lattice formed by laser-written waveguides in bulk glass can be used as an analogue quantum simulator. We explore the integration of SNSPDs with laser-written waveguides with the goal of making this platform fully on-chip.