Integrated optical multi-ion quantum logic

Practical and useful quantum information processing (QIP) requires significant improvements with respect to current systems, both in error rates of basic operations and in scale. Individual trapped-ion qubits' fundamental qualities are promising for long-term systems, but the optics involved in their precise control are a barrier to scaling. Planar-fabricated optics integrated within ion trap devices can make such systems simultaneously more robust and parallelizable, as suggested by previous work with single ions. Here we use scalable optics co-fabricated with a surface-electrode ion trap to achieve high-fidelity multi-ion quantum logic gates, often the limiting elements in building up the precise, large-scale entanglement essential to quantum computation. Light is efficiently delivered to a trap chip in a cryogenic environment via direct fibre coupling on multiple channels, eliminating the need for beam alignment into vacuum systems and cryostats and lending robustness to vibrations and beam pointing drifts. This allows us to perform ground-state laser cooling of ion motion, and to implement gates generating two-ion entangled states with fidelities $>99.3(2)\%$. This work demonstrates hardware that reduces noise and drifts in sensitive quantum logic, and simultaneously offers a route to practical parallelization for high-fidelity quantum processors. Similar devices may also find applications in neutral atom and ion-based quantum-sensing and timekeeping

Metallic, magnetic and molecular nanocontacts

Scanning tunnelling microscopy and break-junction experiments realize metallic and molecular nanocontacts that act as ideal one-dimensional channels between macroscopic electrodes. Emergent nanoscale phenomena typical of these systems encompass structural, mechanical, electronic, transport, and magnetic properties. This Review focuses on the theoretical explanation of some of these properties obtained with the help of first-principles methods. By tracing parallel theoretical and experimental developments from the discovery of nanowire formation and conductance quantization in gold nanowires to recent observations of emergent magnetism and Kondo correlations, we exemplify the main concepts and ingredients needed to bring together ab initio calculations and physical observations. It can be anticipated that diode, sensor, spin-valve and spin-filter functionalities relevant for spintronics and molecular electronics applications will benefit from the physical understanding thus obtained

2022 Roadmap on integrated quantum photonics

AbstractIntegrated photonics will play a key role in quantum systems as they grow from few-qubit prototypes to tens of thousands of qubits. The underlying optical quantum technologies can only be realized through the integration of these components onto quantum photonic integrated circuits (QPICs) with accompanying electronics. In the last decade, remarkable advances in quantum photonic integration have enabled table-top experiments to be scaled down to prototype chips with improvements in efficiency, robustness, and key performance metrics. These advances have enabled integrated quantum photonic technologies combining up to 650 optical and electrical components onto a single chip that are capable of programmable quantum information processing, chip-to-chip networking, hybrid quantum system integration, and high-speed communications. In this roadmap article, we highlight the status, current and future challenges, and emerging technologies in several key research areas in integrated quantum photonics, including photonic platforms, quantum and classical light sources, quantum frequency conversion, integrated detectors, and applications in computing, communications, and sensing. With advances in materials, photonic design architectures, fabrication and integration processes, packaging, and testing and benchmarking, in the next decade we can expect a transition from single- and few-function prototypes to large-scale integration of multi-functional and reconfigurable devices that will have a transformative impact on quantum information science and engineering

Two-photon absorption detection of infrared light in silicon optical resonators

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. [107]-115).The challenge of overcoming energy efficiency and bandwidth limitations in interconnects between components in computer systems (e.g. between memory and processors) has motivated the development of short-range optical interconnects, which in many approaches require optical devices and waveguides fabricated within the same CMOS environments as the electronics. This thesis centers on developing photodetectors for infrared light within the silicon of commercial CMOS processes; silicon's lack of strong absorption at the wavelengths of interest makes this challenging. The approach uses defect-state mediated linear absorption and two-photon absorption (TPA) in small mode-volume resonators to generate photocarriers. Such resonators allow efficient linear absorption in short devices despite low absorption coefficients, and a greater TPA rate than in bulk material due to the large energy densities achievable. The devices here are made in the polysilicon layer of a commercial DRAM process, and characterization of this material, different from crystalline Si in both its linear and nonlinear absorption, forms a starting point. The design, fabrication, and testing of electrically addressable photonic crystal resonators subject to the constraints associated with working in a CMOS process are then presented. The best resonators made were able to reach Qs of 70,000, limited by linear loss in the polysilicon. Linear absorption is dominant in the devices made to date, and allowed quantum efficiencies of a few tens of percent on resonance. However, high biases of around -20 V were required to achieve these QEs, and the bandwidth of the devices was limited to only approximately 500 MHz. Improvements to the electrical structure of the devices are likely to improve these characteristics. The ability to fabricate high-Q photonic crystal resonators within full CMOS flows, and the QEs allowed by defect-assisted absorption in the devices measured, indicate promise for this approach to photodetection in integrated CMOS photonic systems.by Karan K. Mehta.S.M

Integrated optical quantum manipulation and measurement of trapped ions

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages [165]-183).Individual atomic ions confined in designed electromagnetic potentials and manipulated via lasers are strong candidates as physical bases for quantum information processing (QIP). This is in large part due to their long coherence times, in distinguishability, and strong Coulomb interactions. Much work in recent years has utilized these properties to implement increasingly precise quantum operations essential for QIP, as well as to conduct increasingly sophisticated experiments on few-ion systems. Many questions remain however regarding how to implement the significant classical apparatus required to control and measure many ions (and indeed any physical qubit under study) in a scalable way that furthermore does not compromise qubit quality. This work draws on techniques in integrated optics to address this question. Planar-fabricated waveguides and gratings integrated with planar ion traps are demonstrated to allow optical addressing of individual 88Sr+ions 50 [mu]m above the chip surface with distraction-limited focused beams, with advantages in stability and scalability. Motivated by the requirement for low crosstalk in qubit addressing, we show also that intuitively designed devices can generate precisely tailored intensity profiles at the ion locations, with distraction-limited side lobe intensities characterized to the 5x10-6 level in relative intensity up to 25 [mu]m from the focus. Such devices can be implemented alongside complex systems in complementary metal-oxide-semiconductor (CMOS) processes. We show in addition that the multiple patternable metal layers present in CMOS processes can be used to create complex planar ion traps with performance comparable to simple single-layer traps, and that CMOS silicon avalanche photodiodes may be employed for scalable quantum state readout. Finally we show initial results on integrated electro-optic modulators for visible light. These results open possibilities for experiments with trapped ions in the short term, and indicate routes to achieving large-scale systems of thousands or more ions in the future. Though ion qubits may seem isolated from scalable solid-state technologies, it appears this apparent isolation may uniquely allow a cooperation with complex planar-fabricated optical and electronic systems without introducing additional decoherence.by Karan K. Mehta.Ph. D

Precise and diffraction-limited waveguide-to-free-space focusing gratings

We present the design and characterization of waveguide grating devices that couple visible-wavelength light at λ = 674 nm from single-mode, high index-contrast dielectric waveguides to free-space beams forming micron-scale diffraction-limited spots a designed distance and angle from the grating. With a view to application in spatially-selective optical addressing, and in contrast to previous work on similar devices, deviations from the main Gaussian lobe up to 25 microns from the focus and down to the 5 × 10[superscript -6] level in relative intensity are characterized as well; we show that along one dimension the intensity of these weak sidelobes approaches the limit imposed by diffraction from the finite field extent in the grating region. Additionally, we characterize the polarization purity in the focal region, observing at the center of the focus a low impurity < 3 × 10[superscript -4] in relative intensity. Our approach allows quick, intuitive design of devices with such performance, which may be applied in trapped-ion quantum information processing and generally in any systems requiring optical routing to or from objects 10 s-100 s of microns from a chip surface, but benefitting from the parallelism and density of planar-fabricated dielectric integrated optics.National Science Foundation (U.S.) (Program ECCS-1408495

Optimization and implementation of a surface-electrode ion trap junction

We describe the design of a surface-electrode ion trap junction, which is a key element for large-scale ion trap arrays. A bi-objective optimization method is used for designing the electrodes, which maintains the total pseudo-potential curvature while minimizing the axial pseudo-potential gradient along the ion transport path. To facilitate the laser beam delivery for parallel operations in multiple trap zones, we implemented integrated optics on each arm of this X-junction trap. The layout of the trap chip for commercial foundry fabrication is presented. This work suggests routes to improving ion trap junction performance in scalable implementations. Together with integrated optical addressing, this contributes to modular trapped-ion quantum computing in interconnected 2-dimensional arrays.Comment: 29 pages, 14 figure

Optimization and implementation of a surface-electrode ion trap junction

We describe the design of a surface-electrode ion trap junction, which is a key element for large-scale ion trap arrays. A bi-objective optimization method is used for designing the electrodes, which maintains the total pseudo-potential curvature while minimizing the axial pseudo-potential gradient along the ion transport path. To facilitate the laser beam delivery for parallel operations in multiple trap zones, we implemented integrated optics on each arm of this X-junction trap. The layout of the trap chip for commercial foundry fabrication is presented. This work suggests routes to improving ion trap junction performance in scalable implementations. Together with integrated optical addressing, this contributes to modular trapped-ion quantum computing in interconnected two-dimensional arrays.ISSN:1367-263

Pure circularly polarized light emission from waveguide microring resonators

Circularly polarized light plays a key role in many applications including spectroscopy, microscopy, and control of atomic systems. Particularly in the latter, high polarization purity is often required. Integrated technologies for atomic control are progressing rapidly, but while integrated photonics can generate fields with pure linear polarization, integrated generation of highly pure circular polarization states has not been addressed. Here, we show that waveguide microring resonators, perturbed with azimuthal gratings and thereby emitting beams carrying optical orbital angular momentum, can generate radiated fields of high circular polarization purity. We achieve this in a passive device by taking advantage of symmetries of the structure and radiated modes, and directly utilizing both transverse and longitudinal field components of the guided modes. On the axis of emission and at maximum intensity, we measure an average polarization impurity of $1.0 \times 10^{-3}$ in relative intensity across the resonance FWHM, and observe impurities below $10^{-4}$ in this range. This constitutes a significant improvement over the ${\sim}10^{-2}$ impurity demonstrated in previous work on integrated devices. Photonic structures allowing high circular polarization purity may assist in realizing high-fidelity control and measurement in atomic quantum systems