Coherent Ising machines are a type of optical accelerators that can solve different optimization tasks by encoding the problem in the connection matrix of the network. So far, experimental realizations have been limited to time multiplexed solutions, in which one nonlinear node is present in a feedback loop. In Hewlett Packard Labs, we investigate the implementation of a spatially multiplexed solution, with an array of nominally identical nonlinear nodes. As this avoids the need for a long delayline, this makes the system more suitable for integration and hence mass production. HPE investigated two material platforms with good bulk nonlinearity properties: a-Si and GaAs. For the CMOS compatible a-Si platform, HPE demonstrated a design approach that allows to fabricate 1000 component all-optical computational circuits in a scalable way. In addition, to be able to do layout of Ising machines with ~1000 components, HPE developed highly capable photonic layout that will help across interconnects, sensors, and computation. In the GaAs platform, we focused on reducing the energy per elementary operation down to 1 fJ. The optical gates are designed with a bus-waveguide connectivity using a multi-level layered architecture design that allows waveguide connectivity between optical gates. This allows to separate computation and communication into their own dedicated layers increasing overall performance. Finally, we will highlight how both drastic automation at the layout stage and a tight integration between the electronic control layer (used for tuning of resonances and phase-shifters) and the photonic layer are key to achieve actual scalability to larger circuits.
We present a hybrid photonic architecture using gallium arsenide photonic crystals coupled to silicon nitride waveguides. Chrome microheaters are integrated in the system for tuning the cavities. The combination of low-energy switching elements, combined with low loss photonic waveguides provides an ideal architecture for applications in dedicated optical computing and machine learning applications.
A key enabler of the IT revolution of the late 20th century was the development of electronic design automation (EDA) tools allowing engineers to manage the complexity of electronic circuits with transistor counts now reaching into the billions. Recently, we have been developing large-scale nonlinear photonic integrated logic circuits for next generation all-optical information processing. At this time a sufficiently powerful EDA-style software tool chain to design this type of complex circuits does not yet exist. Here we describe a hierarchical approach to automating the design and validation of photonic integrated circuits, which can scale to several orders of magnitude higher complexity than the state of the art.
Most photonic integrated circuits developed today consist of a small number of components, and only limited hierarchy.
For example, a simple photonic transceiver may contain on the order of 10 building-block components,
consisting of grating couplers for photonic I/O, modulators, and signal splitters/combiners. Because this is relatively
easy to lay out by hand (or simple script) existing photonic design tools have relatively little automation in
comparison to electronics tools. But demonstrating all-optical logic will require significantly more complex photonic
circuits containing up to 1,000 components, hence becoming infeasible to design manually.
Our design framework is based off Python-based software from Luceda Photonics which provides an environment to describe components, simulate their behavior, and export design files (GDS) to foundries for fabrication. At a fundamental level, a photonic component is described as a parametric cell (PCell) similarly to electronics design. PCells are described by geometric characteristics of their layout. A critical part of the design framework is the implementation of PCells as Python objects. PCell objects can then use inheritance to simplify design, and hierarchical designs can be made by creating composite PCells (modules) which consist of primitive building-block PCells (components). To automatically produce layouts, we built on a construct provided by Luceda called a PlaceAndAutoRoute cell: we create a module component by supplying a list of child cells, and a list of the desired connections between the cells (e.g. the out0 port of a microring is connected to a grating coupler). This functionality allowed us to write algorithms to automatically lay out the components: for instance, by laying out the first component and walking through the list of connections to check to see if the next component is already placed or not. The placement and orientation of the new component is determined by minimizing the length of a connecting waveguide. Our photonic circuits also utilize electrical signals to tune the photonic elements (setting propagation phases or microring resonant frequencies via thermo-optical tuning): the algorithm also routes the contacts for the metal heaters to contact pads at the edge of the circuit being designed where it can be contacted by electrical probes.
We are currently validating a test run fabricated over the summer, and will use detailed characterization results to prepare our final design cycle in which we aim to demonstrate complex operational logic circuits containing ~50-100 nonlinear resonators.
KEYWORDS: Quantum dots, Photonic crystals, Molecular photonics, Picosecond phenomena, Molecules, Polaritons, Energy transfer, Chemical species, Molecular energy transfer, Control systems
Vacuum Rabi oscillation is a damped oscillation in which energy can transfer between an atomic excitation and a photon when an atom is strongly coupled to a photonic cavity. This process is challenging to be coherently controlled due to the fact that interaction between the atom and the electromagnetic resonator needs to be modulated in a quick manner compared to vacuum Rabi frequency. This control has been achieved at microwave frequencies, but has remained challenging to be implemented in the optical domain. Here we demonstrated coherent control of energy transfer in a semiconductor quantum dot strongly coupled to a photonic crystal molecule by manipulating the vacuum Rabi oscillation of the system. Instead of using a single photonic crystal cavity, we utilized a photonic crystal molecule consisting two coupled photonic crystal defect cavities to obtain both strong quantum dot-cavity coupling and cavityenhanced AC stark shift. In our system the AC stark shift modulates the coupling interaction between the quantum dot and the cavity by shifting the quantum dot resonance, on timescales (picosecond) shorter than the vacuum Rabi period. We demonstrated the ability to transfer excitation between a quantum dot and cavity, and performed coherent control of light-matter states. Our results provides an ultra-fast approach for probing and controlling light-matter interactions in an integrated nanophotonic device, and could pave the way for gigahertz rate synthesis of arbitrary quantum states of light at optical frequencies.
Generating strong interactions between single quanta of light and matter is central to quantum information science, and a key component of quantum computers and long-distance quantum networks. In quantum information processing, these interactions are required to create elementary logic operations (quantum gates) between stationary matter quantum bits (qubits) and photonic qubits that can be transmitted over long distances. Efficient quantum gates between photonic and matter qubits are a crucial enabler for a broad range of applications that include robust quantum networks, nondestructive quantum measurements, and strong photon-photon interactions. So far these qubit-photon gates have been achieved using single atoms and at microwave frequencies in circuit QED systems. Their implementation with solidstate quantum emitters, however, has remained a difficult challenge. We demonstrate that the qubit state of a photon can be controlled by a single solid-state qubit composed of a quantum dot (QD) strongly coupled to an optical nanocavity. We show that the QD acts as a coherently controllable qubit system that conditionally flips the polarization of a photon reflected from the cavity mode on picosecond timescales. This operation implements a controlled NOT (cNOT) logic gate between the QD and the incident photon, which is a universal quantum operation that can serve as a general light-matter interface for remote entanglements and quantum computations. Our results represent an important step towards an all solid-state implementation of quantum networks and quantum computers, and provide a versatile approach for controlling and probing interactions between a photon and a single quantum emitter on ultra-fast timescales.
Low power optical nonlinearities are a crucial requirement for data routing and next generation all-optical processing.
The majority of nonlinear optical devices to date exploit weak nonlinearities from a large ensemble of atomic systems,
resulting in both high power dissipation and a large device footprint. Quantum dots (QDs) coupled to photonic crystals
can provide significant reduction in both device size and power dissipation. The interaction between these two systems
creates extremely strong light-matter interaction owing to the tight optical confinement of photonic crystals and large oscillator strengths of QDs. Such interactions enable optical nonlinearities near the single photon level. In this work we investigate the nonlinear properties of QDs coupled to photonic crystals. We demonstrate large optical Stark shift with only 10 photons. We then propose and demonstrate a novel photonic circuit that can route light on a chip with extremely low optical powers.
We study the interaction of silicon photonic crystal nanocavities with infiltrated colloidal PbS nanocrystals as a viable and efficient source for achieving indistinguishable and single photons. Nanocrystal-nanocavity coupling is predicted at near-infrared wavelengths, suggesting the possibility towards exciting silicon-based nanophotonic lasers, and novel efficient sources for fiber and silicon-based quantum information networks and systems. Two effective designs for nanocrystal-nanocavity coupling are illustrated that exhibit moderate to high cavity quality factors, and ultra-small modal volumes for spontaneous emission enhancements. It is shown that in principle our system can approach the observation of strong exciton-cavity coupling in a solid-state implementation at room temperature.
We show that introducing anisotropy into periodic dielectric structures leads to new optical phenomena as well as to a new approach to a variety of applications. One-dimensional anisotropic structures allow a new type of chiral twist defect resulting in a localized photonic mode with unusual properties. Unlike isotropic layers of alternating index of refraction, where the periodicity can be destroyed only by changing the refractive index or thickness of a layer, a defect can be created in anisotropic media by introducing an additional rotation between consecutive layers. Computer simulations show that introducing an additional rotation in the middle of a sample with cholesteric ordering produces a localized state whose frequency can be tuned from one edge of the photonic stop band to the other by varying the angle of rotation from 0 to 180 degrees. Most of the energy of this mode exists as a circularly polarized standing wave with the same handedness as the structure, independent of the polarization of the exciting wave. This localized mode gives rise to a crossover in the nature of propagation. Below a crossover thickness, the localized mode is excited only by a wave with the same handedness as the structure and exhibits a peak in transmission at the defect frequency. Above the crossover, however, the defect mode can be excited only by the oppositely polarized wave and a resonant peak appears in reflection. Simulations for lengths below the crossover are in agreement with measurements of microwave transmission through stacks of overhead transparencies, ordered in the same way as the molecular layers of a cholesteric liquid crystal. Three types of defect are introduced: (1) an additional 90 degrees rotation, (2) an additional 45 degrees rotation, and (3) a combination of a 45 degrees rotation and a quarter-wavelength separation.
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