Integrated photonic computing promises revolutionary strides in processing power, energy efficiency, and speed, propelling us into an era of unprecedented computational capabilities. By harnessing the innate properties of light, such as high-speed propagation, inherent parallel processing capabilities, and the ability to carry vast amounts of information, photonic computing transcends the limitations of traditional electronic architectures. Furthermore, silicon photonic neural networks hold promise to transform artificial intelligence by enabling faster training and inference with significantly reduced power consumption. This potential leap in efficiency could revolutionize data centers, high-performance computing, and edge computing, minimizing environmental impact while expanding the boundaries of computational possibilities. The latest research on our silicon photonic platform for next-generation optical compute accelerators will be presented and discussed.
This work investigates the effects of the confinement factor on the linewidth enhancement factor in hybrid silicon quantum dot comb lasers, which is a key parameter involved in frequency comb generation. Experiments are performed on two laser devices sharing the same gain material with slightly different cavity designs resulting in different confinement factors. The results highlight that a lower confinement factor leads to a smaller carrier-induced refractive index variation and a larger differential gain, together resulting in a smaller linewidth enhancement factor, which in turn translates into different sets of performance regarding the feedback applications. This paper brings novel insights on the fundamental aspects of quantum dot comb lasers and provides new guidelines of future on-chip light sources for integrated wavelength-division multiplexing applications.
We present a novel technology wherein memristors are heterogeneously integrated with optoelectronic devices on a silicon photonic platform. We present results on memristor integrated microring modulators and lasers with non-volatile memory. Furthermore, multiple devices are combined with optical waveguides to create photonic integrated circuits with neuromorphic computing. By pairing non-volatile memory devices directly with silicon photonics, we can integrate memory, computing, and high-speed optical interconnects all together on the same chip.
Increasing the throughput of a transmitter by scaling out to multiple wavelengths in high bandwidth density links, such as those found in data centers or high performance compute clusters, is beginning to gain traction for two reasons. First, increasing the data rate per wavelength by both increasing the baud rate (>50G) and by increasing the number of bits per symbol (PAM-4) consumes more power and increases latency, from having to use a powerful FEC. Second, a technological advantage, in achieving tight integration of lasers with silicon based photonics, has reduced laser coupling losses and tighter control on laser wavelengths, which in turn allows for greater utilization of the available optical bandwidth. In this talk, we will review our current and past efforts in realizing multi-wavelength laser sources heterogeneously integrated on silicon as a means to generate fully integrated transmitters with bandwidth capacity in excess of 1Tbps over 40 channels. We will also discuss some of the recent results from our MOSCAP microring modulators and Si-Ge/ quantum dot based avalanche photodetectors that enable a fully integrated heater-free compact transceiver. The estimated power consumption for the optical components in such a transmitter is around 1pJ/bit, which is roughly a 10x reduction compared to the state-of-the-art.
We present an avalanche photodetector (APD) behavioral modeling methodology for Silicon Photonics process design kit. The proposed APD behavioral model can describe nonlinearity of multiplication factors versus both bias voltage and input optical power, and it can also cover process-voltage-temperature (PVT) variations. Inside the APD model, built-in lookup tables that contain multiple coefficients are implemented, therefore nonlinear multiplication factor curves can be easily calculated with an automated coefficient setting algorithm which helps to streamline the verification process and reduce model parameter setting time without sacrificing accuracy. Specifically, as coefficients are derived from different PVT conditions, the proposed APD behavioral model has wide verification coverage.
On-chip integration of semiconductor lasers have shown a growing interest in recent years, especially for the development of photonic integrated circuits (PICs) which are of paramount importance for high-speed communication within and between data centers, and fast on-board data exchanges. For all these applications, a key challenge remains the stability of the laser sources integrated on a PIC in presence of external optical feedback with the view to avoid integrated bulky and costly optical isolation. In this study, the effects of external optical feedback are investigated in hybrid InAs/InP quantum dot comb lasers on silicon. The design of the cavity includes a semiconductor optical amplifier section, a saturable absorber and an on-chip external cavity incorporating a vertical coupler. We measured the resulting feedback properties with respect to the operation conditions (bias current and voltage) and to the length of the saturable absorber. We show that under most operating conditions, the laser remains stable against optical feedback, only few regimes of operation occur, which either improve or degrade the frequency comb and/or the radio-frequency beatnote power of the laser.
Recent experimental results show how classical accelerators based on analog computing can outperform quantum annealing alternatives in benchmark tasks that require dense connection matrices. In Hewlett Packard Labs, we have been studying two alternatives: integrated coherent Ising machines and mem-HNNs (based on memristive crossbar arrays). An important challenge for commercial viability is that different industrial workloads typically benefit from the availability of a variety of optimization algorithms and require a broad range of template combinatorial optimization problems. In this talk, we will discuss our recent progress in going beyond Max-Cut, and we will propose a broader range of algorithms. This flexibility in algorithm choices and template problems is an important step forward to address the wide variety of enterprise-level use-cases such as airline scheduling, supply chain optimization, real-time bandwidth management, gene sequencing, etc.
Coherent Ising machines have been proposed as a promising platform for combinatorial optimization. Initial fiber-based, FPGA-assisted instantiations experimentally outperform quantum annealers based on superconducting qubits in speed and energy-efficiency due their ability to have programmable all-to-all connectivity between the Ising nodes. Since then, multiple flavors of coherent Ising machines have been proposed based on silicon photonics. In this talk, we will compare and contrast these integrated Ising machines with their table-top setup counterparts and their upcoming competitors in digital and analog electronics. Moreover, we will explain how large-scale problems can be mapped to small-scale integrated Ising cores.
We present room-temperature continuous-wave lasing of 1.31 μm multi-quantum well lasers on a novel defect-free heterogeneous III-V-on-silicon integration platform. The epitaxially grown laser structure on the platform shows significantly low dislocation density of 9.5×104 cm-2, leading to a minimal threshold current density of 813 A/cm2. These results bring promise to create multi functionalities like source, modulation and detection, etc. on such a defect-free, low-cost, large-scale substrate for Datacom applications.
Optical interconnect is essential for massive data communication in rapidly developed data center and high-performance computing infrastructures. Large bandwidth, high energy efficiency and low latency are intrinsic advantages in optics. But they are also present R&D challenges under new requirements such as low total solution cost and reliable operation in harsh computing environment. Recently we have developed hybrid microring lasers on silicon to enable high integration density, compact chip size, and potentially volume and cost-effective production in a CMOS foundry. Novel structures such as thermal shunts and hybrid metal-oxide-semiconductor (MOS) capacitors were integrated into the laser cavity to allow over 100 oC cw operation and "zero-power" laser wavelength and power control. Special CMOS driver with equalization functionality for direct microring laser modulation with good signal integrality was also designed and fabricated in a 65 nm foundry process. For the first time, we integrated all these designs and chips together to demonstrate a 5-channel hybrid transmitter with 0.5 nm channel spacing and overall 70 Gb/s direct modulation rate. A novel direct photon lifetime modulation with much larger bandwidth than conventional injection current modulation by modulating bias on the MOS capacitor is demonstrated for the first time as well. Finally we review our on-going progress on migrating the similar design from a standard quantum well laser active region to a superior quantum-dot one for further improved temperature and dynamic performance.
Initially, we’ll discuss an SOI based, carrier injection micro-ring modulator. The static optical and electrical characteristics of this device will be reviewed and described. Thermal control and modulation mechanisms with pre-emphasis will be outlined. Automated wafer-lever optical/electrical results from volume foundries (Leti and STMicro) used for PDK/Verilog model development will be reviewed, along with experimental data on direct modulation to 25 Gb/s, crosstalk at various DWDM channel separations, and demonstrations with an external quantum-dot based comb laser with 80 and 50GHz channel spacing.
Following this, our work on directly modulated hybrid quantum-well ring lasers will be reviewed in design, fabrication. Experimental results for modulation at 12.5 Gb/s/channel, integration with MOS capacitor for wavelength control and modulation and a thermal shunt for temperature management will highlight the advantages of this technology that may be exploited. Subsequently, our work on hybrid quantum-dot based comb lasers for on-chip DWDM sources will be discussed in their details of physical operation, demonstrating successful mode-locking and noise-free operation across the 20-80C thermal range. Our work on the integration of on-chip APDs from a CMOS-compatible processes will also be reviewed, demonstrating error-free operation at 12.5 Gb/s and 25 Gb/s with a sensitivity of -26dBm and -16dBm, respectively. The use of APDs will drastically decrease the overall power consumption of the interconnect, lowering total cost of ownership. Finally, our most recent progress on integration of the silicon photonics with CMOS by a flip-chip will be reviewed showing high-speed modulation and thermal control for a multi-channel DWDM transceiver.
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.
We report a 200 mm silicon photonic platform integrating a set of devices dedicated on HPC applications. PiN microring modulator layout and process are optimized together. Active tuning through heating section is investigated using either doped silicon or metal resistors. This technology is supported by a dedicated process design kit (PDK) compatible with conventional CMOS EDA tools. The PDK includes optical device models that will be described and compared with experimental results. A focus will be done on the PiN micro-ring modulator models which covering a wide range of geometries. DC mode and RF behaviors are supported.
Increased data rates have motivated the investigation of advanced modulation schemes, such as four-level pulseamplitude modulation (PAM4), in optical interconnect systems in order to enable longer transmission distances and operation with reduced circuit bandwidth relative to non-return-to-zero (NRZ) modulation. Employing this modulation scheme in interconnect architectures based on high-Q silicon photonic microring resonator devices, which occupy small area and allow for inherent wavelength-division multiplexing (WDM), offers a promising solution to address the dramatic increase in datacenter and high-performance computing system I/O bandwidth demands. Two ring modulator device structures are proposed for PAM4 modulation, including a single phase shifter segment device driven with a multi-level PAM4 transmitter and a two-segment device driven by two simple NRZ (MSB/LSB) transmitters. Transmitter circuits which utilize segmented pulsed-cascode high swing output stages are presented for both device structures. Output stage segmentation is utilized in the single-segment device design for PAM4 voltage level control, while in the two-segment design it is used for both independent MSB/LSB voltage levels and impedance control for output eye skew compensation. The 65nm CMOS transmitters supply a 4.4Vppd output swing for 40Gb/s operation when driving depletion-mode microring modulators implemented in a 130nm SOI process, with the single- and two-segment designs achieving 3.04 and 4.38mW/Gb/s, respectively. A PAM4 optical receiver front-end is also described which employs a large input-stage feedback resistor transimpedance amplifier (TIA) cascaded with an adaptively-tuned continuous-time linear equalizer (CTLE) for improved sensitivity. Receiver linearity, critical in PAM4 systems, is achieved with a peak-detector-based automatic gain control (AGC) loop.
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.
Interconnect architectures based on high-Q silicon photonic microring resonator devices offer a promising solution to
address the dramatic increase in datacenter I/O bandwidth demands due to their ability to realize wavelength-division
multiplexing (WDM) in a compact and energy efficient manner. However, challenges exist in realizing efficient
receivers for these systems due to varying per-channel link budgets, sensitivity requirements, and ring resonance
wavelength shifts. This paper reports on adaptive optical receiver design techniques which address these issues and have
been demonstrated in two hybrid-integrated prototypes based on microring drop filters and waveguide photodetectors
implemented in a 130nm SOI process and high-speed optical front-ends designed in 65nm CMOS. A 10Gb/s powerscalable
architecture employs supply voltage scaling of a three inverter-stage transimpedance amplifier (TIA) that is
adapted with an eye-monitor control loop to yield the necessary sensitivity for a given channel. As reduction of TIA
input-referred noise is more critical at higher data rates, a 25Gb/s design utilizes a large input-stage feedback resistor
TIA cascaded with a continuous-time linear equalizer (CTLE) that compensates for the increased input pole. When
tested with a waveguide Ge PD with 0.45A/W responsivity, this topology achieves 25Gb/s operation with -8.2dBm
sensitivity at a BER=10-12. In order to address microring drop filters sensitivity to fabrication tolerances and thermal
variations, efficient wavelength-stabilization control loops are necessary. A peak-power-based monitoring loop which
locks the drop filter to the input wavelength, while achieving compatibility with the high-speed TIA offset-correction
feedback loop is implemented with a 0.7nm tuning range at 43μW/GHz efficiency.
The evolution of computing infrastructure and workloads has put an enormous pressure on datacenter networks. It is
expected that bandwidth will scale without increases in the network power envelope and total cost of ownership.
Networks based on silicon photonic devices promise to help alleviate these problems, but a viable development path for
these technologies is not yet fully outlined. In this paper, we report our progress on developing components and
strategies for datacenter silicon photonics networks. We will focus on recent progress on compact, low-threshold hybrid
Si lasers and the CWDM transceivers based on these lasers as well as DWDM microring resonator-based transceivers.
The negatively-charged nitrogen-vacancy centers in diamond has motivated many groups building scalable quantum information processors based on diamond photonics. This is owning to the long-lived electronic spin coherence and the capability for spin manipulation and readout of NV centers.1-4 The primitive operation is to create entanglement between two NV centers, based on schemes such as 'atom-photon entanglement' proposed by Cabrillo et al.5To scale this type of scheme beyond two qubits, one important component is an optical switch that allows light emitted from a particular device to be routed to multiple locations. With such a switch, one has choices of routing photons to specified paths and has the benefit of improving the entanglement speed by entangling multiple qubits at the same time. Yield of the existing diamond cavities coupled with NV centers are inevitably low, due to the nature of randomness for NV placement and orientation, variation of spectral stability, and variation of cavity resonance frequency and quality factor. An optical switch provides the capability to tolerate a large fraction of defective devices by routing only to the working devices. Many type of switching devices were built on conventional semiconductor materials with mechanisms from mechanical, thermal switching to carrier injection, photonics crystal, and polymer refractive index tuning .6-8 In this paper, we build an optical-thermal switch on diamond with micro-ring waveguides, mainly for the simplicity of the diamond fabrication. The the switching function was realized by locally tuning the temperature of the diamond waveguides. Switching efficiency of 31% at 'drop' port and 73% at 'through' port were obtained.
We demonstrate coupling between the zero phonon line (ZPL) of nitrogen-vacancy centers in diamond and the
modes of optical micro-resonators fabricated in single crystal diamond membranes sitting on a silicon dioxide
substrate. A more than ten-fold enhancement of the ZPL is estimated by measuring the modification of the
spontaneous emission lifetime. The cavity-coupled ZPL emission was further coupled into on-chip waveguides
thus demonstrating the potential to build optical quantum networks in this diamond on insulator platform.
The combination of the long electron state spin coherence time and the optical coupling of the ground electronic
states to an excited state manifold makes the nitrogen-vacancy (NV) center in diamond an attractive candidate
for quantum information processing. To date the best spin and optical properties have been found in centers
deep within the diamond crystal. For useful devices it will be necessary to engineer NVs with similar properties
close to the diamond surface. We report on properties including charge state control and preferential orientation
for near surface NVs formed either in CVD growth or through implantation and annealing.
Nitrogen-vacancy centers in diamond are widely studied both as a testbed for solid state quantum optics and for
their applications in quantum information processing and magnetometry. Here we demonstrate coupling of the
nitrogen-vacancy centers to gap plasmons in metal nano-slits. We use diamond samples where nitrogen-vacancy
centers are implanted tens of nanometers under the surface. Silver nano-slits are patterned on the sample such
that diamond ridges tens of nanometers wide fill the slit gap. We measure enhancement of the spontaneous
emission rate of the zero photon line by a factor of 3 at a temperature of 8K.
Hybrid silicon platform provides a solution to integrate active components (lasers, amplifiers, photodetectors, etc.) with
passive ones on the same silicon substrate, which can be used for building an optical interconnect system. Owing to the
advantages in footprint, power consumption, and high-speed modulation, hybrid silicon microring lasers have been
demonstrated as a potential candidate for on-chip silicon light source. In this paper we review the progress to improve
the performance of recently demonstrated compact microring lasers with ring diameter of 50 μm. A simple approach to
enhance optical mode and electron-hole recombination, which results in threshold reduction and efficiency improvement
is developed. This is done by appropriately undercutting the multiple quantum well (MQW) region to force carriers to
flow towards the outer edge of the microring for better gain/optical mode overlap. We observe a reduction of the
threshold of over 20% and up to 80% output power enhancement. The model and the experimental results highlight the
benefits, as well as the negative effects from excessive undercutting, including lower MQW confinement, higher modal
loss and higher thermal impedance. A design rule for MQW undercutting is therefore provided. Application as on-chip
optical interconnects is discussed from a system perspective.
We present DWDM nanophotonics architectures based on microring resonator modulators and detectors. We
focus on two implementations: an on chip interconnect for multicore processor (Corona) and a high radix network
switch (HyperX). Based on the requirements of these applications we discuss the key constraints on the photonic
circuits' devices and fabrication techniques as well as strategies to improve their performance.
We have devised and fabricated high-speed silicon-on-insulator resonant microring photodiodes. The detectors comprise a p-i-n junction across a silicon rib waveguide microring resonator. Light absorption at 1550 nm is enhanced by implanting the diode intrinsic region with boron ions at 350 keV with a dosage of 1 × 1013 cm−2. We have measured 3-dB bandwidths of 2.4 and 3.5 GHz at 5 and 15 V reverse bias, respectively, and observed an open-eye diagram at 5 gigabit/s with 5 V bias.
In this paper we review the recent progress in developing compact microring lasers on the hybrid silicon platform.
A simplified self-aligned process is used to fabricate devices as small as 15 μm in diameter. The optically-pumped,
continuous wave (cw) devices show low threshold carrier density, comparable to the carrier density to reach material
transparency. In the electrically-pumped lasers, the short cavity length leads to the minimum laser threshold less than 5
mA in cw operation. The maximum cw lasing temperature is up to 65 °C. Detailed studies in threshold as a function of
coupling coefficient and bus waveguide width are presented. Surface recombination at the dry-etched exposed interface
is investigated qualitatively by studying the current-voltage characteristics. Ring resonator-based figures of merits
including good spectral purity and large side-mode suppression ratio are demonstrated. Thermal impedance data is
extracted from temperature-dependent spectral measurement, and buried oxide layer in silicon-on-insulator wafer is
identified as the major thermal barrier to cause high thermal impedance for small-size devices. The demonstrated
compact hybrid ring lasers have low power consumption, small footprint and dynamic performance. They are promising
for Si-based optical interconnects and flip-flop applications.
The understanding of the coherence properties of photons emitted from negatively charged nitrogen-vacancy (NV)
centers in diamond is essential for the success of quantum information applications based on indistinguishable
photons. Here we study both the polarization of photons emitted from and the linewidth of photons absorbed by
single NV centers as a function of temperature T. We find that for T < 100 K the main dephasing mechanism
contributing to the linewidth broadening is phonon-mediated population transfer between the two excited orbital
states. The observed T5 temperature dependence of the population transfer rate and linewidth is experimental
evidence of a dynamic Jahn-Teller effect in the excited states.
A compact electrically-pumped hybrid silicon microring laser is realized on a hybrid silicon platform. A simplified, selfaligned,
deep-etch process is developed to result in low-loss resonator with a high quality factor Q>15,000. Small
footprint (resonator diameter=50 μm), electrical and optical losses all contribute to lasing threshold as low as 5.4 mA and
up to 65 °C operation temperature in continuous-wave (cw) mode. Outcoupling- and bus waveguide width-dependent
studies are conducted for optimizing device structure. A simple qualitative study in current-voltage (IV) characteristic
shows that dry etching through active region leads to <3× more leakage current at the same reverse bias than wet etch
counterpart. It indicates a relatively good interface with tolerable surface recombination from deep dry etch. The
spectrum is single mode with large extinction ratio (>40 dB) and small linewidth (<0.04 nm) observed. The unique
bistability operation in ring resonator structure is also demonstrated.
We present a novel quantum communication protocol for "Private Data Sampling", where a player (Bob) obtains
a random sample of limited size of a classical database, while the database owner (Alice) remains oblivious as
to which bits were accessed. The protocol is efficient in the sense that the communication complexity per query
scales at most linearly with the size of the database. It does not violate Lo's "no-go" theorem for one-sided twoparty
secure computation, since a given joint input by Alice and Bob can result in randomly different protocol
outcomes. After outlining the main security features of the protocol, we present our first experimental results.
High-channel-count WDM will eventually be used for short reach optical interconnects since it maximizes link bandwidth and efficiency. An impediment to adoption is the fact that each WDM wavelength currently requires its own DFB laser. The alternative is a single, multi-wavelength laser, but noise, size and/or expense make existing options impractical. In contrast, a new low-noise, diode comb laser based on InAs/GaAs quantum dots provides a practical and timely alternative, albeit in the O-band. Samples are being evaluated in short reach WDM development systems. Tests show this type of Fabry-Perot laser permits >10 Gb/s error-free modulation of 10 to over 50 separate channels, as well as potential for 1.25 Gb/s direct modulation. The paper describes comb laser requirements, noise measurements for external and direct modulation, O-band issues, transmitter photonic circuitry and components, future CMP applications, and optical couplers that may help drive down packaging costs to below a dollar.
We describe research on new optical structures in diamond for quantum information and sensing applications
based on the nitrogen-vacancy (NV) center. Results include etching experiments that reveal the vertical distribution
of NV centers produced by ion implantation and annealing, and gallium phosphide waveguides fabricated
on diamond with evanescent coupling to NV centers close to the diamond surface.
We observe the coupling of nitrogen-vacancy centers in single-crystal diamond to GaP waveguides on the diamond
surface. We describe the fabrication procedure and characterize the waveguide performance. Our results
indicate that the GaP/diamond hybrid system is a promising system for coupling nitrogen-vacancies to optical
microstructures for quantum information processing and sensing applications.
Spatial and k-space properties of subwavelength cross-section GaP waveguides supported by a diamond substrate are
analyzed theoretically. These waveguides are suitable for optically coupling to nitrogen vacancy centers located near the
surface of a single crystal diamond sample.
We present an unconditionally secure Oblivious Transfer protocol relying on two rounds of entanglement-free
quantum communication. When played honestly, the protocol only requires the ability to measure a single qubit
in a fixed basis, and to perform a coherent bit-flip (Pauli X) operation. We present a generalization to a "Private
Data Sampling" protocol, where a player (Bob) can obtain a random sample of fixed size from a classical database
of size N, while the database owner (Alice) remains oblivious as to which bits were accessed. The protocol is
efficient in the sense that the communication complexity per query scales at most linearly with the size of the
database. It does not violate Lo's "no-go" theorem for one-sided two-party secure computation, since a given
joint input by Alice and Bob can result in randomly different protocol outcomes. Finally it could be used to
implement a practical bit string commitment protocol, among other applications.
Nanophotonic structures can be used to dramatically enhance interactions between light and matter. We describe some of
our recent progress in fabricating optical nanostructures suitable for both classical and quantum information processing.
In particular, we present our progress using nanoimprint lithography, a low cost nanoreplication method, to fabricate low
loss photonic crystals.
Scalable quantum information processing using nitrogen-vacancy (NV) centers in diamond will be difficult without
the ability to couple the centers to optical microcavities and waveguides. Here we present our preliminary
result of coupling a single NV center in a nanoparticle to a silica microdisk at cryogenic temperatures. The
cavity-coupled NV photoluminescence is coupled out of the cavity through a tapered fiber. Although the current
system is limited by the spectral properties of the NV center and the Q of the cavity, efficient particle-cavity
and cavity-waveguide coupling should lead to the realization of a "one-dimensional atom" as needed for CQED,
enable single-shot electron-spin readout, and increase the probability of success in entanglement schemes based
on single-photon detection.
General requirements for single-photon devices in various applications are presented and compared with experimental
progress to date. The quantum information applications that currently appear the most promising require
a matter qubit-enabled single-photon source, where the emitted photon state is linked to the state of a long-lived
quantum system such as an electron spin. The nitrogen-vacancy center in diamond is a promising solid-state
system for realizing such a device due to its long-lived electron spin coherence, optical addressability, and ability
to couple to a manageable number of nuclear spins. This system is discussed in detail, and experimental results
from our laboratory are shown. A critical component of such a device is an optical microcavity to enhance the
coupling between the nitrogen-vacancy center and a single photon, and we discuss theoretically the requirements
for achieving this enhancement.
We present two experiments geared toward the realization of a robust and intense source of polarization-entangled
photons. First, we describe a novel source of polarization-entangled pairs that uses periodically-poled potassium
titanyl phosphate (PPKTP) and an interferometer based on polarization beam displacers. The source emits an
intense flux of high-quality single-mode entangled photons and is stable, robust, and easy to align. Second, we
report on sources of correlated photons generated in PPKTP waveguides. Waveguide sources of correlated pairs
have been shown to generate high fluxes of pairs: we theoretically and experimentally investigate spontaneous
parametric down-conversion generation of photon pairs in waveguides at different wavelengths.
We propose a novel design for a guided-mode resonance (GMR) grating sensor that is optimized for detecting
small average index changes in an extended region of space, retaining sensitivity up to several tens of microns
away from the grating surface at optical detection frequencies. This kind of sensors has high sensitivity in the
half-space above the grating, close to the theoretical limit, together with a controllable - potentially very high - quality factor. It relies on a resonance with a "confined" mode of a sub-wavelength thick grating slab, a mode
that is largely expelled from the grating itself. The small thickness assumption allows us to derive analytical
expressions for many properties of these sensors, expressions that are then tested numerically using a rigorous
coupled-wave analysis (RCWA) method, and in preliminary experiments.
We propose a novel design for a guided-mode resonance (GMR) grating sensor that extends the sensitivity to a
large region of space, possibly several tens of microns away from the grating surface. This type of sensors has
high sensitivity in the half-space above the grating, close to the theoretical limit, together with a controllable -
potentially very high - quality factor. It relies on a resonance caused by a "confined" mode of a sub-wavelength
thick grating slab, a mode that is largely expelled from the grating itself into the grating environment. The small
thickness assumption allows us to derive a simple yet accurate analytical model for the sensor behavior, which
is tested numerically using a rigorous coupled-wave analysis (RCWA) method as well as in preliminary grating
transmission measurements.
Nitrogen-vacancy centers in diamond typically have spin-conserving optical transitions, a feature which allows
for optical detection of the long-lived electronic spin states through fluorescence detection. However, by applying
stress to a sample it is possible to obtain spin-nonconserving transitions in which a single excited state couples to
multiple ground states. Here we describe two-frequency optical spectroscopy on single nitrogen-vacancy centers
in a high-purity diamond sample at low temperature. When stress is applied to the sample it is possible to
observe coherent population trapping with a single center. By adjusting the stress it is possible to obtain a
situation in which all of the transitions from the three ground sublevels to a common excited state are strongly
allowed. These results show that all-optical spin manipulation is possible for this system, and we propose that
that by coupling single centers to optical microcavities, a scalable quantum network could be realized for photonic
quantum information processing.
We present an experimental approach to study low light level absorption in a tapered optical fiber embedded
in a rubidium atomic vapor medium. Our initial measurements demonstrates the potential of the system to
realize extremely low light level quantum interference effects in the ultra small mode volume of the thin fiber,
which is promising for many practical integrated device applications. The measurement shows saturated
probe absorption using a low optical power of only ten nanowatt. Efforts are underway to use the fiber in a
cloud of trapped rubidium atoms, which will circumvent the transit time limit for demonstrating a low photon
optical switch via quantum interference.
Moore's Law has set great expectations that the performance/price ratio of commercially available semiconductor
devices will continue to improve exponentially at least until the end of the next decade. Although the physics
of nanoscale silicon transistors alone would allow these expectations to be met, the physics of the metal wires
that connect these transistors will soon place stringent limits on the performance of integrated circuits. We
will describe a Si-compatible global interconnect architecture - based on chip-scale optical wavelength division
multiplexing - that could precipitate an "optical Moore's Law" and allow exponential performance gains until
the transistors themselves become the bottleneck. Based on similar fabrication techniques and technologies, we
will also present an approach to an optically-coupled quantum information processor for computation beyond
Moore's Law, encouraging the development of practical applications of quantum information technology for
commercial utilization. We present recent results demonstrating coherent population trapping in single N-V
diamond color centers as an important first step in this direction.
We report on two experiments implementing quantum communications primitives in linear optics systems: a
secure Quantum Random Bit Generator (QRBG) and a multi-qubit gate based on Two-Photon Multiple-Qubit
(TPMQ) quantum logic. In the first we use photons to generate random numbers and introduce and implement
a physics-based estimation of the sequence randomness as opposed to the commonly used statistical tests. This
scheme allows one to detect and neutralize attempts to eavesdrop or influence the random number sequence. We
also demonstrate a C-SWAP gate that can be used to implement quantum signature and fingerprinting protocols.
A source of momentum-entangled photons, remote state preparation, and a C-SWAP gate are the ingredients
used for this proof-of-principle experiment. While this implementation cannot be used in field applications due to the limitations of TPMQ logic, it provides useful insights into this protocol.
We describe how a quantum non-demolition device based on electromagnetically-induced transparency in solidstate atom-like systems could be realized. Such a resource, requiring only weak optical nonlinearities, could potentially enable photonic quantum information processing (QIP) that is much more efficient than QIP based on linear optics alone. As an example, we show how a parity gate could be constructed. A particularly interesting physical system for constructing devices is the nitrogen-vacancy defect in diamond, but the excited-state structure for this system is unclear in the existing literature. We include some of our latest spectroscopic results that indicate that the optical transitions are generally not spin-preserving, even at zero magnetic field, which allows the realization of a Λ-type system.
Processing information quantum mechanically is known to enable new
communication and computational scenarios that cannot be accessed with conventional information technology (IT). It is known that such quantum processing can be performed with linear optical techniques, measurement and feed-forward. However, here the gates are intrinsically probabilistic and so scaling up such an approach requires considerable qubit resources. Here we present an alternative approach to optical QIP, based on the use of weak cross-Kerr non-linearities and highly efficient homodyne measurements. This approach enables a parity gate between optical qubits, mediated by an additional optical probe mode, enabling near-deterministic Bell state measurement and entangling gates. Our approach is therefore the basis for very efficient, scalable optical QIP, and provides a natural method for distributing such processing, combining it with quantum communication.
We review our work on electromagnetically induced transparency (EIT) as a potentially key enabling science for few-qubit Quantum Information Technology (QIT). EIT systems capable of providing two-qubit phase shifts as large as pi are possible in a condensed matter system such as NV-diamond, but the potentially large residual absorption necessarily arising under this condition significantly reduces the fidelity of a nonlinear optical gate based on EIT. Instead, we emphasize that a universal set of quantum gates can be constructed using EIT systems that provide cumulative phase shifts (and residual absorptions) that are much smaller than unity. We describe a single-photon quantum nondemolition detector and a two-photon parity gate as basic elements of a nonlinear optical quantum information processing system.
A narrow band, tunable Cr:LiSAF laser oscillator using self injection- locking for line narrowing was developed. With two etalons and one prism in the master oscillator arm of the composite resonator, output energies of up to 75 mJ/pulse were obtained with a linewidth of 1.5 pm FWHM at 830 nm, and a pulsewidth of 70 ns. This linewidth was achieved without the use of a separate injection seed source. Up to 530 mJ output energy was obtained in long pulse, free-running mode. The laser was tuned from 800 nm to 960 nm. the Q-switched laser data was consistent with the performance predicted by our model.
The technique of Type I quadrature doubling has been used to generate 266 nm output from 532 nm input. Using a simple flashlamp pumped Nd:YAG laser doubled in KTP, the best conversion efficiency from the second harmonic to the fourth harmonic was 42% using two KDP crystals. With a second harmonic conversion efficiency of 60%, the total conversion efficiency from 1.064 micrometers to 266 nm was 25%. Output energies at 266 nm of up to 11.5 mJ in a nearly diffraction-limited beam, and 22 mJ in a multimode beam, were obtained, limited by the pump laser. The transient nonlinear absorption of 266 nm induced in the KDP crystals was measured in our samples, and the effect was accurately modelled to predict performance of a harmonic conversion system using improved nonlinear crystals.
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