Quantum information processing relies on the fundamental property of quantum interference, where the quality of the interference directly correlates to the indistinguishability of the interacting particles. The creation of these indistinguishable particles, photons in this case, has conventionally been accomplished with nonlinear crystals and optical filters to remove spectral distinguishability, albeit sacrificing the number of photons. This research describes the use of an integrated aluminum nitride microring resonator circuit to selectively generate photon pairs at the narrow cavity transmissions, thereby producing spectrally indistinguishable photons in the ultraviolet regime to interact with trapped ion quantum memories. The spectral characteristics of these photons must be carefully controlled for two reasons: (i) interference quality depends on the spectral indistinguishability, and (ii) the wavelength must be strictly controlled to interact with atomic transitions. The specific ion of interest for these trapped ion quantum memories is Ytterbium which has a transition at 369.5 nm with 12.5 GHz offset levels. Ytterbium ions serve as very long lived and stable quantum memories with storage times on the order of 10’s of minutes, compared with photonic quantum memories which are limited to 10-6 to 10-3 seconds. The combination of the long lived atomic memory, integrated photonic circuitry, and the photonic quantum bits are necessary to produce the first quantum information processors. In this article, I will present results on wavelength operation, dispersion analysis, and second harmonic generation in aluminum nitride waveguides.
Ring resonators are used as photon pair sources by taking advantage of the materials second or third order non- linearities through the processes of spontaneous parametric downconversion and spontaneous four wave mixing respectively. Two materials of interest for these applications are silicon for the infrared and aluminum nitride for the ultraviolet through the infrared. When fabricated into ring type sources they are capable of producing pairs of indistinguishable photons but typically suffer from an effective 50% loss. By slightly decoupling the input waveguide from the ring, the drop port coincidence ratio can be significantly increased with the trade-off being that the pump is less efficiently coupled into the ring. Ring resonators with this design have been demonstrated having coincidence ratios of 96% but requiring a factor of ~10 increase in the pump power. Through the modification of the coupling design that relies on additional spectral dependence, it is possible to achieve similar coincidence ratios without the increased pumping requirement. This can be achieved by coupling the input waveguide to the ring multiple times, thus creating a Mach-Zehnder interferometer. This coupler design can be used on both sides of the ring resonator so that resonances supported by one of the couplers are suppressed by the other. This is the ideal configuration for a photon-pair source as it can only support the pump photons at the input side while only allowing the generated photons to leave through the output side. Recently, this device has been realized with preliminary results exhibiting the desired spectral dependence and with a coincidence ratio as high as ~ 97% while allowing the pump to be nearly critically coupled to the ring. The demonstrated near unity coincidence ratio infers a near maximal heralding efficiency from the fabricated device. This device has the potential to greatly improve the scalability and performance of quantum computing and communication systems.
Quantum information processing relies on the fundamental property of quantum interference, where the quality of the interference directly correlates to the indistinguishability of the interacting particles. The creation of these indistinguishable particles, photons in this case, has conventionally been accomplished with nonlinear crystals and optical filters to remove spectral distinguishability, albeit sacrificing the number of photons. This research describes the use of an integrated aluminum nitride microring resonator circuit to selectively generate photon pairs at the narrow cavity transmissions, thereby producing spectrally indistinguishable photons. These spectrally indistinguishable photons can then be routed through optical waveguide circuitry, concatenated interferometers, to manipulate and entangle the photons into the desired quantum states. Photon sources and circuitry are only two of the three required pieces of the puzzle. The final piece which this research is aimed at interfacing with are trapped ion quantum memories, based on trapped Ytterbium ions. These ions serve as very long lived and stable quantum memories with storage times on the order of 10’s of minutes, compared with photonic quantum memories which are limited to 10-6 to 10-3 seconds. The caveat with trapped ions is the interaction wavelength of the photons is 369.5nm and therefore the goal of this research is to develop entangled photon sources and circuitry in that wavelength regime to interact directly with the trapped ions and bypass the need for frequency conversion.
Presented here are results on a silicon ring resonator photon pair source with a high heralding efficiency. Previous ring resonator sources suffered from an effective 50% loss because, in order to generate the photons, the pump must be able to couple into the resonator which is an effective loss channel. However, in practice the optical loss of the pump can be traded off for a dramatic increase in heralding efficiency. This research found theoretically that the heralding efficiency should increase by a factor of ∼ 3:75 with a factor of 10 increase in the required pump power. This was demonstrated experimentally by varying the separation (gap) between the input waveguide and the ring while maintaining a constant drop port gap. The ring (R = 18:5μm, W = 500nm, and H = 220nm) was pumped by a tunable laser (λ ≈ 1550nm). The non-degenerate photons, produced via spontaneous four wave mixing, exited the ring and were coupled to fiber upon which they were filtered symmetrically about the pump. Coincidence counts were collected for all possible photon path combinations (through and drop port) and the ratio of the drop port coincidences to the sum of the drop port and cross term coincidences (one photon from the drop port and one from the through port) was calculated. With a 350nm pump waveguide gap (2:33 times larger than the drop port gap) we confirmed our theoretical predictions, with an observed improvement in heralding efficiency by a factor of ∼ 2:61 (96:7% of correlated photons coupled out of the drop port). These results will enable increased photon flux integrated photon sources which can be utilized for high performance quantum computing and communication systems.
InAs quantum dot (QD) laser heterostructures are grown by molecular beam epitaxy (MBE) system on GaAs substrates and fabricated. The InAs QD lasers exhibit comparable properties of the state-of-the-art QD lasers with the threshold current density Jth and efficiency ηi of 475A/cm2 and 72.6%, respectively, at room temperature. The quantum dot laser emission is butt-joint coupled into silicon photonics waveguides by aligning the laser and silicon photonics chips with two translation stages. Due to the optical feedback to the laser cavity at the air/Si interface, the laser power self-pulsation and reduced threshold current density are observed. And the effective facet reflectivity, Reff, of 62.7% is obtained from the theoretically analysis of the laser characteristics. Furthermore, the silicon photonics waveguides interface is coated with the SiO2/TiO2 antireflection (AR) coating layers, and no laser performance interference is observed owing the reduced optical feedback.
Here we present the experimental demonstration of a Silicon ring resonator photon-pair source. The crystalline Silicon ring resonator (radius of 18.5μm) was designed to realize low dispersion across multiple resonances, which allows for operation with a high quality factor of Q~50k. In turn, the source exhibits very high brightness of >3x105 photons/s/mW2/GHz since the produced photon pairs have a very narrow bandwidth. Furthermore, the waveguidefiber coupling loss was minimized to <1.5dB using an inverse tapered waveguide (tip width of ~150nm over a 300μm length) that is butt-coupled to a high-NA fiber (Nufern UHNA-7). This ensured minimal loss of photon pairs to the detectors, which enabled very high purity photon pairs with minimal noise, as exhibited by a very high Coincidental-Accidental Ratio of >1900. The low coupling loss (3dB fiber-fiber) also allowed for operation with very low off-chip pump power of <200μW. In addition, the zero dispersion of the ring resonator resulted in the production of a photon-pair comb across multiple resonances symmetric about the pump resonance (every ~5nm spanning >20nm), which could be used in future wavelength division multiplexed quantum networks.
The need for bright efficient sources of entangled photons has been a subject of tremendous research over the last decade. Researchers have been working to increase the brightness and purity to help overcome the spontaneous nature of the sources. Periodic poling has been implemented to allow for the use of crystals that would not normally satisfy the phase matching conditions. Utilizing periodic poling and single mode waveguide confinement of the pump field has yielded extremely large effective nonlinearities in sources easily producing millions of photon pairs. Here we will demonstrate these large nonlinearity effects in a periodically poled potassium titanyl phosphate (PPKTP) waveguide as well as characterizing the source purity.
We present a quantum optical analysis of waveguides directionally coupled to ring resonators, an architecture realizable using silicon nanophotonics. The innate scalability of the silicon platform allows for the possibility of “on-chip” quantum computation and information processing. In this paper, we briefly review a comprehensive method for analyzing the quantum mechanical output of such a network for an arbitrary input state of the quantized, traveling electromagnetic field in the continuous wave (cw) limit. Specifically, we briefly review a recent theoretical result identifying a particular device topology that yields, via Passive Quantum Optical Feedback (PQOF), dramatic and unexpected enhancements of the Hong-Ou-Mandel Effect, an effect central to the operation of many quantum information processing systems. Next, we extend the analysis to our proposal for a scalable, on-chip realization of the Nonlinear Sign (NS) shifter essential for implementation of the Knill-Laflamme-Milburn (KLM) protocol for Linear Optical Quantum Computing (LOQC). Finally, we discuss generalizations to arbitrary networks of directionally coupled ring resonators along with possible applications is the areas of quantum metrology and sensitive photon detection.
The research detailed in this paper describes a Periodic Cluster State Generator (PCSG) consisting of a monolithic
integrated waveguide device that employs four wave mixing, an array of probabilistic photon guns, single mode
sequential entanglers and an array of controllable entangling gates between modes to create arbitrary cluster states.
Utilizing the PCSG one is able to produce a cluster state with nearest neighbor entanglement in the form of a linear or
square lattice. Cluster state resources of this type have been proven to be able to perform universal quantum
Here we present a fully quantum mechanical transfer function model for travelling wave whispering gallery mode
resonators. Micro-resonators, such as ring and disk resonators, have been key to the development of high performance
chip-scale photonic systems due to their compact footprint, sensitivity and low power operation. In this work we present
the first understanding of these resonators to any arbitrary multi-photon state. This was achieved by developing a model
that utilizes an efficient scheme for determining the quantum electrodynamic transfer functions relating the Bosonic
input/output mode operators in the resonator. This approach has been applied to the understanding of both single photon
and two-photon states. In this work we will present a key result on a resonant Hong-Ou-Mandel effect that is inherently
realized for any resonator-waveguide coupling constants and can operate over a wide range of resonance conditions.
Furthermore, the transfer function approach allows for the straightforward understanding of any resonator-waveguide
network with arbitrary modes. This will directly enable the application of quantum resonators to the realization of robust,
scalable and efficient Linear Optical Quantum Computing (LOQC) gates. Consequently, it is expected that resonators
can be used for both Nonlinear Sign Shift and CNOT gates. And these gates can robustly controlled and efficiently tuned
using standard electro-optic effects available in a variety of material systems, such as, Silicon.
In this paper, ultra low cross talk is achieved by using a resonant cavity at the intersection between two strip waveguides
formed in a square lattice photonic crystal structure (PhC). Two PhC structures are studied: one consists of cylindrical
rods and another consists of cubic rods. The Q-Factor of the cavity is changed by increasing the number of rods that form
the cavity and by decreasing the spacing between the waveguide and the cavity. Our two dimensional simulation results
show that the latter method resulted in cross talk reduction of more than 21 dB for both structures. The overall cross talk
was -90.50 dB for the cylindrical rods structure and -105.0 dB for the cubic rods structure. The optimized PhC structures
were fabricated on a silicon-on-insulator platform. The rods were buried in silicon oxide in order to maximize the
photonic band gap and provide index guiding in the vertical direction.
We use evolutionary algorithms to search the space of two-dimensional photonic crystals with maximum bandgaps for a given index contrast. The unit-cells of the photonic crystal are represented as bitmaps which are either directly encoded or generated through bottom-up and top-down construction trees. The fitness criterion rewards for partial band gaps and is evaluated by solving for the photonic crystals bands using an eigensolver in a planewave basis. Starting from random patterns and with no prior knowledge, the process discovered a number of novel photonic crystals, some with bandgaps that are 12.5% larger than any previously reported human-designed crystals.
We present experimental demonstration of fast all-optical switching in a one-dimensional photonic crystal nanocavity embedded in a Silicon waveguide. The transmission of the device is tuned by injecting free carriers into the nanocavity region using an optical pump beam. By strongly confining light in the photonic crystal nanocavity the sensitivity of light to small refractive index changes is enhanced. The small cavity volume (~0.1 μm3) and unpassivated sidewalls enable ultra-fast switching speeds with low pulse energies. Using a pulse energy of only 60pJ, a refractive index change of approximately 10-2 is obtained. This small index change, due to the high confinement nature of the cavity structure, leads to a strong change in transmission spectrum. Consequently, the resonance is shifted up to its full-width-at half-maximum (~7.5nm), and the transmission of the device is modulated by 71% with a time response of less than 1.5 ns. Such a device could open the door to the large-scale integration of ultra-fast modulators and switches.