68 research outputs found
Photon correlation studies of single GaN quantum dots
We present measurements of the second-order coherence function on emission
from single GaN quantum dots. In some cases a large degree of photon
antibunching is observed, demonstrating isolation of a single quantum system.
For a selected quantum dot, we study the dependence of photon antibunching on
excitation power and temperature. Using pulsed excitation, we demonstrate an
ultraviolet triggered single-photon source operating at a wavelength of 358 nm.Comment: 3 pages, 4 figure
Tensile strain engineering of germanium micro-disks on free-standing SiO2 beams
Tensile strain is required to enhance light-emitting direct-gap recombinations in germanium (Ge), which is a promising group IV material for realizing a monolithic light source on Si. Ge micro-disks on free-standing SiO2 beams were fabricated using Ge-on-Insulator wafers for applying tensile strain to Ge in a structure compatible with an optical confinement. We have studied the nature of the strain by Raman spectroscopy in comparison with finite-element computer simulations. We show the impacts of the beam design on the corresponding strain value, orientation, and uniformity, which can be exploited for Ge light emission applications. It was found that the tensile strain values are larger if the length of the beam is smaller. We confirmed that both uniaxial and biaxial strain can be applied to Ge disks, and maximum strain values of 1.1 and 0.6% have been achieved, as confirmed by Raman spectroscopy. From the photoluminescence spectra of Ge micro-disks, we have also found a larger energy splitting between the light-hole and the heavy-hole bands in shorter beams, indicating the impact of tensile strain
Tensile strain of germanium micro-disks on freestanding SiO2 beams
Tensile strain is crucial to expect the direct recombination in germanium (Ge), towards monolithic light sources on silicon (Si). Freestanding beams of Ge are known to produce strong tensile strain, however, it is not trivial to construct a cavity in a freestanding structure. Here, we fabricated Ge micro-disks on freestanding oxide beams, and observed Whispering-Gallery-Modes (WGM) by photoluminescence. The tensile strain was larger in shorter beams, which is consistent with simulations
Mean-field Coherent Ising Machines with artificial Zeeman terms
Coherent Ising Machine (CIM) is a network of optical parametric oscillators
that solves combinatorial optimization problems by finding the ground state of
an Ising Hamiltonian. In CIMs, a problem arises when attempting to realize the
Zeeman term because of the mismatch in size between interaction and Zeeman
terms due to the variable amplitude of the optical parametric oscillator pulses
corresponding to spins. There have been three approaches proposed so far to
address this problem for CIM, including the absolute mean amplitude method, the
auxiliary spin method, and the chaotic amplitude control (CAC) method. This
paper focuses on the efficient implementation of Zeeman terms within the
mean-field CIM model, which is a physics-inspired heuristic solver without
quantum noise. With the mean-field model, computation is easier than with more
physically accurate models, which makes it suitable for implementation in FPGAs
and large-scale simulations. Firstly, we examined the performance of the
mean-field CIM model for realizing the Zeeman term with the CAC method, as well
as their performance when compared to a more physically accurate model. Next,
we compared the CAC method to other Zeeman term realization techniques on the
mean-field model and a more physically accurate model. In both models, the CAC
method outperformed the other methods while retaining similar performance.Comment: 8 pages, 4 figure
Experimental investigation of performance differences between Coherent Ising Machines and a quantum annealer
Physical annealing systems provide heuristic approaches to solving NP-hard
Ising optimization problems. Here, we study the performance of two types of
annealing machines--a commercially available quantum annealer built by D-Wave
Systems, and measurement-feedback coherent Ising machines (CIMs) based on
optical parametric oscillator networks--on two classes of problems, the
Sherrington-Kirkpatrick (SK) model and MAX-CUT. The D-Wave quantum annealer
outperforms the CIMs on MAX-CUT on regular graphs of degree 3. On denser
problems, however, we observe an exponential penalty for the quantum annealer
() relative to CIMs () for fixed anneal times, on both the SK model and on 50%-edge-density
MAX-CUT, where the coefficients and
are problem-class-dependent. On instances with over vertices, a
several-orders-of-magnitude time-to-solution difference exists between CIMs and
the D-Wave annealer. An optimal-annealing-time analysis is also consistent with
a significant projected performance difference. The difference in performance
between the sparsely connected D-Wave machine and the measurement-feedback
facilitated all-to-all connectivity of the CIMs provides strong experimental
support for efforts to increase the connectivity of quantum annealers.Comment: 12 pages, 5 figures, 1 table (main text); 14 pages, 12 figures, 2
tables (supplementary
Scaling advantages of all-to-all connectivity in physical annealers: the Coherent Ising Machine vs. D-Wave 2000Q
Physical annealing systems provide a heuristic approach to solve NP-hard Ising optimization problems. It is believed that the connectivity between spins in such annealers significantly impacts the machine's computational effectiveness. In this paper we study the performance of two types of annealing machines that have very different connectivity -- a commercially available quantum annealer built by D-wave Systems, which has sparse connectivity, and coherent Ising machines based on optical parametric oscillator networks, which have all-to-all connectivity. We demonstrate an exponential (e^(āO(N^2))) penalty in performance for the D-wave quantum annealer relative to coherent Ising machines when solving Ising problems on dense graphs, which is attributable to the differences in internal connectivity between the machines. This leads to a several-orders-of-magnitude time-to-solution difference between coherent Ising machines and the D-wave system for problems with over 50 vertices. Our results provide strong experimental support to efforts to increase the connectivity of physical annealers
Experimental investigation of performance differences between coherent Ising machines and a quantum annealer
Physical annealing systems provide heuristic approaches to solving combinatorial optimization problems. Here, we benchmark two types of annealing machinesāa quantum annealer built by D-Wave Systems and measurement-feedback coherent Ising machines (CIMs) based on optical parametric oscillatorsāon two problem classes, the Sherrington-Kirkpatrick (SK) model and MAX-CUT. The D-Wave quantum annealer outperforms the CIMs on MAX-CUT on cubic graphs. On denser problems, however, we observe an exponential penalty for the quantum annealer [exp(āĪ±_(DW)N^2)] relative to CIMs [exp(āĪ±_(CIM)N)] for fixed anneal times, both on the SK model and on 50% edge density MAX-CUT. This leads to a several orders of magnitude time-to-solution difference for instances with over 50 vertices. An optimalāannealing time analysis is also consistent with a substantial projected performance difference. The difference in performance between the sparsely connected D-Wave machine and the fully-connected CIMs provides strong experimental support for efforts to increase the connectivity of quantum annealers
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