The main ingredients for chips dedicated to quantum sensing, quantum computation, quantum measurement, etc, are non-classical states of light like single and entangled photons.
Effect of a defect on the cavity eigenfrequencies. (a) Schematics of a microring resonator, a 2D photonic crystal cavity, and corresponding mode field intensities in the azimuthal/longitudinal directions. The grey “cloud” represents a defect. (b) Consequence on the eigenfrequencies ω (red arrows). Horizontal green lines show the same (different) spectral shift δω(δω,k ) in the resonance frequencies for ring resonators (PhC) caused by the defect on the top (bottom) figure, depending on the spatial profile of the modes. Image Credit: Institute of Electrical and Electronics Engineers.
The fabrication of a conventional chip is hard, but with billions of dollars of specialized equipment and guys in white bunny suits it could be done. The fabrication of a quantum chip is very hard. Besides, sources of non-linear light are required, making such light sources fabricable.
Construction of around ≈ 20 µm non-classical light sources has been demonstrated in recent research.
The study hails from the four corners of France plus the US National Institute of Standards and Technology (NIST) in Maryland. They start by elaborating on quantum resonators that produce non-classical states of light, similar to the microring and photonic crystal (PhC) cavity.
The scientists had made the “exotic” choice, the photonic crystal. They developed the first Optical Parametric Oscillator (OPO) functioning at room temperature with the microwatt-level continuous wave pump.
Indium Gallium Phosphide (InGaP) is utilized instead of silicon. The test vehicle was developed to function in the telecom spectral range even when the emission spectrum could be engineered.
They illustrated repeatability in the fabrication process and the potential to reach effective parametric conversion with the help of very low pump power (≈ 40 µW), a key to conserving energy.
The device generates correlated photons and a quadrature-squeezed vacuum below and near a threshold, both of which are resources for quantum information. The OPO creates correlated beams of coherent light over the threshold by efficiently converting the pump's power.
The research offers measurements on the OPO and analyzes issues like quality, tuning, tolerances, and scaling.
Photons from the squeezed light, pump decay, whispering gallery modes, degenerate cases. However, there is time-energy entangled photon pairs and “gentle” confinement conditions.
The charming language of photonic combined circuits for quantum computing belies its seriousness. There is a cause that NIST, the Commerce Department Lab, has been involved. NIST oversees cybersecurity technology. If bad actors get their quantum chips made, then it is possible to break any code.
As the authors describe, the quantum benefit vis-a-vis today’s digital chips is that quantum mechanics inside a crystal enables non-exponential scaling where fantastically complicated math is worried.
A deep dive into the idea of canonical resonant Four-Wave-Mixing (FWM), implies FWM occurs in a cavity enabling the interaction of just three modes (four in the non-degenerate case).
In this context, the usage of “canonical” relates to the details of a Hamiltonian matrix transformation (math). They defend their option of a PhC cavity visa vie ring resonators when considering the structural disorder. They display how to make a resonator with a prescribed number of modes and not more. This is significant since additional modes imply a bigger volume for the resonator.
More significantly, each of such modes could be regulated separately, that is, their frequency spacing and quality factor could be developed to be different. This translates into excellent control over the parametric processes, guaranteeing that only the preferred interactions occur efficiently, repressing parasitic effects. Practically obtaining this degree of control is extremely difficult.
The authors then matched the properties of three geometries of PhC multimode resonators. The photonic crystal comprises a 200 nm thin layer of In0.5Ga0.5P with a two-dimensional pattern of holes.
The detailed statistical analysis of a batch of new devices. Display that structural disorder induces unrelated fluctuations of the modes of the same resonator.
Investigators also compared the theory and experiment on a parametric oscillation in 11 OPOs, with good agreement on slope efficiency and threshold.
Journal Reference
Chopin, A., et al. (2022) Canonical Resonant Four-Wave-Mixing in Photonic Crystal Cavities: Tuning, Tolerances and Scaling. IEEE Journal of Selected Topics in Quantum Electronics. https://doi.org/10.1109/JSTQE.2022.3229164.