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     Current Focus: "Quantum Optical Circuits"- with Single Photon Sources and Light Manipulating Circuits


  Why do we care about Single Photons?

    A single photon can act as qubit--a carrier for quantum information. The qubit may be encoded in the photon's polarization, number, phase, etc. Manipulation of single photons and their interference/ entanglement can result in Quantum Communication, Quantum Metrology, and Quantum Computation for Quantum Information Science.

  What is so special about a Single Photon Source?

    Single photon sources emit only one photon at a time of only one color (/frequency).

Everyday photon sources:
-- Emit lots of photon of different (random) color.
Single Photon Sources (SPSs):
-- Emit only one photon at a time.
-- All the photons emitted are of the same color so that they can interfere/ entangle.

  Material Realization of Single Photon Sources-Quantum Dots:

    One of the ways to implement a single photon source is to create a 3D confined structure of a lower bandgap semiconductor surrounded by a higher bandgap one- Referred to as Quantum Dots. In a Quantum Dot, the electron and hole energy levels are discrete, thus enforcing it to emit only one photon at a time. For example the following picture schematically depicts a InGaAs QD surrounded by GaAs.

  Dominant approach to implement QD Single Photon Sources:

    Currently dominant approach to form QD Single Photon Sources is to form the quantum dots using lattice mismatch strain between two semiconductors during epitaxial growth such as MBE. This class of QDs are referred to as self-assembled island quantum dots.

Major Limitation:
-- --QDs are formed randomly on substrate, with locations.
-- High degree of size and shape fluctuation amongst the QDs, thus the emitted photons by different QDs are of random different frequency.
-- No way to couple the SPS to an optical circuit deterministically.

  Our approach: Single Quantum Dots formed on Mesas in regular arrays:

    We have pioneered a new approach to form the single QDs in regular arrays with uniform size and shape- exploiting engineered stress-controlled growth on Mesas- that allows integration to and scaling of the optical circuits. We name this new class of QDs as Mesa-top Single Quantum Dots (MTSQDs).

  Demonstrated Array of MTSQDs as Single Photon Sources:

    We have recently demonstrated this new class of QDs as spatially regular array of single photon sources!

    Check out our paper for more details:
    J. Zhang et.al, J. Appl. Phys.120,243103(2016) [Link]
    J. Zhang et.al, Appl. Phys. Lett.114,071102(2019) [Link]

  Manipulating the single photons emitted from the ordered array of SPSs for Quantum Information Processing:

    Let us now consider what is needed to build an on-chip MTSQD Single Photon Source based quantum information processing system. - We need an optical circuit to manipulate the emitted photons. Specifically-

  1. Enhancing the single photon emission rate.
  2. Enhancing single photon emission directionality to a specific direction
  3. On-chip propagation of the emitted photons
  4. On-chip splitting of a photon into different branches
  5. On-chip recombining of two photons from different SPSs- resulting in photon interference.

Such photon interference from different SPSs form the quantum information processing in the optical circuits.

   Two approaches: There are two dominant approaches in the literature to manipulate the emitted photons.

   (1) Bragg Scattering: Photonic crystals that exploit Bragg scattering of light by array of holes in a semiconductor membrane surrounding the QD to trap the photons in space-3D confined (Cavity), or 2D confined (waveguide).

<< This is a schematic of a single QD coupled to a photonic crystal waveguide.
Major Limitation:

--Need different components for the different light manipulating functions mentioned above.
-- Need to match between these components to form a larger optical circuit.

   (2) Mie Resonance: Exploiting collective Mie resonance of array of dielectric building blocks (DBBs):
Mie resonance are optical resonances in dielectric blocks when the size of the block is comparable to the wavelength/ refractive index. Coupling the Mie resonances of arrays of dielectric building blocks (DBBs) provide a unique way to achieve manipulation of light.
For example, following is a DBB array that functions as a nano-scale Yagi-Uda antenna (Nanoantenna)-enhancing the emission rate of the SPS, enhancing directionality and also propagating the photons.

  Multiple Functions using a collective Mie Resonance:

   In larger optical circuits based on DBBs, a single Mie mode can actually provide all the needed light manipulating functions for a working quantum optical circuit based on the single photon source. This provides a path to quantum information processing system based on photons as qubits- using on-chip interference and entanglement of the photons.

    Check out our paper for more details:
    S. Chattaraj, J. Opt. Soc. Am. B. 33, 12(2016) [Link]
    S. Chattaraj et.al, arXiv1811.06652v1(2018) [Link]

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