Tech.4 - Optional - Photon Quantum Technology

Source: Atharva Vidwans, https://medium.com/predict/a-parallel-approach-race-towards-building-a-perfect-quantum-computer-b9f7fc642865 Links to an external site.

Photonic Systems

Light is made up of small packets called Photons. These photons carry information, which acts as qubits. When these photons interact with each other, this information is processed, thus allowing computation.

In quantum mechanics, light behaves as both wave and particle. This can form a major hindrance to Photonics. Because if light acts as a wave, then it is challenging to guide the wave through a path. To address this issue in Integrated Optics, is to use Wave Guides. These are small cable-like structures called fiber optics. Once a photon enters through one end of a fiber optics cable, it travels through the cable without leakage. Like macro-size fiber optics cables, smaller waveguides are embedded into a photonic circuit such that photons interact in a controlled way.

The Photonic Qubit

It is necessary for photons to interact with each other to perform logical operations. This is where linear optical crystals come into the picture.

Researchers can engineer cavities and temporarily trap photons. This can act as two possible states that a qubit can hold:

  • First is when the photon is absent, which represents |0> state.
  • And second is when the photon is present, which represents |1> state.

These qubits can then form logic gates.

This type of computing is called ‘Linear Optical Quantum Computing’(LOQC). This technique relies on individual stationary photons trapped in cavities. It manipulates photons using lasers, beam splitters, and phase shifters. After applying gates and computations, photon detectors read the individual photons and analyze the results.

The major issue with LOQC is that single photons are difficult to experiment with and thus limit this technique to only a handful of photons. Furthermore, it requires a lot of technologies like synchronization of pulses, beam splitters, phase shifters, and accurate and fast single photodetectors. The feedback control of these detectors must also be extremely fast to select proper state preparation before photon loss becomes an issue.

To overcome the drawback of this system, scientists and engineers have come up with an innovative system for Photonic Quantum Computing called ‘Continuous Variable Quantum Computing’ (CVQC).

Unlike LOQC, the CVQC model requires passive linear optics (i.e. beamsplitters and phase-shifters) and photodetection. No quantum memory or feedforward is required. At the heart of CVQC lies the Hong-Ou-Mandel Effect. L

Continuous Variable Quantum Computing & the Photonic Chip

The best example of CVQC is Xanadu’s photonic chips. The typical Xanadu Photonic chip is shown below.

The chip is divided into different modules:

  • Pump Distribution
  • Squeezing
  • Filtering
  • Interferometer
  • Photon detection.

Pump Distribution

Pump Distribution is a module that distributes photons to various waveguides present on the chip. The bright classical laser light drives this module through waveguides.

Squeezers

Squeezing of photons contains small ring resonators, which create a small quantum state called Squeezed State. They act as a qubit and are carried to the next module through waveguides. 

Interferometers

These squeezed states of light enter a network array of beam splitters, and phase shifters called an interferometer. This is where computation takes place. User code is translated into voltage levels, leading the qubit to interact with one another. How qubits behave at the beam splitter is based on Hong-Ou-Mandel Effect.

A beam splitter is a 50–50 silver mirror. When two photons enter each waveguide of the beam splitter, the two photons interfere with each other, which is complete quantum. When they interfere at the neck, it purports that the output should be one of the four, as below.

But that is not the case. According to the Hong-Ou-Mandel Effect, both photons will appear either on Output1 or Output2. The first two cases shown in the table above will not appear. Only a single photon detector at the end will click, not both. This takes place at each junction of beam splitters. This quantum interference effect is called the Hong-Ou-Mandel Effect.

Along with beam splitters, phase shifters are used in the Interferometer module, which introduces the required phase in a certain qubit. The output from this module contains a highly entangled quantum state which encodes quantum information. These Photons are later measured using photon detectors.

Photon Detection

After passing through the interferometer, the quantum state is ready for readout. This task is carried out in this phase called Photon Detection. For this purpose, special photon detectors are used called Transition Edge Sensors. These are special detectors that count the number of photons present in each waveguide. This array of photon counts is reported back to the user on the screen as the output of the algorithm. These photon detectors operate at low temperatures.