Module 2 - Photonic Qubit Sources

The fundamental building block of a classical information theory is the unit of information: the bits. Being a binary digit, it can take on the values 0 or 1 which we refer to as the two possible states of a bit. Likewise, within quantum information, we have a unit of information, now called a quantum bit or a qubit, but unlike a classical bit, be in any coherent superposition of 0 and 1. The superposition, interference and entanglement (in which multiple qubits share a common quantum state) is the basis of a quantum technology and make qubits fundamentally different and much more powerful than classical bits.

To build quantum computers and other quantum information technologies we need quantum objects that will act as qubits, any two-level quantum-mechanical system can be used as a qubit. Multilevel systems can be used as well, if they possess two states that can be effectively decoupled from the rest (e.g., ground state and first excited state of a nonlinear oscillator). Researchers are exploring more than half a dozen ways to implement qubits, with different approaches. Popular ones are superconducting circuits, trapped atoms and ions, photons and spins.

Photons as qubits

A photonic qubit is a qubit made from photons. In photonic quantum computing, the unit of light in a given mode or photon is used as information carriers and to represent a qubit. Photons have some properties that make them extremely attractive for use in quantum computers, their Light speed transmission and low noise properties make photons indispensable for quantum technology. Photons interact relatively weakly with their environment and with each other, this is the reason why photons can travel quite far in many materials without being scattered or absorbed, giving photonic qubits good coherence properties and making them useful for transmitting quantum information over long distances.

Single photons are largely free of the noise, or decoherence, that plagues other systems; can be easily manipulated to realize one-qubit logic gates; and enable encoding in any of several degrees of freedom, for example, polarization, time bin, or path. When using photons, there are various possible ways of qubit implementation, three of which are explained below. Superpositions of quantum states can be easily represented, encrypted, transmitted and detected using photons.

Polarization

Each photon carries an electromagnetic field with a specific direction, known as its polarization.

A qubit can be encoded as the polarization of a single photon. Using the polarization of the photons is a common choice for photonic qubits mainly due to the relative ease with which polarization can be manipulated in an experimental setup. The qubits are individual photons, with the two different photon polarizations (up-down and left-right) serving as the two qubit states. The two states used to define qubits are horizontal polarization and vertical polarization. Horizontal = 0, vertical = 1.

Image 1 shows how a qubit can be encoded in the polarization of a single photon. An arbitrary state of a single qubit 𝛼|𝐻〉+𝛽|𝑉〉 (|𝛼|2+|𝛽|2=1) can be represented on the Poincaré (or Bloch) sphere. Single qubit gates can be implemented with standard passive optical components used to rotate the polarization, but two-qubit gates require a low-loss nonlinearity, which is difficult to achieve.

Path

The path a photon takes is another way to define a qubit. We can actually put a photon in a superposition of being “here” and “there”, by using beam-splitters. Top path = 0, Bottom path = 1.

Time

It is also possible to build a photon qubit using its time of arrival (Timing Information). We can create a quantum superposition of a “photon arriving early” and a “photon arriving late”. Photon arrives early = 0, Photon arrives later = 1.

Where then two time-bins correspond to the logic basis states |0〉 and |1〉. Hence, the basis states can be written |1〉⊗|0〉 and |0〉⊗|1〉, where |1〉⊗|0〉 denotes having one photon in the first time-bin and none in the second. The qubits are created by sending photons through an unbalanced interferometer, creating a superposition between two different time-bins corresponding to whether the photon traversed the short or the long arm in the interferometer. Time coding is usually used for long distance communication over optical fibers, since it is fairly robust against decoherence effects in the fibers.

Why are Photons Suitable for Quantum Computing

• They have only weak interaction with the environment, which means a good coherence.
• Photons have high-speed, low-loss transmission and can be used for long distance quantum communication.
• Can achieve good single qubit control with standard optical components (waveplates, beamsplitters, mirrors, etc.).
• There are efficient photon detectors (e.g photodiodes).

Disadvantage

Weak two-photon interactions: Light quanta (photons) in a quantum computer must interact with each other. Under normal conditions, however, light does not interact with itself and so the challenge is to create correlations between them. (requires non-linear medium -> two-qubit gates are hard)