We have discussed the different types of qubits which forms the building blocks of quantum computer. In order to create a novel quantum computer, one has to have powerful quantum processors which could handle complex computations.
One of type of qubit which is becoming more mature in its stage to achieve the near-term mature quantum computer is spin qubits. Why do I say this qubit to be more mature is because of the already available methods of fabrication in semiconductor industry. Due to the abundance of Silicon (Si), semiconductor fabrication is far easier. Only thing we need to make sure is to create and manipulate spin qubits.
Oh yes! If you want to know more about the various fields in quantum, I will strongly suggest you go through my previous article,
Various fields in Quantum computing | by Shalini D | Apr, 2024 | Medium
Ok, now lets start!
Before that knowing the basics of electron is mandatory.
Fundamentals of electrons:
See, electrons have two fundamental angular momenta. I am aware that this concept is chaotic for us. I have tried to keep it as simple as possible.
- Intrinsic Spin Angular Momentum is an inherent property of all electrons (and other elementary particles) that does not arise from spatial motion but is an intrinsic characteristic of the particle, analogous to its mass or charge. The electron’s spin has only two possible orientations in an external magnetic field, commonly referred to as “spin up” and “spin down.”
- Orbital Angular Momentum relates to the motion of electrons around the nucleus of an atom, similar to how planets have angular momentum due to their orbit around the Sun. This form of angular momentum is quantized and determined by the electron’s quantum state within the atom.
In short, Intrinsic angular momentum is the spin of electrons. In terms of quantum, spin is quantized. If you imagine electrons spin as a literal movement as is the case for classical angular momentum, its false. But why? Electron spin, as a fundamental quantum property, is quantized, meaning it can take on only two intrinsic states, commonly referred to as spin up and spin down and that is also termed as pure states. It can’t take more value than this. But these two states can be in superposition until we measure in any of the axis. When states are in superposition, it's called as mixed states (in physics term).
See, electron spin is more intrinsic in nature like Earth’s rotation similar where we have night and day just like spin up and spin down, electron orbital is more analogous to the earth’s revolution around sun. Simple!!
You can also visualize this in terms of Bloch Sphere. Spin up is typically represented by a vector pointing to the north pole of the sphere. This corresponds to the quantum state ∣0⟩ or ∣↑⟩ (Below image, ∣+z⟩).
Spin down is represented by a vector pointing to the south pole of the sphere. This corresponds to the quantum state ∣1⟩ or ∣↓⟩ (Below image, ∣-z⟩).
Image source: Bloch sphere visualization of spin-1/2 precession around a vertical… | Download Scientific Diagram (researchgate.net)
Ok, that's too much about details about spin!
Do you know that spin qubit comes under solid state qubits?
Let’s understand the meaning of solid-state qubits in short.
Solid state systems are those that are based on electronic / magnetic properties of solid. For example, Si is an agreed example of solid-state system because of its electronic or magnetic properties allowing it to be using as a semi-conducting material.
Also, semiconductors have crystalline structure. This means that their atoms are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. The crystalline structure of semiconductors is crucial for their electrical properties and behavior. For example, Silicon and germanium has diamond cubic crystal structure. Other semiconductors have different crystal structure.
Image source: Diamond Cubic Unit Cell — Materials Science & Engineering (msestudent.com)
Ok, coming back to the question how to justify that spin qubits comes under solid state category?
I can think of 2 reasons straightaway.
- Material basis and integration into solid state devices:
Spin qubits are made to form by creating an electrostatic potential inside the nano-structure material called nanowire. Usually, nanowire is made up of silicon. The very fact that these qubits are created and manipulated within solid materials places them firmly within the category of solid-state qubits.
2. Manipulation using solid-state electronics:
The control and readout mechanisms of spin qubits rely on advanced solid-state electronics. Techniques such as electron spin resonance (ESR) or nuclear magnetic resonance (NMR) are used to manipulate the spin states through magnetic fields generated by currents in nearby gates or wires embedded in the solid matrix. Additionally, the spin state of a qubit can be coupled to the charge state in a quantum dot or similar structure, which can then be controlled and read out using electrical gates. These methods are deeply rooted in solid-state physics and electronics, leveraging the properties of the solid-state environment to achieve precise control over the qubits.
So, from discussion we understood basics of electron spin, spin qubits and solid-state qubits.
P.S. This is just a beginning of spin qubits and there are a lot of details about spin qubits one should know which I will exactly do in my upcoming articles.
