All About Qubits

I recently heard about an online quantum computer that IBM has made available to the public and I talk a bit about it in this post.

The fact that you can play with a quantum computer online is cool both figuratively and literally since the computer is chilled down to 0.015 K.

Up to that moment, quantum computers were interesting to me and something I intended to study eventually but now that I know that I can actually play with one, well I just have to figure it out for real now.

In another post of mine I also discussed the Feynman technique of learning. I intend to use this technique and go through the basic concepts of quantum computing in a series of posts.

Basically I am going to follow my own advice and learn about quantum computing by writing about it. Let's see how it goes.

The Classic Computing Bit

First let's start with the classic information bit which is just a representation of the numbers 0 and 1.

Many things can be used to represent a bit:

• On a hard drive we use magnetic north versus magnetic south.
• On CD's, DVD's and Blurays we use non-reflective pits versus reflective materials (usually aluminum).
• In an fiber optic cable we use light on versus light off.
• In an electronic computer circuit we use 5V versus 0 V.

The thing about a classic bit is that it will only represent either a 0 or a 1 at any given time. Never both at the same time.

The Qubit

A qubit, which is pronounced "cue-bit", is a quantum representation of a 0 or a 1. It is often written down as "|0>" and "|1>" to represent the quantum state. You can already tell that this is quite unusual and a bit weird. More about that later.

Just as for classical bits, a number of objects can be used to represent a qubit:

• An electron with spin up versus spin down.
• An atomic nucleus with spin up versus spin down (but you must choose a nucleus with half-integer spin though).
• A photon with vertical polarization versus horizontal polarization.
• Some more macro-scale device like a Superconducting Josephson Junction.

The important part is that the object you choose must be able to behave in a "quantum manner".

Essentially this means that both superposition of states needs to be possible for the qubit and entanglement of 2 or more qubits needs to be a property of the device.

Superposition

Superposition is a property of a quantum object which can be in the two states, 0 and 1, at the same time (until you 'read' them at which point they collapse into either a 0 or a 1 like a classic bit).

Example:

Examples are helpful so imagine the double slit experiment in which a single photon is emitted at a time and goes towards and through a double slit and then hits a screen behind that. We will call the holes in the slit 'hole 0' and 'hole 1'.

If you do this over and over for thousands of individual photons you will eventually build up the classic interference wave pattern. At least one interpretation of quantum mechanics says that the photon passed through both slits at the same time and that it 'interfered with itself' to produce the interference pattern on the screen.

This is one example of superposition, the photon was in two positions at the same time, i.e. hole 0 and hole 1.

This is quantum weirdness at its most fundamental.

Entanglement

Entanglement means that if you have two entangled quantum objects you can only describe them as a whole system and not as separate systems.

Example:

An electron has a property called 'spin'. It can be either spin 'up' or spin 'down'. Now imagine that some sub-atomic particle with zero spin decays into two electrons which go flying out from it in different directions.

The total spin before the decay is zero, and since spin must be conserved, the total spin of the subsequent two electron system must also add up to zero. This means that one electron will be spin up and the other will be spin down.

These two electrons are therefore entangled. Say you measure one electron to be spin up, you will always find that the other electron will be spin down.

It turns out that you can use entanglement to bump a quantum state of one object with another object to change its value without losing the special quantum superposition property (the second quantum object can take on the properties of the first one without losing coherence and becoming just an ordinary bit).

The Incredible Instability of Qubits

Quantum objects are small and are very very easily perturbed which means that qubits are incredibly unstable.

Things that usually destabilize qubits are heat, light and depending on the system, magnetic fields. This is why quantum computers are cooled down to near absolute zero and kept in dark, magnetically shielded conditions.

Even with all of these precautions qubits will stay unperturbed for only very short period on the order of 50 microseconds or so. This means that any quantum program must necessary also be short and reach its finish before decoherence occurs.

Closing Words

That's enough for one day. There is so much more to go over and learn and I hope to do that over the course of the next days and weeks in a series of blog posts.

Thank you for reading my post.