How does a Qubit work?

The Qubit

A Qubit is the basic component of a Quantum Computer. It's like a Bit from Classical Computing in the same way a Lamborghini Veneno and a supermarket trolley can both carry groceries. (OK, maybe stretching it a bit, but allow me some poetic license.)

In my time at Cisco, the question I often got asked was what does a Qubit look like? How does it work? So this article focuses on the mechanics of a Qubit, not the whole shebang about superposition, entanglement and interference and all the other stuff that Quantum Computing involves.

The Reality of Quantum

Reality comes in different flavours. There is actual reality, where if you poke a bear, it will end your existence. Then there's virtual reality, which is a much safer way to poke a bear, and finally we have what I will call Quantum Reality. This is a very bizarre realm where, although you are certain you poked the bear, you never really touched it. Unfortunately for you, quantum or not, your demise at the paws of the bear is still certain (100% probability).

It is in this weird world of Quantum that the Qubit exists. Where sub-atomic particles like electrons do their thing. Where humans are not really welcome, but we can't help ourselves and have to poke around.

The Probabilities of Quantum Computing

OK, to set the scene I do need to quickly explain the difference between Quantum Computing and Classical Computing

Quantum computing is all about the probability of an outcome as opposed to classic digital computer which produces a discrete outcome. With a QC you run a specialised algorithm maybe thousands of times. Each run (called a shot) improves the probability of a correct answer of the previous run. Eventually you arrive to an answer with a high degree of probability of it being correct. A classic computer will produce an answer after a single run.

However, when solving very complex exponential type problems, such as calculating the best path every bus can take across every street in New York to efficiently cover the city, or finding the prime factors of a encryption key, it would take a classical computer more time than there have been humans on earth. Forget the bus and walk...

An appropriately sized (depends on the number of Qubits) Quantum Computer using a quantum algorithm could solve the problem in less than a day.

The Anatomy of a Qubit

A Qubit is a two-level quantum mechanical version of a classical data bit. Any quantum particle that can be measured in two discrete states could be used.

You could use a quantum particle from an atom, maybe a photon or an Ion.

In our example we are going to use an electron from a phosphorus atom (Atomic Qubit). At this point you may be thinking, why did I start reading this, it sort of made sense, now it doesn't. That's OK, you're human.

Recall I said, a Quantum Computer is all about probabilities. So by extension the basic component of a quantum machine, the Qubit has to work on probabilities, not just a 0 or 1. It does this through a characteristic of an electron called spin.

The Spin of an Electron

Not only do humans like to poke things, we like to spin things too, like yoyos and spinning tops. For that reason scientists refer to an electron's "spin" in a way that makes us all warm and fuzzy and reminds us of our childhood.

Alas, electrons don't spin like a top. No one can reliably say what the heck they do, but the correct term for spin is Intrinsic Angular Momentum. I'm no physicist, so at this point all you scientists out there are welcome to correct any misinformation in the comments.

You will often hear it quoted that a Qubit can be 0 AND 1 at the same time!

The statement is most likely influenced by Schr?dinger's cat paradox whereby if a cat is put into a box, you don't know whether it is dead or alive until you open the box. So the assertion is that the cat can be both dead and alive until we look.

Unfortunately it confuses the bejesus out of all those millions of folk who have been brought up in a discrete binary world where a computer bit is either 0 OR 1.

For those confused people may I offer that it isn't a 0 AND 1 at the same time.

It is SOME PROBABILITY of being a 0 OR 1 at the same time. It some cases the Qubit could have a higher probability of being a 0 and in other cases it may have a higher probability of being a 1 when measured (aka, opening the box).

This probabilistic state the Qubit finds itself in is called "Superposition", where it is somewhere between a 0 and a 1. But that is a article for another time.

So the secret to getting a Qubit to be in this superposition state where it might be a 0 or a 1 is the spin.

Electrons have a North and a South pole, if North is facing UP then the electron is considered to represent a 0, if North is facing DOWN then the electron represents a 1. In the diagram above you will see this Quantum 0 or 1 state is represented as ∣0? or ∣1?. This is called "bra-ket" notation.

The advantage of Bra-ket notation allows you to define the electron in a state of superposition using the notation α∣0? + β∣1? which means a bit of both. Alpha (α) is the probability of 0 and Beta (β) is the probability of 1. Therefore you can manipulate the spin in such a way that it is somewhere in between 0 and 1 and then when you measure it it will immediately point UP or DOWN based on the probability of (α) and (β).

But how do you actually do this manipulation? And how do you measure it?

The Anatomy of a Qubit

OK, we finally got to the point of this article. What does a Qubit technically look like?

Firstly, I have to acknowledge Professor Andrea Morello from UNSW Quantum Engineering for help and clarification he provided several years ago. The Qubit described below was based on the UNSW Quantum Computer.

Low Energy State

In the diagram below the Qubit is in the steady state (no energy applied). There is one atom of Phosphorus contained within a bed of pure silicon. Why Phosphorus? Because it has one extra electron that the Silicon atoms that surround it and it is this electron that represents the Qubit. A strong magnetic field controls the spin of the electron. There is a highly sensitive transistor on top of the Silicon chip that detects any current change from the electron.

High Energy State

To change the spin of the electron towards UP you need to provide it with a microwave energy pulse. The longer the pulse the more the spin will turn UP (higher probability of a 0), however you can stop the pulse at any moment to achieve the desired superposition state.

When you measure a Qubit (read its state), it will collapse to either a 0 (energy) or a 1 (no energy). You can't measure it in a half-way state. (See Schr?dinger).

When the electron is on the spin up state it will have more energy that the surrounding electrons in the transistor. Because of this, the additional electron from the Phosphorus atom (additional compared to the Silicon around it) will jump to the transistor.

This will leave the P atom with a positive charge because there are more protons (15) than electrons (14) in the atom. This additional positive charge is transferred to the transistor which will appear as a bust of current which represents a ∣0?. (You could say it represents a 100% probability of a 0).

By the way the whole thing has to be kept really cold because Qubits are very nervous. They jump at the slightest interference or noise, just like a cat (Schr?dinger again). Technically this is called coherence, how long can you keep the Qubit in superposition (whilst you setup your calculation) before it freaks out and collapses to a 0 or a 1. Ideally it will not collapse until you measure it.

The Physical Appearance

In case you were wondering what these things physically look like... A Quantum Computer would not be out of place in the Tate Modern.

The Conclusion

I thank you for getting this far. In terms of Quantum, you either get it, don't get it, or wish you never started. Apologies for any mistakes I may have included or areas that may have caused more confusion. All I can say is, join the club :-)

By the way, what I don't apologise for is writing in Australian/British English despite Linked-In's best efforts to replace 's' with 'z'.