How To Make a Quantum Bit

To find the prime factors of a 2048 number, it would take a classical computer millions of years; a quantum computer could do it in just minutes. And that is because a quantum computer is built on qubits, these devices which take advantage of quantum superposition to reduce the number of steps required to complete the computation. But how do you actually make a qubit in practice, and how do you read and write information on it?

I met up with researchers who are using the outermost electron in a phosphorous atom as a qubit. This single phosphorous atom is embedded in a silicon crystal right next to a tiny transistor. Now the electron has a magnetic dipole called its spin. And it has two orientations, up or down, which are like the classical one and zero. Now to differentiate the energy state of the electron when it is in spin up and spin down, you need to apply a strong magnetic field.

And to do that we use a superconducting magnet. So, a superconducting magnet is a large solenoid. It is a coil of superconducting wire that sits inside that vessel that is full of liquid helium.

So now the electron will line up with its spin pointing down. That is its lowest energy state. And it would take some energy to put it into the spin up state. But actually not that much energy, and if it were at room temperature, the electron would have so much thermal energy that it would be bouncing around from spin up to spin down and back. And so you need to cool down the whole apparatus to only a few hundredths of a degree above absolute zero. That way you know that the electron will definitely be spin down.

There is not enough thermal energy in the surroundings to flip it the other way.

Now if you want to write information onto the qubit, you can put the electron into the spin up state by hitting it with a pulse of microwaves. But that pulse needs to be a very specific frequency, and that frequency depends on the magnetic field that the electron is sitting in.

So what you see here is the frequency that is being produced by this microwave source, and it is 45.021 gigahertz, which in the magnetic field that we are applying now is the resonance frequency of the electron.

So the electron is a little bit like a radio that can only tune in to one station. And when that station is broadcasting, the electron gets all excited and turns to the spin up state.

But you can stop at any point. So if you just make a new tape and stop your pulse at some specific point, what you have created is a special quantum superposition of the spin up and spin down states with a specific phase between the two superpositions.

And how do you read out the information? Well, you use the transistor that this phosphorous atom is embedded next to.

The spin down has the lower energy. And the spin up has the higher energy. Now in this transistor, there is, in fact, a little bundle of electrons. This bundle of electrons is filled up to a certain axis. This vertical axis here is energy. And here we have got all these electrons that line up in energy just like the electrons on the shells of an atom.

So now if the electron is pointing up, it can jump into the transistor, right? Because it has more energy than all the others. It leaves behind the bare nuclear charge of the phosphorous, right? The phosphorous has one more positive charge in the nucleus compared to silicon, but normally it is neutralized by the extra electron, so the two things cancel out. But if you take the electron away, then the phosphorous has a positive charge. So it is as if you have a positive voltage, a more positive voltage applied to this gate.

It doesn’t come from the gate. It comes from the atom, but it is the same. It is just a positive voltage.

It is like the transistor has been switched more on. And so you see a pulse of current, and that indicates that the electron was in the spin up state.

In this measurement phase, if you find one of these spikes of current, it is because you had an electron spin up. So it can play catching a spin up or a spin down event. You use, there was no current here. That was a spin down event. And try again. Again a spin down electron. Spin up electron.

Now these researchers have actually gone further, using the nucleus of the phosphorous atom as a qubit. Like an electron, the nucleus has a spin, although it is 2000 times weaker than the spin of the electron. But you can still write to it the same way using electromagnetic radiation; only it needs to be a longer wavelength and a longer pulse in order to get the spin to flip.

Because it is so small, so weakly magnetic, and so perfectly isolated from the rest of the world, it is a qubit that lives for a very long time.

But how do you read out the spin of the nucleus? Well, you use the electron. Remember that the electron’s frequency that it will respond to depends on the magnetic field that it is sitting in.

So that magnetic field is the external magnetic field that is produced by the superconducting magnet, but there is also an internal magnetic field coming from the nucleus. But that internal magnetic field can have two directions. Right? The nucleus can be pointing up or down itself. So what it means is that there are two frequencies at which the electron can respond, depending on the direction of the nucleus.

So the nucleus actually acts as a little selector. It tells the electron, basically, which radio station it can listen to.

So what you are looking at now is an experiment where we actually flip the nucleus every five seconds. So for five seconds you will see that the electron always responds, because the nucleus is always in the right direction to make the electron respond to the frequency we are applying to the electron. And then for the other five seconds, the electron will not respond because we have flipped the nucleus the other way.

So now watch. You see?

So in this period of time the nucleus has been flipped down.

Yeah.

And now after five seconds it will flip up, and then...

Yeah, you see?

And then the electron starts responding.

Yeah. So you are watching on the oscilloscope screen in real time the measurement of the direction of a single nucleus and our ability to flip it at will every five seconds.

The spin of the single nucleus.

Nucleus.

Now because all of this depends so sensitively on magnetic fields, you need to make sure to eliminate all spin from the silicon crystal. But, unfortunately, natural silicon contains about five percent of the isotope silicon 29, and that does have a spin.

But, in fact, the beauty of silicon is that it has this isotope called silicon 28 that has no nuclear spin. The nuclear spin is zero. So it is a completely non-magnetic atom.

But where are you going to find a pure crystal of silicon 28? Oh, wait.

These isotopically purified silicon 28 crystals are being produced anyway for a purpose completely different from particle computing. They are being produced to redefine the kilogram through the Avogadro project.

So the off cuts from that silicon sphere are actually being used as the home for qubits. That, I think, is incredible. There is no waste in this science.

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