Kind: captions
Language: en
To find the prime factors of a48 bit
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 cubits. 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 cubit 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 phosphorus atom as a
cubit. This single phosphorus atom is
embedded in a silicon crystal right next
to a tiny transistor. Now the electron
has a magnetic dipole called it 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's 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 is a coil of superconducting
wire that sits inside of that vessel
that is full of liquid helium.
So now the electron will line up with
its spin pointing down. That's 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 cubit, 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's 45.021 021 GHz 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 into 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 it mutate and stop your pulse
at some specific point, what you've
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
phosphorus atom is embedded next to.
The spin down has the lower energy and
the spin up has the higher energy. Now
in this [music] transistor there is in
fact uh a little puddle of electrons.
This puddle of electrons is filled up up
to a certain [music] energy. So this
vertical axis here is energy. And here
we have 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
[music] into the transistor, right?
Because it has more energy than all the
others. It leaves behind the bare
nuclear charge of the phosphorus. Right?
The phosphorus has one more positive
charge in the nucleus as compared to
silicon. But normally it's neutralized
by the extra electron. So the [music]
two things cancel out. But if you take
the electron away, then the phosphorus
has a positive charge. So it's as if you
have a positive voltage, a more positive
voltage applied to this gate. It doesn't
come from the gate. It [music] comes
from the atom, but it's the same. It's
just a positive voltage. It's 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's
because you had an electron spin up. So
you can play catching a spin up or a
spin down electron. You see there was no
current here. That was a spin down
electron. Can try again. Again, a spin
down electron.
Spin up electron. Now, these researchers
have actually gone further using the
nucleus of the phosphorus atom as a
cubit. Like an electron, the nucleus has
a spin, although it's 2,000 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's so small, so weakly
magnetic, and so perfectly isolated from
the rest of the world, it's a cubid 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's 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're looking at now
is an experiment where we actually flip
the nucleus every 5 seconds. So for 5
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
were applying to the electron. And then
for the other 5 seconds the electron
will not respond because we have flipped
the nucleus the other way. So now watch
in this period of time the nucleus has
been flipped down.
Yeah.
And now after 5 seconds it'll flip up
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 5 seconds.
Spin of a 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 5% 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's a completely
nonmagnetic 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 quantum computing.
They're being produced to redefine the
kilogram through the Avagadro project.
So the offcuts from that silicon sphere
are actually being used as the home for
cubits. That I think is incredible.
There is no waste in this science.
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