7102 put it in a “one state” or
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.
A classical computer performs operations using classical
bits, which can be either zero or one. In contrast, a quantum computer uses
quantum bits (qubits), and they can be both zero and one at the same time. It
is this that gives a quantum computer its superior computing power.
What is a qubit ?
There are a number of physical objects that can be used as a
qubit. A single photon, a nucleus or an
electron. Some researchers used the outermost electron in phosphorous as a
qubit. But how does that work ?
All electrons have magnetic fields, so they basically like
tiny bar magnets, and this property is called spin. If you place them in a
magnetic field, they will align with that field, and this would be the lowest
energy state “zero state” which is called for the electron “spin
down”. You can put it in a “one state” or “spin up”,
but that takes some energy to reach that highest energy state. If you were so
delicate to really put it exactly against the magnetic field, it would stay
So far, this is basically just
like a classical bit. It has got two states, spin up and spin down, which are
like the classical one and zero. But what is interesting about the quantum
objects is that they can be in both states at once. When you measure the spin,
it will be either up or down. But before you measure it, the electron can exist
in a quantum superposition.
coefficients a & b indicate the
relative probability of finding the electron in one state or the other.
It is hard to imagine how this
enables this incredible computing power of quantum computers without
considering two interacting qubits. Now there are four possible states of these
In the case of a
& a , this quantum superposition of up down and
down up is what we call an entangled state. The two electrons
no longer have a direction of their own, so you cannot say that one electron is
down and the other one is up. The two electrons are in a state where they have
the opposite direction, but they do not have a direction of their own.
So it is only when
you measure one of them and you find it is “down”, the other ends up
being “up”. But until you measure them, they are in a state where their
only property is being opposite to each other.
Now if you have two bits in a classical computer, you can
There is four numbers, but they are still just two bits of
information. All I need to determine which one of the four numbers you have in
your computer code is the value of the first bit and the value of the second
In a qubit, instead, quantum
mechanics allows us to make a super position of each one of the four possible
states. So I can write a quantum mechanical state, that is some coefficients
times each one of the four states, i.e.
So to determine the state of this two spin system, I need to
give you four numbers, four coefficients. Whereas in the classical example of
the two bits, I only need to give you two bits. This how to understand why two
qubits actually contain four bits of information. I need to give you four
numbers to tell you the state of this system.
Now if we make three spins, we would have eight different
states and it could give you eight different numbers to define the state of
those three spins, whereas in classical it is just three bits. If you keep
going, what you will find is that the amount of equivalent classical
information contained by N qubits is classical bits. And, of course, the power of
exponentials tells you that once you have, for example, 300 of those qubits in
what we call fully entangled state, you will be able to create states where
there is a superposition of all 300 qubit states, then you will have like classical bits, which is as many particles as
there are in the universe.
Although the qubits can exist in any combination of states,
when they are measured they must fall into one of the basis states. And all the
other information about the state before the measurement is lost. So you do not
want generally to have as the final result of your quantum computation
something that is a very complicated super positional state, because you cannot
measure a superposition. You can only measure one of the basis states. So what
you want is to design the logical operations that you need to get to the final
computational result in such a way that the final result s something you are
able to measure, just a unique state.
This is the reason why quantum computers are not a
replacement of classical computers. They are not universally faster. They are
only faster for special types of calculations where you can use the fact that
you have all these quantum superpositions available to you at the same time, to
do some kind of computational parallelism. If you just want to watch a video in
high definition or browse the internet or write some documenting work, they are
not going to give you any particular improvement if you need to use a classical
algorithm to get the result.
You should not think of a quantum computer as something
where every operation is faster. In fact, every operation is probably going to
be slower than in the computer you have at your desk. But it is a computer
where the number of operations required to arrive at the result is
exponentially small. So the improvement is not in the speed of the individual
operation, it is in the total number of operations you need to arrive at the
result. But that is only the case in particular types of calculations,
particular algorithms. It is not universally, which is why it is not a
replacement of a classical computer.
How to make a quantum bit ?
How do you actually make a qubit in practice? and how do you
read and write information on it ?
Some researchers use the outermost electron in a phosphorous
atom as a qubit. A single phosphorous atom is embedded in a silicon crystal
right next to a tiny transistor. 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 using a super conducting
magnet, which is a large solenoid.
The electron will line up with its spin pointing down in 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 and 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. And
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 super positions.
To read out the information, you use the transistor that the
phosphorous atom is embedded next to. In this transistor there is, in fact, a
little bundle of electrons. This bundle of electrons is filled up to a certain
energy and all these electrons line up in energy levels just like the electrons
on the shells of an atom. So now if the electron is pointing up, it can jump
into the transistor, because it has more energy than all the others. It leaves
behind the bare nuclear charge of the phosphorous with a positive charge. So it
is as if you have a positive voltage applied to the gate which comes from the
atom. It is like the transistor has been switched more on, and so you see a
pulse of current that indicates that the electron was in the spin up state.
A quantum computer consists of quantum bits (qubits), which
are quantum devices that can exist in different superpositions and can be used
to store information. Each qubit can be given a known initial state, then a
quantum logic gate acts on it to give the result that can be read out by making
a quantum measurement.