The high-stakes race to make quantum computers work - Chiara Decaroli
The contents of this metal cylinder could
either revolutionize technology
or be completely useless—
it all depends on whether we can harness
the strange physics of matter
at very, very small scales.
To have even a chance of doing so,
we have to control the environment
precisely:
the thick tabletop and legs guard against
vibrations from footsteps,
nearby elevators, and opening
or closing doors.
The cylinder is a vacuum chamber,
devoid of all the gases in air.
Inside the vacuum chamber is a smaller,
extremely cold compartment,
reachable by tiny laser beams.
Inside are ultra-sensitive particles
that make up a quantum computer.
So what makes these particles
worth the effort?
In theory, quantum computers could
outstrip the computational limits
of classical computers.
Classical computers process
data in the form of bits.
Each bit can switch between two states
labeled zero and one.
A quantum computer uses something
called a qubit,
which can switch between zero, one,
and what’s called a superposition.
While the qubit is in its superposition,
it has a lot more information
than one or zero.
You can think of these positions as
points on a sphere:
the north and south poles of the sphere
represent one and zero.
A bit can only switch between
these two poles,
but when a qubit is in its superposition,
it can be at any point on the sphere.
We can’t locate it exactly—
the moment we read it, the qubit resolves
into a zero or a one.
But even though we can’t observe the
qubit in its superposition,
we can manipulate it to perform
particular operations while in this state.
So as a problem grows more complicated,
a classical computer needs correspondingly
more bits to solve it,
while a quantum computer will
theoretically be able to handle
more and more complicated problems
without requiring as many more qubits as a
classical computer would need bits.
The unique properties of quantum computers
result from the behavior of atomic
and subatomic particles.
These particles have quantum states,
which correspond to the
state of the qubit.
Quantum states are incredibly fragile,
easily destroyed by temperature
and pressure fluctuations,
stray electromagnetic fields,
and collisions with nearby particles.
That’s why quantum computers need
such an elaborate set up.
It’s also why, for now,
the power of quantum computers
remains largely theoretical.
So far, we can only control a few qubits
in the same place at the same time.
There are two key components involved
in managing these fickle quantum
states effectively:
the types of particles a quantum
computer uses,
and how it manipulates those particles.
For now, there are two leading approaches:
trapped ions and superconducting qubits.
A trapped ion quantum computer uses
ions as its particles
and manipulates them with lasers.
The ions are housed in a trap made
of electrical fields.
Inputs from the lasers tell the ions what
operation to make
by causing the qubit state
to rotate on the sphere.
To use a simplified example,
the lasers could input the question:
what are the prime factors of 15?
In response, the ions may release photons—
the state of the qubit determines whether
the ion emits photons
and how many photons it emits.
An imaging system collects these photons
and processes them to reveal the answer:
3 and 5.
Superconducting qubit quantum computers
do the same thing in a different way:
using a chip with electrical circuits
instead of an ion trap.
The states of each electrical circuit
translate to the state of the qubit.
They can be manipulated with electrical
inputs in the form of microwaves.
So: the qubits come from either ions
or electrical circuits,
acted on by either lasers or microwaves.
Each approach has advantages
and disadvantages.
Ions can be manipulated very precisely,
and they last a long time,
but as more ions are added to a trap,
it becomes increasingly difficult to
control each with precision.
We can’t currently contain enough ions
in a trap to make advanced computations,
but one possible solution might be to
connect many smaller traps
that communicate with each
other via photons
rather than trying to create one big trap.
Superconducting circuits, meanwhile, make
operations much faster than trapped ions,
and it’s easier to scale up the number
of circuits in a computer
than the number of ions.
But the circuits are also more fragile,
and have a shorter overall lifespan.
And as quantum computers advance,
they will still be subject to the
environmental constraints
needed to preserve quantum states.
But in spite of all these obstacles,
we’ve already succeeded at making
computations
in a realm we can’t enter or even observe.
