40 | New Scientist | 6 November 2021
through a slit one at a time to see if they behave
like waves by forming interference patterns,
to show that proteins obey quantum rules.
This approach has its problems. When
you are working with big, complex objects,
their quantumness quickly disappears as a
result of interaction with the surrounding
environment – a phenomenon called
decoherence. Quantum states are fragile.
They easily break under bombardment from
gas molecules, stray photons of light and
even delicate electric and magnetic fields.
“Any quantum object can behave classically
if you’re not treating it properly,” says Chiara
Marletto at the University of Oxford. This
is especially troublesome for double-slit
experiments because it takes a long time to
build up the double-slit interference pattern –
time in which decoherence can run riot.
Leggett-Garg experiments are just as tricky.
They have their own sources of decoherence,
but researchers must also find ways to measure
a system without disturbing it. Only by doing
this can you say for sure whether the object is
in a quantum superposition or not. “You have
to do the measurement in a clever way,” says
Sinha. “You’re trying to measure something,
but on the other hand, you want to ensure
that the act of measurement doesn’t leave
any invasive mark.”
It is impossible to drag most quantum
systems – which move in discrete steps –
into the classical world, where movement is
continuous. This makes it hard to examine
quantum objects and the stuff we would usually
think of as classical in the same experiment.
But Sougato Bose, a theorist at University
College London, has a plan. He proposes using
an experimental set-up that can transcend the
classical and quantum worlds.
The set-up he has in mind, a simple harmonic
oscillator, comprises an object trapped inside
a well, moving back and forth like a swinging
pendulum. Precisely how it oscillates depends
on whether it obeys quantum or classical rules.
And as there is theoretically no limit to how
big a simple harmonic oscillator can be, Bose
and his collaborators hope to use it to take
a leap into the macroscopic world – using
a nanocrystal 100,000 times more massive
than objects tested by Arndt’s team.
To do this, the researchers’ idea is to look for
the swinging nanocrystal when they expect it
to be exactly in the middle of the oscillator, on
the border between left and right (see “Reality
in the balance,” right). “We don’t observe,and then we suddenly take a snapshot
observation,” says Bose. But crucially, the
detector will only be able to see one half
of the oscillator. If it sees the nanocrystal,
the researchers know it is in that side. If the
detector doesn’t, they know it is in the other.
If the crystal is behaving in a classical way,
it should be there half of the time on this first
measurement. Then, after waiting the time it
takes to complete half a swing and so return
to the centre of the system, the researchers
would measure again and would expect
to see it half the time. But if the particle is
quantum, the act of not seeing it in one half of
the oscillator would collapse its so-called wave
function – the mathematical description of the
quantum state. Even though we don’t see the
nanocrystal, we now know its position, and
because of quantum uncertainty, this injects
the particle with momentum and changes the
way it is oscillating. By repeating measurementsat set intervals, the researchers hope to be able
to build up correlations that tell them whether
the nanocrystal is behaving in a quantum way
or classically. The trick in all of this is to throw
out the measurements in which the nanocrystal
is seen and only keep the ones in which it isn’t,
so that the measurements are non-invasive.
Since Bose and his collaborators proposed
the experiment in 2018, advances in the
trapping and cooling of nanocrystals to avoid
decoherence, alongside new precision lasers,
mean the idea can now be realised. Teaming
up with Hendrik Ulbricht, an experimentalist
at the University of Southampton, UK, Bose
plans to carry out the test on a nanocrystal
made of about a billion atoms. “It’s a big
jump,” says Ulbricht.
Only recently have lasers become sharp
enough to determine which side of the trap the
nanocrystal is oscillating in. Bigger particles
are described by smaller waves and so for these