Almost Quantum
Theory
“This new
version of quantum theory is even stranger than the original
Science Hub Editorial
TeamMarch 03, 2023
An idea called
almost quantum theory predicts particles could have stronger correlations than
we've ever observed. If tests show it to be true, it would be a huge scientific
upset.
“EINSTEIN
attacks quantum theory.” That was the headline in The New York Times on 4 May
1935. The world’s most famous scientist and two collaborators had discovered
what they saw as a fatal flaw at the heart of our greatest theory of nature.
They had found that particles separated by kilometres seemed to be able to
interact instantaneously with each other. Albert Einstein called it “spooky
action at a distance”.
Even though he
had helped lay the foundations of quantum theory, Einstein felt it must be
missing something. That spookiness just didn’t feel right – there must be
something we weren’t seeing that could explain it. No idea this strange could
be true, surely?
We now know that
it is. That is the lesson from most of the past century of physics, as quantum
theory, including spooky action at a distance, passed every experimental test
thrown at it. At the tiniest scales, reality really is as strange as our best
theory of the subatomic world suggests.
What we haven’t
figured out is why quantum theory is so strange. Physicists like me have long
been examining its foundations in search of answers. Recently, these efforts
have turned up a major surprise: a new hypothesis called “almost quantum
theory” that is even more bizarre than the original. What really excites me is
that we might be on the cusp of putting it to the test. If it passes, the
newspapers will be reporting the scientific upset of the century.
Quantum theory
deals with the subatomic world of particles, and it describes their behaviour
with peerless accuracy. It is often spoken of as the most bulletproof
scientific theory. But that doesn’t make its ideas any easier to digest. Among
its strange facets is that subatomic particles can exist in a cloud of possible
states called a superposition before they are measured – the counterintuitive
nature of which is most famously captured by Schrödinger’s cat, the thought
experiment about a feline that is simultaneously dead and alive. Then, there is
the fact that light, say, can behave as both a particle and a wave.
But it is
Einstein’s spooky action at a distance, more properly known as non-locality,
that bamboozles us most. Take two particles, prepared using a special procedure
known as quantum entanglement, and send them far apart. If you peek at one, you
will immediately be able to discern some of the quantum properties of the
other. It seems that they influence each other instantaneously over large
distances, even though no influence takes place. “Spooky” really is the word.
The Bell test
To grasp
non-locality more fully, it helps to consider an anecdote about odd socks first
told by the Irish physicist John Bell, who greatly advanced our understanding
of the quantum world. It was inspired by Reinhold Bertlmann, who worked with
Bell in the late 1970s. Bell realised his colleague had a habit of wearing a
different coloured sock on each foot. This meant that as soon as you saw that
one of Bertlmann’s socks was pink, for instance, then you knew the other one
wouldn’t be pink.
Bell thought
that sounded suspiciously similar to entanglement. It made him wonder if
entanglement was as odd as it seemed. The socks anecdote can be explained
easily enough by Bertlmann’s choices as he dressed. Could the correspondence
between entangled particles be similarly predetermined – thus explained by
everyday, non-quantum physics?
Bell’s genius
was to answer this question with what has come to be known as the Bell test. It
involves entangling two particles and sending them far apart, to labs where
they can be measured in two different ways. Each lab makes one measurement, not
knowing which one the other lab has chosen, and uses that to predict things
about the result of the other lab’s measurement. Think of it as the quantum
version of looking at the pink sock and predicting that the other sock isn’t
pink. They do this lots of times and count up the number of correct
predictions. Bell showed that if entanglement can be explained by everyday,
non-quantum physics, you would get the right answer in a Bell test no more than
75 per cent of the time. When the test is conducted on quantum entangled
particles, however, the right answer emerges 85 per cent of the time.
Bell’s test,
then, was a way to quantify how weird the correlations between quantum
particles are – and it showed that they really do exceed anything we can
explain using classical physics. This is what we really mean when we talk about
“non-locality”.
Reading about
this is what first got me interested in becoming a physicist. The fact you
could ask such deep questions about reality and get a clear answer fascinated
me. Now, Bell’s test is playing a key role in the development of a set of ideas
even stranger than quantum theory.
These ideas had
their genesis 30 years ago, when researchers wondered if there were a single
principle at the heart of quantum theory. To see why that matters, compare
quantum theory with Einstein’s theory of special relativity. This was built
chiefly from the basic principle that nothing can travel faster than light. If
quantum theory can be similarly derived from one principle, a kind of essence
of quantum, it would not only be highly elegant, it might also show us where
the weirdness ultimately springs from.
In 1994, Sandu
Popescu at the University of Bristol, UK, and Daniel Rohrlich at Ben-Gurion
University of the Negev in Israel were mulling this over. They came up with a
potential theory of physics that mathematically formalised just two simple
principles. First, no signals can go faster than the speed of light. Second,
non-locality applies to reality. It all seemed routine. But they were in for a
shock.
It turned out
their idea, known as PR boxes, allowed for much stronger correlations than we
observe. A Bell test would produce the right answer 100 per cent of the time.
It seems obviously mistaken, but PR boxes started from reasonable assumptions,
so why was it wrong? “It was a huge surprise,” says Mirjam Weilenmann at the
Institute for Quantum Optics and Quantum Information in Austria.
This result went
largely unnoticed for a time. “Their work appeared in a somewhat obscure
journal,” says Matty Hoban at Quantinuum, a quantum computing company in
Oxford, UK. But a little over a decade ago, some physicists began to
investigate further.
One was Miguel
Navascués, also at the Vienna institute. In 2009, he too decided to reformulate
the rules of quantum theory, this time starting from the principle that nothing
travels faster than light and a new principle called macroscopic locality. The
latter says that, as we move from particle-sized objects to the larger,
macroscopic world, the rules of classical physics emerge and non-locality
vanishes. A Bell test under these assumptions showed the right answers for entangled
particles must occur less than 100 per cent of the time. It suggested that PR
boxes had gone off the rails because it left out the principle of macroscopic
locality. There was now a feeling that this kind of research might inch us
closer to finding the essence of quantum theory.
In the same
year, a team led by Marcin Pawłowski at the University of Gdansk in Poland
tried the reformulation trick again, this time starting from a single principle
called information causality. This says that when two people exchange
information, one can’t receive more than the other sent. This proved decisive.
A Bell test performed under the resulting formulation would produce the right
answers 85 per cent of the time, the maximum level of accuracy observed in real
experiments.
Spooky action at
a distance
This caused
quite a stir. “Information causality was an enormous success, it was amazing,”
says Navascués. Some thought that we might finally have hit on the essence of
quantum theory. “People said maybe this principle encapsulates all of quantum
mechanics,” says Navascués. But he wasn’t so sure. He didn’t think the authors
had done enough to show that their framework could describe all the nuances of
quantum physics, not least the other strange phenomena beyond non-locality.
So, Navascués,
Hoban and their collaborators came up with yet another proposal in 2015. It
misses out some of the information contained in quantum theory proper, which is
why it has become known as almost quantum theory. But it seems to come with everything
we know to be true about quantum theory baked in. What’s more, when you work
through the result you would get from a Bell test under almost quantum theory,
it again comes out as about 85 per cent. Navascués and his collaborators had
achieved their aim of showing the flaws in information causality because that
principle didn’t uniquely reproduce quantum theory.
It might seem a
downer that information causality had been found wanting. But when you think it
through, there is an exciting alternative: what if almost quantum theory is
actually the true description of reality?
In almost all
situations, it makes the same predictions as regular quantum theory. Yet there
are some unusual instances where, in a surprising twist, it predicts that there
would be correlations between particles that are stronger than plain vanilla
quantum theory does. None of these situations has so far been experimentally
investigated. So that leaves us in a historic position. We have a potentially
viable theory of reality that we can’t rule out, and it suggests that, in some
circumstances, quantum theory isn’t weird enough to do justice to reality.
As if that
weren’t thrilling enough, there is another reason to get excited about almost
quantum theory. One of the biggest missions of physics is to find a more
unified description of reality. At the moment, our theories of gravity and the
quantum world are separate beasts, and a promising way of uniting them would be
to find a quantum version of general relativity. It turns out that almost
quantum theory has a similar mathematical structure to one candidate for a
theory of quantum gravity, known as the consistent histories formulation of
quantum gravity. The building blocks of this hypothesis, proposed by Nobel
prizewinner Murray Gell-Mann, correspond to sequences of particle interactions.
The idea isn’t currently popular and this could all be a coincidence. Or it
could be telling us something. “I thought it was a really cool connection,”
says Hoban.
Quantum
entanglement
It is vital that
we find out if almost quantum theory stands up. But it won’t be easy. It
predicts that, in certain situations, particles can have stronger correlations
than we have ever observed. But, by definition, the systems of particles
involved will be harder to control and work with. One way to put it to the test
might be to conduct a version of the Bell test with three particles instead of
two, says Ana Belén Sainz, also at the University of Gdansk. “I would love to
see these experiments,” she says.
The only trouble
is, we don’t yet know what kinds of particles would be best for such tests.
Familiar ones like electrons or photons aren’t likely to be hiding much. But
Navascués says there are systems of quantum particles that we have always
struggled to control – particles like kaons, which are composed of quarks
bundled together in an unusual way. He thinks these might be hiding
post-quantum physics.
Another place to
look for this is inside quantum computers, says Hoban. Within these machines,
lots of particles interact in ways we can’t always understand. “I would love it
if we start building these quantum computers and, suddenly, they’re not
behaving as they should,” says Hoban. This could be a sign of almost quantum
theory. Navascués agrees that looking at systems where large numbers of
particles are interacting might be fertile ground. He is talking with a group
of experimentalists in China to explore how they could design systems like this
and test them.
If almost
quantum theory turns out to be true, there will be major implications. The
ability to entangle particles underpins quantum computing and quantum
cryptography. Quantum computing promises a revolution by providing a totally
new way to do calculations. Quantum cryptography offers a reliable way to secure
communications and could form the basis of a quantum internet. If almost
quantum theory is true and we can harness it, it could supercharge both
technologies.
Even if all this
turns out to be smoke and mirrors, the search for new principles of physics is
valuable. The more we learn about quantum theory, the better the chance we
might find a way to reconcile it with general relativity, Einstein’s theory of
gravity. “Quantum theory is already super old compared to other theories, but
there are so many new avenues people explore all the time,” says Weilenmann.
Speaking of
Einstein, you have to spare a thought for him in all of this. He fervently
hoped that spooky action at a distance was a flaw that would end up showing
quantum mechanics was wrong. Little did he know that 90 years later we might be
about to find an even spookier theory of physics.”
Ray Gruszecki
March 4, 2023