The 2022 Nobel Prize in physics recognized three
scientists who made groundbreaking contributions in understanding one of the
most mysterious of all natural phenomena: quantum entanglement.
In the simplest terms, quantum entanglement means
that aspects of one particle of an entangled pair depend on aspects of the
other particle, no matter how far apart they are or what lies between them.
These particles could be, for example, electrons or photons, and an aspect
could be the state it is in, such as whether it is "spinning" in one
direction or another.
The strange part of quantum entanglement is that
when you measure something about one particle in an entangled pair, you
immediately know something about the other particle, even if they are millions
of light years apart. This odd connection between the two particles is
instantaneous, seemingly breaking a fundamental law of the universe. Albert
Einstein famously called the phenomenon "spooky action at a
distance."
Having spent the better part of two decades
conducting experiments rooted in quantum mechanics, I have come to accept its
strangeness. Thanks to ever more precise and reliable instruments and the work
of this year's Nobel winners, Alain Aspect, John Clauser and Anton Zeilinger,
physicists now integrate quantum phenomena into their knowledge of the world
with an exceptional degree of certainty.
However, even until the 1970s, researchers were still
divided over whether quantum entanglement was a real phenomenon. And for good
reasons — who would dare contradict the great Einstein, who himself doubted it?
It took the development of new experimental technology and bold researchers to
finally put this mystery to rest.
Particles Exist in Multiple States at Once
To truly understand the spookiness of quantum
entanglement, it is important to first understand quantum superposition.
Quantum superposition is the idea that particles exist in multiple states at
once. When a measurement is performed, it is as if the particle selects one of
the states in the superposition.
For example, many particles have an attribute called
spin that is measured either as "up" or "down" for a given
orientation of the analyzer. But until you measure the spin of a particle, it
simultaneously exists in a superposition of spin up and spin down.
There is a probability attached to each state, and
it is possible to predict the average outcome from many measurements. The
likelihood of a single measurement being up or down depends on these
probabilities, but is itself unpredictable.
Though very weird, the mathematics and a vast number
of experiments have shown that quantum mechanics correctly describes physical
reality.
The Reality of Quantum Superposition
The spookiness of quantum entanglement emerges from
the reality of quantum superposition, and was clear to the founding fathers of
quantum mechanics who developed the theory in the 1920s and 1930s.
To create entangled particles you essentially break
a system into two, where the sum of the parts is known. For example, you can
split a particle with spin of zero into two particles that necessarily will
have opposite spins so that their sum is zero.
In 1935, Albert Einstein, Boris Podolsky and Nathan
Rosen published a paper that describes a thought experiment designed to
illustrate a seeming absurdity of quantum entanglement that challenged a
foundational law of the universe.
A simplified version of this thought experiment, attributed
to David Bohm, considers the decay of a particle called the pi meson. When this
particle decays, it produces an electron and a positron that have opposite spin
and are moving away from each other. Therefore, if the electron spin is
measured to be up, then the measured spin of the positron could only be down,
and vice versa. This is true even if the particles are billions of miles apart.
This would be fine if the measurement of the
electron spin were always up and the measured spin of the positron were always
down. But because of quantum mechanics, the spin of each particle is both part
up and part down until it is measured. Only when the measurement occurs does
the quantum state of the spin "collapse" into either up or down —
instantaneously collapsing the other particle into the opposite spin. This
seems to suggest that the particles communicate with each other through some
means that moves faster than the speed of light. But according to the laws of
physics, nothing can travel faster than the speed of light. Surely the measured
state of one particle cannot instantaneously determine the state of another
particle at the far end of the universe?
Physicists, including Einstein, proposed a number of
alternative interpretations of quantum entanglement in the 1930s. They
theorized there was some unknown property — dubbed hidden variables — that
determined the state of a particle before measurement. But at the time,
physicists did not have the technology nor a definition of a clear measurement
that could test whether quantum theory needed to be modified to include hidden
variables.
Did Quantum Theory Need to Be Modified?
It took until the 1960s before there were any clues
to an answer. John Bell, a brilliant Irish physicist who did not live to
receive the Nobel Prize, devised a scheme to test whether the notion of hidden
variables made sense.
Bell produced an equation now known as Bell's
inequality that is always correct — and only correct – for hidden variable
theories, and not always for quantum mechanics. Thus, if Bell's equation was
found not to be satisfied in a real-world experiment, local hidden variable
theories can be ruled out as an explanation for quantum entanglement.
The experiments of the 2022 Nobel laureates,
particularly those of Alain Aspect, were the first tests of the Bell
inequality. The experiments used entangled photons, rather than pairs of an
electron and a positron, as in many thought experiments. The results
conclusively ruled out the existence of hidden variables, a mysterious attribute
that would predetermine the states of entangled particles. Collectively, these
and many follow-up experiments have vindicated quantum mechanics. Objects can
be correlated over large distances in ways that physics before quantum
mechanics cannot explain.
Importantly, there is also no conflict with special
relativity, which forbids faster-than-light communication. The fact that
measurements over vast distances are correlated does not imply that information
is transmitted between the particles. Two parties far apart performing
measurements on entangled particles cannot use the phenomenon to pass along
information faster than the speed of light.
Today, physicists continue to research quantum entanglement and investigate potential practical applications. Although quantum mechanics can predict the probability of a measurement with incredible accuracy, many researchers remain skeptical that it provides a complete description of reality. One thing is certain, though. Much remains to be said about the mysterious world of quantum mechanics.
Reference: The coversation
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