The "local causality" theory, developed by Albert Einstein in reaction to
quantum physics, has been refuted by a team of researchers led by Andreas
Wallraff, Professor of Solid State Physics at ETH Zurich.
The researchers have further supported quantum mechanics by demonstrating
that distant quantum mechanical objects may be far more tightly connected
with one another than is feasible in traditional systems. This experiment is
unique in that it was carried out for the first time utilizing
superconducting circuits, which are thought to be viable prospects for
creating potent quantum computers.
Old disagreement
A Bell test is based on an experimental setting that British physicist John
Bell first developed as a thought experiment in the 1960s. Bell sought to
answer a dispute that the pioneers of physics had already engaged in during
the 1930s: Are the predictions of quantum mechanics, which are wholly at
odds with common sense, accurate, or do they also hold true in the atomic
microcosm, as Albert Einstein believed?
Bell suggested making a random measurement on two entangled particles
simultaneously and comparing it to Bell's inequality to get the answer to
this query. These experiments always meet Bell's inequality if Einstein's
theory of local causation is correct. Quantum mechanics, on the other hand,
predicts that they will go against it.
The last skepticism was removed
John Francis Clauser, who received the Physics Nobel Prize last year, and
Stuart Freedman performed the first real Bell test in the early 1970s. The
two scientists were able to demonstrate through their tests that Bell's
inequality is definitely broken. But in order to perform their tests in the
first place, they had to make a number of presumptions. Therefore,
hypothetically, Einstein's skepticism of quantum mechanics may still have
been justified.
But as time goes on, more and more of these gaps could be filled. Finally,
in 2015, many groups achieved the first genuinely error-free Bell tests,
putting an end to the long-running argument.
Applications that show promise Wallraff's team may now use a brand-new
experiment to confirm these findings. Despite the original confirmation
being seven years ago, the study by the ETH researchers published in Nature
demonstrates that more research has to be done on this issue. This is due to
a number of factors.
For starters, despite being far larger than minuscule quantum particles
like photons or ions, the ETH researchers' experiment proves that
superconducting circuits follow the rules of quantum physics as well. The
term "macroscopic quantum objects" refers to the few hundred
micrometer-sized superconducting-based microwave-operated electronic
circuits.
Bell tests also have a practical importance, to mention another issue.
According to Simon Storz, a doctorate student in Wallraff's lab, "Modified
Bell tests can be used in cryptography, for example, to show that
information is actually transmitted in encrypted form." "With our method, we
can demonstrate that Bell's inequality is broken considerably more quickly
than we could in prior experimental configurations. Because of this, it is
especially intriguing for practical applications.
Trying to reach a compromise
But for this, the researchers require a high-tech testing laboratory.
Because they need to make sure that no data can be transferred between the
two entangled circuits before the quantum measurements are finished for the
Bell test to be really bug-free. Since light is the only speed at which
information can be sent, the measurement must be completed in less time than
it takes a light particle to go from one circuit to another.
Therefore, it's crucial to find a balance while setting up the experiment:
the farther apart the two superconducting circuits are, the more time is
available for the measurement, but the more complicated the experimental
setup is. This is due to the fact that the entire experiment must be carried
out in a vacuum at very low temperatures.
The ETH researchers have found that the smallest distance over which to
carry out a successful loophole-free Bell test is roughly 33 meters, since a
light particle takes this distance in a vacuum about 110 nanoseconds. That's
a few of nanoseconds longer than the researchers' experiment time.
30 meters of vacuum
The ETH campus's subterranean tunnels have been home to an outstanding
facility constructed by Wallraff's team. It has a cryostat with a
superconducting circuit at each of its two ends. A 30-meter-long tube that
is internally chilled at a temperature slightly above absolute zero
(-273.15°C) connects these two cooling devices.
One of the two superconducting circuits transmits a microwave photon to the
other before the beginning of each measurement, entailing the two circuits.
The Bell test then uses random number generators to determine which
measurements are done on the two circuits. The results of the measurements
on both sides are then compared.
A significant entanglement
The researchers have demonstrated that Bell's inequality is broken in this
experimental design with extremely high statistical certainty after
analyzing more than one million observations. In other words, they have
demonstrated that superconducting circuits may be entangled across a great
distance and that quantum theory also permits non-local correlations in
macroscopic electrical circuits. This offers up some intriguing potential
uses for quantum cryptography and distributed quantum computing.
It was difficult to construct the laboratory and conduct the test,
according to Wallraff. It takes a lot of work just to chill the complete
experimental apparatus to a temperature near absolute zero.
Our machine has 14,000 screws, 1.3 tons of copper, and a ton of physics and
technical expertise, according to Wallraff. He thinks that it would
theoretically be conceivable to create infrastructure that could traverse
much bigger distances. For example, using this technique to connect
superconducting quantum computers across vast distances.
Provided by
ETH Zurich
