Unprecedented measurements could help realize quantum-reliant technologies

May 6, 2021

In this artistic representation, the quantum entanglement of two mechanical resonators is represented by the twisting of two tuning forks, so that they cannot be separated from one another. The fully separated shadows, showing no sign of entanglement, represent the information usually available to physicists during experiments. (Jack Bertram/NIST)

Two groups of researchers have independently quantum-entangled tiny drum-like structures — a feat difficult to achieve on objects larger than subatomic particles — and performed unprecedented measurements. One group also sidestepped a fundamental quantum limit on measurement uncertainty.

The advances demonstrated in the studies are important for realizing in-development technologies that would rely on quantum properties, such as quantum computing, quantum communications and high-precision measurements.

An experiment conducted at the National Institute of Standards and Technology entangled microscopic drum-like structures and made direct measurements of the system. Another study, led by Finnish physicists, entangled two similar drums and also measured the state of one of the oscillators with precision exceeding the usual limit set by the Heisenberg uncertainty principle. Both studies will be published Friday in Science.

"Apart from practical applications, these experiments address how far into the macroscopic realm experiments can push the observation of distinctly quantum phenomena," physicists Koi-Kwan Lau and Aashish Clerk wrote in a commentary, which will be published in the same issue of Science.

Once described as "spooky action at a distance" by Albert Einstein, quantum entanglement occurs when the states of two or more objects become dependent on each other and cannot be described separately. Knowing the position, the momentum or another attribute of one object directly implies the attributes of others entangled with it, regardless of the space between them. If two coins were entangled and flipped, for instance, looking at the result of one coin would provide enough information to know which side the other landed on.

While any object could theoretically be placed in quantum entanglement, demonstrating it on things larger than particles such as electrons or photons has rarely been accomplished, given the challenge of stabilizing the state. Physicists have previously entangled objects as large as small diamonds three millimeters across.

"At the macroscopic scale, being bigger and more massive gets harder and harder, but it's also potentially more powerful," said John Teufel, a physicist at NIST and a lecturer at the University of Colorado Boulder. "That's the constant balance with quantum technology, between how fragile and delicate the states you're working with are, with how powerful they are."

Along with his colleagues, Teufel, a senior author of the NIST paper, used two small aluminum "drums," around 15 micrometers long, roughly the width of a thin human hair. The drumheads, tuned to different frequencies, were connected to a circuit with a different resonance frequency. Pulses of microwaves were fired at the system in a way that was intended to trigger strong correlations between the drums and create entanglement.

The system was then measured by detecting how the microwaves were altered by the Doppler effect when they were reflected back, the waves' frequency changing based on the movement of the drum they struck. Lead author Shlomi Kotler, an assistant professor of physics at The Hebrew University of Jerusalem, compared the method to radar guns used by police officers to monitor the speed of vehicles. The researchers directly measured that the drums were in quantum entanglement.

"For me, that was the main motivation: Can we do it? Is it realistic?" Kotler said. "And, if so, it will be the springboard, the first thing you would do to make complex quantum systems or multidrum devices."

The research represented a scientific advance on multiple levels, Lau and Clerk said in their commentary. While other macroscopic systems have been entangled, the study's authors were the first to create entanglement deterministically, having directly caused the phenomenon with the microwave pulses. The team also measured the state directly rather than inferring it from other observations, and with clarity that cut through experimental noise present in these kinds of experiments. 

A related accomplishment was made in a study involving physicists from Aalto University and the University of New South Wales Canberra, who entangled two similar drums on a circuit within a cavity by using four microwave tones. Unlike the NIST experiment, the second study did not preserve information from the initial state of its experimental setup, which is useful in quantum computing, according to Lau and Clerk, but its entangled state was stabilized as long as the circuit was energized.

The researchers made another significant demonstration with their setup: evading Heisenberg's uncertainty principle with measurements of usually impossible precision. 

In quantum physics, the uncertainty principle states a limit on the precision of some pairs of measurements, such as position and momentum. The more accurately a particle's position is known, the less precisely measurable its momentum inherently becomes, mandating an inherent amount of fuzziness in observing quantum systems regardless of the technology being used.

But by causing the drums to vibrate in phases opposite to each other, the physicists created a quantum mechanics-free subsystem that behaves according to classical physics, which is not constrained by the uncertainty principle. Mathematically, the two vibrating drums can be depicted as two "effective" oscillators distinct from either drum, which don't physically exist, but respond to outside forces as if they did.

The researchers then successfully measured the position and momentum of one effective oscillator better than the uncertainty principle would allow. This was possible because the effective oscillator was made fully classical from the opposite behaviors of the drums, and the quantum uncertainty from the measurements was piled onto the second, unmeasured effective oscillator.

"We cannot really break the laws of nature, but we can create some effective systems that do not follow exactly the same laws," said senior author Mika Sillanpää, an associate professor of applied physics at Aalto University.

Sillanpää wants to use the quantum mechanics-free subsystem to pursue one of the greatest unsolved mysteries in physics: whether there is a model that accounts for both quantum mechanics and gravity. He said he wants to try to look for evidence of a gravitational force between two oscillating drums, which would be so weak that he would need to evade the Heisenberg uncertainty principle to detect it. 

The system could also be used to make very precise measurements for basic science research, Sillanpää said, by allowing the limits set by the uncertainty principle to be evaded.

Teufel and Kotler said that, if improved, their system could be used to underlie parts of quantum computers — which use the quantum states of particles to store information, rather than the transistors found in modern computers — or quantum communication technologies that rely on quantum properties to better encrypt messages.

It even represents a step toward quantum teleportation, the instant transfer of information across space through entangled particles that someday could be used alongside other quantum technologies to create a "quantum internet" that transfers data more securely.

The study, "Direct observation of deterministic macroscopic entanglement," published May 7 in Science, was authored by Shlomi Kotler, The Hebrew University of Jerusalem; Gabriel Peterson, Ezad Shojaee, Florent Lecocq, Alex Kwiatkowski, Shawn Geller and Emanuel Knill, National Institute of Standards and Technology and University of Colorado Boulder; and Katarina Cicak, Scott Glancy, Raymond Simmonds, José Aumentado and John Teufel, National Institute of Standards and Technology.

The study, "Quantum mechanics–free subsystem with mechanical oscillators," published May 7 in Science, was authored by Laure Mercier de Lépinay and Mika Sillanpää, Aalto University; Caspar Ockeloen-Korppi, IQM Finland Oy; and Matthew Woolley, UNSW Canberra.

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