Like the conductors of a spooky symphony, researchers at the National Institute of Standards and Technology (NIST) “tangled” two small mechanical drums and precisely measured their related quantum properties. Entangled pairs like this could one day perform calculations and transmit data in large-scale quantum networks.

The NIST team used microwave pulses to attract the two tiny aluminum drums into a quantum version of Lindy Hop, with one partner hopping in a calm, cold pattern while the other trembled a bit more. The researchers analyzed radar-like signals to verify that the steps of the two drums formed an entangled pattern – a duet that would be impossible in the everyday classical world.

What is new is not so much the dance itself, but the researchers’ ability to measure the drumbeats, rising and falling only a quadrillionth of a meter, and to verify their fragile entanglement by detecting subtle relationships. statistics between their movements.

The research is described in the May 7 issue of *Science*.

“If you independently analyze the position and momentum data for the two drums, they each look just plain hot,” said NIST physicist John Teufel. “But looking at them together, we can see that what looks like a random movement of one drum correlates strongly with the other, in a way that is only possible through quantum entanglement.”

Quantum mechanics was originally conceived as the rule book for light and matter on the atomic scale. However, in recent years, researchers have shown that the same rules can apply to increasingly large objects such as drums. Their back-and-forth motion makes them a type of system known as a mechanical oscillator. Such systems were first entangled at NIST about ten years ago, and in this case, the mechanical elements were single atoms.

Since then, Teufel’s research group has demonstrated quantum control of drum-shaped aluminum membranes suspended above sapphire mats. By quantum standards, NIST drums are massive, 20 micrometers wide by 14 micrometers long and 100 nanometers thick. They each weigh about 70 picograms, which is about 1 trillion atoms.

The entanglement of massive objects is difficult because they strongly interact with the environment, which can destroy delicate quantum states. Teufel’s group developed new methods to control and measure the movement of two drums simultaneously. Researchers adapted a technique first demonstrated in 2011 to cool a single drum by switching from fixed to pulsed microwave signals to separately optimize the cooling, entanglement, and state measurement steps. To rigorously analyze entanglement, experimenters also worked more closely with theorists, an increasingly important alliance in the global effort to build quantum networks.

The NIST battery assembly is connected to an electrical circuit and enclosed in a cryogenic cavity. When a microwave pulse is applied, the electrical system interacts with and controls the activities of the drums, which can maintain quantum states like entanglement for about a millisecond, long in the quantum world.

For the experiments, the researchers applied two simultaneous microwave pulses to cool the drums, two more simultaneous pulses to entangle the drums, and two final pulses to amplify and record the signals representing the quantum states of the two drums. The states are encoded in a reflected microwave field, similar to radar. The researchers compared the reflections to the original microwave pulse to determine the position and momentum of each drum.

To cool the drums, the researchers applied pulses at a frequency lower than the natural vibrations of the cavity. As in the 2011 experiment, the drumbeats converted the photons applied to the higher frequency of the cavity. These photons leaked out of the cavity as it filled. Each departing photon carried with it a mechanical unit of energy – a phonon or a quantum – of the movement of the drum. This eliminated most of the heat-related drum movement.

To create an entanglement, the researchers applied microwave pulses between the frequencies of the two drums, above Drum 1 and below Drum 2. These pulses entangled the phonons in Drum 1 with the photons in the cavity, generating pairs photon-phonon correlated. The pulses also further cooled Drum 2, the photons exiting the cavity being replaced by phonons. What remained were mainly pairs of entangled phonons shared between the two drums.

To entangle the pairs of phonons, the duration of the pulses was crucial. The researchers found that these microwave pulses had to last longer than 4 microseconds, ideally 16.8 microseconds, to strongly entangle the phonons. During this time, the tangle got stronger and the movement of each drum increased because they moved in unison, a kind of sympathetic reinforcement, Teufel said.

The researchers looked for patterns in the returned signals or radar data. In the classical world, the results would be random. Plotting the results on a graph revealed unusual patterns suggesting the drums were tangled. To be sure, the researchers ran the experiment 10,000 times and applied a statistical test to calculate correlations between various sets of results, such as the positions of the two drums.

“Basically, we measured the correlation between two variables – for example, if you measured the position of one drum, how can you predict the position of the other drum,” Teufel said. “If they don’t have any correlations and they’re both perfectly cold, you can only guess the average position of the other drum within half a quantum of motion uncertainty. When they are entangled, we can do better, with less uncertainty. the only way this is possible. “

“To verify that entanglement is present, we do a statistical test called ‘entanglement control,’ ‘NIST theorist Scott Glancy said. from classical physics we know that the drums must have been entangled. Radar signals measure position and momentum simultaneously, but Heisenberg’s Uncertainty Principle says this cannot be done with perfect precision. Therefore, we pay an additional cost of randomness in our measurements. We manage this uncertainty by collecting a large data set and correcting the uncertainty during our statistical analysis. “

Highly entangled massive quantum systems like this could serve as long-lasting nodes of quantum networks. The high-efficiency radar measurements used in this work could be useful in applications such as quantum teleportation – transfer of data without a physical link – or entanglement exchange between nodes in a quantum network, as these applications require decisions based on measurements. entanglement results. The entangled systems could also be used in fundamental tests of quantum mechanics and force detection beyond standard quantum limits.

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