What are the limits of quantum entanglement

Entanglement can be measured macroscopically

Quantum effect in the micrometer range: For the first time, researchers have succeeded in quantum-physically entangling macroscopic objects and also measuring this directly. To do this, they linked the vibration patterns of two 20 micrometer small aluminum drums and used a type of microwave radar to measure the correlation of the excitation patterns. The possibility of measurably entangling objects of this size allows new research approaches and applications, according to the scientists in the specialist magazine "Science".

In quantum-physical entanglement, two objects are coupled with one another in such a way that the change in state of one automatically causes that of the partner. This happens instantaneously and regardless of the distance. For particles in the quantum world such as photons, atoms or molecules, this is almost routine.

But what is the upper size limit for the entanglement? Does the quantum physical effect also have an effect on everyday objects - without us noticing or being able to prove it? Or is the entanglement limited to the microcosm because the disruptive effects are too strong with larger objects? So far, this question has not been answered clearly, which is why physicists are trying to further explore the upper limit of entanglement.

The limit of the possible pushed further

A team led by Shlomi Kotler from the US National Institute of Standards and Technology in Boulder has now taken an important step in this direction. You have entangled two objects 20 micrometers in size, thereby shifting the limit of macroscopic entanglement by another small piece. In addition, for the first time they measured this entanglement directly with a kind of “microwave radar”. “We can observe the entanglement directly in the measured variables,” explain the researchers.

The possibility of directly measuring a macroscopic entanglement opens up completely new application possibilities for this quantum physical phenomenon. “Such entangled macroscopic systems could be used for fundamental tests of quantum mechanics, as sensors allow measurements beyond the quantum limit and function as long-lived nodes in future quantum networks,” explain Kotler and his colleagues.

Vibrating aluminum cones

Specifically, their entangled system consists of two round aluminum membranes 20 micrometers in diameter and 100 nanometers thick, which can vibrate freely like tiny eardrums. Each of these two wafer-thin membranes weighs around 70 picograms and contains around a trillion atoms - according to quantum physical standards, this is at the limit of the macro world.

In the experiment, the team first used microwave pulses to cool these microtrums down to a temperature just above absolute zero. With further pulses they then made the two membranes vibrate and at the same time caused an entanglement of these movements. In physical terms, the drums now share pairs of entangled phonons - their elementary excitation patterns were linked.

Microwave pulse reveals entanglement

The researchers determined that the two microtrums were entangled using two additional microwave pulses that were reflected by the membranes and then amplified. "Roughly speaking, we measure how correlated two variables are: if you measure the position of one drum, for example, how well the position of the second can be derived from this measurement," explains Kotler's colleague John Teufel.

The results of the test runs, which were repeated around 10,000 times, showed that the movements of the two microtrums were each entangled for one millisecond - a long time according to quantum standards. "If you analyze the position and momentum of both drums individually, they appear independently of one another," explains Teufel. “But if you look at them together, you can see that the supposedly random movement of one drum is highly correlated with that of the other. This happens in a way that is only possible through quantum entanglement. "(Science, 2021; doi: 10.1126 / science.abf2998)

Source: National Institute of Standards and Technology (NIST)

May 10, 2021

- Nadja Podbregar