Uncovering the interplay between two famous quantum effects

July 23, 2018, Delft University of Technology
Artist’s impression of two strings covered in superconducting material, with the Casimir force pushing them together. In the center of the beams are arrays of holes that form an optical resonator trapping an optical field, which is used to measure the force very accurately at any temperature. Credit: Moritz Forsch, Kavli Institute of Nanoscience, Delft University of Technology

The Casimir force and superconductivity are two well-known quantum effects. These phenomena have been thoroughly studied separately, but what happens when these effects are combined in a single experiment? Now, Delft University of Technology have created a microchip on which two wires were placed in close proximity in order to measure the Casimir forces that act upon them when they become superconducting.

Is vacuum really empty? Quantum mechanics tells us that it's actually swarming with particles. In the 1940s, Dutch physicists Hendrik Casimir and Dirk Polder predicted that when two objects are placed in very close proximity, about a thousandth of the diameter of a human hair, this sea of 'vacuum particles' pushes them together – a phenomenon known as the Casimir effect. This attractive force is present between all objects and even sets fundamental limits to how closely we can place components together on microchips.

Superconductivity is another well-known phenomenon, also discovered by a Dutchman, Heike Kamerlingh Onnes, in the early 20th century. It describes how certain materials, such as aluminum or lead, allow electricity to flow through them without any resistance at . Over the last 100 years, superconductors have revolutionized our understanding of physics and are responsible for magnetically levitated trains, MRI scans and even mobile phone stations.

Out of reach

While the Casimir effect and superconductivity are both widely studied quantum phenomena, almost nothing is known about the interplay between the two, and this is where some physicists think some of the next scientific breakthroughs could lie. The Casimir force has been conclusively demonstrated between various materials. However, using superconductors to measure the effect has remained out of reach due to immense technological challenges at ultra-cold temperatures.

In a new publication in Physical Review Letters, researchers from Delft University of Technology have introduced a novel state-of-the-art sensor that allows them to measure the forces between closely spaced superconductors for the first time. The sensor consists of a microchip on which two strings are placed in close proximity. These wires can then be cooled down to cryogenic temperatures, making them superconducting. "The strings have holes in the centre that act as an optical resonator," said group leader Simon Gröblacher. "Laser light of a certain wavelength gets trapped in there. We can use this light to measure small displacements between the two wires, which means that we can measure the forces that are acting upon them at any temperature."

Additional tests

With their unprecedented force sensitivity, the researchers are also able to probe some highly speculative theories of quantum gravity at temperatures near absolute zero—a holy grail of physics. "We could disprove one of the more unlikely and controversial quantum gravity theories, which predicted that we should see a strong Casimir-like effect due to gravitational fields bouncing off the superconductors," said Richard Norte, the first author of the paper. "We measured no such effect with our current sensitivity." If there is a gravitational Casimir effect, it is more subtle than this theory predicted.

The new microchips pave the way for further experiments in an uncharted territory of science where these two famous quantum effects collide. The researchers hope to further increase the sensitivity of their microchip sensors in the near future and potentially probe the Casimir effect between high-temperature superconductors. It remains an open question how, exactly, superconductivity works in these exotic materials, and Casimir experiments could illuminate the underlying physics.

Explore further: Physicists’ ‘light from darkness’ breakthrough named a top 2011 discovery

More information: Richard A. Norte, Moritz Forsch, Andreas Wallucks, Igor Marinković, and Simon Gröblacher, Platform for measurements of the Casimir force between two superconductors, Phys. Rev. Lett. 121, 030405 (2018). doi.org/10.1103/PhysRevLett.121.030405

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Denys Picard
2.3 / 5 (3) Jul 23, 2018
Interesting, at a time where we are experiencng with the first Quantum President, who can speak a negation and affirmation at the same time....as in "in I wouldn't and and I also meant would at the same time..". This president is amazingly quantum in that he can say the truth and lie at the same time...be absolutly unintelligent and a genius at the same time...but here, everyhthing has to do with the audience. I wonder if Quantum physics is the same, do one audience see a "zero" where another one sees a "1"?
1 / 5 (1) Jul 23, 2018
one of the more unlikely and controversial quantum gravity theories predicted that we should see a strong Casimir-like effect due to gravitational fields bouncing off the superconductors, we measured no such effect with our current sensitivity
Interesting experiment but its interpretation looks confused for me. They could possibly have this theory on mind: Do Mirrors for Gravitational Waves Exist? (see also MIT follow-up here). But this theory talks about gravitational waves, not gravitational fields, which aren't mediated by gravitational waves but gravitons. Even if gravitational fields would get somehow shielded, I seriously doubt they could measure it with two lightweight pieces of superconductor. And the Cassimir effect is the result of shielding of virtual photons, not gravitons. So maybe they talk about another theory.
1 / 5 (1) Jul 23, 2018
Most of all this experiment resembles quite old Podkletnov/Poher/Tajmar experiments with gravitomagnetic field around superconductors for me. It would be great to attempt for replication at least one of these experiments finally, if they all found some anomalous effect. The contemporary science needs more replications rather than new blind shots. Anyway, such an inquisitive experiment is still better than nothing. I also appreciate the publishing of negative result, which also deserves incentives in contemporary scientific climate oriented to success. I just don't think, that its interpretation is relevant for theory doubted. The gravity field is long distance effect, it should manifest at large distances than Casimir force (and Podkletnov really observed these effects at quite large distances between walls and buildings).
1 / 5 (1) Jul 23, 2018
BTW It's also worth to realize, that superconductors as such (at their rest state) aren't supposed to shield gravitational or whatever else field. They (or better to say their fast moving electrons in Dirac fermion state) must get some acceleration first for to interact with scalar fields and these interactions are always temporal i.e. transient effects. So that superconductors as such are supposed to be inert to scalar wave effects and Poher and Podkletnov tested superconductor junctions loaded with electric impulses whereas Tajmar tested mechanical impulse around fast braking superconductor disks. The measurement of dynamic Casimir force could possibly reveal some anomaly - but statical one?

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