It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Friday, January 19, 2024
QUANTUM
Quantum particles can’t separate from their properties, after all
What actually happens is much weirder, and may help us understand more about quantum mechanics
THE SIMPLE INTERFEROMETER USED IN THE QUANTUM CHESHIRE CAT SCENARIO, WHERE A PHOTON IS PREPARED IN THE PATH-POLARISATION ENTANGLED STATE ECC,BUT IS ONLY CONSIDERED IF IT ARRIVES ON OUTPUT PATH + WITH POLARISATION D. THE PARADOX ARISES WHEN WE CONSIDER THE PHOTON’S PATH, POLARISATION, AND PATH-POLARISATION CORRELATION, WHILE IT IS INSIDE THE INTERFEROMETER.
CREDIT: JONTE R HANCE ET AL 2023 NEW J. PHYS. 25 113028
The quantum Cheshire cat effect draws its name from the fictional Cheshire Cat in the Alice in Wonderland story. That cat was able to disappear, leaving only its grin behind. Similarly, in a 2013 paper, researchers claimed quantum particles are able to separate from their properties, with the properties travelling along paths the particle cannot. They named this the quantum Cheshire cat effect. Researchers since have claimed to extend this further, swapping disembodied properties between particles, disembodying multiple properties simultaneously, and even “separating the wave-particle duality” of a particle.
However, recent research, published in the New Journal of Physics on November 17, 2023, shows that these experiments don’t actually show particles splitting from their properties, but instead display another counterintuitive feature of quantum mechanics — contextuality.
Quantum mechanics is the study of the behavior of light and matter at the atomic and subatomic scale. By its nature, quantum mechanics is counterintuitive. The research team set out to fundamentally understand this counterintuitive nature, while exploring practical benefits.
“Most people know that quantum mechanics is weird, but identifying what causes this weirdness is still an active area of research. It has been slowly formalized into a notion called contextuality — that quantum systems change depending on what measurements you do on them,” said Jonte Hance, a research fellow at Hiroshima University and the University of Bristol.
A sequence of measurements on a quantum system will produce different results depending on the order in which the measurements are done. For instance, if we measure where a particle is, then how fast it is travelling, this will give different results to first measuring how fast it travels, then where it is. Because of this contextuality, quantum systems can be measured as having properties which we would expect to be mutually incompatible. “However, we still don’t really understand what causes this, so this is what we wanted to investigate, using the paradoxical quantum Cheshire cat scenario as a testbed,” said Hance.
The team notes that the problem with the quantum Cheshire cat paradox is that its original claim, that the particle and its property, such as spin or polarization, separate and travel along different paths, may be a misleading representation of the actual physics of the situation. “We want to correct this by showing that different results are obtained if a quantum system is measured in different ways, and that the original interpretation of the quantum Cheshire cat only comes about if you combine the results of these different measurements in a very specific way, and ignore this measurement-related change,” said Holger Hofmann, a professor at Hiroshima University.
The team analyzed the Cheshire cat protocol by examining the relation between three different measurements regarding the path and polarization of a photon within the quantum Cheshire cat protocol. These would have seemed to result in a logical contradiction, were the system not contextual. Their paper discusses how this contextual behavior links to weak values, and the coherences between prohibited states. Through their work, they showed that instead of a property of the particle being disembodied, the quantum Cheshire cat instead demonstrates the effects of these coherences, typically found in pre- and post-selected systems.
Looking ahead the team wants to expand this research, to find a way to unify paradoxical quantum effects as manifestations of contextuality, and to explain once and for all how and why measurements change quantum systems. “This will not only help us finally explain why quantum mechanics is so counterintuitive, but will also help us develop ways to use this weirdness for practical purposes. Given contextuality is inherently linked to scenarios where there is a quantum advantage over classical solutions to a given problem, only by understanding contextuality will we be able to realize the full potential of, for instance, quantum computing,” said Hance.
The research team includes Jonte R. Hance, Ming Ji, and Holger F. Hofmann from the Graduate School of Advanced Science and Engineering, Hiroshima University. Hance is also a research associate in the Department of Electrical and Electronic Engineering at the University of Bristol.
The research was funded by Hiroshima University’s Phoenix Postdoctoral Fellowship for Research, the University of York’s EPSRC DTP grant, the Quantum Communications Hub that is funded by EPSRC grants, and a JST SPRING grant.
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About Hiroshima University
Since its foundation in 1949, Hiroshima University has striven to become one of the most prominent and comprehensive universities in Japan for the promotion and development of scholarship and education. Consisting of 12 schools for undergraduate level and 4 graduate schools, ranging from natural sciences to humanities and social sciences, the university has grown into one of the most distinguished comprehensive research universities in Japan. English website: https://www.hiroshima-u.ac.jp/en
ELECTRONS THAT INTERACT WITH MAGNETIC SPINS IN HEAVY FERMION MATERIALS HAVE A HEAVIER-THAN-USUAL EFFECTIVE MASS. IN ADDITION TO BEING A HEAVY FERMION, CESII IS A VAN DER WAALS CRYSTAL THAT CAN BE PEELED INTO ATOM-THIN LAYERS.
Researchers at Columbia University have successfully synthesized the first 2D heavy fermion material. They introduce the new material, a layered intermetallic crystal composed of cerium, silicon, and iodine (CeSiI), in a research article published today in Nature.
Heavy fermion compounds are a class of materials with electrons that are up to 1000x heavier than usual. In these materials, electrons get tangled up with magnetic spins that slow them down and increase their effective mass. Such interactions are thought to play important roles in a number of enigmatic quantum phenomena, including superconductivity, the movement of electrical current with zero resistance.
Researchers have been exploring heavy fermions for decades, but in the form of bulky, 3D crystals. The new material synthesized by PhD student Victoria Posey in the lab of Columbia chemist Xavier Roy will allow researchers to drop a dimension.
“We’ve laid a new foundation to explore fundamental physics and to probe unique quantum phases,” said Posey.
One of the latest materials to come out of the Roy lab, CeSiI is a van der Waals crystal that can be peeled into layers that are just a few atoms thick. That makes it easier to manipulate and combine with other materials than a bulk crystal, in addition to possessing potential quantum properties that occur in 2D. “It’s amazing that Posey and the Roy lab could make a heavy fermion so small and thin,” said senior author Abhay Pasupathy, a physicist at Columbia and Brookhaven National Laboratory. “Just like we saw with the recent Nobel Prize to quantum dots, you can do many interesting things when you shrink dimensions.”
With its middle sheet of silicon sandwiched between magnetic cerium atoms, Posey and her colleagues suspected that CeSiI, first described in a paper in 1998, might have some interesting electronic properties. Its first stop (after Posey figured out how to prepare the extremely air-sensitive crystal for transport) was a Scanning Tunneling Microscope (STM) in Abhay Pasupathy’s physics lab at Columbia. With the STM, they observed a particular spectrum shape characteristic of heavy fermions. Posey then synthesized a non-magnetic equivalent to CeSiI and weighed the electrons of both materials via their heat capacities. CeSiI’s were heavier. “By comparing the two—one with magnetic spins and one without—we can confirm we’ve created a heavy fermion,” said Posey.
Samples then made their way across campus and the country for additional analyses, including to Pasupathy’s lab at Brookhaven National Laboratory for photoemission spectroscopy; to Philip Kim’s lab at Harvard for electron transport measurements; and to the National High Magnetic Field Laboratory in Florida to study its magnetic properties. Along the way, theorists Andrew Millis at Columbia and Angel Rubio at Max Planck helped explain the teams’ observations.
From here, Columbia’s researchers will do what they do best with 2D materials: stack, strain, poke, and prod them to see what unique quantum behaviors can be coaxed out of them. Pasupathy plans to add CeSiI to his arsenal of materials in the search for quantum criticality, the point where a material shifts from one unique phase to another. At the crossover, interesting phenomena like superconductivity may await.
“Manipulating CeSiI at the 2D limit will let us explore new pathways to achieve quantum criticality,” said Michael Ziebel, a postdoc in the Roy group and co-corresponding author, “and this can guide us in the design of new materials.”
Back in the chemistry department, Posey, who has perfected the air-free synthesis techniques needed, is systematically replacing the atoms in the crystal—for example, swapping silicon for other metals, like aluminum or gallium—to create related heavy fermions with their own unique properties to study. “We initially thought CeSiI was a one-off,” said Roy. “But this project has blossomed into a new kind of chemistry in my group.”
Two-dimensional heavy fermions in the van der Waals metal CeSiI
ARTICLE PUBLICATION DATE
17-Jan-2024
Two atoms playing ping-pong
A team at TU Wien has developed a "quantum ping-pong": Using a special lens, two atoms can be made to bounce a single photon back and forth with high precision.
Atoms can absorb and reemit light - this is an everyday phenomenon. In most cases, however, an atom emits a light particle in all possible directions - recapturing this photon is therefore quite hard.
A research team from TU Wien in Vienna (Austria) has now been able to demonstrate theoretically that using a special lens, a single photon emitted by one atom can be guaranteed to be reabsorbed by a second atom. This second atom not only absorbs the photon though, but directly returns it back to the first atom. That way, the atoms pass the photon to each other with pinpoint accuracy again and again – just like in ping-pong.
How to tame a wave
"If an atom emits a photon somewhere in free space, the direction of emission is completely random. This makes it practically impossible to get another distant atom to catch this photon again," says Prof. Stefan Rotter from the Institute of Theoretical Physics at TU Wien. "The photon propagates as a wave, which means that nobody can say exactly in which direction it is travelling. It is therefore pure chance whether the light particle is reabsorbed by a second atom or not."
The situation is different if the experiment is not carried out in free space, but in an enclosed environment. Something quite similar is known from so-called whispering galleries in acoustics: if two people place themselves in an elliptical room exactly at the focal points of the ellipse, they can hear each other perfectly – even when only whispering quietly. The sound waves are reflected by the elliptical wall in such a way that they meet again exactly where the second person is standing – this person can therefore hear the quiet whisper perfectly.
"In principle, something similar could be built for light waves when positioning two atoms at the focal points of an ellipse," says Oliver Diekmann, the first author of the current publication. "But in practice, the two atoms would have to be positioned very precisely at these focal points."
The Maxwell fish-eye lens
The research team therefore came up with a better strategy based on the concept of the fish-eye lens, which was developed by James Clerk Maxwell, the founder of classical electrodynamics. The lens comprises a spatially varying refractive index. While light travels in straight lines in a uniform medium such as air or water, light rays are bent in a Maxwell fish-eye lens.
"In this way, it is possible to ensure that all rays emanating from one atom reach the lens’s edge on a curved path, are subsequently reflected and then arrive at the target atom on another curved path," explains Oliver Diekmann. In this case, the effect works much more efficiently than in a simple ellipse and deviations from the ideal positions of the atoms are less harmful.
"The light field in this Maxwell fish-eye lens consists of many different oscillatory modes. This is reminiscent of playing a musical instrument where different harmonics are generated at the same time," says Stefan Rotter. "We were able to show that the coupling between the atom and these different oscillating modes can be adapted in such a way that the photon is transferred from one atom to the other one almost certainly – quite different from what would be the case in free space."
Once the atom has absorbed the photon, it is left in a state of higher energy until it reemits the photon after a very short time. Then the game starts over: the two atoms swap roles and the photon is returned from the receiver atom to the original sender atom - and so on.
Optimal control for quantum technologies
So far, the effect has been demonstrated theoretically, but practical tests are possible with today’s technology. "In practice, the efficiency could be increased even further by using not just two atoms, but two groups of atoms," says Stefan Rotter. "The concept could be an interesting starting point for quantum control systems to study effects at extremely strong light-matter interaction."
Maxwell fish-eye lens with two atoms. A photon (green) is travelling between the two atoms along the curved light rays (white).
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