Can neutrons be located at a different place than their own spin? A quantum experiment, carried out by a team of researchers from the Vienna University of Technology, demonstrates a new kind of quantum paradox
The Cheshire Cat featured in Lewis Caroll's novel "Alice in Wonderland" is a remarkable creature: it disappears, leaving its grin behind. Can an object be separated from its properties? It is possible in the quantum world. In an experiment, neutrons travel along a different path than one of their properties – their magnetic moment. This "Quantum Cheshire Cat" could be used to make high precision measurements less sensitive to external perturbations.
The basic idea of the Quantum Cheshire Cat: In an interferometer, an object is separated from one if its properties -- like a cat, moving on a different path than its own grin.
Credit: TU Vienna / Leon Filter
At Different Places at Once
According to the law of quantum physics, particles can be in different physical states at the same time. If, for example, a beam of neutrons is divided into two beams using a silicon crystal, it can be shown that the individual neutrons do not have to decide which of the two possible paths they choose. Instead, they can travel along both paths at the same time in a quantum superposition.
"This experimental technique is called neutron interferometry", says Professor Yuji Hasegawa from the Vienna University of Technology. "It was invented here at our institute in the 1970s, and it has turned out to be the perfect tool to investigate fundamental quantum mechanics."
To see if the same technique could separate the properties of a particle from the particle itself, Yuji Hasegawa brought together a team including Tobis Denkmayr, Hermann Geppert and Stephan Sponar, together with Alexandre Matzkin from CNRS in France, Professor Jeff Tollaksen from Chapman University in California and Hartmut Lemmel from the Institut Laue-Langevin to develop a brand new quantum experiment.
The experiment was done at the neutron source at the Institut Laue-Langevin (ILL) in Grenoble, where a unique kind of measuring station is operated by the Viennese team, supported by Hartmut Lemmel from ILL.
Where is the Cat …?
Neutrons are not electrically charged, but they carry a magnetic moment. They have a magnetic direction, the neutron spin, which can be influenced by external magnetic fields.
First, a neutron beam is split into two parts in a neutron interferometer. Then the spins of the two beams are shifted into different directions: The upper neutron beam has a spin parallel to the neutrons' trajectory, the spin of the lower beam points into the opposite direction. After the two beams have been recombined, only those neutrons are chosen, which have a spin parallel to their direction of motion. All the others are just ignored. "This is called postselection", says Hermann Geppert. "The beam contains neutrons of both spin directions, but we only analyse part of the neutrons."
These neutrons, which are found to have a spin parallel to its direction of motion, must clearly have travelled along the upper path - only there, the neutrons have this spin state. This can be shown in the experiment. If the lower beam is sent through a filter which absorbs some of the neutrons, then the number of the neutrons with spin parallel to their trajectory stays the same. If the upper beam is sent through a filter, than the number of these neutrons is reduced.
… and Where is the Grin?
Things get tricky, when the system is used to measure where the neutron spin is located: the spin can be slightly changed using a magnetic field. When the two beams are recombined appropriately, they can amplify or cancel each other. This is exactly what can be seen in the measurement, if the magnetic field is applied at the lower beam – but that is the path which the neutrons considered in the experiment are actually never supposed to take. A magnetic field applied to the upper beam, on the other hand, does not have any effect.
"By preparing the neurons in a special initial state and then postselecting another state, we can achieve a situation in which both the possible paths in the interferometer are important for the experiment, but in very different ways", says Tobias Denkmayr. "Along one of the paths, the particles themselves couple to our measurement device, but only the other path is sensitive to magnetic spin coupling. The system behaves as if the particles were spatially separated from their properties."
High Hopes for High-Precision Measurements
This counter intuitive effect is very interesting for high precision measurements, which are very often based on the principle of quantum interference. "When the quantum system has a property you want to measure and another property which makes the system prone to perturbations, the two can be separated using a Quantum Cheshire Cat, and possibly the perturbation can be minimized", says Stephan Sponar.
The idea of the Quantum Cheshire Cat was first discovered by Prof. Yakir Aharonov and first published by Aharonov's collaborator, Prof. Jeff Tollaksen (both now from Chapman University), in 2001. The measurements which have now been presented are the first experimental proof of this phenomenon. The experimental results have been published in the journal "Nature Communications".
Prof. Yuji Hasegawa
Institute of Atomic and Subatomic Physics,
Stadionallee 2, 1020 Vienna
Florian Aigner | Eurek Alert!
Listening to the Extragalactic Radio
13.10.2015 | Max-Planck-Institut für Radioastronomie
Scientists paint quantum electronics with beams of light
12.10.2015 | University of Chicago
Physicists of TU Berlin and mathematicians of MATHEON are so successful that even the prestigious journal “Nature Communications” reported on their project.
Security in data transfer is an important issue, and not only since the NSA scandal. Sometimes, however, the need for speed conflicts to a certain degree with...
Having a light touch can make a hefty difference in how well animals and robots move across challenging granular surfaces such as snow, sand and leaf litter. Research reported October 9 in the journal Bioinspiration & Biomimetics shows how the design of appendages – whether legs or wheels – affects the ability of both robots and animals to cross weak and flowing surfaces.
Using an air fluidized bed trackway filled with poppy seeds or glass spheres, researchers at the Georgia Institute of Technology systematically varied the...
Nondestructive material testing (NDT) is a fast and effective way to analyze the quality of a product during the manufacturing process. Because defective materials can lead to malfunctioning finished products, NDT is an essential quality assurance measure, especially in the manufacture of safety-critical components such as automotive B-pillars. NDT examines the quality without damaging the component or modifying the surface of the material. At this year's Blechexpo trade fair in Stuttgart, Fraunhofer IZFP will have an exhibit that demonstrates the nondestructive testing of high-strength automotive body parts using 3MA. The measurement results are available in a matter of seconds.
To minimize vehicle weight and fuel consumption while providing the highest level of crash safety, automotive bodies are reinforced with elements made from...
The MICADO camera, a first light instrument for the European Extremely Large Telescope (E-ELT), has entered a new phase in the project: by agreeing to a Memorandum of Understanding, the partners in Germany, France, the Netherlands, Austria, and Italy, have all confirmed their participation. Following this milestone, the project's transition into its preliminary design phase was approved at a kick-off meeting held in Vienna. Two weeks earlier, on September 18, the consortium and the European Southern Observatory (ESO), which is building the telescope, have signed the corresponding collaboration agreement.
As the first dedicated camera for the E-ELT, MICADO will equip the giant telescope with a capability for diffraction-limited imaging at near-infrared...
Self-driving cars will be on our streets in the foreseeable future. In Graz, research is currently dedicated to an innovative driver assistance system that takes over control if there is a danger of collision. It was nature that inspired Dr Manfred Hartbauer from the Institute of Zoology at the University of Graz: in dangerous traffic situations, migratory locusts react around ten times faster than humans. Working together with an interdisciplinary team, Hartbauer is investigating an affordable collision detector that is equipped with artificial locust eyes and can recognise potential crashes in time, during both day and night.
Inspired by insects
01.10.2015 | Event News
30.09.2015 | Event News
17.09.2015 | Event News
13.10.2015 | Physics and Astronomy
13.10.2015 | Health and Medicine
13.10.2015 | Press release