Researchers from the Vienna University of Technology have performed the first separation of a particle from one of its properties. The study, carried out at the Institute Laue-Langevin (ILL) and published in Nature Communications, showed that in an interferometer a neutron's magnetic moment could be measured independently of the neutron itself, thereby marking the first experimental observation of a new quantum paradox known as the 'Cheshire Cat'. The new technique, which can be applied to any property of any quantum object, could be used to remove disturbance and improve the resolution of high precision measurements.
The idea of a Quantum Cheshire Cat was proposed theoretically last year. It is based on the well known character from Alice in Wonderland who can vanish leaving his smile behind. In quantum physics, the term refers to an object whose properties can be separated from its physical location so that the two can be measured at different places. While this is clearly not possible in our everyday experience, where objects are spatially linked to their properties, the laws of Quantum Mechanics allow it to be achieved.
Quantum mechanics already tells us that particles can be in different physical states at the same time, a phenomenon known as superposition. For example if a neutron beam is divided in two using a crystal, individual neutrons do not have to decide which of the two paths to take. 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 the Atominstitut in the 1970s, and it has turned out to be the perfect tool to investigate the foundations of 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 colleagues Tobis Denkmayr, Hermann Geppert and Stephan Sponar from Vienna, 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.
Their aim was to get neutrons at the ILL to travel along a different path from its magnetic moment - a property describing the particle's coupling strength to an external magnetic field. The neutron's magnetic moment has a directional preference, a property called spin. In the experiment the neutron beam was split into two paths with different spin directions. The upper beam path had a spin parallel to the neutrons' direction of flight whilst the spin of the lower beam pointed in the opposite direction.
After the two beams were recombined the experimental detector was set up so that only neutrons with spin parallel to the direction of motion - implying that those travelling along the upper path - are detected. "This is called postselection", says Hermann Geppert. "The beam contains neutrons of both spin directions, but we only detect a selection of the neutrons."
The team then introduced a filter, which absorbs some of the neutrons, in the lower beam path. This did not change the number of detected particles. However, when the very same filter was introduced in the upper beam path, the number of detected neutrons was reduced.
Things get tricky, when the location of the neutron spin is measured: 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 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 neutrons in a special initial state and then postselecting them, we can achieve a situation in which both possible paths in the interferometer are important for the experiment, but in very different ways", says Tobias Denkmayr. "Along one of the paths, only an interaction with the particles themselves has an effect, but the other path is only sensitive to a magnetic spin coupling. The system behaves as if the particles were spatially separated from their properties."
The success of this unique type of quantum experiment was dependent on making so called 'weak measurements' to avoid the collapse of the superposition in accordance with the laws of quantum mechanics.
"These weak measurements give you less information," explains Hartmut Lemmel, instrument leader on S18, the ILL's crystal thermal neutron interferometer on which the Cheshire Cat was observed. "As a result you need to do lots of observations to achieve any sort of certainty that you have seen what you think you have seen. This was only possible due the strength of the neutron source available at the ILL which can uniquely provide the numbers of neutrons required to run these repeat experiments."
With their landmark observation suitably vindicated, questions turn to the potential impact of their fundamental discovery. One application might high precision measurements of quantum systems which are often affected by disturbance.
"Consider a quantum system that has two properties: you want to measure the first one very precisely but the second 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.
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