As Alice declared when the Cheshire Cat vanished leaving only its grin behind: “It’s the most curious thing I ever saw in my life.”
Scientists may have felt much the same way after a “landmark observation” demonstrating, for the first time, a weird quantum phenomenon known as the ‘Cheshire Cat’ effect.
In an experiment every bit as surreal as Lewis Carroll’s famous scene in Alice’s Adventures in Wonderland, they succeeded in separating a particle from one of its properties.
The ‘cat’ in question was a subatomic neutron particle, and the ghostly ‘grin’ the particle’s magnetic moment, which describes the strength of its coupling to an external magnetic field.
In the familiar ‘macro’ world, an object and its properties are always bound together and inseparable. It would be crazy to imagine a rotating ball, for instance, becoming separated from its spin.
Yet this is exactly what the international team of Austrian, French and US physicists managed to achieve.
Using an apparatus called an ‘interferometer’, they split a beam of neutrons and sent them along two paths, each with an opposite ‘spin’ − the directional preference of their magnetic moment.
The experiment was set up in such a way that only neutrons with a spin parallel to their direction of motion − those travelling along an ‘upper’ path − were detected, a process known as ‘post-selection’.
Subtle tweaking and measurements using a magnetic field led to the strange conclusion of the experiment.
While the physical particles themselves were observed flying along the upper path, the magnetic moment of the same particles could be detected emerging from the lower path.
The extraordinary results are published in the journal Nature Communications.
Tobias Denkmayr, from the Vienna University of Technology, said: “By preparing the neutrons in a special initial state and then post-selecting them, we can achieve a situation in which both possible paths in the interferometer are important for the experiment.
One possible application could be high-precision measurements of quantum systems that are affected by disturbance
“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 the experiment depended on making so-called ‘weak measurements’ that avoided the collapse of the quantum system.
Just as a spun coin comes up either heads or tails when caught, different quantum properties that exist at the same time in a ‘superposition’ are collapsed into a single state by the act of observation.
“These weak measurements give you less information,” said Hartmut Lemmel from the Institut Laue-Langevin in Genoble, France, where the experiment took place. “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.”
Whether the research has any practical potential remains unclear at present. One possible application could be high-precision measurements of quantum systems that are often affected by disturbance.
Stephan Sponar, another member of the Vienna University of Technology team, said: “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 perturbation. The two can be separated using a quantum Cheshire Cat, and possibly the perturbation can be minimised.”
The quantum Cheshire Cat follows in the paw steps of Schrodinger’s cat, the most famous feline to make an impact on the world of physics.
In Austrian physicist Erwin Schrodinger’s celebrated 1935 thought experiment, a cat that is both alive and dead was used to illustrate the apparent paradox of quantum superposition.
The imaginary experiment involved a cat, a flask of poison and a radioactive source placed in a sealed box. If an internal monitor detects radioactivity, caused by the decay of a single atomic particle, the flask is shattered, releasing the poison that kills the cat.
Theoretically the particle can be in a superposition of a decayed and non-decayed state at the same time, logically leading to the cat being both dead and alive. But this state of affairs only lasts until the box is opened. Once an observer peers inside the box, a cat that is either alive or dead is revealed.