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"Well I've often seen a cat without a grin... but a grin without a cat is a most curious thing"
Can you have a ring without a bell? A bark without its dog? In the weird quantum world you can. The Quantum Cheshire Cat Experiment, named after the mischievously grinning feline Alice encounters in Wonderland, has turned from theory to reality the remarkable feat of separating particles from their properties. Physicists were able to separate neutrons from their spins, like the Cheshire cat and its grin, and in doing so further question our understanding of the world around us.
We are taught that the universe is made up of many different particles - light of photons, atoms of electrons and protons and neutrons - and that these all have specific properties such as charge and spin. The idea that somehow these particles could exist without their properties, and ever stranger that these properties could exist without their particles, is a notion that goes against our intuition though one that is permitted by quantum theory.
Quantum physics emerged a century ago, and whilst contrary at the time to our classical understanding, has proven to be hugely successful. No experiment has disagreed with its predictions. Though it provides an accurate description of the microscopic world, this is a strange world, where particles can be in different places at the same time, spin clockwise and anticlockwise at the same time, and instantaneously influence each other from across the universe. Quite why the behaviour in the microscopic and macroscopic worlds differs so much is yet to be fully explained, with physicists still lacking a deep understanding of QM. This has resulted in experiments that have, and continue to, reveal a wonderfully weird picture of our world.
One of the most fascinating theories was provided by Aharonov half a century ago, showing that time in quantum mechanics is, at least mathematically, two way - it does not have to flow from past to present. This let to the time-symmetric formulation of quantum mechanics in the 1960s, which showed that not only did time flow both ways, but that events in both the past and the future could affect the state of a quantum system in the present. In the 1990s Tollaksen and Aharonov found that these influences could lead to a particle and its properties being separated. The Quantum Cheshire Cat analogy was founded.
It was a nice thought experiment, but the real intrigue has been whether in reality it can occur. Testing any quantum theory though encounters a serious problem. In the quantum world particles can spin clockwise and anticlockwise at the same time, but we only ever see them spinning one way or the other, due to the act of measuring them destroying the delicate superposition of states. Aharonov and colleagues theorised however in the late 1980s that by measuring with a device that interacts extremely weakly with the particle, its quantum state is preserved, and information about it can be gathered. But this insight into the particle's quantum nature comes with a great uncertainty. So studying one particle is not really any use - but the trick is to make these weak measurements on numerous identical particles, thereby providing through averaging the data information on the quantum state with a reduced uncertainty.
At the time this approach was met with disbelief and scepticism. But with the advancements in technology in the decades since, theory has become reality, and physicists have been able to study the quantum world in depth. To now investigate, Tollaksen and a team of researchers at Austria's Vienna University of Technology set about attempting to make such weak measurements on neutrons, a feat never previously achieved. They did so using an experimental setup involving weak magnetic fields and a weakly interacting neutron absorber to make the measurements. When the absorber was placed in one of the paths of the interferometer, there was an affect on the output, however placing it in the other path resulted in no such effect. This showed that the neutrons were travelling along only one of the two paths. Along each path of the interferometer, a weak magnetic field was established to interact with the spin of the neutrons. On one of the paths the magnetic field interacted with the neutron's spin and produced a change in the interferometer's output, however there was no such interaction on the other path. This therefore confirmed that the neutrons and their spin had taken different paths. The cat and its grin had been separated.
An amazing bit of experimental physics. But where do we go from here? Speculation has begun into the possibilities resulting from this ability to split particles from their properties, with particular intrigue into measuring the electric dipole moment of the neutron, which is crucial in theories explaining the matter-antimatter composition of the universe. Whilst the experiment involved neutrons and their spin, it isn't limited to this setup. In theory other particles such as photons and electrons, and other properties such as charge and magnetic moment, could all be studied. The only thing that can't be done is to separate particles from their mass. The Cheshire cat phenomenon could perhaps be best put to use in quantum computing, to overcome the major problem of shielding particles from external disturbances that destroy their important superposition of states that is key to the system. Another possibility is in high-precision metrology. Numerous other ways to take advantage of this physics are surely to be found.
It's important not to get too carried away with this new take on reality. The strangeness of the quantum world is clearly far from our everyday experience of the world around us. The post-selection process of the experiment is a neat trick to give an insight into the quantum nature of particles, but is not analogous to the sci-fi tales of future actions changing the past, such as squishing a bug today causing the extinction of the dinosaurs millions of years ago. This quantum take on the Cheshire cat and its grin only makes sense if you give physical meaning to these weak measurements and accept the conclusions based on averaging many results to reduce the high uncertainties. Furthermore, and fundamentally to the whole concept, it is unclear what it actually means for particles and their properties to be separated.
What is obvious is that the quantum Cheshire cat, the separation of particles and their properties, is a fascinating theory now become reality that is further proving insight into the bizarre quantum world and continuing to challenge our understanding of reality. Curiouser and curiouser indeed.
Image Credits:
Cheshire Cat & Alice In Wonderland - Wikipedia, URL: http://upload.wikimedia.org/wikipedia/en/2/25/Cheshire_Cat_Tim_Burton.jpg
Seperating The Cat & Its Grin - L. Filter, URL: http://www.bbc.co.uk/news/science-environment-28543990
