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Particles can be physically separated from own properties, scientists say

July 31, 2014
Courtesy of Vienna University of Technology, 
Chapman University
and World Science staff

“‘Well! I’ve of­ten seen a cat with­out a grin,’ thought Al­ice, ‘but a grin with­out a cat! It’s the most cu­ri­ous thing I ev­er saw in my life!’” So goes the sto­ry of the Chesh­ire cat in the Lew­is Car­roll nov­el Al­ice in Won­der­land, a smil­ing fe­line that dis­ap­pears leav­ing its own grin be­hind.

Al­ice’s sur­prise stems from her ex­pe­ri­ence that an ob­ject and its prop­er­ty can’t ex­ist in­de­pend­ent­ly. It seems to be im­pos­si­ble to find a grin with­out some­one grin­ning.

The diagram shows an ob­ject can be sep­arated from one if its pro­pert­ies in an inter­fer­o­meter -- like a cat, moving on a diff­erent path than its own grin. (Credit: TU Vien­na / Leon Fil­ter)


Yet the strange laws of quan­tum me­chan­ics, the the­o­ry which gov­erns the mi­cro­scop­ic world of atoms and their parts, sug­gest it’s pos­si­ble to sep­a­rate a par­t­i­cle from its prop­er­ties—a phe­nom­e­non strik­ingly anal­o­gous to the Chesh­ire cat sto­ry. 

Ac­cord­ing to quan­tum me­chan­ics, par­t­i­cles can be in dif­fer­ent phys­i­cal states at the same time. 

If, for ex­am­ple, a beam of neu­trons, sub­a­tom­ic par­t­i­cles, is di­vid­ed in­to two beams, it can be shown that the in­di­vid­ual neu­trons don’t have to de­cide which of the two pos­si­ble paths they choose. In­stead, a par­t­i­cle can trav­el along both paths at the same time, a situa­t­ion called quan­tum su­per­po­si­tion.

In a new stu­dy, phys­i­cist Yuji Hasegawa from the Vi­en­na Uni­vers­ity of Tech­nol­o­gy brought to­geth­er a team of sci­en­tists to study wheth­er a par­t­i­cle can al­so sep­a­rated from its prop­er­ties.

Neu­trons aren’t elec­tric­ally charged, but they have a mag­net­ic di­rec­tion, called neu­tron spin. This al­so means they have a prop­er­ty called mag­net­ic mo­ment, the twist­ing force they un­der­go in a mag­netic field.

In the new type of ex­pe­ri­ment, a neu­tron beam is split in­to two parts in an in­stru­ment called an 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 in 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 ig­nored.

The idea is that these neu­trons, which are found to have a spin par­al­lel to their di­rec­tion of mo­tion, must have trav­eled along the up­per path.

Things get tricky when the sys­tem is used to meas­ure where the 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, has no ef­fect.

The re­sults “sug­gest that the sys­tem be­haves as if the neu­trons go through one beam path,” the up­per, “while their mag­net­ic mo­ment trav­els along the oth­er,” the re­search­ers wrote, re­port­ing their find­ings July 29 in the jour­nal Na­ture Com­mu­nica­t­ions.

The ex­pe­ri­ment achieves “a situa­t­ion in which both the pos­si­ble paths in the in­ter­fer­om­eter are im­por­tant for the ex­pe­ri­ment, but in very dif­fer­ent ways,” said col­la­bo­ra­tor To­bi­as Denk­mayr of the uni­vers­ity. “Along one of the paths, the par­t­i­cles them­selves cou­ple to our meas­urement de­vice,” or inter­act with it, “but only the oth­er path is sen­si­tive to mag­net­ic spin coup­ling,” the change in spin.


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“‘Well! I’ve often seen a cat without a grin,’ thought Alice, ‘but a grin without a cat! It’s the most curious thing I ever saw in my life!’” So goes the story of the Cheshire cat in the Lewis Carroll novel Alice in Wonderland, a smiling feline that disappears leaving its own grin behind. Alice’s surprise stems from her experience that an object and its property can’t exist independently. It seems to be impossible to find a grin without someone grinning. Yet the strange laws of quantum mechanics, the theory which governs the microscopic world of atoms and their parts, suggest it’s possible to separate a particle from its properties—a phenomenon strikingly analogous to the Cheshire cat story. According to quantum mechanics, particles can be in different physical states at the same time. If, for example, a beam of neutrons, subatomic particles, is divided into two beams, it can be shown that the individual neutrons don’t have to decide which of the two possible paths they choose. Instead, a particle can travel along both paths at the same time, a situation called quantum superposition. In a new study, physicist Yuji Hasegawa from the Vienna University of Technology brought together a team of scientists to study whether a particle can also separated from its properties. Neutrons aren’t electrically charged, but they have a magnetic direction, called neutron spin, which also means they have a property called magnetic moment. In the new type of experiment, a neutron beam is split into two parts in an instrument called an 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 in 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 ignored. These neutrons, which are found to have a spin parallel to its direction of motion, must clearly have traveled along the upper path. Things get tricky when the system is used to measure where the 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. The results “suggest that the system behaves as if the neutrons go through one beam path,” the upper, “while their magnetic moment travels along the other,” the researchers wrote, reporting their findings July 29 in the journal Nature Communications. The experiment achieves “a situation in which both the possible paths in the interferometer are important for the experiment, but in very different ways,” said collaborator Tobias Denkmayr of the university. “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.”