TIME CRYS­TALS

They change shape by them­selves, have their own rhythm and should not ex­ist at all. But now, sci­en­tists have cre­ated the bizarre time crys­tals, which may make fu­ture com­put­ers work at ex­treme speeds with­out us­ing en­ergy.

Science Illustrated - - FRONT PAGE -

(Noth­ing To Do With Astrol­ogy)

Imag­ine a ball ly­ing on the ground. Like all other ob­jects, the ball has a num­ber of phys­i­cal prop­er­ties, which de­scribe its three- di­men­sional shape: It is com­pletely round and has a cer­tain di­am­e­ter, which is de­ci­sive for its cir­cum­fer­ence – just like any other round ob­ject. Nev­er­the­less, this ball is com­pletely dif­fer­ent, as its shape also de­pends on the fourth di­men­sion: Time. Ev­ery ten sec­onds, the ball changes shape by it­self and be­comes eggshaped , and then, ten sec­onds later, it turns back into a ball. It is a 4D ball.

Un­til re­cently, any physi­cist would have de­nied the ex­is­tence of such a ball, as this would break with some of the most fun­da­men­tal sci­en­tific laws. But now, in­de­pen­dently of each other, two teams of sci­en­tists have made par­ti­cles in mi­cro­scopic crys­tals turn and change pat­terns by them­selves as time passes. The crys­tals are a com­pletely new state of mat­ter, which is nei­ther fixed, nor liq­uid, gaseous or plasma, but de­pen­dent on time. Time crys­tals, as the sci­en­tists have named them, can be the key that com­puter en­gi­neers have been miss­ing in or­der to make fu­ture quan­tum com­put­ers sta­ble and, not least, ex­tremely en­ergy-sav­ing.

SCI­EN­TISTS IN­VENT PER­PET­UAL "MO­TION" MA­CHINE

Orig­i­nally, time crys­tals only ex­isted as an idea in the head of No­bel Prize-win­ning physi­cist Frank Wil­czek. His idea was based on or­di­nary 3D crys­tals like salt or ice.

At the level of the atom, crys­tals are in­ter­est­ing to physi­cists, be­cause they break with spa­tial sym­me­try. Spa­tial sym­me­try can be found in a cup of liq­uid wa­ter, for ex­am­ple: Wa­ter mol­e­cules fill up the cup in a ho­mo­ge­neous pat­tern, which means that two sam­ples taken from dif­fer­ent places in the cup have the ex­act same molec­u­lar pat­tern. When the wa­ter in the cup freezes and turns into ice crys­tals, the mol­e­cules ar­range them­selves in a re­peated pat­tern of fixed units, which are spa­tially asym­met­ri­cal. Thus, two ran­dom sam­ples taken from the ice crys­tal will not have the same pat­tern.

The dif­fer­ence is sim­i­lar to cut­ting swatches out of two car­pets – one plain-- coloured and one pat­terned. No mat­ter where the scis­sors cut in the plain-coloured one, the swatches will be sim­i­lar in ap­pear­ance, while two pieces from the pat­terned one will al­most never be 100 % iden­ti­cal.

Frank Wil­czek was study­ing these crys­tal struc­tures in 2012, when he had an idea: What if there are sub­stances, which are not only spa­tially asym­met­ri­cal like crys­tals, but also asym­met­ri­cal in time? This would mean that an ob­ject, where en­ergy is nei­ther added nor taken, might change its char­ac­ter­is­tic, just be­cause time passes. In the ex­am­ple, the pat­tern in a piece of time-re­lated asym­met­ri­cal car­pet would not only de­pend on where the piece was cut, but also when it was cut.

The idea was re­ceived with fas­ci­na­tion, but also in­dig­na­tion. Par­ti­cles, which change by them­selves over time, break with one of the ba­sic prin­ci­ples of physics: All en­ergy in the uni­verse is con­stant. This means that en­ergy will nei­ther ap­pear nor dis­ap­pear, but only change from one state to an­other, e.g. from light to heat. If Wil­czek’s time crys­tals changed shape with­out en­ergy be­ing added, they would have to cre­ate en­ergy out of noth­ing. So the time crys­tals would be per­pet­ual mo­tion ma­chines, which, ac­cord­ing to the laws of physics, can­not ex­ist.

TIME CRYS­TALS MET WITH RE­SIS­TANCE

The idea was not left untested. In 2015, two sci­en­tists from Univer­sity of Cal­i­for­nia and The Univer­sity of Tokyo ap­peared to defini­tively bring the im­pos­si­ble per­pet­ual mo­tion ma­chines to their graves when, the­o­ret­i­cally, they proved that, ac­cord­ing to the laws of physics, time crys­tals can­not ex­ist in a so-called ther­mal equi­lib­rium.

When an ob­ject is in ther­mal equi­lib­rium, it can­not give off or re­ceive heat from its sur­round­ings. In the world of physics, heat is a mea­sure of the ki­netic en­ergy of par­ti­cles. The sci­en­tists dis­cov­ered that time crys­tals could only move if they were “pushed” by their sur­round­ings. Thus, it was phys­i­cally im­pos­si­ble for the time crys­tals to change shape with­out any help from the out­side, which was the very ba­sis for Wil­czek’s idea.

But other physi­cists re­fused to give in. If the time crys­tals could not ex­ist in ther­mal equi­lib­rium, it may be pos­si­ble to cre­ate them in a state of dis­e­qui­lib­rium. In re­cent years, quan­tum physi­cists have stud­ied a phe­nom­e­non called many-body lo­cal­i­sa­tion, which oc­curs when a group of atoms is not in ther­mal equi­lib­rium. Atoms in this state are in­vis­i­bly con­nected and can af­fect each other.

In a con­tainer filled with air, the atoms would nor­mally fill up the bulk of the con­tainer evenly and move ran­domly among each other. How­ever, by means of many-body lo­cal­i­sa­tion, the atoms can af­fect each other and cause them to gather on the one side of the con­tainer or move around in a spe­cial pat­tern.

FROM IM­POS­SI­BLE THE­ORY TO RE­AL­ITY

The ma­jor break­through came in 2015 when re­searchers at Princeton Univer­sity proved how, in the­ory, the “im­pos­si­ble” crys­tals could ex­ist if they moved at fixed time in­ter­vals us­ing many-body lo­cal­i­sa­tion. The cru­cial point in the sci­en­tists’ new idea was that the atoms would not move all by them­selves, as this goes against the fun­da­men­tal laws of physics. Nei­ther would they move be­cause they were af­fected from the out­side. In­stead, they would make each other move.

This loop­hole in the world of physics in­spired sci­en­tists to go to their labs and test the the­ory in prac­tice, and in early 2017, two teams from Univer­sity of Mary­land and Har­vard Univer­sity, used dif­fer­ent ap­proaches, but still man­aged to get the same end re­sult.

In Mary­land, sci­en­tists shot laser pulses at a chain of ions of the sub­stance yt­ter­bium. The laser pulses pushed at the ions and made them change the di­rec­tion of their mag­net fields up­side down and back again in sync. Quite re­mark­ably, the fre­quency in the change of the ions’ mag­net fields re­mained un­changed, even though the fre­quency of the laser pulses changed. The chain of yt­ter­bium ions had its own rhythm, which could be con­sid­ered one of its fun­da­men­tal char­ac­ter­is­tics, like its mass or elec­tric charge. At Har­vard, sci­en­tists used mi­crowave pulses to push at small par­ti­cles inside a di­a­mond. The par­ti­cles turned around at pre­cise in­ter­vals – just like in the Mary­land test.

QUAN­TUM COM­PUT­ERS GET SUPERPOWERS

The new phe­nom­e­non, which has now been proved in the lab­o­ra­tory, has aroused ex­cite­ment in the world of physics. The time crys­tals are the first ev­i­dence that mat­ter is ca­pa­ble of or­gan­is­ing it­self in a time di­men­sion. The crys­tals may be seen as the clock­work of the uni­verse, which only needs a push to get go­ing and which will then move in a set rhythm of its own – for­ever.

One area, which may ben­e­fit from these clock­works, is quan­tum com­put­ers, which com­puter en­gi­neers are still strug­gling to make fit for use. In quan­tum com­put­ers, quan­tum bits will re­place the tran­sis­tors in or­di­nary com­put­ers. The tran­sis­tors are small, phys­i­cal switches, which are ei­ther on or off and used by the com­puter soft­ware to rep­re­sent 1s and 0s. The reg­u­lar changes of the time crys­tal mag­net fields can as­sume that func­tion, but with­out us­ing en­ergy like tran­sis­tors. Also, they will be far smaller, so more com­put­ing power can be gath­ered in less space. At the same time, the abil­ity of time crys­tals to main­tain the rhythm in spite of out­side in­flu­ences from e.g. laser pulses is also good for their use as quan­tum bits. So far, it has been a prob­lem to find par­ti­cles that can be used as quan­tum bits, which were not too frag­ile to use in prac­tice.

The ex­per­i­ments with time crys­tals may mark the be­gin­ning of a com­pletely new field within physics, says one of the sci­en­tists behind the Mary­land ex­per­i­ment. Although the crys­tals in the ex­per­i­ments only ex­isted briefly and in very small sizes, the ba­sic con­cept has been proved. And just like salt crys­tals are nat­u­rally found in rel­a­tively large pieces, like the ones we use in cook­ing, Mon­roe thinks that time crys­tals may be nat­u­rally oc­cur­ring. In other words, the Uni­verse may be full of four-di­men­sional crys­tals that no one thought could ex­ist.

CLAUS LUNAU

Time crys­tals are four-di­men­sional and thus can­not be shown in three di­men­sions.

GE­ORG KUCSKO/HAR­VARD UNI.

In an ex­per­i­ment, sci­en­tists made small par­ti­cles in a di­a­mond be­have like time.

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