Time crystals: the discovery of a previously theoretical matter

This week, two research groups at different universities in the US have created a new type of matter: Time Crystals.

Normal crystals, such as salt and snowflakes, have an ‘asymmetrical ground state’ which means that when they have no energy (are in their ground state), the structure may look different from different angles (asymmetrical).

The time crystals created, however, look the same from any angle at any one time, but over time and without energy being added, they change molecular organisation. In other words, the particles in time crystals move even though there is no energy to give them motion.

This appears to break the rules of physics when it comes to motion. Normally one would have to transfer energy to an object for movement to occur.

However, the research groups have not created tiny everlasting energy producers or broken the law of energy conservation, as there is no energy involved in the movements at all.

The teams used an approach outlined by Norman Yao from University of California, Berkeley, after the existence of time crystals was predicted by Nobel Prize winner Frank Wilczek in 2012.

The experiments involved entangled particles. Entangled particles are pairs of linked particles where something done to one particle has a direct effect on the other particle in the entangled pair. A magnetic field held each particle in place in a line. A laser was pulsed at the first particle in the line. This caused the particle to flip over, influencing its entangled partner and causing the neighbouring particles to flip down the line, like a row of falling dominos. This part was expected because the laser gives the particles the energy to flip over.

But the research groups at the University of Maryland and Harvard University noticed that the particles had started to flip at a different rate than they should if it was due to the laser’s energy. Instead, the particles were flipping at half the rate of the laser pulses. In other words, they were flipping without the energy input to flip.

This can be understood by using jelly as an analogy, where your finger is the laser and the jelly is the particles. When you tap the jelly repeatedly it wobbles at the same rate as your taps. But in these experiments, the jelly started to wobble at a different rate than the taps.

This shows a break in time symmetry, the idea that particles should flip over at the same rate as the laser.

Time crystals can be thought of as continually jiggling jelly. Not only do they wobble at different rates than the taps, but they wobble constantly even without the presence of a tap or laser to give them the energy to jiggle. This is different from the theory of perpetual motion, as there is no energy either lost or gained.

The different teams used different materials. The University of Maryland team used ions of the chemical element ytterbium lined up in a magnetic field, while Harvard University utilised the impurities in diamonds called ‘nitrogen vacancy centres’.

Since two different teams were able to come to the same discovery using the same instructions but different materials, there is a strong indication that their results are not chance happenings. They also indicate that time crystals are a new phase of matter, not just very specific physical systems.

Phases of matter are regions of space in which all physical properties are the same. Known phases of matter include solid, liquid, gas, plasma, and Bose-Einstein condensate. For example, ice, water, and water vapour are all different phases of the same material.

This discovery is expected to be useful in the application of quantum computers – computers with the potential to allow scientists to quickly create memory and retain information using the physical properties of subatomic particles.

However, it is important to note that the papers from both universities are yet to undergo peer review.

Image: Maxim Bilovitskiy

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