Physicists have just taken an incredible step towards quantum devices that look like something out of science fiction.
For the first time, isolated groups of particles behaving like bizarre states of matter known as time crystals have been linked into a single, scalable system that could be incredibly useful in quantum computing.
After the first observation of the interaction between two time crystals, detailed in a paper two years ago, this is the next step towards the potential exploitation of time crystals for practical purposes, such as processing quantum information.
Time crystals, discovered and officially confirmed a few years ago in 2016, were once considered physically impossible. They are a phase of matter very similar to normal crystals, but for an additional, particular and very special property.
In regular crystals, the atoms are arranged in a fixed three-dimensional grid structure, like the atomic lattice of a diamond or quartz crystal. These repetitive networks may differ in configuration, but any movement they exhibit comes exclusively from external pushes.
In time crystals, the atoms behave a little differently. They exhibit patterns of movement over time that cannot be so easily explained by external pushing or pushing. These oscillations – called “tick-tock” – are locked on a regular and particular frequency.
Theoretically, time crystals operate at their lowest possible energy state – known as the ground state – and are therefore stable and consistent over long periods of time. Thus, where the structure of regular crystals repeats in space, in time crystals it repeats in space and time, thus presenting a perpetual motion of the ground state.
“Everyone knows that perpetual motion machines are impossible,” says physicist and lead author Samuli Autti from Lancaster University in the UK.
“However, in quantum physics, perpetual motion is acceptable as long as we keep our eyes closed. By weaving through this crack, we can create time crystals.”
The time crystals the team worked with are made up of quasiparticles called magnons. Magnons are not true particles, but consist of a collective spin excitation of electrons, like a wave propagating through a spin network.
Magnons emerge when helium-3 – a stable isotope of helium with two protons but only one neutron – is cooled to less than one ten thousandth of a degree from absolute zero. This creates what is called a B-phase superfluid, a fluid with zero viscosity at low pressure.
In this medium, time crystals formed as spatially distinct Bose-Einstein condensates, each consisting of a trillion magnon quasiparticles.
A Bose-Einstein condensate is formed from bosons cooled to just a fraction above absolute zero (but not reaching absolute zero, at which point the atoms stop moving).
This causes them to drop into their lowest energy state, moving extremely slowly and getting close enough to overlap, producing a high density cloud of atoms that acts like a “super atom” or matter wave.
When the two time crystals were allowed to touch, they exchanged magnons. This exchange influenced the oscillation of each of the time crystals, creating a unique system with the option of operating in two discrete states.
In quantum physics, objects that can have more than one state exist in a mixture of those states before they have been determined by clear measurement. Thus, having a time crystal operating in a two-state system provides rich new choices as a basis for quantum-based technologies.
Time crystals are far from being deployed as qubits, as there are a significant number of hurdles to solve first. But the pieces are starting to fall into place.
Earlier this year, another team of physicists announced that they had succeeded in creating room-temperature time crystals that do not need to be isolated from their surrounding environment.
More sophisticated interactions between time crystals, and their fine control, will need to be developed further, as will the observation of time crystals interacting without the need for cooled superfluids. But scientists are optimistic.
“It turns out that setting up two of them works like a charm, even though the Time Crystals shouldn’t exist in the first place,” says Autti. “And we already know that they also exist at room temperature.”
The research has been published in Nature Communication.