Time crystals, romance until the end of the world

In July 2021, Google and a group of scientists used their own sycamore quantum processor to realize a 20-qubit time crystal, and published the research results in Nature. Philip Frey and Stephen Rachel have made another major breakthrough based on Google research, designing the largest time crystal to date. The significance of this achievement is that it demonstrates the ability of quantum computers to simulate complex systems, transforming theoretical models that can only exist in the minds of physicists into objective entities.

What is a time crystal

There is a beautiful poem in "Three-Body Problem": The gift I gave to her was a small piece of frozen time, with beautiful patterns on it, and it felt as soft as the mud in the shallow sea. In sci-fi works, time is a tangible entity, a gift for lovers, in which the romance of two people is preserved, and this romance will not disappear until the end of the universe.

Time crystals are called time crystals in English, also called space-time crystals (space-time crystals), which are four-dimensional crystals with periodic structures in space and time. We are in daily contact with three basic forms of matter: solid, liquid and gas, but with the development of science, the concept of material form has also been expanded, such as plasma state, Bose-Einstein condensate, supercritical fluid, etc. . Time crystal is a brand-new form of matter, and it is also a non-equilibrium phase that breaks the symmetry of time translation.

The concept of time crystals was first proposed by Nobel Laureate in Physics Frank Wilczek in 2012. We are not unfamiliar with crystals in three-dimensional space, such as ice cubes and diamonds. Crystals are geometrically symmetrical structures in which microscopic particles are periodically arranged in space. Wilczek began to think about whether the concept of three-dimensional crystals could be extended to four-dimensional space-time when he was teaching students, so that matter could be arranged periodically in the dimension of time. That is to say, the time crystal has different states at different moments, and the change of this state is periodic. To give a simple example, a time crystal may be white sugar the first second, brown sugar the second, and back to white sugar the third.

Three-dimensional crystals have space-translational symmetry breaking, and similarly, time crystals should also have time-translational symmetry breaking. The so-called symmetry of space translation means that the laws of physics will not change after a physical system is translated by any distance along a certain direction in space. Simply put, it means doing the same experiment in different places and getting the same result. The symmetry of time translation refers to doing the same experiment at different times and getting the same result.

Symmetry is divided into high and low, and the symmetry of a circle is higher than that of a rectangle. Liquid water is isotropic, solid ice is anisotropic, and water is more symmetrical than ice. This process of high symmetry to low symmetry is called symmetry breaking. Three-dimensional crystals have to move an integer number of lattice constants to have the same spatial structure, and space-time crystals also have to go through a certain amount of time to return to their original state. That is to say, what you see in odd-numbered seconds such as 1st, 3rd, and 5th is white sugar, and what you see in even-numbered seconds, such as 2nd, 4th, and 6th, is brown sugar. The difference between brown sugar and white sugar is the time-translational symmetry of the time crystal. Sexually broken.

New perpetual motion machine?

In 1918, German female mathematician Emmy Noether proposed the physics-significant Noether theorem, that every symmetry has an associated conservation law, and vice versa. The spatial translation symmetry corresponds to the conservation of momentum, the spatial rotational symmetry corresponds to the conservation of angular momentum, and the time translation symmetry corresponds to the conservation of energy. The time crystal breaks the time translation symmetry, but it does not violate the law of conservation of energy, because the time crystal has neither energy input nor energy output.

Implementation of a time crystal

It is one thing to propose a theoretical model of time crystals, and another to realize it. The concept of time crystals has been questioned by many scientists, who believe that it is impossible to exist. In 2016, Norman Yao of the University of California, Berkeley, devised a detailed blueprint for making time crystals. Yao likens his blueprint to a bridge connecting theoretical models with experimental methods.

Based on Yao's blueprint, two teams from the University of Maryland and Harvard University independently created time crystals. The two teams used different methods but achieved similar results, confirming that time crystals are indeed an entirely new state of matter.

It should be pointed out that time crystals are divided into continuous time crystals and discrete time crystals. The time crystals implemented so far are all discrete time crystals. Continuous-time crystals are difficult to achieve and are still controversial.

Rachel's time crystal isn't perfect, it currently only lasts 50 cycles. In the future, time crystals could be used in quantum computers as a form of storage. Maybe time crystals, while not as sci-fi as the time gem in Marvel movies, can be as romantic as the poem in "Three-Body Problem": she paints time all over her body, and then pulls me to the edge of existence.

For physicists, discovering time crystals is like discovering a new world, but whether this new land is fertile or desert is not yet known.

The mystery of the time crystal, it will take time to unravel.