By firing a Fibonacci laser pulse at atoms in a quantum computer, physicists have created an entirely new, strange phase of matter that behaves as if it had two dimensions of time.
The new phase of mattercreated by using lasers to rhythmically shake a string of 10 ytterbium ions, allows scientists to store information in a much more error-proof way, thus paving the way for quantum computers which can hold data for a long time without being deformed. The researchers outlined their findings in a paper published July 20 in the journal Nature (opens in new tab).
Incorporating a theoretical “extra” dimension of time “is a completely different way of thinking about the phases of matter,” lead author Philip Dumitrescu, a researcher at the Center for Computational Quantum Physics at the Flatiron Institute in New York, it said in a statement. “I worked on those theory ideas for over five years, and it’s exciting to see them actually come to fruition in experiments.”
Connected: The unearthly ‘time crystal’ made in Google’s quantum computer could change physics forever
Physicists did not set out to create a phase with a theoretical extra time dimension, nor did they seek a method that would allow better quantum data storage. Instead, they were interested in creating a new phase of matter—a new form in which matter could exist, beyond the standard solid, liquid, gasplasma.
They set about building the new phase in quantum computing company Quantinuum’s H1 quantum processor, which consists of 10 ytterbium ions in a vacuum chamber that are precisely controlled by lasers in a device known as an ion trap.
Ordinary computers use bits or zeros and 1s to form the basis of all calculations. Quantum computers are designed to use qubits, which can also exist in a state of 0 or 1. But that’s pretty much where the similarities end. Thanks to the strange laws of the quantum world, qubits can exist in a combination or superposition of both 0 and 1 states until they are measured, after which they arbitrarily collapse into either 0 or 1.
This strange behavior is the key to the power of quantum computing, as it allows qubits to bind together through quantum entanglementa process which Albert Einstein called “spooky action at a distance.” Entanglement binds two or more qubits together, binding their properties so that any change in one particle will cause a change in the other, even if they are separated by vast distances. This gives quantum computers the ability to perform multiple calculations simultaneously, exponentially increasing their processing power over that of classical devices.
But the development of quantum computers has been held back by a major flaw: qubits don’t just interact and entangle each other; since they cannot be completely isolated from the environment outside the quantum computer, they also interact with the external environment, thereby causing them to lose their quantum properties and the information they carry in a process called decoherence.
“Even if you keep everything atoms under tight control, they can lose their ‘quantumness’ by talking to the environment, heating up, or interacting with things in ways you didn’t plan for,” Dumitrescu said.
To bypass these annoying decoherence effects and create a new, stable phase, physicists have sought a special set of phases called topological phases. Quantum entanglement not only enables quantum devices to encode information in the individual, static positions of qubits, but also to weave them into the dynamic motions and interactions of the entire material—into the very form or topology of the material’s entangled states. This creates a “topological” qubit that encodes information in the form of multiple parts rather than a single part on its own, making the phase much less likely to lose its information.
A major hallmark of phase transitions is the breaking of physical symmetries—the idea that the laws of physics are the same for an object at any point in time or space. As a liquid, the molecules in water follow the same physical laws at every point in space and in every direction. But if you cool water enough so that it transforms into ice, its molecules will pick the right points along a crystal structure or lattice to line up across. Suddenly, water molecules have preferred points in space to occupy and leave other points empty; the spatial symmetry of water is spontaneously broken.
Creating a new topological phase in a quantum computer also relies on symmetry breaking, but with this new phase, the symmetry is not broken in space, but in time.
Connected: The world’s first multi-node quantum network is a breakthrough for the quantum internet
By giving each ion in the chain a periodic pulse with the lasers, the physicists wanted to break the continuous time symmetry of the ions at rest and impose their own time symmetry – where the qubits remain the same for certain time intervals – which would create a rhythmic topological phase in the material.
But the experiment failed. Instead of inducing a topological phase that is immune to decoherence effects, regular laser pulses amplify the noise outside the system, destroying it less than 1.5 seconds after switching on.
After revisiting the experiment, the researchers realized that to create a more stable topological phase, they would need to bind more than one time symmetry in the ion filament to reduce the chances of the system being scrambled. To do this, they focus on finding a pulse pattern that does not repeat simply and regularly, but nevertheless exhibits some kind of higher symmetry in time.
This led them to Fibonacci sequence, in which the next number in the sequence is created by adding the previous two. While a simple periodic laser pulse can simply alternate between two laser sources (A, B, A, B, A, B, etc.), their new pulse train is instead performed by combining the two pulses that came before (A, AB, ABA, ABAAB, ABAABABA, etc.).
This Fibonacci ripple created a time symmetry that, just like a quasi-crystal in space, was ordered without ever repeating itself. And just like a quasicrystal, Fibonacci pulses also compress a higher-dimensional pattern onto a lower-dimensional surface. In the case of a three-dimensional quasicrystal like the Penrose tile, a piece of a five-dimensional lattice is projected onto a two-dimensional surface. When we look at the Fibonacci momentum pattern, we see two theoretical time symmetries flatten into one physical one.
“The system essentially gets a bonus symmetry from a nonexistent extra time dimension,” the researchers wrote in the statement. The system appears to be material that exists in some higher dimension with two dimensions of time – even if this is physically impossible in reality.
When the team tested it, the new quasi-periodic Fibonacci pulse created a topographic phase that protected the system from data loss for the entire 5.5 seconds of the test. Indeed, they had created a phase that was immune to decoherence for much longer than others.
“With this quasi-periodic sequence, there is a complex evolution that cancels out all the errors that live on the edge,” Dumitrescu said. “Therefore, the edge remains quantum-mechanically coherent for much, much longer than you would expect.”
Although the physicists have achieved their goal, one hurdle remains in making their phase a useful tool for quantum programmers: integrating it with the computational side of quantum computing so that it can be entered with calculations.
“We have this direct, irritating application, but we have to find a way to incorporate it into the calculations,” Dumitrescu said. “This is an open issue that we are working on.”
Originally published on Live Science.