Science

A new phase of matter in two time dimensions created on a quantum computer

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Many researchers are now working on the creation of functional quantum computers. Endowed with enormous computing power, they promise many advances. However, they still face a major hurdle: their core computing units, known as “qubits”, cannot maintain consistency for a very long time. By creating two “time dimensions”, the researchers are exploring a solution to this problem.

Their goal is to succeed in “protecting” quantum information in a more efficient way than in modern quantum systems. Their results were published in the journal Nature. To better understand how they got there, we must remember what a “qubit” is in principle. A qubit is primarily the most elementary unit of information. In a classical computer, we find “bits” instead of these qubits. Their value is 0 or 1, and they allow you to compose all the “codes” necessary for computer calculations.

In a quantum computer, qubits can (in addition to these two states) be both 0 and 1 at the same time. This state, which is rather difficult to understand, is called “quantum superposition”. This is what allows you to push the boundaries of the standard calculation. The density of additional information, as well as the interactions of qubits with each other, make it possible to perform calculations of extreme complexity, and much faster.

However, this quantum state is also very difficult to maintain. Indeed, more specifically, qubits are made up of atoms. In this particular case, the scientists chose to work with 10 ytterbium atoms. In their system, each of these ions is held and controlled individually by electric fields generated by an ion trap. It can be manipulated or measured with laser pulses. This configuration is relatively classical in quantum computing.

What makes it complex is the interaction of these ions with the environment. Indeed, in order to perform quantum computations, ions must, in particular, interact with each other. But the interaction they can have with their environment is exactly what can disrupt their quantum state. “Even if you keep all the atoms under tight control, they can lose their quantum state by talking to the environment, heating up, or interacting with things in ways you didn’t expect,” explains Philippe Dumitrescu, one of the authors of the study. , in the statement. “In practice, experimental devices have many sources of errors that can degrade coherence after just a few laser pulses.”

The researchers wanted to explore the path of an additional “time dimension” in their work. In their opinion, adding this dimension will reduce the risk of decoherence of atoms. To understand this idea, one can draw a parallel with “quasicrystals”. In a classical crystal, we observe a regular and repeating structure: there is a regular pattern, somewhat reminiscent of the cells of a beehive, the cells of a chessboard … A quasicrystal is a little different: it has a well-ordered structure. , and yet its structure never repeats. A good example of this is the tile from the so-called “Penrose” paving, which can often be seen in everyday life.

Penrose tiles. © Simons Foundation

Time Crystals

However, the so-called “temporary crystals” can be obtained in a little the same way. It is believed that there are repeating patterns, but this time over time. In a specific case, in this case, we are talking about periodic stimulation of atoms with lasers to create movements that repeat indefinitely in time (for example, a particle moves and returns to the same place). This method, already in use, provides a temporal “symmetry” that has been shown to enhance the coherence of the qubits. This time, instead of the usual laser pulses, the scientists decided to send “quasi-rhythmic” pulses. That is, these pulses are ordered, but not repeated: as in the case of a quasicrystal.

These impulses were sent according to the Fibonacci sequence. In such a sequence, each part of the sequence is the sum of the two previous parts (A, AB, ABA, ABAAB, ABAABABA, etc.). A schematically correct sequence would give, on the contrary, A, B, A, B … Thus, we are in an ordered sequence, but not repeating. Thus, by “bombarding” qubits, scientists somehow get two “patterns” of different temporality. In response to the laser rhythm, the qubits also perform a quasi-periodic motion, but different from the laser motion. This is why researchers talk about two simultaneous “time dimensions”.

However, according to them, it is these movements that are able to cancel errors that may arise due to the interaction of qubits with the environment. In short, it would make them more “sustainable”. “The use of an “extra” time dimension in the approach is a completely different way of understanding the phases of matter,” explains Philippe Dumitrescu. “I have been working on these theoretical ideas for more than five years, and it is very interesting for me to see how they come to life in experiments.”

To test their theory, the researchers ran tests on a row of 10 atoms that form qubits. First, they sent laser pulses at regular intervals, and then at a rate corresponding to Fibonacci numbers. They focused on qubits at the ends of their 10-atom alignment. Their results seem pretty convincing. Indeed, with periodic pulses, the qubits remained in the quantum state for about 1.5 seconds. In the quasi-periodic model, they could be maintained throughout the entire experiment, i.e., about 5.5 seconds. “Because the extra temporal symmetry provided more protection,” says Philippe Dumitrescu.

If the resistance has indeed increased, it remains to find a way to use this new method in a functional quantum calculation. “We have this direct and attractive application, but we have to find a way to integrate it into the calculations,” says Philippe Dumitrescu. “This is an open issue that we are working on.”

Nature

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