There is a quantum artificial intelligence.

Symmetry-protected Majorana edge modes produced on Google's quantum computer
An artist's depiction of Majorana edge modes on a chain of superconducting qubits. Credit: Google Quantum AI

Physicists at Google have used their quantum computer to study a type of particle that is more resistant to environmental changes that can affect quantum calculations. The Majorana edge modes are effective because of a collective excitation of multiple individual particles. Majorana edge modes can protect quantum states from noise in the environment, which is of interest to quantum computing applications.

It is only slightly overstating the case to say that physics is the study of symmetry. Physical phenomena and their relationship to underlying symmetries have been studied for hundreds of years. Symmetries are statements about what changes a system can make. Problems can be simplified and laws can be understood. Symmetries can prevent the seemingly inexorable quantum process from happening.

When running a calculation on a quantum computer, we usually want the quantum bits to be in a single quantum state. When external electric fields or other noise disturb these states by jumbling them up with other states, it's called decoherence. If a state has a certain symmetry, it can be possible to create an island of stability that is impossible to mix with other states. Since the noise can't connect the symmetric state to the others, it could preserve the state's coherence.

The physicist created a model to make quantum states. fermions are a chain of particles. Two particles at the end of the chain could be connected in such a way. The particles were delocalized in space and appeared at both ends of the chain at the same time.

The Majorana edge modes were called theMEMs. Under parity transformation, the two modes had distinct behaviors. There was a symmetry of the state under this transformation. One picked up a sign. The difference in parity between the two states made it impossible to mix them with other noise sources.

Xiao Mi, Pedram Roushan, Dima Abanin and their colleagues at Google realized these MEMs with superconducting qubits for the first time in a paper published in Science. The model Kitaev had considered to be the 1D kicked-Ising model was mapped using the Jordan-Wigner transformation. Each qubit in a 1D chain is connected to each of its two nearest neighbors. The chain is disturbed by a kick.

Mi and his colleagues compared the behavior of the edge qubits to those in the middle of the chain. The states of those on the edge lasted longer than the ones in the middle. The indication for the resilience of the MEMs was given by this.

The noise resilience of the MEMs was studied. They measured the energies corresponding to the different quantum states of the system and compared them to the textbook example of the Kitaev model. As the system size grew, the two MEMs at the opposite ends of the chain became harder to mix.

The team added low-frequency noise to the control operations. They found that the MEMs were immune to such changes. The team found that the MEMs are resistant to some noise that breaks the Ising model. The mechanism called "prethermalization" arises from the large energy cost required to change the MEMs into other possibilities.

The team measured the wave functions of the MEMs. It's necessary to simultaneously measure the states of different qubits close to either end of the chain. They discovered that the decay time was the same regardless of how many qubits were included. Measurement involving up to 12 qubits decayed over the same timescale as those of just one qubit. The noise resilience of the MEMs was highlighted by the fact that larger quantum observables decay slower in the presence of noise.

In the future, Mi and Roushan think they can use MEMs to enable symmetry-protected quantum gates. This is a promising route to make more robust gates in a quantum processor due to the fact that the MEMs are sensitive to both low- and high-frequency noise.

The researchers plan to continue to improve the level of protection these MEMs experience, hoping to rival some of the leading techniques used to fight against decoherence in quantum computers There is a question of whether these techniques can be extended to achieve the levels of protection comparable to active error-correction codes.

There are noise-resilient edge modes on a chain of qubits. There is a science.abq5769.

Journal information: Science

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