fast multi-qubit
The setup for realizing a three-qubit gate through two simultaneous two-qubit gates is a chain of three qubits; q1, q0, and q2. Two so called CZ gates are applied simultaneously between qubit q0 and q1 and q0 and q2. The result is a swap of states between qubit q1 and q2, conditioned on the state of q0.​​
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Fast multi-qubit gates enhance the performance of quantum computers

Available quantum computers struggle with noise that causes the qubits to quickly forget their values. Therefore, it is desirable to execute the algorithms swiftly. A team of WACQT researchers have now shown how two-qubit gates can be run simultaneously to create multi-qubit gates, which are more powerful – but still take less time to execute – than the constituent two-qubit gates.

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Quantum algorithms may outperform classical ones on important computational tasks in chemistry, optimization, and many other fields. However, to run on current quantum computers, these algorithms must be compiled into long sequences of elementary operations (gates) on one or two qubits. 

Since the available quantum hardware still struggles to protect qubits from noise, it is desirable to execute the algorithms as swiftly as possible. In a recent publication, a team of WACQT researchers at Chalmers shows how two-qubit gates on existing quantum hardware can be run simultaneously to create new, powerful multi-qubit gates, which surprisingly take less time to execute than the two-qubit gates from which they are constructed.

“This is not entirely intuitive, but it arises from interference between the simultaneous two-qubit gates,” says Anton Frisk Kockum who led the study.

The team has specifically explored how to create three-qubit gates. The key in their scheme is two-qubit gates that swap excitations between two neighbouring qubits. When the middle qubit in a chain of three qubits simultaneously interacts in that manner with both its neighbours, a pathway is created for swapping states between the two outer qubits, conditioned on the state of the middle qubit. And thus a three-qubit gate is created.

Through extensive numerical simulations, the team has shown that such three-qubit gates can be constructed for multiple quantum computer architectures and that they can be implemented with high reliability in available experimental setups. The results also suggest that additional multi-qubit gates can be discovered using similar constructions with other two-qubit gates. 

“This new way of creating multi-qubit gates opens up for re-compiling many quantum algorithms into shorter gate sequences, enhancing the performance of existing quantum computers without needing to upgrade the hardware,” says Frisk Kockum.

Read more in the paper Fast Multiqubit Gates through Simultaneous Two-Qubit Gates​​​​, published in PRX Quantum.

Text: Ingela Roos

Page manager Published: Wed 22 Dec 2021.