**Liberating Quantum Processors from Parasitic Interactions**

Creating perfect entanglement – a basic prerequisite for the success of quantum computers – requires full control over all qubit-qubit interactions. Until now, this goal has been hindered by the presence of an always-on and fundamental parasitic interaction that disturbs entanglement. Now, researchers at Forschungszentrum Jülich and RWTH Aachen University in collaboration with IBM T.J. Watson Research Center and Syracuse University both in the USA, have developed a theory-motivated idea and successfully implemented it to eliminate these interactions between two qubits. Their work results in a better understanding of the physics behind the error which also allows more precise entanglement to be engineered, as well as the unentanglement of two qubits.

Imagine a computer processor with no interaction between bits. Such a device could not compute anything because it cannot process AND and OR logical operations needed to add and multiply numbers. A quantum computing processor is no exception. Quantum bits need to interact with one another to create entanglement, however they also need as much isolation from the environment as possible in order to be stable.

Notable quantum processors have so far been made using superconducting circuits. On such circuits, qubits can interact with one another via shared couplers. Responding to the request for entanglement between two qubits, a two-qubit gate operation is activated to let them interact in a controllable way. However, up to now there has been a problem: desired entanglements could not be made with more than 99% accuracy. Such an imperfection is an insurmountable barrier to the ambition of scaling up the number of qubits in quantum processors. A relatively small computation that requires a series of two-qubit gates accumulates the error in every step, and eventually fails.

Experiments had shown that one of the main elements in the error is a fundamental parasitic interaction that is always-on. The theoretical research group of Dr. Mohammad Ansari at the Peter Grünberg Institute at Forschungszentrum Jülich and RWTH Aachen University in Germany, in collaboration with researchers at IBM T.J. Watson Research Center and the group of Prof. Britton Plourde at Syracuse University in the USA, have now developed a theory-motivated idea and implemented it to eliminate parasitic interactions between two qubits.

”A real qubit does not have just two computational levels, it has more. When two qubits are put together, the computational states increase to four, but in real life there are many non-computational levels around them and they repel one another. The parasitic interaction between two qubits originates from these repulsions. The key point is that in all circuits made so far, all qubits have similar anharmonicity signs, either negative with higher excited levels that move closer as we go higher in energy, or positive with higher excited levels that move further apart. We combined the two types of qubits on a circuit and noticed an interesting theoretical symmetry that makes it possible to cancel the repulsions and set qubits free from parasitic interaction,” Ansari explains.

Motivated by the idea, the researchers designed the first interacting qubit-qubit circuit with dissimilar qubits. In their circuit, a superconducting qubit with negative anharmonicity called a transmon is coupled to another superconducting qubit with positive anharmonicity, a so-called capacitively-shunted flux qubit (CSFQ), via a resonator. This circuit successfully demonstrated no parasitic interaction between the two qubits.

“We were surprised by the accuracy of the experimental verification of the symmetry! We knew that in general there are two dissimilar species of qubits with positive or negative anharmonicity signs. What we did not know, however, was that the dissimilar qubits are like the yin and yang of quantum computation: together they make qubits free from level repulsions,” Ansari continued.

The researchers performed two-qubit gate operations on the two qubits and showed that for boosting the gate fidelity, zeroing the parasitic interaction is as important as enhancing qubit coherence times. Their theory predicts that their architecture is not far off from achieving 99.9% fidelity in a two-qubit gate.

“This work not only brings us an important step closer to engineering perfect entanglement, but also accomplishes this in an elegant manner interesting to a broad physics audience”, enthuses Prof. David DiVincenzo, director at the Peter Grünberg Institute. “This is almost a textbook example of physics intuition being confirmed by experiment. It also opens up tremendous opportunities in quantum metrology, cryptography, internet, and computation as well as thermodynamics and nonequilibrium systems – in brief, wherever entanglement is useful.”

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