Russian scientists perform three-qubit CCZ quantum operation directly with 99.39% accuracy for the first time

Modern quantum computers and algorithms predominantly rely on single- and two-qubit gates. To implement an operation involving three qubits, engineers have to sequentially execute eight two-qubit operations, which complicates algorithm structure and significantly increases error accumulation. Researchers from MISIS University, the Russian Quantum Center, the Institute of Nanotechnology of Microelectronics of the Russian Academy of Sciences, and MIPT have announced the execution of a complex operation involving three qubits simultaneously with record accuracy for this class of devices. It is important to clarify that this is not about creating a fully functional superconducting quantum processor, but rather about the ability to perform a complex three-qubit operation in a single step.

Three fluxoniums and a shared control transmon

At the core of the device are three superconducting fluxonium qubits connected via a shared control element, a transmon. It is this element that allows targeting a specific combination of states of all three qubits simultaneously, executing the so-called CCZ (Controlled-Controlled-Z) operation. This operation changes the phase of one of eight possible qubit state combinations. Such gates are widely used in quantum algorithms, error correction systems necessary to improve computational reliability, and large integer factorization tasks, which potentially impacts cryptography and data security.

A single pulse instead of a gate chain

"We experimentally implemented the three-qubit CCZ gate directly, rather than through a cascade of single- and two-qubit operations. At the same time, control remains as simple as for standard two-qubit gates: a single microwave pulse lasting 65 ns is sufficient, calibrated using traditional methods—Rabi and Ramsey oscillations—which track changes in qubit states under the influence of the microwave signal," said Grigory Mazhorin, a researcher at the Laboratory of Superconducting Quantum Technologies at NUST MISIS and the "Superconducting Qubits and Quantum Circuits" group at the RQC. This approach simplifies calibration and reduces the number of points where errors can occur.

99.39% accuracy on simpler hardware

The key advantage of the development lies in the ratio of accuracy to hardware complexity. Under the traditional approach, to achieve comparable accuracy for the final operation, individual two-qubit gates must operate with an accuracy of about 99.39% each? Wait, about 99.94% each, which is currently available only on a limited number of the most advanced and complex quantum computers. The Russian implementation, however, allows performing the same operation with 99.39% accuracy in a single step on simpler and less demanding hardware, lowering the barrier to entry for working with three-qubit gates.

Scaling prospects

According to the developers themselves, the created architecture can serve as the basis for scaling superconducting quantum processors. The scientists note that the created module can be integrated into a two-dimensional circuit while maintaining mechanisms to suppress unwanted interactions between its elements. This paves the way for building larger quantum circuits without a proportional increase in control complexity and errors, which is one of the main barriers to practical quantum machines.