Why 'qudits' (not qubits) may be the key to quantum computing

Building quantum computers is difficult because the qubits they're made with tend to be highly unstable. Qubits are the quantum counterpart of the bits used in traditional computing. While traditional bits represent data as 0s or 1s, qubits are distinguished by what's known as superposition, or the ability to be both 0 and 1 at once.

Superposition is the heart of quantum computing's exciting potential, but it's also proved a thorny challenge. While calculations require that qubits not only maintain their state but also interact with one another, the quantum objects that have been used to create qubits -- ions or electrons, for example -- have so far only been able to maintain a certain quantum state for a short time. In a system with dozens or hundreds of qubits, the problem gets even trickier.

That's where physicists from the Moscow Institute of Physics and Technology and the Russian Quantum Center are proposing a different approach. Rather than trying to maintain the stability of a large qubit system, they sought instead to increase the capacity of the units doing the calculations. For that, they turned to the "qudit," a qubit alternative.

Qudits are quantum objects for which the number of possible states is greater than two. Included among them are qutrits, which have three potential states, and ququarts, which boast four. Because of those additional potential states, it takes fewer qudits than qubits to do the same amount of work.

“A qudit with four or five levels is able to function as a system of two 'ordinary' qubits, and eight levels is enough to imitate a three-qubit system," explained Aleksey Fedorov, a researcher at the Russian Quantum Center.

Fedorov and his colleagues demonstrated that on one qudit with five levels, created using an artificial atom, it is possible to perform full quantum computations.

“We are making significant progress because in certain physical implementations it is easier to control multilevel qudits than a system of the corresponding number of qubits," Fedorov said. "This means that we are one step closer to creating a full-fledged quantum computer."

The researchers' results were recently published in a series of papers in Physical Review A, Physics Letters A, and Quantum Measurements and Quantum Metrology.