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

They could make it easier to build a working device, scientists say

Quantum computers may promise a giant leap forward in performance and efficiency, but none of that can happen until we figure out a practical way to build them. Russian scientists just announced what they say is a major advance.

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.

quantum qudit ququart Moscow MIPT Moscow Institute of Physics and Technology and the Russian Quantum Center

A multilevel quantum system known as a 'ququart.'

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.

Join the PC World newsletter!

Error: Please check your email address.

Our Back to Business guide highlights the best products for you to boost your productivity at home, on the road, at the office, or in the classroom.

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Katherine Noyes

IDG News Service
Show Comments

Most Popular Reviews

Latest News Articles


GGG Evaluation Team

Kathy Cassidy


First impression on unpacking the Q702 test unit was the solid feel and clean, minimalist styling.

Anthony Grifoni


For work use, Microsoft Word and Excel programs pre-installed on the device are adequate for preparing short documents.

Steph Mundell


The Fujitsu LifeBook UH574 allowed for great mobility without being obnoxiously heavy or clunky. Its twelve hours of battery life did not disappoint.

Andrew Mitsi


The screen was particularly good. It is bright and visible from most angles, however heat is an issue, particularly around the Windows button on the front, and on the back where the battery housing is located.

Simon Harriott


My first impression after unboxing the Q702 is that it is a nice looking unit. Styling is somewhat minimalist but very effective. The tablet part, once detached, has a nice weight, and no buttons or switches are located in awkward or intrusive positions.

Featured Content

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?