**So far, all computers have calculated with binary data in the form of zeros and ones – including quantum computers. But their computing units can actually assume more states. Physicists have now designed and tested a quantum computer that calculates with eight “quantum digits” instead of binary quantum bits. They use a series of calcium ions in ion traps as arithmetic units. With the help of a magnetic field, these particles can be brought into eight different, easily distinguishable energy states, seven of which were used for calculations. In the test, this new “Qudit” computer calculated as reliably as conventional quantum computers, as the researchers report.**

From smartphones to PCs to the most powerful supercomputers: all common computers calculate in binary and use sequences of zeros and ones to encode information. This standard for everything digital has so far also applied to the latest computer technology – the quantum computer. They solve tasks using atoms, ions or virtual particles, which, thanks to their quantum nature, show the phenomenon of quantum physical superposition and entanglement. This enables them, for example, to check all possible solutions to an arithmetic operation in parallel, making them faster than conventional electronic computers. The first quantum computers have already demonstrated this superiority over supercomputers, if only for certain tasks.

### Quantum digits instead of quantum bits

So far, however, the current quantum computers have been working with limited performance, so to speak. Because in order to be able to transfer logical structures and processes from conventional computers to quantum computers more easily, they too have been working in binary mode up to now. “However, the physical building blocks of the quantum computer can do much more than just zero and one,” explains first author Martin Ringbauer from the University of Innsbruck. “The restriction to binary systems deprives these computers of their real potential.” In order to make the quantum particles binary, two easily distinguishable physical states are usually chosen as zero and one – for example two energy states of an ion. This also makes reading out comparatively reliable and less error-prone. However, depending on the excitation, such a particle can assume many other energy states.

Ringbauer and his colleagues have now developed and tested a quantum computer that can fully exploit this potential and thus achieve more computing power with fewer quantum particles. “In the Innsbruck quantum computer, information is stored in individual trapped calcium atoms, each of which has eight states, of which only two have been used for calculations so far,” explains Ringbauer’s colleague Thomas Monz. For their experiment, however, the physicists developed a technique that allows all eight states to be used to store information. Instead of binary quantum bits, the ions act as quantum digits, or qudits for short. Seven states served as active information carriers, the eighth energy level was reserved for multi-step reading. The energy level of the qudits is gradually queried via fluorescence measurements.

### Advantage for many applications

For practical calculations with their Qudit quantum computer, they coupled the ions into logic gates that made various calculations possible. The first tests showed that the new quantum computer works just as reliably as one with only zeros and ones. “We are thus demonstrating a universal Qudit quantum processor that works on the basis of common ion trap hardware,” write Ringbauer and his colleagues. It could bring advantages for many applications, because many of the tasks for quantum computers, such as in physics, chemistry or materials science, first have to be rewritten for binary computing and are therefore more complicated. “Calculating with more than zero and one is not only ideal for quantum computers, but also much more natural for many applications,” says Ringbauer. “This approach allows us to exploit the full potential of our quantum computers.”

Source: Martin Ringbauer (University of Innsbruck) et al., Nature Physics, doi: 10.1038/s41567-022-01658-0