A physical qubit with integrated error correction

Significant progress has been made in the field of quantum computing. Large global players, such as Google and IBM, already offer cloud-based quantum computing services. However, quantum computers cannot yet solve the problems that arise when standard computers reach the limits of their capabilities, because the availability of qubits or quantum bits, i.e. the basic units of quantum information , is still insufficient.

One reason for this is that bare qubits are not of immediate use for running a quantum algorithm. While the binary bits of typical computers store information in the form of fixed values ​​of 0 or 1, qubits can represent 0 and 1 at the same time, making probability play out as to their value. This is called quantum superposition.

This makes them very susceptible to outside influences, meaning the information they store can easily be lost. To ensure that quantum computers provide reliable results, it is necessary to generate true entanglement to bring together multiple physical qubits to form a logical qubit. If one of these physical qubits fails, the other qubits will retain the information. However, one of the main difficulties preventing the development of functional quantum computers is the large number of physical qubits required.

Advantages of a photon-based approach

Many different concepts are used to make quantum computing viable. Large companies currently rely, for example, on semiconductor superconducting systems, but these have the disadvantage of only operating at temperatures close to absolute zero. Photonic concepts, on the other hand, operate at room temperature.

Single photons generally serve as physical qubits here. These photons, which are like tiny particles of light, inherently work faster than solid-state qubits, but, at the same time, are more easily lost. To avoid qubit losses and other errors, it is necessary to couple multiple single-photon light pulses to construct a logical qubit, as in the case of the superconductor-based approach.

A qubit with inherent error correction capability

Researchers from the University of Tokyo and colleagues from Johannes Gutenberg University Mainz (JGU) in Germany and Palacký University Olomouc in the Czech Republic recently demonstrated a new way to build a photonic quantum computer. Rather than using a single photon, the team used a laser-generated light pulse that can be made up of multiple photons. The research is published in the journal Science.

“Our laser pulse has been converted into a quantum optical state which gives us an inherent ability to correct errors,” said Professor Peter van Loock from the University of Mainz. “Although the system consists only of a laser pulse and is therefore very small, it can, in principle, eliminate errors immediately.” Thus, there is no need to generate individual photons as qubits via many light pulses and then have them interact as logical qubits.

“We only need a single pulse of light to obtain a robust logical qubit,” van Loock added. In other words, a physical qubit is already equivalent to a logical qubit in this system – a remarkable and unique concept. However, the logic qubit produced experimentally at the University of Tokyo was not yet of sufficient quality to provide the necessary level of error tolerance. Nevertheless, researchers have clearly demonstrated that it is possible to transform non-universally correctable qubits into correctable qubits using the most innovative quantum optics methods.