Quantum Computers Could Read Qubits Without Wrecking Them, Study Says

Physicists in China and Brazil say they have found a way, on paper, to check the value of a quantum bit without erasing the very property that makes it useful. Instead of measuring the qubit directly, their method reads the electrical current the qubit produces, according to ScienceAlert.

The work appeared in Physical Review B. It is a theoretical study with no experiments behind it, and the team's modeling suggests the technique might retrieve qubit data with an accuracy of up to 99.85 percent.

The problem with looking at a qubit

A classical computer bit is either 1 or 0. A qubit can be 1, 0 or a blend of both, a state known as superposition, and that flexibility is why quantum computing is expected to be so powerful.

The catch is that measuring a qubit forces it to settle on 1 or 0. As ScienceAlert put it, the most valuable trait of the qubit disappears the moment someone checks on it.

Jose Carlos Egues, a physicist at the University of São Paulo in Brazil, explained that a direct measurement of a qubit prepared in superposition tends to destroy that superposition and the information it held.

How the proposed method works

Today's quantum computers often get around the problem with a helper known as an ancilla, an auxiliary qubit that takes on the information of the original so it can be read without disturbing it. ScienceAlert noted that this works but adds time and complexity.

The new proposal skips the helper. It relies on Andreev qubits, which are built from Andreev states that form between superconducting materials and can encode 0s, 1s and superpositions.

According to the researchers' calculations, an Andreev qubit in superposition generates an oscillating electrical current with no outside voltage needed. The quantum behavior itself drives the current. That current, which the team calls a supercurrent, produces a magnetic field, and the field can reveal the qubit's state.

In the system the team studied, the key region is a quantum dot, a nanoscale structure that confines electrons. It sits in an indium arsenide nanowire connected to superconducting materials on both sides, and the Andreev qubit's states form there, Egues said.

A second device detects how the current varies. ScienceAlert described the arrangement as two compasses, one that points the way and one that interprets the direction. By tracking how the current changes over time, Egues said, it is possible to reconstruct the qubit's original state.

The researchers also suggest the qubit would not need the reset that is normally required after a measurement.

Why it matters for quantum computing

Reading out qubits quickly and reliably is one of the practical hurdles for building useful quantum computers. A readout method that leaves the quantum state intact and does not require extra qubits could, in principle, make the process simpler and more dependable.

ScienceAlert reported that Andreev qubits have been studied extensively before, but using their electrical behavior as a readable signal is new. It is the first time this kind of oscillating signature has been proposed as a way to decode qubits.

For ordinary readers, this is not a development that changes anything today. No device exists yet, and the result has not been tested in a lab.

What the researchers admit it does not solve

Egues was careful about the claims. He said the method does not remove the fundamental problem of quantum measurement, but that it can come close to a non-destructive readout while avoiding the ancilla qubits that other non-destructive protocols use.

The accuracy figure of up to 99.85 percent comes from the team's own modeling, not from a physical device, so real-world performance may differ.

What happens next

The obvious next step, according to ScienceAlert, is to build the device the researchers have described and put it through real tests. Only then will it become clear whether the supercurrent approach holds up outside the equations.

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Around Prague, based on reporting by ScienceAlert

Cover photo: Anita Fors (Chalmers) / Wikimedia Commons, CC BY-SA 4.0, cropped