Physicists Propose Reading a Qubit's State From a Supercurrent Without Destroying It

A new theoretical study proposes a way to read the information stored in a quantum bit without wiping it out, by letting the qubit announce its own state through an electric current. The work, published in the journal Physical Review B, was led by José Carlos Egues of the São Carlos Institute of Physics at the University of São Paulo in Brazil and Yugui Yao of the Beijing Institute of Technology in China.

The problem it targets is one of quantum computing's oldest. A classical bit can be read over and over without changing it. A qubit, which can sit in a superposition of 0 and 1, usually cannot. "In the conventional formulation of quantum mechanics, a projective measurement causes the system to assume one of the states associated with the measured quantity," Egues said. Measure a qubit directly and the superposition tends to collapse, taking the information with it.

Physicists have long worked around this with what are called quantum nondemolition, or QND, measurements. Many of those protocols copy the qubit's information onto an auxiliary quantum system known as an ancilla, then measure the ancilla instead. The method works, but it means preparing ancillas, coupling them to the qubit, transferring the information and making repeated measurements. In machines with many qubits, that adds a great deal of experimental overhead. "Our strategy is different," Egues said. "Instead of transferring the information to an auxiliary qubit, we seek to obtain it from a signal produced by the qubit under study itself."

The qubit in question is an Andreev qubit, named for the Soviet physicist Alexander Andreev, who in 1964 described a quantum process at the boundary between normal and superconducting materials. Its states are confined to a nanoscale region between two superconductors. In this proposal, that region is a quantum dot formed in an indium arsenide nanowire connected to superconductors on both sides.

The team showed that when the qubit is in a superposition, the supercurrent flowing through the device, an electric current that travels without resistance, acquires an oscillating component. That oscillation comes from quantum interference between the two states in the superposition. The constant part of the current and the size of the oscillation reveal the proportion of 0 and 1 in the superposition, and the moment the oscillation begins reveals the phase relationship between them. Those two quantities are the two angles that fix a qubit's position on the Bloch sphere, so measuring how the current changes over time is enough to reconstruct the qubit's initial state.

There is a second surprise. In most superconducting junctions, a supercurrent only oscillates when an external voltage is applied, a phenomenon known as the AC Josephson effect. Here, the current oscillates with no voltage at all. "Our system generates a supercurrent that varies over time like an alternating current but without voltage," Egues said. "It's the very evolution of the qubit's state that produces that oscillation."

To read the signal, the qubit would sit inside a closed superconducting loop, with a SQUID, a superconducting quantum interference device that is extremely sensitive to weak magnetic fields, placed nearby. The circulating current produces a magnetic field that the SQUID detects. Egues likened the pair to two compasses. Because the coupling between them is very weak, the measuring instrument's effect on the qubit "tends to be very small." That matters because in quantum physics, the instrument is never a passive observer.

The study is a proposal rather than an experiment, and no device has yet been built to test it. If it holds up in the lab, it would offer a simpler route to reading out qubits at scale.