Supercurrent Offers a Way to Read Qubits Without Destroying Their Quantum State

Reading a qubit is a problem of its own making. A conventional bit can be checked as many times as you like without changing what it holds. A qubit cannot. Measure it directly and its superposition of 0 and 1 typically collapses into one outcome before the full state can be determined. Reading the information alters it.

A theoretical study in Physical Review B proposes a way around this. It was led by José Carlos Egues, a full professor at the São Carlos Institute of Physics at the University of São Paulo in Brazil, and Yugui Yao of the School of Physics at the Beijing Institute of Technology in China. Their idea is to read a particular kind of qubit, the Andreev qubit, through an oscillating electric supercurrent that the qubit generates through its own quantum dynamics.

The existing workaround is the quantum nondemolition measurement, or QND. In many such protocols the qubit's information is transferred to an auxiliary system called an ancilla, and the measurements are made on that instead, sparing the original qubit as much as possible. It works, but it means preparing ancillas, coupling them to the qubit, transferring the information and measuring repeatedly. In systems with many qubits, that adds considerable experimental overhead. Egues describes his group's approach as different in kind: rather than moving the information elsewhere, they draw it from a signal the qubit itself produces.

The Andreev qubit takes its name from the Soviet physicist Alexander Andreev (1939–2023), who in 1964 described a quantum process at the interface between normal and superconducting materials. The resulting Andreev states are confined to the nanoscale region between two superconductors and can encode the basic states 0 and 1 as well as their superpositions. In the proposed system, that region is a quantum dot formed in an indium arsenide nanowire connected to superconductors on both sides.

The key finding is that when the qubit is in a superposition, the supercurrent, which flows without resistance, acquires a component that oscillates in time. The oscillation arises from quantum interference between the states making up the superposition. Any qubit state can be pictured as a point on the Bloch sphere, fixed by two angles: one giving the mix of 0 and 1, the other their phase relationship. Egues explains that the constant part of the current and the amplitude of the oscillation reveal the mix, while the moment the oscillation begins reveals the phase. Measuring how the current varies over time therefore allows the initial state to be reconstructed.

The effect also differs from the familiar AC Josephson effect, where an external voltage is normally needed to make a supercurrent oscillate. Here, the evolution of the qubit's own state drives the oscillation, with no voltage applied.

To read it, the Andreev qubit sits in a closed superconducting circuit carrying the supercurrent, with a SQUID, an instrument highly sensitive to weak magnetic fields, placed nearby. The current produces a varying magnetic field, the SQUID tracks it, and the qubit's state is inferred without being measured directly. Egues compares it to two compasses, one the qubit and one the SQUID. Because their coupling is very weak, the instrument's effect on the qubit should be very small. Feedback from the measuring device cannot be eliminated, but the study's simulations indicated it can be reduced to negligible values. Egues is explicit that the method does not remove the fundamental problem of quantum measurement; it approaches the nondestructive limit without ancillas. Nor does the qubit need resetting after each reading, so measurements can follow the system as it evolves.

The team also tested robustness by simulating measurement noise and the feedback from the small detection coil. Depending on the number of measurements, accuracy reached 99.52%, 99.81% and 99.85%, within standard thresholds used in quantum error correction schemes.

The study is entirely theoretical, though Egues says the parameters come from existing systems and SQUID detection of tiny currents is established technology. What does not yet exist is the proposed geometry. The next test is building it and seeing whether the predicted oscillating supercurrent appears in a real device.