Supercurrent readout could spare quantum bits from measurement collapse

Reading a quantum bit without destroying the information it carries has long been one of the central obstacles standing between today’s experimental quantum processors and the fault-tolerant machines researchers hope to build. A theoretical study led by physicists at the University of São Paulo in Brazil and the Beijing Institute of Technology in China now proposes a way to sidestep the problem entirely, by turning the qubit’s own electric current into a faithful mirror of its quantum state. The proposal, published in the journal Physical Review B Letters, describes how an oscillating supercurrent flowing through an Andreev qubit can be monitored from outside to reconstruct the qubit’s state with simulated accuracies exceeding 99.5 percent, all without the auxiliary qubits that current non-destructive readout schemes demand.

The difficulty the researchers set out to address is rooted in the foundations of quantum mechanics. A classical bit can be interrogated again and again without altering its content, but a qubit is different. When a qubit is prepared in a superposition of its two basis states, labeled 0 and 1, a direct projective measurement forces the system to assume one of those states, collapsing the superposition and destroying the information encoded in it. In the conventional formulation of quantum mechanics, the act of measuring a quantity makes the system take on one of the states associated with that quantity, so the measurement itself typically rewrites what it was meant to reveal.

To work around this, researchers have developed what are known as quantum nondemolition measurements, or QND protocols. In many of these schemes, the qubit’s information is first transferred to an auxiliary quantum system called an ancilla, a term derived from the Latin word for servant, and the measurements are then carried out on the ancilla instead. The original qubit is preserved as much as possible, but the procedure comes at a cost. Each readout requires preparing the ancilla, coupling it to the qubit, transferring the information, and performing successive measurements. In processors with many qubits, this ancillary infrastructure adds considerable experimental overhead in time and complexity.

“Our strategy is different. Instead of transferring the information to an auxiliary qubit, we seek to obtain it from a signal produced by the qubit under study itself,” says José Carlos Egues, a full professor at the São Carlos Institute of Physics at the University of São Paulo, who led the study together with Yugui Yao of the School of Physics at the Beijing Institute of Technology. The signal in question is a supercurrent, an electric current that flows without resistance, generated by the qubit’s own quantum dynamics.

The qubit at the center of the proposal is an Andreev qubit, named after the Soviet physicist Alexander Andreev, who in 1964 described a quantum process characteristic of the interface between normal and superconducting materials. That process gives rise to so-called Andreev states, quantum states confined to the nanoscale region separating two superconducting materials. In the system considered by the researchers, this region is a quantum dot, a nanoscale structure in which electrons are spatially confined, formed in an indium arsenide nanowire connected to superconducting materials on both sides. The Andreev states in this environment can encode the basis states 0 and 1, along with arbitrary superpositions of the two.

What makes this platform remarkable for readout is that the supercurrent flowing through the system is not static when the qubit is in a superposition. The study showed that the current develops an oscillating component over time, produced by quantum interference between the states that make up the superposition. That oscillation carries information about the qubit’s state. “It’s precisely that property that we use to read the qubit. It’s that supercurrent that lies at the heart of our proposal,” Egues points out.

The mapping between the current and the qubit’s state can be described geometrically. Any qubit state corresponds to a point on the Bloch sphere, where each point represents a specific superposition defined by two angles: one indicating the proportions in which the states 0 and 1 contribute to the superposition, and the other capturing the phase relationship between them. Knowing these two angles is equivalent to knowing the state. The researchers showed that this information is encoded in the supercurrent itself. The current has a constant component and a fluctuating component; the value of the constant part and the amplitude of the oscillation reveal the proportions of the two basis states, while the moment the oscillation begins reveals their phase relationship. By measuring how the current varies over time, the initial state of the qubit can in principle be reconstructed. Notably, in superconducting systems of this kind a voltage is usually required to make a supercurrent oscillate, a phenomenon known as the AC Josephson effect. Here, however, the current oscillates without any applied external voltage, because it is the evolution of the qubit’s own state that produces the oscillation.

To actually detect the current, the researchers embed the Andreev qubit in a closed superconducting loop through which the supercurrent circulates. Near this loop they place a SQUID, a superconducting quantum interference device that is extraordinarily sensitive to weak magnetic fields. As the supercurrent flows, it generates a magnetic field that varies along with the current, and the SQUID registers those variations. Egues offers a simple analogy: the qubit is one compass and the SQUID is another. The current in the circuit produces a magnetic field, and the SQUID senses that field. Because the coupling between the two is very weak, the influence of the measuring instrument on the qubit tends to be very small. This matters because in quantum physics the measuring device is never a passive observer; its interaction with the system produces feedback that can alter what is being measured. The proposed device is subject to the same back-action, but simulations carried out in the study indicated that the feedback can be reduced to values so small that they become negligible for the readout.

The researchers also tested whether the approach would survive the imperfections of a real laboratory. They ran simulations that incorporated noise into the measurements and accounted for the feedback caused by the small detection coil. Depending on the number of measurements considered, the simulations yielded state-reconstruction accuracies of 99.52 percent, 99.81 percent, and 99.85 percent, results that fall within the standard thresholds adopted in quantum error correction schemes. “Our method doesn’t eliminate the fundamental problem of quantum measurement. But it can approach the limit of a non-destructive readout and, at the same time, avoid the use of auxiliary qubits – the ancillas – employed in other non-destructive quantum measurement protocols,” Egues notes. A further advantage is that the qubit does not need to be reset to its initial state after each measurement, so successive measurements can track the system’s dynamics as it evolves over time.

The study is entirely theoretical, but the authors argue that it does not rest on speculative technology. “All the numbers we used are realistic. In principle, this can be achieved. In fact, we used parameters from existing systems in our calculations. What doesn’t yet exist is that geometry, that setup that we proposed,” Egues says. Detecting extremely small currents and magnetic fields with SQUIDs is an established capability, which distinguishes this proposal from theoretical schemes that would depend on future advances in materials or device fabrication. If experiments confirm the prediction, the method could offer a simpler architecture for reading out Andreev qubits, dispensing with ancillas and repeated resets while exploiting an appealing feature of these systems: the microscopic quantum information becomes accessible through a macroscopic electric current generated by the qubit itself. The work thus joins two results, one fundamental, the prediction of an oscillating supercurrent akin to the AC Josephson effect but driven purely by the qubit’s coherent evolution without external voltage, and one technological, the use of that current’s characteristics as a signature from which the qubit’s state can be reconstructed with minimal perturbation.

Subject of Research: Non-destructive quantum readout of Andreev qubits using an oscillating interference supercurrent

Article Title: Supercurrent enables the reading of quantum information without destroying it

Article References: Supercurrent enables the reading of quantum information without destroying it. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum computing, Andreev qubit, supercurrent, quantum nondemolition measurement, SQUID, Josephson effect, quantum dot, indium arsenide nanowire, superconductivity, qubit readout, Bloch sphere, quantum error correction