Physicists have proposed a way to read quantum information while barely disturbing a qubit's state. Rather than measuring it directly or using an additional intermediary qubit, the researchers suggest analyzing the electrical supercurrent generated within the quantum system itself. Computer simulations showed state reconstruction accuracy of up to 99.85%, although the method has yet to be tested experimentally.

The work focuses on a particular type of quantum bit: Andreev qubits. The study, supported by the FAPESP foundation, was published in Physical Review B Letters. Its authors are 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, Science Daily reports.

Why quantum information is difficult to measure

A conventional computer stores data in bits that take one of two values: 0 or 1. They can be read repeatedly without changing the original information. Quantum computers use qubits, which can exist not only in the states 0 and 1, but also in a superposition—a special quantum combination of the two possibilities.

Superposition is precisely what underlies the potential advantages of quantum computing. But it also creates a problem: measuring a qubit can change its state. Direct measurement usually destroys the superposition, and with it, information about the original quantum state is lost.

To get around this limitation, physicists have developed methods known as quantum nondemolition measurements. Many of them use an additional quantum system called an ancilla. Information is first transferred from the main qubit to the auxiliary one, and the ancilla is then measured.

This approach reduces the impact on the original qubit but makes the experiment more complicated. An additional system must be prepared, coupled to the main qubit, used to transfer information, and measured. As the number of qubits increases, these operations require more and more equipment and time.

The authors of the new study propose eliminating this intermediary by obtaining information about the qubit's state from a signal produced by the quantum system itself.

An Andreev qubit generates its own electrical signal

The method is based on an Andreev qubit. It is named after physicist Alexander Andreev, who in 1964 described a distinctive quantum process at the interface between a normal material and a superconductor.

Under certain conditions, superconductors conduct electrical current without resistance. Special quantum states known as Andreev states can arise at the interfaces of such materials. In the system under consideration, they form in an extremely small region between two superconductors.

This region is a quantum dot—a nanostructure in which electrons are spatially confined. In the study, it is formed in an indium arsenide (InAs) nanowire connected to superconducting materials on both sides. The quantum states in such a structure can encode the values 0 and 1, as well as their superpositions.

A key role is played by the supercurrent—an electrical current that flows through a superconducting system without resistance. According to the researchers' calculations, when an Andreev qubit is in a superposition, a component that oscillates over time appears in the supercurrent.

These oscillations arise from quantum interference between the states that make up the superposition. They are not random: the pattern of changes in the current depends on the qubit's state. The electrical signal can therefore serve as a kind of signature of the information stored in it.

How current oscillations can reveal a qubit's state

Physicists often use the Bloch sphere to describe a qubit's state. Each pure state of a single qubit corresponds to a point on the surface of this sphere. Its position is determined by two angles: one describes the relative contributions of states 0 and 1 to the superposition, while the other describes their relative phase.

The study's authors showed that both parameters can be obtained from the characteristics of the supercurrent. Its constant component and oscillation amplitude reveal the proportions in which states 0 and 1 contribute to the superposition. The timing of the oscillations, in turn, contains information about their relative phase.

Thus, observing how the current changes over time makes it possible to reconstruct the qubit's original state. This does not require directly measuring the quantum bit itself.

The proposed effect has another distinctive feature: the supercurrent oscillations arise without an external voltage. An alternating supercurrent in such superconducting systems is usually associated with a voltage—a phenomenon known as the AC Josephson effect. In the researchers' model, the oscillations have a different source: they emerge naturally during the qubit's own quantum evolution.

In other words, the system itself converts changes in its quantum state into an electrical signal that can be recorded by an external instrument.

Readout using an ultrasensitive sensor

For a practical implementation, the authors propose placing an Andreev qubit in a closed superconducting circuit through which the supercurrent can circulate continuously. A SQUID—a superconducting quantum interference device capable of detecting extremely weak magnetic fields—would be positioned nearby.

Any electrical current generates a magnetic field. Since the supercurrent in the proposed system oscillates, its surrounding magnetic field will also change. The SQUID could detect these weak oscillations and use them to determine the current's characteristics, then reconstruct the qubit's state.

A key requirement is weak coupling between the sensor and the system being measured. This allows information to be obtained while minimizing the measuring device's backaction on the qubit. Egues compares the setup to two compasses: one corresponds to the qubit, and the other to the SQUID, which picks up the magnetic field generated by the current in the circuit.

The method does not promise to eliminate the effects of measurement entirely. The sensor still interacts with the system, and its influence may slightly change the qubit's state. However, computer simulations showed that this backaction can be made so weak that its effect becomes practically negligible.

Another advantage of the proposed setup is the ability to observe changes in the state repeatedly without having to return the qubit to its initial state each time. This would allow the system's quantum evolution to be tracked over time, rather than producing separate results after cycles of measurement and preparation.

Accuracy of up to 99.85%—so far only in calculations

To assess how well the method would work under conditions close to those of a real experiment, the researchers conducted computer simulations. The calculations accounted for measurement noise and slight backaction caused by the measurement coil.

Depending on the number of measurements, state reconstruction accuracy reached 99.52%, 99.81%, and 99.85%. According to the authors, these figures fall within the standard thresholds used in quantum error correction schemes.

Nevertheless, these results cannot yet be regarded as a demonstration of a working device. The study is based on theoretical analysis and computer calculations. Whether comparable accuracy can be achieved in a physical system remains to be tested experimentally.

The authors emphasize, however, that their proposal does not require fundamentally new materials or instruments. The calculations use parameters from existing quantum systems, and SQUIDs are already used to detect extremely weak magnetic fields and currents. The main unresolved challenge is to assemble the components in the proposed configuration and confirm the system's predicted behavior.

If experiments confirm the results, reading information from Andreev qubits could become simpler, without additional intermediary qubits or repeated state-reset cycles. But the significance of the work extends beyond potentially reducing the complexity of quantum computers. It offers a way to connect a system's internal quantum dynamics to a measurable electrical signal—and thus move closer to reading fragile quantum information with minimal disturbance.