Supercurrent could help scientists read quantum information without destroying it

A collaborative research project between scientists at the University of São Paulo (IFSC-USP) in Brazil and the Beijing Institute of Technology in China has solved a long-pending problem in quantum technology: reading a qubit without destroying the information it contains. Their approach avoids the use of ancillas or auxiliary qubits that are used in non-destructive quantum readout methods too, making it quite a special feat.

As quantum technologies advance, scientists are looking for ways to measure quantum bits, or qubits that store quantum information. The nature of qubits is that they can store a large number of values between 0 and 1 but are also extremely prone to external disturbances. While this property lets qubits store large amounts of information at once, the latter makes computations with that information error-prone.

To minimize errors, one could read the information stored in the qubits from time to time, but this destroys or changes the information, presenting different challenges for researchers.

To address this, scientists have developed non-destructive quantum measurement methods that typically involve ancillas or auxiliary qubits. These ancillas must be paired with the qubit, after which information is transferred and then read. This increases the time required and the complexity of the whole measurement process.

So, a team led by Jose Carlos Euges at IFSC-USP and Yugui Yao at the Beijing Institute of Technology turned to Andreev qubits to determine if they could be measured directly.

What are Andreev Qubits?

Named after Soviet physicist Alexander Andreev, the Andreev qubits are a quantum characteristic of the interface between normal and superconducting materials. Also known as Andreev states, these occur in the nanoscale region between superconducting materials and can encode states like 0 and 1 and superpositions.

A system has another phenomenon called supercurrent, where electric current flows without any resistance. The researchers incorporated an Andreev qubit in a closed superconducting circuit and placed a SQUID (superconducting quantum interference device) near it to measure changes in weak magnetic fields.

When supercurrent flows through the circuit, the SQUID can measure the changes in the magnetic field and determine how the current is behaving. The researchers also found that the supercurrent has two components, a constant one and a fluctuating one.

Measuring these components reveals the proportion of states 0 and 1 contributing to the superposition, while the other reveals their phase relationship. Knowing these is equivalent to knowing the state of the qubit.

Reading without destroying

“We can think of the qubit as one compass and the SQUID as another compass. The current in the circuit produces a magnetic field, and the SQUID senses that field,” explained Egues in a press release. “Since the coupling between the two is very weak, the effect of the measuring instrument on the qubit tends to be very small.”

Quantum measurements always change the information on the qubit, and this time too; it’s no different. The device the team is proposing will give feedback that will alter the state of the qubit. However, in the team’s simulations, these state changes are so small that they are irrelevant. With accuracies of 99.52 percent, 99.81 percent, and 99.85 percent, the results are within the limits of the margin of error.

“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,” added Egues.

Moreover, the approach allows the researchers to work without resetting the qubit to its initial state, which could help develop more advanced systems in the near future.

The researchers published the findings in Physical Review B.