RRI team uses timed single gate to steady fragile quantum states

Researchers at the Quantum Information and Computing (QuIC) laboratory of the Raman Research Institute in Bengaluru have demonstrated a technique that could help make quantum computations more dependable by improving the stability of the states on which they rely.

Quantum computers differ fundamentally from conventional machines, which store and process information as 0s and 1s. They exploit counter-intuitive quantum properties, among them entanglement — the phenomenon in which two particles that have interacted become linked and behave as a single system, so that measuring one reveals something about the other no matter how far apart they are. Combined with superposition, the ability of quantum particles to occupy multiple states simultaneously, this property allows such machines to tackle certain highly complex computations efficiently.

The states that give quantum computers their advantage are, however, extremely delicate. An entangled state breaks down quickly when it meets the external environment, a process called decoherence. In some cases entanglement disappears abruptly even before that gradual decay sets in, a condition known as entanglement sudden death. The resulting uncertainty undermines confidence in the integrity of calculations.

The usual response has been to apply corrective interventions repeatedly in order to postpone decoherence. Such repeated operations carry their own penalties: they are expensive and can themselves introduce errors.

The team led by Urbasi Sinha, group leader of the laboratory and a senior professor at RRI, has taken a different route. Rather than relying on repeated interventions, it has developed a single-shot operation to address decoherence. Depending on the moment at which it is applied, the operation can delay decoherence and entirely prevent sudden death. Applied at another instant, it can accelerate decoherence — an outcome that is not scientifically desirable but one that illustrates the degree of control the method provides.

Sinha described the timing of the operation as the core of the finding. In her view, the instant of application is not merely an experimental detail but can serve as a control resource. She characterised the experiments as a proof of concept relevant to the problem of decoherence rather than a complete solution, adding that different quantum computing systems would need to test it to establish how useful it proves in varied environments.

The results have been published in Physical Review A, a journal of the American Physical Society. Sinha said the work does not claim to have solved decoherence or to replace quantum error correction. Instead, it identifies timing as an additional control parameter that future quantum processors could exploit alongside improved materials, better gates and error-correction protocols. The experiment, she said, shows that decoherence need not always be fought with long sequences of corrective operations, since the timing of a single local gate can redirect the entanglement trajectory and extend the period over which a state remains useful.