Scientists find single-shot way to protect fragile quantum states

Bengaluru: Scientists at the Raman Research Institute (RRI) have demonstrated a novel way to protect the fragile quantum states that power quantum computers, potentially opening another route to making quantum calculations more reliable.

The research, carried out by the Quantum Information and Computing (QuIC) laboratory at RRI in Bengaluru, shows that a single, precisely timed operation can delay the breakdown of quantum entanglement and, in certain conditions, prevent its sudden collapse.

The finding is significant because quantum computers derive their extraordinary computing capabilities from properties of the quantum world that are both powerful and extremely delicate.

Unlike conventional computers, which process information using electrical states represented as 0s and 1s, quantum computers use phenomena such as superposition and quantum entanglement to perform certain complex calculations more efficiently.

Quantum entanglement occurs when two particles interact and become linked in such a way that they behave as a single quantum system. Information about one particle can reveal corresponding information about the other, regardless of the distance separating them.

Superposition, meanwhile, allows quantum systems to exist in multiple possible states at the same time. Together, these properties form the foundation of many quantum computing technologies.

But there is a fundamental problem: the very quantum states that give these machines their power are also highly fragile.

Interaction with the surrounding environment can cause an entangled state to deteriorate rapidly, a process known as decoherence. In some cases, the entanglement can disappear abruptly even before this normal decay occurs — a phenomenon scientists refer to as “entanglement sudden death.”

Such instability can undermine the reliability of calculations performed by quantum computers.

A single-shot approach

Scientists have traditionally explored repeated corrective operations to slow decoherence and preserve fragile quantum states. While such interventions can help, repeatedly manipulating a quantum system can be costly and may itself introduce additional errors.

The RRI team, led by senior professor and QuIC laboratory group leader Urbasi Sinha, has demonstrated a different approach.

Instead of relying on a sequence of corrective operations, the researchers showed that a single operation performed at the right moment can alter the course of the quantum system.

The timing of that operation becomes crucial.

Performed at the appropriate instant, the operation can delay decoherence and completely avoid entanglement sudden death in the experimental setting. At a different point in time, the same operation can instead accelerate decoherence — an undesirable outcome, but one that demonstrates the degree of control the method can provide.

The researchers say this makes timing more than simply an experimental consideration. It can itself become a tool for controlling fragile quantum states.

“To me, the heart of the result is that the timing of the operation is not just an experimental detail, it can be a control resource,” Sinha said.

She stressed, however, that the work represents a proof of concept, rather than a complete solution to the long-standing problem of decoherence.

The technique will need to be tested on different quantum computing platforms and under different environmental conditions to determine how effectively it can be integrated into practical quantum systems.

The findings have been published in a research paper in the American Physical Society journal Physical Review A.

Sinha said the work should not be seen as a replacement for existing quantum error-correction techniques.

“We are not claiming to have solved decoherence or replaced quantum error correction, rather the work identifies timing itself as another control parameter that future quantum processors could exploit alongside better materials, better gates, and error-correction protocols,” she said.

The researchers believe the experiment points to a potentially simpler way of manipulating quantum systems, where a carefully chosen moment can be as important as the operation itself.

“Our experiment shows that one does not always have to fight decoherence with long sequences of corrective operations. The timing of a single local gate can redirect the entanglement trajectory and extend the regime over which the state remains useful,” Sinha said.

The finding adds a new dimension to efforts to overcome one of quantum computing’s biggest challenges: keeping delicate quantum information intact long enough for useful calculations to be completed.