Scientists from India and the US have demonstrated a way to preserve quantum entanglement for longer by applying a single, well-timed flip operation, showing that the timing of an intervention can be as important as the operation itself in controlling quantum systems.
The findings could help extend the lifetime of fragile quantum links inside future quantum computers without requiring changes to the underlying hardware.
The study was carried out by researchers from the Raman Research Institute (RRI), an autonomous institution under the Department of Science and Technology (DST), the University of Calgary and Louisiana State University. It was partly funded by DST’s National Quantum Mission.
Entanglement is a key resource in quantum systems in which the states of two particles remain correlated even when they are separated. However, interactions with the environment can cause entanglement to weaken and eventually disappear.
In some cases, entanglement can vanish abruptly at a finite point in time, even before the decay of the individual particles is complete. This phenomenon is known as “entanglement sudden death”.
To investigate whether this loss could be delayed, the researchers developed an optical setup to mimic a two-level quantum system.
In the experiment, the researchers used the polarisation of light as an analogue for the two states of a particle. Vertical polarisation was treated as the excited state and horizontal polarisation as the ground state.
Using a waveplate, an optical device that changes the polarisation of light, the researchers were able to control the transition between the two states.
Instead of allowing the excited-state population to naturally decay into the ground state, the researchers applied a single operation that swapped the populations of the ground and excited states.
The impact of the operation depended on when it was applied during the decay process. By selecting the appropriate timing, the researchers were able to delay the decay of the particles and, in some cases, delay the loss of entanglement.
“To me, the heart of the result is that timing is not just an experimental detail; it can be a control resource,” said Urbasi Sinha, group leader of the Quantum Information and Computing (QuIC) lab at RRI and senior professor at the institute.
The researchers found that the timing of the flip could determine whether entanglement sudden death was delayed, avoided or hastened.
“When you are applying, after how much of an evolution through the decay, you’re applying this single flip operation will determine if you avoid or delay or hasten the sudden death,” said Saumya Ranjan Behera, a quantum scientist at the QuIC lab and lead author of the study.
The study was published in the American Physical Society’s Physical Review A in July.
The findings are particularly relevant to quantum computing, where quantum information can be lost because of interactions with the environment. The experiment indicates that carefully timed operations could influence how long quantum information and entanglement survive.
The researchers also encountered an unexpected result during the experiment. The observed behaviour did not initially fit either of two standard textbook models describing how quantum systems lose information to their environment.
Researchers in the QuIC lab spent nearly a year examining different theoretical descriptions of the experimental data.
They eventually found that the experimental results lay on a curve connecting the two established frameworks. This led them to identify a “tuning parameter” that allows the same experimental setup to move between the two models and represent different variants depending on its value.
The researchers said the finding indicates that the two models need not necessarily be treated as separate experimental scenarios, as a single setup can capture both noise models and the range of behaviour between them through the tuning parameter.
The study demonstrates that controlling the timing of an operation can provide a new way of influencing the evolution of entanglement, offering a potential approach to managing quantum information loss in future quantum technologies.
