RRI Scientists Delay Entanglement Sudden Death with Timed Flip

Scientists have found a way to keep quantum entanglement alive for longer. They apply a single flip operation at the right moment. The finding says that in a quantum system, when an operation is applied matters as much as what operation is applied. The hardware itself is not changed at all.

What entanglement is

Entanglement ties the state of one particle to the state of another particle far away. An operation on one affects the other in the same way, even when the two sit in different surroundings. Entanglement is the basic resource of every quantum system. It also weakens when a particle interacts with its environment.

Entanglement sudden death

Take two entangled particles. Each can be in an excited state or in a ground state. Left alone, a particle slips from the excited state to the ground state, the way an atom loses energy to its surroundings. As it does, the entanglement between the pair weakens. Physicists call the abrupt disappearance of entanglement, at a fixed moment and well before the decay itself is over, entanglement sudden death.

The experiment

The team came from the Raman Research Institute (RRI), an autonomous institution of the Department of Science and Technology, together with the University of Calgary and Louisiana State University. The study was funded in part by the Department's flagship National Quantum Mission. The group built a two-level system out of light. Polarisation tells the direction in which a light wave oscillates. Vertical polarisation stood for the excited state and horizontal polarisation for the ground state. A waveplate, an optical device that alters polarisation, let the team control the decay. Instead of letting the excited population decay on its own, they applied one operation that swapped the ground and excited populations.

Delay, hasten or avoid

Urbasi Sinha, group leader of the Quantum Information and Computing lab at RRI and senior professor there, said timing is not merely an experimental detail but a control resource. Saumya Ranjan Behera, the lead author, said the point in the decay at which the flip is applied decides whether sudden death is avoided, delayed or hastened. The paper appeared in Physical Review A of the American Physical Society in July. The results fitted neither of the two textbook models of how a quantum system loses information, and the theorists took nearly a year to find the tuning parameter that lets one setup slide between both models.