The intriguing phenomenon of quantum superposition has enabled scientists to surpass the limitations imposed by fundamental quantum mechanics, equipping quantum objects with properties advantageous for long-term quantum computing.

For over a century, physicists have wrestled with the challenge of distinguishing between the minuscule quantum world and the larger macroscopic universe. In 1985, physicists Anthony Leggett and Anupam Garg introduced a mathematical assessment for determining the size threshold at which an object transcends its quantum characteristics. Quantum objects are recognized by remarkably strong correlations of their properties over time, akin to surprising connections between actions of yesterday and tomorrow.

Objects that achieve a sufficient score in this assessment are classified as quantum, with the scores traditionally held back by a value known as the temporal Zirelson limit (TTB). Theorists believed that even distinctly quantum objects could not surpass this threshold. However, Arijit Chatterjee and his colleagues from the Indian Institute of Science Education and Research in Pune have discovered a method to significantly exceed the TTB using one of the most basic quantum elements.

They centered their research on qubits, the essential building blocks of quantum computers and other quantum information systems. While qubits can be produced through various methods, the team utilized a carbon-based molecule incorporating three qubits. The first qubit was employed to control the behavior of the second “target” qubit over time, with the third qubit employed to extract properties from the target.

Though three-qubit configurations are generally believed to be constrained by the TTB, Chatterjee and his team discovered a method to push the target qubits beyond this limitation dramatically. In fact, their technique resulted in one of the most significant deviations from mathematical plausibility. The key was for the first qubit to govern the target qubit while it was in a state of quantum superposition, where it can effectively embody two states or actions that seem mutually exclusive. For instance, in their experiment, the first qubit directed the target qubit to rotate both clockwise and counterclockwise simultaneously.

Qubits are usually susceptible to decoherence over time, diminishing their capacity to store quantum information. However, after the target qubit surpassed the TTB, decoherence set in, yet the ability to encode information persisted five times longer due to its time-controlled behavior influenced by superposition.

According to Chatterjee, this resilience is advantageous in any context requiring precise qubit control, such as in computational applications. Team member HS Kartik from Poland’s University of Gdańsk mentions that procedures in quantum metrology, including accurate sensing of electromagnetic fields, could benefit significantly from this level of qubit control.

Rakura and their colleagues from China’s Sun Yat-sen University indicate that this research not only has clear potential for enhancing quantum computing practices but also fundamentally broadens our comprehension of how quantum objects behave over time. This is significant because immensely surpassing the TTB indicates that the properties of the qubit are highly interconnected at two divergent time points, a phenomenon absent in non-quantum entities.

The substantial breach of the TTB strongly demonstrates the extent of quantum characteristics present throughout the three-qubit configuration and exemplifies how researchers are advancing the frontiers of the quantum domain, says Karthik.