Qubits break quantum limit to encode information for longer

The strange phenomenon of quantum superposition has helped researchers overcome the fundamental quantum mechanical limit and impart properties to quantum objects that make them useful for quantum computing over longer periods of time.

For a century, physicists have been puzzled by exactly where to draw the line between the small quantum world and the macroscopic world we experience. In 1985, physicists Anthony Leggett and Anupam Garg developed math test this could be applied to objects and their behavior over time to diagnose whether they are large enough to escape quantumness. Here, quantum objects are identified by unusually strong correlations between their properties at different points in time, just as their behavior yesterday and tomorrow are unexpectedly related.

Objects that score high enough on this test are considered quantum, but these scores were thought to be limited by a number called the Tsirelson Time Bound (TTB). Theorists believed that even quantum objects could not cross this boundary. But now, Arijit Chatterjee from the Indian Institute of Science Education and Research in Pune and his colleagues have developed a way to fundamentally disrupt TTB using one of the simplest quantum objects.

They focused on qubitswhich are the basic building blocks of quantum computers and other quantum information processing devices. Qubits can be created in a variety of ways, but the researchers used a carbon-based molecule containing three qubits. They used the first qubit to control the behavior of the second “target” qubit over time. They then used the third qubit to extract the properties of the target.

A three-qubit system is expected to be limited by the TTB, but Chatterjee and his colleagues found a way to push the target qubit beyond this limit in an extreme way. In fact, their method resulted in one of the largest violations that seems mathematically plausible. Their secret was to have the first qubit control the target qubit using quantum superposition state. Here, an object can effectively embody two states or behaviors that appear to be mutually exclusive. For example, the team's experiment was similar to the first qubit, which actually instructed the target qubit to rotate clockwise and counterclockwise at the same time.

Over time, a qubit typically falls victim to something called decoherence, meaning its ability to encode quantum information is destroyed. But when the target qubit violated the TTB, decoherence occurred later, and it retained the ability to encode information five times longer because its temporal behavior was controlled by superposition.

Chatterjee says this reliability desirable and useful in any situation where qubits must be precisely controlled, such as for computation. Team Member HS Karthik from the University of Gdansk in Poland say there are procedures in quantum metrology—for example, for extremely precise measurements of electromagnetic fields—that can be improved by this kind of qubit control.

Le Luo at Sun Yat-sen University in China say that, in addition to having clear potential for improving quantum computing protocols, the new research also fundamentally expands our understanding of how quantum objects behave over time. This is because a sudden violation of the TTB means that the properties of the qubit will change. extremely correlated between two different moments in time, which simply cannot happen with non-quantum objects.

So the extreme TTB violation is strong evidence of just how much quantumness there was in the entire three-qubit system, says Karthik—and an example of how researchers are still pushing the boundaries of the quantum world.