INL researchers have explored a fascinating quantum ability which lacks a classical explanation: the capacity to detect objects without traditional physical interaction. Rafael Wagner and Anita Camillini, INL PhD candidates, alongside the research group-leader Ernesto F. Galvão, have published a paper describing the revolutionary approach that challenges conventional concepts of detection.

Imagine being able to identify something without actually touching it – similarly to diagnosis a hidden fracture without an X-ray or recognising a distant voice without hearing it directly. This study explores how quantum computers can achieve that, and detect objects without direct interaction.

Rafael Wagner explains “It has been known for a long time that it is possible to detect things without interactions, as a result of the famous thought experiment introduced by Elitzur and Vaidman. They imagined an extreme situation where either there is a bomb in one of the arms of an interferometer, or there is no bomb.”

Interferometers are tools used in many fields of science and engineering. The working principle of interferometry consists on splitting the light into two beams that travel different optical paths and are then combined to produce an interference pattern.

Their scenario presents an unusual premise: within an interferometer setup, one arm may contain a potentially dangerous object such as a bomb, while the other remains inert. This experiment highlights the intriguing notion that detection can occur without physical interaction, sparking significant curiosity within the scientific community.

Wagner continues “suppose now that a single photon enters the interferometer. If there is no bomb, photons are always detected at only one detector – this implies that detector 1 always clicks. Each click corresponds to the photon being detected; therefore, when there is no bomb, detector 2 never clicks. However, in case there is a bomb, when the photons interact with it, the bomb explodes!”

“If the photons do not interact with it, there is a chance that the second detector 2 clicks. This is because of a property known as quantum superposition. In this case, one learns that there must have a bomb inside the interferometer, since otherwise detector 2 would never click, and that the photon never interacted with the bomb (since it didn’t explode).”

The researchers from the Quantum and Linear-Optical Computation group at INL demonstrated that classical or statistical mechanics are not capable, even in theory, of reproducing the same effect. Recent theories, which resemble statistical mechanics but accommodate quantum-like phenomena such as superpositions, also fall short in reproducing the functionality of quantum computers. These classical models, known technically as ‘noncontextual’, fail to fully capture the intricacies of quantum computing.

Detection of an object without direct interaction occurs statistically, with varying success rates. In the interferometric setup explored by INL researchers, efficiency is quantified by the efficiency parameter, which represents the proportion of successful detections without direct interaction. The team’s findings reveal that quantum computers exceed classical models in terms of capability.

As the scientific community continues to unlock the mysteries of quantum computing, this study marks another step to harness the full power of quantum mechanics for practical applications.

Text by Rafael Wagner & Catarina Moura