Quantum History

The renowned double-slit experiment, conducted by Thomas Young in 1801, revealed that light behaves as a wave. When a laser beam is directed at a double slit, the resulting pattern differs from what would be expected if light were merely particles. Instead of observing two bright blocks corresponding to the slits, the outcome consists of a series of light blocks arranged in a specific pattern. A similar effect occurs with water waves passing through a double slit.

In 1881, Heinrich Hertz made a puzzling discovery: when he applied a high voltage between two electrodes, sparks appeared. When light was shone on these electrodes, the voltage changed. This was explained by the light dislodging electrons from the material. However, the speed of these ejected electrons was found to depend on the light's frequency rather than its intensity, contradicting wave theory. Albert Einstein resolved this in 1905 by proposing that light is, in fact, composed of particles.

Despite these findings, scientists prefer a singular theory that consistently explains phenomena rather than one that is only occasionally accurate. They sought clarity on when to treat light or other objects as waves or particles. While they recognized certain triggers for wave-like behavior, such as slit edges, a comprehensive explanation remained elusive.

The perplexing concept of wave-particle duality continues to challenge researchers. However, new investigations may provide insight. Scientists at Korea's Institute for Basic Sciences have demonstrated that the characteristics of a light source influence its particle and wave properties. Their innovative approach could lead to advancements in quantum computing.

How to Make Particles and Waves

In their experiments, researchers utilized a semi-reflective mirror to divide a laser beam into two. Each beam was directed at a crystal, producing two photons from each. Consequently, four photons were generated in total.

One photon from each crystal was sent into an interferometer, which merges two light sources and creates an interference pattern. This pattern, first observed in Young’s double-slit experiment, resembles the ripples formed when two stones are dropped into water, with overlapping waves either amplifying or canceling each other out. Thus, the interferometer illustrates light's wave nature.

The remaining two photons were assessed for their particle characteristics. Although the researchers did not disclose their method, this is typically achieved by passing the photon through a medium that indicates its trajectory. For instance, passing it through gas reveals its path. If the exact location of the photon is known at all times, it behaves as a particle, unable to exhibit wave-like interference.

This illustrates how measurement can impact outcomes in quantum physics. Therefore, in this part of the experiment, no interference pattern was observed at the photon’s endpoint, emphasizing its particle-like nature. The challenge was to quantify the degree to which it exhibited particle versus wave properties.

Since both photons from one crystal share a collective quantum state, a mathematical representation can describe them simultaneously. By quantifying the "particleness" and "waviness" of these photons, the findings can be applied to the entire beam arriving at the crystal.

The researchers successfully achieved this by analyzing the interference pattern's visibility. High visibility indicated a wave-like photon, while low visibility suggested a more particle-like nature. Notably, visibility peaked when both crystals received equal laser intensity. Conversely, significant intensity disparity resulted in diminished visibility, correlating with particle-like behavior.

This result is particularly noteworthy, as most experiments historically measure light as either waves or particles. This dual measurement approach simplifies the determination of a light source's characteristics.

Theoretical Physicists Are Excited

These results align with previous theoretical predictions, which suggested that a quantum object's wave and particle properties depend on the purity of the source. In this context, purity refers to the likelihood of a specific crystal emitting light. The relationship can be expressed mathematically as: V² + P² = µ², where V represents visibility, P indicates path distinguishability, and µ denotes source purity.

This implies that a quantum object, such as light, can exhibit both wave and particle characteristics, constrained by the purity of its source. A quantum object appears wave-like when an interference pattern is discernible, while it appears particle-like if the path is distinguishable.

Furthermore, the relationship indicates that high quantum-path entanglement corresponds with low purity, and vice versa. The researchers validated these theoretical predictions through mathematical modeling. By adjusting the purity of the crystals and observing the outcomes, they demonstrated that their findings were indeed accurate.

Faster Quantum Computers?

The link between a quantum object's entanglement and its wave-particle characteristics is particularly promising. Quantum devices, which may one day facilitate the quantum internet, depend on entanglement. This quantum version of the internet is analogous to the classical internet for traditional computers. By connecting multiple quantum computers, scientists aim to unlock greater computational power than a single device could offer.

Instead of transmitting bits through optical fibers, as done with classical networks, qubits must be entangled to create a quantum internet. The ability to measure entanglement via a photon's particle and wave properties could lead to enhanced methods for controlling the quality of the quantum internet.

Additionally, the principles of particle-wave duality could improve quantum computing. A proposal from researchers at Tsinghua University in China suggests that passing a small quantum computer through a multi-slit could amplify its capabilities. This device, comprising multiple atoms functioning as qubits, already exists.

This process mirrors the double-slit experiment but involves greater complexity. By allowing these atoms to traverse a multi-slit, a greater variety of quantum states can be generated, enhancing the power of the "shot" computer. While the underlying mathematics is intricate, one significant outcome is that such a dual-quantum computer may outperform standard quantum computers in parallel computing tasks, which enable simultaneous calculations and enhance overall speed.

While this research is foundational, practical applications are on the horizon. Although it is too early to confirm, these discoveries could expedite the development of quantum computers and the realization of the quantum internet.

Very Fundamental, but Very Exciting

Despite the solid research, these findings should be approached with caution. The work is fundamental, and the path from basic research to real-world applications is often lengthy.

The researchers from Korea identified a compelling insight: the enigma of wave-particle duality is deeply ingrained in quantum objects and is unlikely to dissipate soon. Instead, it may be beneficial to leverage this complexity. With a new quantitative framework linked to source purity, understanding and applying these principles will become more feasible.

Potential applications may emerge in quantum computing. The connection between quantum entanglement and wave-particle duality could allow scientists to quantify waviness and particle-ness instead of solely measuring entanglement. This could assist those working on the quantum internet or enhance quantum computer performance. Exciting times in the quantum realm appear to be on the horizon.

This article was originally published on Built In.