Quantum Physics Redefines Cause and Effect with Indefinite Order

Recent explorations in quantum physics are challenging long-held beliefs about the nature of cause and effect. This phenomenon, known as indefinite causal order (ICO), suggests that events can exist in a superposition of causal relationships, defying conventional understanding. The implications of this research extend beyond theoretical physics and could influence future technologies.

The principle of cause and effect is fundamental to our daily experiences. For example, when a ball is kicked, it travels forward and strikes a window, resulting in a breakage. This sequence reflects a clear causal order: the kick occurs before the impact. Yet, in the quantum realm, these notions become more complex, raising questions about whether the same rules apply.

Many physicists accept the paradox of Schrödinger’s cat, a thought experiment illustrating quantum superposition, where a cat in a box is simultaneously alive and dead until observed. This concept forms the backbone of ICO, which posits that multiple distinct causal processes can occur simultaneously.

A notable example of ICO is the thought experiment titled the gravitational quantum switch, proposed by Magdalena Zych from the University of Queensland in 2019. In this scenario, two observers, Alice and Bob, are situated near a massive object, such as a star. Due to the effects of Einstein’s general theory of relativity, time flows differently for each observer based on their proximity to the star.

If Alice is closer, her clock ticks slower than Bob’s, creating a superposition where both scenarios coexist. This presents a unique situation: Alice could receive a message first, or Bob could, depending on the state of their clocks. The implications are profound when this messaging system is replaced with quantum particles, such as photons, where the order of operations determines the final outcome.

When Alice and Bob perform different operations on a photon, the interaction is influenced by the order of these operations. If they do not commute—meaning the order matters—the results change based on who acts first. The gravitational quantum switch demonstrates that two causal processes can exist in superposition, leading to various outcomes.

As interesting as these theoretical frameworks are, experimental physicists have begun to unveil practical applications. In recent years, researchers have successfully created light-based quantum switches in the laboratory. These switches utilize a two-level quantum state to dictate the order of events, displaying ICO when the control state exists as a superposition.

An example of this is a photon navigating two paths: first to Alice, then to Bob, or vice versa. The path taken by the photon is determined by a control qubit, which can also exist in superposition. This indicates that the exact sequence of events remains undefined until a measurement is made.

The first successful quantum switch was developed in 2015 by Lorenzo Procopio and his team at the Vienna Center for Quantum Science and Technology. Their experimentation involved directing a photon through a beam splitter, leading to two potential paths, each resulting in different effects based on the sequence of operations performed by Alice and Bob.

In a significant breakthrough in 2017, quantum-information researcher Giulia Rubino verified the existence of ICO using a method called a “causal witness.” This mathematical construct identifies whether a quantum system exhibits definite or indefinite causal order, confirming the capabilities of their quantum switch.

Recent developments have revealed that ICO could enhance computational processes. A team led by renowned physicist Jian-Wei Pan demonstrated that ICO could accelerate data processing rates when distributing tasks between two parties. This could mark a significant advancement in quantum computing, particularly in applications that require rapid processing of long data strings.

Additionally, research from Oxford University indicates that ICO may improve quantum metrology, potentially leading to enhanced precision in measurements compared to traditional methods with definite causal orders.

Despite the challenges in demonstrating ICO in practical applications, its potential is becoming increasingly recognized. Researchers are exploring various configurations and quantum circuits that could exhibit ICO, paving the way for future innovations in quantum technologies.

This ongoing research contributes to the broader efforts of the 2025 International Year of Quantum Science and Technology (IYQ), aimed at promoting global awareness of quantum physics and its applications. As our understanding of quantum mechanics evolves, so too does our perspective on the fundamental nature of reality itself.