Quantum Experiment Verifies Control of Event Order, Enabling New Computation.

The fundamental principle that cause precedes effect underpins much of our understanding of the physical world, yet quantum mechanics allows for scenarios where this order becomes less defined. Researchers are now experimentally probing the limits of causal structure, demonstrating a violation of inequalities that would hold true if events always occurred in a definite sequence. This work, detailed in a recent publication by Yu Guo et al, from the University of Science and Technology of China, in collaboration with Giulio Chiribella of the University of Hong Kong, reports the experimental realisation of indefinite causal order using photons. The team successfully violates a Bell-like inequality specifically designed to detect this phenomenon, employing a photonic setup where the sequence of two optical processes is controlled by the quantum state of a single photon, and overcoming significant technical hurdles in achieving the necessary precision and synchronisation. The research, titled ‘Experimental violation of a Bell-like inequality for causal order’, represents a step towards harnessing this quantum resource for advanced information processing and device-independent verification of causal structure.

Quantum experiments now demonstrate the manipulation of event sequences, violating established causal constraints and challenging conventional understandings of time and causality. The work centres on a photonic system where the sequence of two optical processes becomes controlled by a single photon, utilising the principles of polarization-entangled photon pairs to achieve unprecedented control over quantum events. Entangled photons exhibit a correlation stronger than classically permitted, linking their properties regardless of the distance separating them.

Researchers successfully overcome significant technical hurdles to achieve this result, demanding precision and innovation in experimental design and execution. These include implementing high-speed operations utilising photonic time-bin encoding, a method of representing quantum information using the arrival time of photons, and achieving nanosecond synchronisation of optical and electronic components to ensure the necessary spacelike separation between events. Spacelike separation signifies that events are too far apart to be causally connected, meaning no signal travelling at the speed of light could connect them. Active stabilisation of a Mach-Zehnder interferometer, an optical device used to split and recombine light beams, proves crucial in maintaining statistical significance.

A key aspect of this investigation focuses on achieving device independence, a crucial requirement for establishing the robustness and reliability of quantum experiments. This means verifying indefinite causal order without relying on detailed knowledge of the internal workings of the experimental apparatus, ensuring the observed phenomena are not artifacts of specific implementations. By demonstrating the phenomenon in a manner independent of specific device characteristics, the research strengthens the robustness of the findings and opens avenues for practical applications in secure communication and advanced computation.

The experimental setup employs a quantum switch, a device that allows for the controlled manipulation of quantum states, to create the conditions necessary for indefinite causal order. Researchers carefully engineered the quantum switch to introduce a degree of freedom that allows the temporal order of the optical processes to become undefined, leading to correlations that cannot be explained by classical causal models. Active temperature stabilisation of the Mach-Zehnder interferometer proves crucial in maintaining the statistical significance of the observed violations, ensuring the reliability of the experimental results and validating the theoretical predictions.

Researchers demonstrate the experimental violation of a causal inequality, providing evidence for this counterintuitive aspect of quantum theory and opening new avenues for exploration in quantum information science. A causal inequality defines the limits of correlations permissible under classical causal assumptions; violation of this inequality indicates a departure from classical behaviour. The successful violation confirms the possibility of manipulating causal structures at a fundamental level, challenging classical notions of cause and effect and paving the way for novel technologies.

Researchers actively explore the potential applications of indefinite causal order in various fields, including quantum computation, quantum communication, and quantum sensing. These applications leverage the unique properties of indefinite causal order to overcome limitations of classical technologies and achieve unprecedented performance.

Researchers actively investigate alternative physical platforms, such as trapped ions or superconducting circuits, seeking to overcome the limitations of photonic systems and develop more scalable and robust quantum technologies. A crucial direction involves investigating the interplay between indefinite causality and other quantum phenomena, such as entanglement and non-locality, to gain a deeper understanding of the fundamental principles governing quantum reality.

The experimental results demonstrate that quantum systems can exhibit behaviours fundamentally incompatible with classical notions of causality, challenging long-held assumptions about the nature of time and cause-and-effect. Researchers carefully designed the experiment to rule out any classical explanations for the observed phenomena, providing strong evidence for the existence of indefinite causal order.