Separated Photons Still Interfere, Defying Expectations of Light Behaviour

A new demonstration of the Hong-Ou-Mandel effect using spatially separated photons challenges conventional understanding of two-photon interference. Yuki Kodama and colleagues at Hiroshima University in collaboration with Newcastle University and University of Bristol report observing this effect even when photons travel along paths that never physically intersect at a beam splitter. Their research, detailed in a four-path interferometer, reveals that correlations between detection events define non-local output modes, effectively linking the behaviour of these photons. The findings clarify how linear optics generate entanglement and establish a key connection between multiphoton interference and entanglement, potentially advancing optical quantum technologies.

Non-local photon correlations realised via a four-path interferometer

A Hong-Ou-Mandel (HOM) effect with 50% visibility was achieved, representing a strong improvement over previous limitations restricted to two optical modes. The Hong-Ou-Mandel effect, first demonstrated in 1987, is a fundamental phenomenon in quantum optics where indistinguishable photons exhibit a strong correlation at a beam splitter. Typically, two photons arriving simultaneously at a beam splitter have a zero probability of being detected in the same output port; they either both transmit or both reflect. This arises from the quantum mechanical principle of indistinguishability, meaning that the photons’ wavefunctions interfere destructively in the case of simultaneous detection in a single output port. The Newcastle team’s achievement extends this to scenarios where the photons do not share a common spatial path prior to the effective ‘beam splitting’ event. The team utilised a four-path interferometer, employing post-selection, a technique akin to filtering data, to reveal correlations between spatially separated photons. This demonstrates the effect even when photons travel entirely separate paths, something previously considered impossible due to the conventional requirement for photons to share a route. The four-path interferometer allows for a more complex manipulation of the photons’ spatial modes, enabling the observation of interference patterns even without direct spatial overlap. The interferometer consists of a series of beam splitters and mirrors arranged to create four distinct output paths for the photons, allowing for precise control over their propagation.

The non-local Hong-Ou-Mandel effect is now established, clarifying how linear optics generate entanglement and linking multiphoton interference directly to the creation of entanglement between distant photons. Entanglement, a cornerstone of quantum mechanics, describes a situation where two or more particles become linked in such a way that they share the same fate, no matter how far apart they are. This correlation is stronger than any classical correlation and is essential for many quantum technologies. The ability to generate entanglement using only linear optical elements, beam splitters and mirrors, is particularly significant because these components are relatively easy to manufacture and control. This contrasts with many other entanglement generation schemes that rely on non-linear materials or complex quantum systems. The setup allowed photons to be detected in four distinct output modes despite originating from only two input modes, a result that expands the possibilities for quantum systems. This increased dimensionality of the output space allows for more complex quantum states to be encoded and manipulated, potentially leading to more powerful quantum algorithms and communication protocols.

Further manipulation of this non-local interference using local phase shifts enabled control over the correlations detected at different output locations, confirming the link between interference and entanglement. Phase shifts, introduced by altering the optical path length of one of the photons, allow for precise tuning of the interference pattern. By systematically varying the phase shift and observing the resulting changes in the correlation between the photons, the researchers were able to demonstrate a direct relationship between the interference pattern and the degree of entanglement. This control allows for potential applications in quantum key distribution and advanced sensing technologies. Quantum key distribution (QKD) uses the principles of quantum mechanics to ensure secure communication, while advanced sensing technologies could leverage entanglement to achieve unprecedented levels of precision. The experiment successfully generated an entangled state, where interference in one subsystem directly correlated with interference in another, spatially separated subsystem. However, the current system relies on carefully aligned phase shifts and post-selection, meaning a substantial reduction in photon count rate; scaling this to more complex quantum circuits and maintaining entanglement fidelity remains a significant challenge. Post-selection, while enabling the observation of the effect, discards a significant fraction of the detected photons, reducing the overall efficiency of the system. Scaling to more complex quantum circuits requires maintaining entanglement fidelity, the degree to which the entanglement is preserved, as the number of photons and operations increases. Future work will focus on improving the efficiency of the system and exploring alternative methods to reduce the reliance on post-selection, potentially through the development of more efficient detectors or novel interferometer designs.

Non-overlapping photon paths expand possibilities for strong quantum entanglement

Scientists have long sought methods to generate and manipulate entanglement, a bizarre quantum link between particles, for use in future technologies like secure communication and ultra-powerful computing. The Hong-Ou-Mandel effect, typically needing photons to travel the same path, was achieved with photons on entirely separate trajectories. This demonstration is striking because it reveals entanglement can arise in more configurations than previously understood, challenging conventional assumptions about quantum links. The conventional understanding of the HOM effect relies on the spatial overlap of the photons’ wavefunctions at the beam splitter, allowing for destructive interference. This new work demonstrates that entanglement can be established even when the photons do not physically overlap, suggesting that the correlation arises from a more subtle quantum connection.

This broadened understanding is vital for designing more flexible and robust quantum devices, potentially simplifying the engineering challenges of building practical quantum technologies. The limitations imposed by the need for precise spatial alignment and overlap have been a major obstacle in the development of quantum devices. By demonstrating that entanglement can be generated without these constraints, this research opens up new possibilities for designing more compact and robust quantum systems. Such flexibility will begin to simplify the building of quantum computers and secure communication networks, paving the way for more scalable systems. The demonstration of the effect with spatially separated photons fundamentally alters understanding of how quantum interference and entanglement relate. It suggests that the key ingredient for entanglement is not the physical proximity of the photons, but rather the correlations between their quantum states.

Previously, the effect, where two photons either pass through or bounce off a beam splitter together, required photons to share a physical path; this research bypasses that requirement through the interferometer and careful data selection. The interferometer effectively creates a situation where the photons’ wavefunctions are correlated even though they do not directly interact. By focusing analysis on only spatially separated photon detections, scientists revealed correlations indicative of entanglement, even though the photons never directly interacted. This achievement clarifies that linear optics, standard components manipulating light, can generate entanglement without photons needing to occupy the same space. This is significant because linear optics are relatively easy to implement and control, making them an attractive platform for building quantum technologies. The use of standard optical components simplifies the engineering challenges and reduces the cost of building quantum devices.

The research demonstrated the Hong-Ou-Mandel effect, a form of two-photon interference, using spatially separated photons within a four-path interferometer. This finding matters because it establishes that entanglement can occur even when photons do not share a physical path or directly interact. By revealing correlations between photons detected at different locations, scientists clarified the role of linear optics in generating entanglement. The authors suggest this work establishes a fundamental relationship between multiphoton interference and entanglement, potentially simplifying the design of future optical quantum technologies.