Janus State Achieves Switchable Multi-Photon Correlations and Wigner Function Negativity

The fundamental principle of complementarity, which states that certain properties of a quantum system cannot be known simultaneously, extends surprisingly into the realm of multi-photon correlations, as demonstrated by research led by Arash Azizi from The Institute for Quantum Science and Engineering and the Department of Physics and Astronomy at Texas A and M University. This study reveals that a specifically engineered quantum state, known as a Janus state, functions as a switch for these correlations, responding to whether information about a photon’s path is available. The team proves that while some correlations persist, all higher-order correlations can be driven to zero, representing a transition from chaotic to highly ordered light, and visualised through a key signature of non-classical behaviour. This demonstration of complementarity in multi-photon statistics introduces a new approach to creating highly ordered, non-classical light using readily available quantum resources, potentially impacting fields such as quantum communication and computation.

Researchers have discovered that a specific quantum state acts as a perfect switch for controlling how multiple photons interact, a property governed by whether information about their paths is known. Erasing this path information activates quantum interference, allowing for precise tuning and, remarkably, the complete elimination of correlations between more than two photons. This reveals a hierarchy of suppression; while some correlation between pairs of photons remains, all higher-order correlations can be driven to zero.

Third-Order Coherence in Squeezed Janus States

This is a detailed theoretical analysis of how a Janus state, a combination of two squeezed light sources, behaves when considering the interactions of multiple photons. The analysis rigorously demonstrates the state’s ability to control third-order coherence, a measure of how photons arrive in groups of three. By manipulating squeezing parameters, phase differences, and amplitudes, researchers can precisely predict and control the system’s behavior. The strength of this analysis lies in its comprehensive mathematical framework and exploration of a wide range of parameters. The team derived scaling laws that describe the system’s behavior in different conditions, providing valuable insights into the underlying physical mechanisms.

They clearly explain how quantum interference between the two squeezed states affects the coherence properties, leading to either enhanced or suppressed photon bunching. Numerical results validate the theoretical predictions, bridging the gap between theory and experiment. Key findings reveal that the Janus state exhibits remarkable tunability, allowing for strong suppression or enhancement of photon number fluctuations. Quantum interference plays a crucial role, with destructive interference leading to antibunching and constructive interference leading to bunching. The analysis highlights the importance of a specific mathematical constraint that limits the possible values of the amplitudes and affects the scaling laws. Future research could extend this analysis to even higher-order coherence functions, explore experimental realization of the Janus state using physical systems like superconducting circuits or trapped ions, and investigate potential applications in quantum information processing, metrology, and communication.

Tunable Multi-Photon Correlations via Quantum Control

Researchers have demonstrated remarkable control over light’s quantum properties by creating a “Janus state” that acts as a switch for multi-photon correlations. This state, built from squeezed light sources, precisely regulates how photons interact, controlling the likelihood of detecting multiple photons simultaneously. The key finding is the ability to suppress higher-order photon correlations, the tendency for more than two photons to arrive together, to an unprecedented degree. The research team achieved this control by manipulating information about the photons’ paths. By erasing this information, they induced quantum interference that could be tuned to completely eliminate correlations beyond those of just two photons.

This represents a significant advance, as conventional light sources typically exhibit strong correlations at all orders, meaning photons tend to arrive in bunches. The Janus state, however, transitions from this bunched behavior to a state of profound order, a change confirmed by the negativity observed in a mathematical representation of the light’s quantum state. The implications of this work extend beyond fundamental quantum optics. The ability to engineer light with such precise control over photon correlations opens doors to new technologies in quantum communication and computation. Specifically, suppressing unwanted multi-photon events is crucial for improving the efficiency and fidelity of quantum key distribution protocols, which rely on the secure transmission of information using single photons. Furthermore, the researchers found that the suppression of correlations becomes even more pronounced as the light intensity decreases, demonstrating a powerful level of control over the quantum state of light even at the single-photon level.

Janus State Controls Multi-Photon Correlations

This research investigates the higher-order quantum statistics of the Janus state, a combination of two squeezed light sources, revealing a manifestation of quantum complementarity. The team demonstrates that this state functions as a switch for multi-photon correlations; the presence of path information leads to strong photon bunching, while erasing this information allows for quantum interference that can suppress these correlations. Notably, the researchers prove that all higher-order correlations can be driven to zero, while two-photon correlations remain finite, establishing a remarkable hierarchy of suppression. This complete suppression of higher-order correlations is a direct consequence of the non-classical nature of the Janus state.

The findings establish the Janus state as a promising platform for exploring fundamental quantum principles and engineering highly ordered, non-classical light tailored for advanced quantum technologies. The authors acknowledge that experimental imperfections, such as photon loss, may affect the purity of the state, but their analysis indicates that the core phenomena of tunable bunching and antibunching are robust and observable with readily achievable squeezing parameters. Future work may focus on realizing and applying this state in practical quantum devices.