Engineered Quantum Interaction Creates One-Way Transmission for Light Signals

Researchers are increasingly focused on achieving nonreciprocal light propagation, crucial for developing advanced optical devices such as isolators and routers. Zhenghao Zhang and Qingtian Miao, both from the Department of Physics and Astronomy at Texas A&M University, alongside G. S. Agarwal, demonstrate a pathway to induce such nonreciprocity through the Dzyaloshinskii-Moriya interaction (DMI) in a waveguide quantum electrodynamic system. This collaborative work establishes the DMI as a versatile tool for engineering not only nonreciprocity and transparency, but also quantum entanglement and light correlations within waveguide QED, potentially circumventing the need for complex chiral waveguide designs. Their theoretical study, utilising the full quantum master equation, reveals tunable nonreciprocal behaviour and the creation of pure states exhibiting power-independent perfect transparency, representing a significant step towards realising robust and efficient quantum photonic circuits.

Scientists have demonstrated a new approach to controlling light and matter interactions at the quantum level, potentially simplifying the design of future photonic technologies. This work introduces a method for achieving nonreciprocity, the property of allowing signals to travel easily in one direction but not the other, without relying on complex, chiral waveguides. Traditionally, creating nonreciprocal devices requires carefully engineered structures that guide light in a specific direction, limiting material choices and fabrication processes. Researchers now show that a carefully tuned interaction between quantum bits, mediated by the Dzyaloshinskii-Moriya interaction, can induce strong nonreciprocity even in a standard, non-chiral waveguide system. The study focuses on a theoretical model involving two qubits coupled to a one-dimensional waveguide and driven by a coherent field. Through detailed calculations using the quantum master equation and input-output formalism, researchers demonstrate that an engineered Dzyaloshinskii-Moriya interaction enables strong nonreciprocity. This interaction not only governs the transmission of light but also influences quantum entanglement and the way photons are grouped together. Furthermore, the research reveals that the DMI reshapes the statistical properties of photons, shifting the tendency for photons to bunch together from transmission to reflection as the interaction strength increases. These findings establish the DMI as a versatile tool for engineering a range of quantum phenomena, including nonreciprocity, transparency, entanglement, and correlations, paving the way for the development of isolators, routers, and novel light sources for quantum technologies. The work offers a pathway toward building more flexible and adaptable quantum photonic circuits. Achieving perfect transparency requires a propagation phase that is a multiple of pi (φ = nπ), where n is an integer. This condition, derived from the study of pure states, demonstrates that specific phase relationships between the qubits eliminate decohering channels and enable lossless transmission. The research establishes a direct link between propagation phase and the emergence of these pure states, crucial for maintaining quantum coherence. These pure states are demonstrably achievable through precise control of the system’s parameters, including detuning and exchange interaction. The engineered Dzyaloshinskii-Moriya interaction fundamentally reshapes photon statistics, shifting superbunching from transmission to reflection. Without the DMI, two-photon correlations favour transmission, indicating a tendency for photons to travel together in the same direction. However, the introduction of a finite DMI reverses this behaviour, directing superbunching towards reflection, a clear indication of altered photon pathways. This redistribution of correlations is quantified by analysing the two-photon correlation function, revealing a significant change in the system’s light-emission characteristics. Furthermore, the study reveals that the steady-state entanglement is reciprocal when the system resides in these pure states, meaning the degree of entanglement is independent of the direction of propagation, simplifying the requirements for quantum communication protocols. Away from these pure-state points, however, phase control of the DMI generates strong nonreciprocal entanglement, creating a directional dependence in the quantum correlations. This ability to dynamically tune entanglement reciprocity via the DMI offers a powerful tool for manipulating quantum states. A bidirectional one-dimensional waveguide forms the central element of this study, supporting the coupling of two qubits positioned a distance x ab apart. Each qubit interacts with the waveguide at a rate denoted by Γ, alongside additional loss channels characterised by rates γ a and γ b. The system operates within a rotating frame at frequency ω d, with detunings ∆ j representing the difference between the qubit frequencies ω j and the drive frequency. A crucial aspect of the methodology involves defining a propagation phase φ = ω d x ab /vp, where v p signifies the phase velocity of the drive field. To engineer interactions between the qubits, a complex inter-qubit exchange J = Je iθ is introduced, with the real part representing symmetric exchange and the imaginary component embodying the Dzyaloshinskii-Moriya interaction. This DMI, manifested as a phase-biased coupling, is intended to skew the left-right response of the system despite the nonchiral photonic environment. Coherent drives with amplitudes ε 1→ and ε 2← are applied from the left and right, respectively, enabling precise control over the qubit dynamics. The theoretical framework relies on spin-1/2 operators, S z j and S ± j, to describe the qubit states, and the Hamiltonian incorporates terms representing the detunings, the complex exchange interaction, and the coherent drives. The full quantum master equation and input-output formalism are employed to describe the dynamics of the two-level systems coupled to the waveguide, accounting for both coherent evolution and dissipative effects. By utilising the input-output formalism, the researchers relate the internal state of the qubits to the transmitted and reflected fields, enabling the analysis of nonreciprocal behaviour. A key methodological innovation lies in the deliberate engineering of the complex inter-qubit exchange. By carefully controlling the phase θ of the DMI, the researchers can tune the strength of the antisymmetric coupling and manipulate the left-right response of the system, circumventing the need for chiral waveguides. The use of synthetic realizations of the DMI, such as those found in superconducting parametric circuits, further expands the potential for experimental implementation. The researchers demonstrate that at pure-state points (φ = nπ), the steady-state entanglement becomes reciprocal and admits a closed-form expression. To characterise the entanglement, the concurrence is used as a measure of the quantum correlations between the qubits, allowing for a quantitative assessment of how the magnitude and phase of the DMI influence forward/backward contrasts in entanglement. Furthermore, the researchers analyse photon statistics to understand how the DMI reshapes two-photon correlations, shifting superbunching from transmission to reflection at finite DMI values. Scientists have achieved a significant advance in controlling light-matter interactions, demonstrating a pathway to nonreciprocal quantum systems without relying on complex, chiral waveguide designs. The implications extend beyond fundamental physics, offering the potential to streamline the creation of quantum photonic devices, paving the way for more compact and efficient technologies for secure quantum communication and powerful quantum computation. The demonstration of “pure states”, where unwanted decoherence is suppressed, is particularly noteworthy, enabling perfect transparency under specific conditions. This level of control over light propagation could unlock new possibilities for optical isolators and routers, essential components in complex quantum circuits. However, scaling up these systems remains a considerable challenge, requiring overcoming hurdles in controlling interactions and minimising noise. Future research will likely focus on exploring different materials and architectures to enhance these effects and integrate them into practical devices, benefiting the broader effort to build robust and scalable quantum technologies.