Quantum Networks: New Protocols Boost Remote State Preparation Say QuTech et al.

Remote state preparation allows the transfer of quantum information between distant parties, forming a crucial building block for future quantum networks and secure communication technologies. Janice van Dam, Emil R. Hellebek, and Tzula B. Propp, all from QuTech at Delft University of Technology, alongside collaborators from Q*Bird BV and the University of Copenhagen, have now developed new methods to simplify this process. Their research introduces two innovative protocols, single-click and double-single-click, that dramatically reduce the requirements for successful remote state preparation, moving beyond existing techniques which demand signals from both parties involved. These advancements offer significant potential for building practical quantum communication systems, particularly for long-distance applications like secure key distribution using quantum repeaters, by easing the technical challenges associated with generating and detecting single photons. The team’s work demonstrates that these new protocols not only achieve comparable or improved performance but also offer pathways to reduce the complexity of building a quantum internet.

Long-Distance Quantum Communication via Repeaters

Quantum repeaters are crucial for overcoming the limitations of long-distance quantum communication, where quantum information is lost due to signal attenuation in optical fibers. These devices break long distances into smaller segments, utilizing entanglement and a process called entanglement swapping to extend the range of quantum communication. Entanglement, a core resource in quantum communication, creates a correlation between qubits that enables secure key distribution and other quantum applications. Bell states, maximally entangled states of two qubits, are essential for entanglement swapping, while techniques like bright-state preparation increase the probability of successful entanglement distribution. Researchers also employ photon-number-resolving detectors to identify the number of photons present and the decoy state method to estimate channel parameters and detect potential eavesdropping attempts. These advancements are vital for secure communication methods like quantum key distribution, which uses the principles of quantum mechanics to generate and distribute encryption keys.

Weak Coherent Pulse Remote State Preparation

Researchers have developed new methods for remotely preparing quantum states on another party’s qubit, a process called remote state preparation (RSP). This work focuses on improving RSP by utilizing weak coherent pulses, a practical and readily available alternative to more complex single-photon sources, which simplifies the implementation of RSP in real-world quantum networks. The standard approach, termed the “double-click” protocol, requires detecting a photon from both the sender and receiver, a process that can be inefficient. To address this, the team introduced a “single-click” protocol, which only requires detection of a photon from one party, significantly improving the rate of successful state preparation without sacrificing fidelity.

Surprisingly, this new protocol does not suffer from the inherent fidelity limitations seen in entanglement generation when using weak coherent pulses, offering a substantial advantage. Further refinement led to a “double-single-click” protocol, which repeats the single-click process and applies an additional operation, mitigating the need for precise phase stabilization, a common technical hurdle in quantum communication. These advancements move the field closer to practical quantum communication networks, offering improved efficiency and reduced technical complexity.

Single-Click Beats Double-Click for State Preparation

This research introduces two new protocols, single-click (SC) and double-single-click (DSC), for remote state preparation, a technique allowing one party to prepare a quantum state on another’s qubit using entanglement. These protocols build upon an existing ‘double-click’ method and offer potential advantages in practical quantum networks, particularly for applications like long-distance quantum key distribution. The key finding is that the SC protocol consistently achieves higher preparation rates than the existing double-click method, without sacrificing fidelity, a result that contrasts with typical entanglement generation where increased rate often reduces fidelity. The DSC protocol offers a further refinement, potentially eliminating the need for precise phase stabilization. The authors demonstrate that the optimal protocol choice depends on specific experimental conditions; SC prioritizes maximum rate, DSC balances rate and technical complexity, and the original double-click method remains suitable when maximum fidelity is paramount or phase noise can be minimized. Further investigation is required for a complete security analysis of the DSC protocol.