Ntt Co Team Proposes Half-Beam Splitter Protocol for Rotation-Symmetric Cat Bell State Measurement

Scientists Issa Oe of NTT in Japan and colleagues have achieved a deterministic Bell state measurement (BSM) for rotation-symmetric cat codes under ideal conditions, utilising an arbitrary symmetry order N. The team developed a protocol employing only a half beam splitter and photon-number-resolving detectors. This method extracts both photon-number modulo and phase information, exceeding previous techniques that required more complex nonlinear operations and identifying a loss regime where higher-order codes offer advantages. The work simplifies the process of measuring quantum states using rotation-symmetric cat codes, a technique for encoding and transmitting quantum information, and represents a significant advancement in the field of quantum information science.

This method requires fewer complex operations than previous approaches, relying on identifying both the number and phase of individual photons. The team discovered that employing more intricate cat codes improves performance when signals weaken during transmission, key for long-distance quantum communication and computation. Issa Oe at NTT in Japan and colleagues have demonstrated a simplified method for performing Bell state measurement, a vital process in quantum communication and computation, using rotation-symmetric cat codes. The underlying principle of rotation-symmetric cat codes involves superposing coherent states with different phases, creating a robust quantum state less susceptible to decoherence and photon loss, major obstacles in quantum communication. These codes are considered bosonic codes, differing from traditional qubit-based approaches and offering unique advantages in error correction.

A Bell state measurement determines the relationship between two entangled quantum particles, much like checking if two coins flipped simultaneously both landed on heads or tails. However, in the quantum realm, this determination isn’t a simple observation; it requires a specific measurement protocol that projects the entangled state onto one of four possible Bell states. The team’s protocol relies on a half beam splitter, a device that splits a single photon into two possible paths similar to a partially reflective mirror, and photon-number-resolving detectors, sensors that precisely count individual photons, analogous to a highly sensitive light metre. This approach extracts both photon-number and phase information, streamlining the process compared to previous techniques and revealing that more complex cat codes improve performance when signals weaken. The researchers identified a specific loss regime where using higher-order codes offers distinct advantages, meaning that as photon loss increases during transmission, the higher-order codes maintain a higher fidelity in the BSM. This is crucial for practical quantum communication systems where signal attenuation is inevitable.

High-fidelity Bell state measurement enables scalable quantum error correction with

NTT Co has achieved a Bell state measurement (BSM) success probability exceeding 99.7% for rotation-symmetric cat (RS-cat) codes, a substantial improvement over previous methods limited to around 90% for similar codes. The breakthrough utilises only a half beam splitter and photon-number-resolving detectors, outperforming techniques requiring complex nonlinear operations like controlled-rotations and reducing platform dependency for quantum communication systems. This improvement in BSM fidelity directly translates to a reduction in error rates in quantum communication protocols and enhances the feasibility of implementing quantum error correction. The ability to achieve such high fidelity with a simplified setup is particularly noteworthy, as it lowers the technological barrier to building practical quantum networks. Prior to BSM schemes often relied on intricate optical setups and precise control of nonlinear interactions, making them challenging to scale. The parameter N, representing the symmetry order of the RS-cat code, dictates the complexity of the code and its resilience to noise. The team demonstrated deterministic performance for arbitrary N. The parameter N dictates the complexity of the code and its resilience to noise, and the team demonstrated deterministic performance for arbitrary N.

Numerical evaluations further revealed that higher-order RS-cat codes offer advantages in loss regimes, suggesting improved durability to signal degradation during transmission. The unique photon-number structure inherent in rotation-symmetric cat (RS-cat) codes was exploited, enabling extraction of both photon-number modulo and phase information important for distinguishing Bell states. Specifically, the discrete rotational symmetry of these codes allows for a simplified measurement scheme that focuses on these two key parameters. Post-selection techniques can further enhance the measurement’s success probability, though practical implementation will require addressing challenges in achieving sufficiently high photon-number resolution with photon-number-resolving detectors (PNRDs) and mitigating the impact of realistic photon loss, as the current results are based on ideal, loss-free conditions. Improving PNRD efficiency and reducing detector dark counts are critical areas for future research. These higher-order codes are more resilient to imperfect detection and transmission, offering a pathway towards strong quantum communication, potentially enabling secure communication over extended distances.

Simplified entanglement distribution balances efficiency with scalability challenges

The team’s simplified Bell state measurement offers a potential pathway towards practical quantum communication and computation, addressing a long-standing need for efficient entanglement distribution. Entanglement distribution is the process of sharing entangled quantum states between distant parties, a fundamental requirement for many quantum applications. It was acknowledged that a reliance on post-selection, discarding unsuccessful measurement attempts, is currently necessary to achieve high success rates, a technique that introduces overhead and complicates scaling to larger quantum networks. The overhead arises because only a fraction of measurement attempts yield valid results, requiring repeated attempts to establish entanglement. Alternative strategies to reduce the need for post-selection are being explored, such as optimising measurement parameters and employing more sophisticated error correction schemes, including codes designed to tolerate higher levels of loss and detector inefficiency.

This refined Bell state measurement protocol represents a valuable step forward for applications like long-distance quantum communication and fusion-based quantum computation, by streamlining a key operation within these emerging technologies. Fusion-based quantum computation involves combining multiple smaller quantum processors to create a larger, more powerful quantum computer. Rotation-symmetric cat codes achieve deterministic performance regardless of their complexity, denoted by the parameter N, opening possibilities for more scalable quantum networks. The team simplified the process of entanglement distribution by bypassing the need for complex nonlinear optical components previously considered essential for discriminating between quantum states; photon-number-resolving detectors precisely count individual photons, providing the necessary information for accurate state identification. This simplification reduces the cost and complexity of building and maintaining quantum communication systems, paving the way for wider adoption of this technology. Further research will focus on implementing this protocol in realistic quantum communication channels and exploring its compatibility with different quantum error correction codes.

The researchers demonstrated a deterministic Bell state measurement protocol for rotation-symmetric cat codes, achieving success regardless of the code’s symmetry order, N, under ideal conditions. This matters because efficient Bell state measurements are crucial for long-distance quantum communication and fusion-based quantum computation, enabling entanglement between quantum processors. The protocol utilises photon-number-resolving detectors and a half beam splitter, simplifying the process by avoiding complex optical components previously thought necessary. The team intends to implement this protocol in realistic quantum communication channels and investigate compatibility with various error correction codes.