University of Massachusetts Amherst: Researchers Detail Architecture for 1,000km Quantum Communication Via 9km Repeaters

A new quantum repeater architecture enables quantum communication over 1,000km with repeater nodes spaced just 9km apart. Ryosuke Shiina and colleagues at University of Massachusetts Amherst, in collaboration with Manning College of Information and Computer Sciences and Photon Queue Inc, have demonstrated an all-photonic system. The advancement results from a combination of continuous-variable and discrete-variable quantum error correction, specifically the bosonic Gottesman-Kitaev-Preskill and Steane codes, which work together to enhance resilience against photon loss. Notably, the architecture requires only a few thousand GKP qubits per repeater station, a substantial reduction in resource demands compared to previous third-generation proposals and paving the way for more practical, long-range quantum networks.

Extended quantum range achieved via combined error correction and reduced qubit requirements

A 9km repeater spacing has been achieved, a substantial improvement over prior third-generation all-photonic repeater proposals which required more complex infrastructure. Photons, while ideal for carrying quantum information due to their minimal interaction with the environment and their ability to travel at the speed of light, are easily lost during transmission, previously making distances beyond 1,000km unattainable due to exponential signal attenuation and photon loss. This attenuation arises from both absorption within the transmission medium (such as optical fibre) and scattering, leading to a reduction in the number of photons carrying the quantum signal. The architecture combines continuous-variable and discrete-variable quantum error correction, utilising GKP and Steane codes to create a strong enhancement in durability against these losses. Continuous-variable quantum information is encoded in the amplitude and phase of light, while discrete-variable uses individual photons, offering complementary strengths in error correction.

This combination allows for quantum communication over 1,000km, a distance previously considered impossible. The fundamental principle behind quantum repeaters is to break the long distance into smaller segments, establishing entanglement between adjacent repeater nodes and then ‘distilling’ this entanglement to create a long-distance entangled state. During operation, the system necessitates only a few thousand GKP qubits per repeater station, a substantial reduction compared to earlier designs needing up to 16,400 single photons or 42,000 GKP qubits for equivalent performance. GKP qubits, named after Gottesman, Kitaev, and Preskill, are a type of continuous-variable qubit that encode quantum information in the quantum state of a harmonic oscillator, offering inherent robustness against photon loss. A squeezing level of 15.4 dB was achieved, representing a major improvement in maintaining qubit fidelity. Squeezing reduces the quantum noise in one quadrature of the electromagnetic field, enhancing the signal-to-noise ratio and improving the quality of the quantum information. The architecture incorporates discard windows to mitigate errors arising from imperfect GKP states, allowing correction of shifts less than √π/2. These discard windows operate by selectively accepting only those GKP states that fall within a defined range of parameters, effectively filtering out noisy or corrupted qubits. GKP qubits, which encode quantum information in the quantum state of a harmonic oscillator, contribute to this efficiency by providing durability against photon loss. The demonstrated 9km repeater spacing represents the maximum distance achieved with this specific architecture, and future research will focus on optimising performance under realistic network conditions, including factors like fibre imperfections and temperature fluctuations.

Reduced qubit demands enable feasible long-distance quantum repeaters

Establishing secure quantum links over vast distances promises revolutionary advances in communication and computation. Applications range from unconditionally secure communication networks, impervious to eavesdropping, to distributed quantum computing, where geographically separated quantum processors can collaborate to solve complex problems. Transmitting quantum information across 1,000km is now possible, a feat previously limited by the inherent fragility of quantum states and the inevitable loss of photons during transmission. The proposed architecture relies on generating entangled Bell pairs, which are fundamental resources in quantum communication, but a fundamental trade-off exists in their creation; fewer qubits can be used, but this accepts a higher error rate, or vice versa. This trade-off is inherent in all quantum communication systems and requires careful optimisation to balance performance and resource requirements.

Acknowledging the trade-off between qubit number and error rates is vital for practical implementation. The Steane code, a quantum error-correcting code, protects against bit-flip and phase-flip errors, while the bosonic Gottesman-Kitaev-Preskill code provides resilience against photon loss. By combining these two codes, the architecture achieves a synergistic effect, enhancing overall error correction performance. This reduction in complexity, requiring only a few thousand GKP qubits per repeater station, represents a key step towards building a viable quantum network. The lower qubit requirement translates directly into reduced hardware costs, power consumption, and cooling requirements, making the technology more accessible and scalable. Despite the inherent challenges of maintaining quantum states over vast distances, long-distance quantum communication is now more attainable. The all-photonic quantum repeater architecture establishes a pathway for quantum communication across distances exceeding 1,000 kilometres, utilising repeater stations spaced just 9km apart.

This advancement overcomes limitations imposed by photon loss, a fundamental obstacle to transmitting fragile quantum information over long distances. The bosonic Gottesman-Kitaev-Preskill code and the Steane code work together to enhance signal durability, representing a major reduction in the complex quantum bits, or qubits, needed for operation. The reduction in qubit demands is particularly significant because creating and controlling qubits is one of the most challenging and expensive aspects of quantum technology. Future work will explore the scalability of this architecture and its integration with existing communication infrastructure, including the development of efficient interfaces between quantum and classical networks and the investigation of different modulation and detection schemes to further improve performance. Furthermore, research will focus on characterising the performance of the system under realistic conditions, accounting for factors such as fibre noise, atmospheric turbulence, and detector imperfections.

The researchers demonstrated an all-photonic quantum repeater architecture capable of enabling quantum communication over 1,000km with repeater stations 9km apart. This is significant because it addresses the problem of signal loss that typically limits the range of quantum communication networks. By combining the bosonic Gottesman-Kitaev-Preskill code and the [[7,1,3]] Steane code, the system improves error correction while reducing the number of qubits required to a few thousand per station. The authors intend to further investigate the scalability of this architecture and its integration with current communication systems.