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arXiv:2608.04093

An optical-fibre-integrated buffer for packet-switched quantum networks

Daniel Spegel-Lexne, Joakim Argillander, Martin Clason, Åsa Claesson, Kenny Hey Tow, Gustavo Lima, João M. B. Pereira, Guilherme B. Xavier

quant-phphysics.opticsSubmitted Aug 4, 2026• Announced Aug 6, 2026• 0 citations
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Abstract

Packet-switched quantum networks require buffers that can delay qubit payloads while routing information is read out in real time. Previous approaches have not provided this functionality in a fully fibre-integrated architecture compatible with telecom infrastructure. Here we demonstrate an optical-fibre-integrated buffer, based on a recirculating loop and a fibre storage line, in which the storage time of a polarisation-encoded qubit payload is determined by readout of an attached packet header. The key component behind this achievement is an ultra-low-loss poled fibre phase modulator, which provides fast, polarisation-insensitive switching directly in fibre and allows header and payload to be processed within the buffer. We demonstrate storage and retrieval of polarisation-encoded qubit payloads for storage times up to 47 $μ$s, with an average quantum bit error rate of 1.8% together with stable operation over several hours. These results establish a practical fibre-based architecture for packet-level quantum network buffering that can easily integrate into the current telecommunication infrastructure opening up new paths for deployment of the quantum internet.

Comments
11 pages, 8 figures. Please see the parallel paper "Poled-fibre phase modulator for efficient high-dimensional quantum measurements'' that was submitted today to the arXiv

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Packet-switched quantum networks require buffers that can delay qubit payloads while routing information is read out in real time. Previous approaches have not provided this functionality in a fully fibre-integrated architecture compatible with telecom infrastructure. Here we demonstrate an optical-fibre-integrated buffer, based on a recirculating loop and a fibre storage line, in which the storage time of a polarisation-encoded qubit payload is determined by readout of an attached packet header. The key component behind this achievement is an ultra-low-loss poled fibre phase modulator, which provides fast, polarisation-insensitive switching directly in fibre and allows header and payload to be processed within the buffer. We demonstrate storage and retrieval of polarisation-encoded qubit payloads for storage times up to 47 μμμs, with an average quantum bit error rate of 1.8% together with stable operation over several hours. These results establish a practical fibre-based architecture for packet-level quantum network buffering that can easily integrate into the current telecommunication infrastructure opening up new paths for deployment of the quantum internet.
The realization of a scalable quantum internet relies on the ability to temporally align asynchronous photonic signals through on-demand buffering. While matter-based quantum memories achieve long storage times, their extremely narrow bandwidths and cryogenic requirements pose significant barriers to integration with existing telecommunications infrastructure. Conversely, current all-optical memories operate at room temperature but are hampered by high input/output losses and a lack of universality across different photonic degrees of freedom. Here, we demonstrate a universal, fully fiber-integrated quantum buffer operating over the full telecom C-band that overcomes these fundamental trade-offs. By implementing an actively switched dual-Sagnac cavity driven by cross-phase modulation, we achieve an ultra-low input/output loss of 0.46 dB and a storage time exceeding 18 μμμs. The device exhibits an operational bandwidth exceeding 12.5 THz ($\sim$100 nm), covering the full telecom C-band. We show the simultaneous buffering of over 200 temporal modes with the ability to address them either collectively or one by one. We demonstrate high-fidelity storage for all three degrees of freedom compatible with optical fiber propagation, namely time-bin, frequency-bin, and polarization qubits, along with faithful preservation of entanglement, confirming the platform's true universality. These results provide a robust, room-temperature solution for the high-rate synchronization of multidimensional quantum states, clearing a major hurdle for the deployment of global photonic quantum networks.
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Abstract We report a fiber loop quantum buffer based on a low-loss 2 × 2 switch and a unit delay made of a fiber delay line. We characterize the device by using a two-photon polarization entangled state in which one photon of the entangled photon pair is stored and retrieved at a repetition rate up to 78 kHz. The device, which enables integer multiples of a unit delay, can store the qubit state in a unit of fiber delay line up to 5.4 km and the number of loop round-trips up to 3. Furthermore, we configure the device with other active elements to realize integer multiplier and divider of a unit delay of a qubit. The quantum state tomography is performed on the retrieved photon and its entangled photon. We obtain a state fidelity > 94 % with a maximum storage time of 52 μs with an insertion loss of 5.56 dB. To further characterize the storing and retrieving processes of the device, we perform entanglement-assisted quantum process tomography on the buffered qubit state. The process fidelity of the device is > 0.98. Our result implies that the device preserves the superposition and entanglement of a qubit state from a two-photon polarization-entangled state. This is a significant step towards facilitating applications in optical asynchronous transfer mode based quantum networks.
Future quantum networks require a hybrid platform of dissimilar quantum systems. Within the platform, joint quantum states have to be mediated either by single photons, photon pairs or entangled photon pairs. The photon wavelength has to lie within the telecommunication band to enable long-distance fibre transmission. In addition, the temporal shape of the photons needs to be tailored to efficiently match the involved quantum systems. Altogether, this requires the efficient coherent wavelength-conversion of arbitrarily shaped single-photon wave packets. Here, we demonstrate the heralded temporal filtering of single photons as well as the synchronisation of state manipulation and detection as key elements in a typical experiment, besides of delaying a photon in a long fibre. All three are realised by utilising commercial telecommunication fibre-optical components which will permit the transition of quantum networks from the lab to real-world applications. The combination of these renders a temporally filtering single-photon storage in a fast switchable fibre loop possible.
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Adapting the architecture and protocols of classical networks to quantum networking is challenging due to the quantum mechanical properties. In this paper, we utilize a Quantum Wrapper Networking architecture that enables transparent and interoperable transportation of quantum wrapper (QW) datagrams, consisting of quantum payloads and classical headers, over optical fiber networks. We experimentally demonstrate end-to-end transportation of QW datagrams in a three-node packet-switched optical network testbed. The header and payload of a QW datagram are multiplexed in time and wavelength, i.e., the 1561.41 nm header precedes the L-band payload. A QW switch/router performs packet switching by reading the headers, generating new headers, and routing the datagrams to their destinations without disturbing quantum information in the payload. We use 20 km fiber links from the source to two distinct destinations in the testbed. Our experiments show$>22coincidence−to−accidentalratio(CAR)forbothdestinationsattwodifferentwavelengthchannelsandclearvisibility,22 coincidence-to-accidental ratio (CAR) for both destinations at two different wavelength channels and clear visibility,22coincidence−to−accidentalratio(CAR)forbothdestinationsattwodifferentwavelengthchannelsandclearvisibility,>$79%. We further investigate impairments such as chromatic dispersion and wavelength-dependent polarization rotations in the fiber. We observe that the classical headers' bit-error rate (BER) and the quantum payloads' CAR are correlated under the wavelength-independent channel attenuation. Thus, QW network control and management can utilize a type of performance monitoring. We also discuss the QW datagram design constraints and future fast packet switching implementations.