Quantum secured gigabit optical access networks

Abstract

Optical access networks connect multiple endpoints to a common network node via shared fibre infrastructure. They will play a vital role to scale up the number of users in quantum key distribution (QKD) networks. However, the presence of power splitters in the commonly used passive network architecture makes successful transmission of weak quantum signals challenging. This is especially true if QKD and data signals are multiplexed in the passive network. The splitter introduces an imbalance between quantum signal and Raman noise, which can prevent the recovery of the quantum signal completely. Here we introduce a method to overcome this limitation and demonstrate coexistence of multi-user QKD and full power data traffic from a gigabit passive optical network (GPON) for the first time. The dual feeder implementation is compatible with standard GPON architectures and can support up to 128 users, highlighting that quantum protected GPON networks could be commonplace in the future.

Figure 1. Quantum secured optical access network.

(a) In a passive optical network multiple users (ONU: optical network unit) are connected via drop fibres, an optical power splitter, and a feeder fibre to a network node (OLT: optical line terminal). We integrate QKD into the network using wavelength filters (trapezoid symbols). Quantum transmitters are installed in the ONUs and a shared quantum receiver is installed in the OLT. Each user exchanges individual encryption keys with the network node. (b) Spectrum measured in upstream direction by inserting a 50:50 beam splitter in front of the OLT in an 8-user network. The spectrum shows peaks at 1310 nm and 1490 nm from data signals and a peak at 1610 nm from the synchronisation signal. The quantum signal at 1550 nm is completely obscured by the broad Raman scattering background.

Figure 2. Single feeder fibre network.

(a) Secure key rate for the first quantum transmitter as a function of feeder fibre length F in an 8-user single feeder network for different downstream (DS) data signal launch power. The total distance F + D is kept equal to 20 km. Key transmission over the full length of 20 km is possible only for strongly attenuated downstream power. Shown in grey is the dependence for a 32-user network. Error bars correspond to 1 standard deviation of 3 consecutive measurements. The solid line is calculated using the numerical simulation described in the methods section. (b) Simulation of QBER as a function of the feeder fibre length F and splitting ratio in an idealised multiplexed quantum access network. The simulation only takes Raman photons in the feeder fibre into account as a source of errors.

Figure 3. Dual feeder fibre access network.

(a) Secure key rate per quantum transmitter as a function of feeder fibre distance F in a dual feeder network. The total distance F + D is kept equal to 20 km. Error bars correspond to 1 standard deviation of 3 consecutive measurements. The solid line is calculated using the numerical simulation described in the methods section. Inset: Schematic of the dual feeder network. The power splitter is replaced with a 2 × N splitter connected to two separate feeder fibres. The downstream GPON and synchronisation signal are launched into one feeder fibre, whereas the quantum signals is extracted from the second feeder fibre. (b) Secure key rate per transmitter for varying network capacity with two feeder fibres. Secure transmission is demonstrated up to a splitting ratio of 2 × 128. Inset: Secure key rate over several days in a 128-user network.

Figure 4. Secure key rate in a quantum network with more users.

The secure key rate decreases when more users are added to the network but stays positive for all network capacities considered. The reduction stems from higher afterpulsing noise in a network where more of the detection bandwidth is used. The key rates are overall reduced compared to the data shown in Fig. 3 due to the lower operational speed of the transmitters (see text).

Figure 5. Secure key rate as a function of total drop fibre.

Simulated data for a full quantum and full GPON network with varying capacity. Due to increasing Raman noise the secure key rate decreases with the amount of drop fibre in the network. It stays positive, however, up to total drop fibre lengths of more than N × 10 km for all capacities indicated by the dashed lines. More drop fibre can be tolerated in larger capacity networks because the optical power transmitted per ONU decreases with the number of ONUs due to time-division multiplexing.