Case study: Dynamically Switched Quantum Network Supporting Co-existence of Entanglement, Prepare-and-measure QKD, and Classical Channels

The development of heterogeneous network supporting various quantum technologies and co-existence with high-speed classical communication forms a strong pillar to enable the UK Quantum Network between Bristol and Cambridge. This case study includes several innovations.

The setup for an 8-user heterogeneous quantum network is shown in Fig, 1A. Alice (A), Bob (B), Chloe (C), Dave (D), Faye (F), Gopi (G) and Heidi (H) are capable of receiving entangled photons via a programmable quantum-enabled reconfigurable optical add-drop multiplexer (q-ROADM). Bob* (B*), a prepare-and-measure QKD receiver (IDQuantique Cerberis XGR), co-locating with one of the entanglement distribution users (deemed as the same user) is able to establish a dedicated QKD link with an external node, Alice* (A*).

Fig. 1: (A) The 8-user heterogeneous network experiment set up. (B) The scale of the field-trial testbed, Bob* is a mobile prepare-and-measure QKD receiver (dash blue ellipse) moving between users. (C) The logical view of the connection links and entanglement wavelength assignment scheme. Photons in channel -n are entangled with ones in +n, presented in the same color.

Fig. 1B provides an overview of the physical scale of our heterogeneous quantum network. Alice uses a 5.7 km installed fibre looped back from a museum, We The Curious (WTC), in Bristol city. Bob’s connection to the q-ROADM spans 49.1 km deployed fibre via Bradley Stoke (BST), a site within the National Dark Fibre Facility. Chloe and Dave are linked via 1.7 km of campus fibre, looping back at High Performance Networks (HPN) research lab at the University of Bristol. Faye, Gopi and Heidi are connected to the q-ROADM via tens of meters of fibre in the lab. For entanglement distribution, we employ a type-0 spontaneous parametric down-conversion (SPDC) entanglement source capable of producing the state |Φ+⟩ = √1 2 (|HsHi⟩ + |VsVi⟩) centred at 1550.12 nm. The source is connected to the q-ROADM, where a DEMUX slices the spectrum into multiple 100G spacing channels. Each channel is indexed (n) based on its centre frequency, f_central = 193.40 ± 0.1 × n THz, where n is an integer 0, 1, 2, ..., 15, limited by the size of DEMUX. Energy conservation during the SPDC process (pump laser at 775 nm) ensures down-converted photons in channel n are correlated with ones in channel −n. Inside the q- ROADM, specific wavelength channels are connected to an optical switch directly, while others pass through 1 × 4 beam splitters (BSs), allowing the same wavelength channel to be distributed to up to 4 users. Subsequently, the wavelength channels are multiplexed, to enable each user to generate entanglement with others. The wavelength assignment scheme for entanglement distribution, along with the logical connection graph showing both prepare-and-measure QKD (blue) and entanglement-based (black) links, is presented in Fig. 1C. In particular, D, F, G and H receive wavelengths ±7 and ±8 via 1 × 4 BSs, facilitating full connectivity between them with two wavelength pairs. At the user end, each employs a polarisation analysis module (PAM), which randomly performs measurement in the HV basis (short path) or in the DA basis (long path). After measurement, correlation is performed via a time-digital converter (Swabian Time Tagger Ultra).

The external user, Alice*, sends 1551.72 nm (channel -2) prepare-and-measure QKD photons to the receiver Bob* (co-locating with a user) in a QKD & classical co-existing scheme over 7 or 9 km spool. The classical light consisting of eight 100G PM-QPSK channels (Tx1 − Tx8), with frequencies 192.85 THz - 193.00 THz and 193.40 THz - 193.55 THz at 50 GHz spacing, is dropped by DEMUX 1. The QKD signal is forwarded to the q-ROADM for further co-propagation with entangled photons and 200G PM-16QAM classical light from Tx9 (channel +2). At user D, the classical light is dropped via DEMUX 2 and the rest is directed to DEMUX3, where the prepare-and-measure QKD photons and entangled photons are further separated. Users A, B and C have similar setups, all capable of receiving the prepare-and-measure QKD signal. In Fig. 1C, the blue dash line denotes that Bob* is a mobile receiver moving between users to perform the measurement. This mobility is also depicted as the blue dash ellipses in Fig. 1B.

Funding information: This work has been funded by The EPSRC Quantum Communications Hub, (EP/T001011/1), and European Union’s Horizon RIA project ALLEGRO (No.101092766)

For further information: contact Dr. Rui Wang, ndff@ee.ucl.ac.uk

Published: 28 March 2024