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We show simplified DPS-QKD using a SiN micro-ring resonator operated at 852 nm. A raw-key rate of up to 25.3 kb/s is reached at a QBER suitable for secure-key extraction. Short-reach QKD operation is maintained for zero-touch link layouts with C-band telecom fiber.

The dawning of the quantum age makes quantum key distribution (QKD) an indispensable necessity for our global communication infrastructure. The realization of an all-silicon QKD transmitter supplied by a light source that is native to silicon integration platforms is seen as a disruptive step towards the pervasive introduction of QKD in new applications, which up to now have not been explored due to the missing credentials of quantum optics as a cost-effective and highly miniaturized technology., Comment: to be published in 2024 IEEE International Conference on Photonics, Pulau Pinang, Malaysia, Aug. 2024

The generation and detection of squeezed light through spontaneous parametric down-conversion in a nonlinear crystal up to a frequency of 3.5 GHz is presented. We characterize the quantum state with balanced homodyne detection, leveraging a low-noise die-level receiver.

We evaluate large-core FSO links where excellent coupling pairs with bandwidth fading due to multi-mode propagation. The 10-Gb/s/{\lambda} limit for 105-{\mu}m double-clad fibers is mitigated by spectral launch tuning, restoring 84% of single-clad 25-Gb/s/{\lambda} capacity.

We implement a silicon-on-insulator light emitter as optical supply for a QKD transmitter and transfer it to an electronic BiCMOS wafer. A secure key is established over short reach in co-existence with shortwave data transmission.

We demonstrate QKD and data communication over an out-door free-space link where large-core fiber substitutes active alignment. We further prove E-band QKD as stable and robust under full daylight, despite the loss of spatial filtering.

We demonstrate a novel transmitter concept for quantum key distribution based on the polarization-encoded BB84 protocol, which is sourced by the incoherent light of a forward-biased Ge-on-Si PIN junction. We investigate two architectures for quantum state preparation, including independent polarization encoding through multiple modulators and a simplified approach leveraging on an interferometric polarization modulator. We experimentally prove that the Ge-on-Si light source can accommodate for quantum key generation by accomplishing raw-key rates of 2.15 kbit/s at a quantum bit error ratio of 7.71% at a symbol rate of 1 GHz. We further investigate the impact of depolarization along fiber-based transmission channels in combination with the broadband nature of the incoherent light source. Our results prove the feasibility of a fully-integrated silicon quantum key distribution transmitter, including its light source, for possible short-reach applications in zero-trust intra-datacenter environments.

As quantum key distribution has reached the maturity level for practical deployment, questions about the co-integration with existing classical communication systems are of utmost importance. To this end we demonstrate how the co-propagation of classical and quantum signals can benefit from the development of novel hollow-core fibers. We demonstrate a secure key rate of 330 bit/s for a quantum channel at 1538 nm in the presence of 25 x 10 Gb/s classical channels, transmitted at an aggregated launch power of 12 dBm, spanning over the C+L-band in the same hollow-core fiber link. Furthermore, we show the co-integration of the classical key-distillation channel onto this fiber link, turning it into a bidirectional fiber link and thereby mitigating the need for multiple fibers. We believe this to be an important step towards the deployment and integration of hollow-core fibers together with DV-QKD for the inherently secure telecom network of the future.

We investigate the co-existence of 852-nm and 1550-nm QKD with carrier-grade 4x25-Gb/s/$\lambda$ LANWDM over a short-reach interconnect. Shortwave QKD yields a higher key rate and is insensitive to Raman noise, as opposed to 1550-nm QKD., Comment: 5 pages, 7 figures (including sub-figures), conference paper (submitted)

We investigate a polarization-encoded BB84-QKD transmitter that is simplified from an architectural and technological point-of-view, demonstrating a silicon emitter sourcing a low-complexity polarization modulator for secure-key generation at a raw-key rate of 2.8kb/s and QBER of 10.47%, underpinning the feasibility of an all-silicon QKD transmitter.

We investigate the behavior of a Ge-on-Si light source and demonstrate its feasibility for polarization-encoded discrete-variable quantum key distribution following the BB84 protocol, enabling a potential "all-silicon" QKD scheme which can operate well below the necessary QBER limit and successfully generate secret keys.

We demonstrate a simplified out-door FSO link with modal split for down-/uplink and confirm its long-term stability without active beam tracking. We further prove the duality of modal and directional split through penalty-free full-duplex transmission.

We demonstrate a polarization-encoded BB84 transmitter sourced by a SiGe light source and show that such a potentially "all-silicon" QKD scheme can operate well below the QBER threshold at which secret keys can be established.

Quantum key distribution (QKD) has been researched for almost four decades and is currently making its way to commercial applications. However, deployment of the technology at scale is challenging, because of the very particular nature of QKD and its physical limitations. Among others, QKD is computationally intensive in the post-processing phase and devices are therefore complex and power hungry, which leads to problems in certain application scenarios. In this work we study the possibility to offload computationally intensive parts in the QKD post-processing stack in a secure way to untrusted hardware. We show how error correction can be securely offloaded for discrete-variable QKD to a single untrusted server and that the same method cannot be used for long distance continuous-variable QKD. Furthermore, we analyze possibilities for multi-server protocols to be used for error correction and privacy amplification. Even in cases where it is not possible to offload to an external server, being able to delegate computation to untrusted hardware components on the device could improve the cost and certification effort for device manufacturers., Comment: 17 pages

We successfully integrate coherent one-way QKD at 1538 nm in a 7.7 km long hollow-core fiber link with 17 EDFA-boosted C-band data channels from 1540.56 to 1558.17 nm, aggregating a power of 11 dBm. QKD operation proves successful despite the wideband layout of classical channels.

We experimentally compare a loss-optimized coherent heterodyne and a bandwidth-blessed intradyne CV-QKD architecture. We find the former to prevail performance-wise for medium/long link reach, while the latter features a 5-9 dB higher secure-key rate over short reach.

We investigate the practical network integration of differential phase shift quantum key distribution following a cost-optimized deployment scheme where complexity is off-loaded to a centralized location. User terminal equipment for quantum state preparation at 1 GHz symbol rate is kept technologically lean through use of a directly-modulated laser as optical encoder. Integration in a passive optical network infrastructure is experimentally studied for legacy and modern optical access standards. We analyze the implications that result from Raman scattering arising from different spectral allocations of the classical channels in the O-, S-, C- and L-band, and prove that the quantum channel can co-exist with up to 19 classical channels of a fully-loaded modern access standard. Secure-key generation at a rate of 5.1 times 10e-7 bits per pulse at a quantum bit error ratio of 3.28 percent is obtained over a 13.5 km reach, 2 to 16 split passive network configuration. The high power difference of 93.8 dB between launched classical and quantum signals in the lit access network leads to a low penalty of 0.52 percent in terms of error ratio., Comment: 9 pages, 10 figures, journal

We demonstrate a die-level balanced homodyne receiver for coherent optical access and continuous-variable quantum applications, featuring a 40dB CMRR up to 1GHz and a high quantum-to-classical noise ratio of 26.8dB at 12.3mW of LO power. 500Mb/s QPSK transmission was accomplished with a sensitivity of -55.8dBm., Comment: 4 pages, 3 Figures, ECOC 2021 - European Conference on Optical Communication, paper ID Th2D.4

We demonstrate a die-level balanced homodyne detector with a high quantum-to-classical noise clearance of 19.1dB over 3GHz bandwidth. We evaluate this receiver as a high-quality entropy source for random number generation at >10Gb/s and prove the functional integration of a classical 10Gb/s duobinary communication channel.

We propose a minimally invasive optical overlay for time-interleaved QRNG operation in NG-PON2 transceivers. We experimentally demonstrate that random numbers can be generated at a 0.5 Gb/s rate and validate the randomness through statistical tests.

A coherent transmission methodology for a continuous-variable quantum key distribution (CV-QKD) system based on quantum-heterodyne measurement through a coherent intradyne receiver is experimentally demonstrated in the framework of 5G mobile fronthaul links. Continuous optical carrier synchronization is obtained through training information, which is multiplexing to the quantum signal as pilot tone in both, frequency and polarization. Spectral tailoring by means of optical carrier suppression and single-sideband modulation is adopted to simultaneously mitigate crosstalk into the quantum channel and self-interference for the pilot tone, thus allowing for a high signal-to-noise ratio for this training signal. Frequency offset correction and optical phase estimation for the free-running local oscillator of the receiver is accurately performed and guarantees low-noise quantum signal reception at high symbol rates of 250 MHz and 500 MHz with additional Nyquist pulse shaping. A low excess noise in the order of 0.1% to 0.5% of shot-noise units is obtained for fiber-based transmission over a fronthaul link reach of 13.2 km. Moreover, co-existence with 11 carrier-grade classical signals is experimentally investigated. Joint signal transmission in the C-band of both, quantum signal and classical signals, is successfully demonstrated. Secure-key rates of 18 and 10 Mb/s are obtained under strict security assumptions, where Eve has control of the receiver noise, for a dark and a lit fiber link, respectively. Moreover, rates of 85 and 72 Mb/s are resulting for a trusted receiver scenario. These secure-key rates are well addressing the requirements for time-shared CV-QKD system in densified 5G radio access networks with cloud-based processing.

The integration of quantum communication functions often requires dedicated opto-electronic components that do not bode well with the technology roadmaps of telecom systems. We investigate the capability of commercial coherent transceiver sub-systems to support quantum random number generation next to classical data transmission, and demonstrate how the quantum entropy source based on vacuum fluctuations can be potentially converted into a true random number generator for this purpose. We discuss two possible implementations, building on a receiver- and a transmitter-centric architecture. In the first scheme, balanced homodyne broadband detection in a coherent intradyne receiver is exploited to measure the vacuum state at the input of a 90-degree hybrid. In our proof-of-principle demonstration, a clearance of >2 dB between optical and electrical noise is obtained over a wide bandwidth of more than 11 GHz. In the second scheme, we propose and evaluate the re-use of monitoring photodiodes of a polarization-multiplexed inphase/quadrature modulator for the same purpose. Time-interleaved random number generation is demonstrated for 10 Gbaud polarization-multiplexed quadrature phase shift keyed data transmission. The availability of detailed models will allow to calculate the extractable entropy and we accordingly show randomness extraction for our two proof-of-principle experiments, employing a two-universal strong extractor.

We demonstrate cost-effective QKD integration for GPON and NG-PON2. Operation at 5.1e-7 secure bits/pulse and a QBER of 3.28% is accomplished for a 13.5-km reach, 2:16-split PON, with 0.52% co-existence penalty for 19 classical channels.

We leverage spectral assets of entanglement and spatial switching to realize a flexible distribution map for cloud-to-edge and edge-to-edge quantum pipes that seed IT-secure primitives. Dynamic bandwidth allocation and co-existence with classical control are demonstrated.

We demonstrate a phase-selective and colorless quantum receiver assisted by a silicon-on-insulator microring, enabling a low 1.3% QBER at 5.3kb/s secure-key rate. No penalty incurs compared to a delay interferometer. BiCMOS 3D-integration is proven feasible.

The practical roll-out of quantum communication technologies in optical networks and the adoption of novel quantum applications demand the distribution of single or entangled photons. Flexibility and dynamicity are paramount for the provision of quantum resources, in order to scale with the number of users and to meet the demand of complex network architectures. We present a quantum network architecture that features this degree of reconfigurability, without being restricted to a rigid physical-layer network based on purely passive multiplexing componentry. We leverage spectral assets of photon-pair sources, from the short-wavelength band to the L-band, and agile spatial switching at a remote optical network node, in order to realize a flexible distribution map that features different flavors, reaching from cloud-centric to user-centric quantum connectivity. Photon pair distribution is experimentally demonstrated between five users in a 17-km reach, tree-shaped optical network, with high visibility entanglement shared between three users of the network. Simultaneous distribution of photons to more than one user is enabled and the delivered photon rate can be dynamically adjusted. Penalty-free operation is confirmed for integrating a co-existing classical control channel within the quantum., Comment: 10 pages, 8 figures, 2 tables

Coherent homodyne detection requires a precise matching of emission wavelengths between transmitter and local oscillator at the receiver. Injection-locking can provide all-optical synchronization of the emission frequencies, even under wavelength-swept emission. By adapting the sweep parameters to the conditions in the optical fiber plant, transmission impairments can be mitigated. In this regard I experimentally demonstrate that a wavelength-hopping yet locked transceiver pair, which builds on conceptually simple externally modulated laser technology, features a much higher robustness to reflection crosstalk. The reception penalty due to distortions arising at a Fresnel reflection in the transmission path can be reduced by >90% at a low optical signal-to-reflection ratio of ~0 dB.

A cost-effective QKD transmitter is evaluated in a 16km reach, 2:16-split PON and yields 5.10-7secure bits/pulse. Co-existence with 20 down-and 1 upstream channel is possible at low QBER degradation of 0.93% and 1.1%.

We present a pilot-assisted coherent intradyne reception methodology for CV-QKD with true local oscillator. An optically phase-locked reference tone, prepared using carrier-suppressed optical single-sideband modulation, is multiplexed in polarisation and frequency to the 250 Mbaud quantum signal in order to provide optical frequency- and phase matching between quantum signal and local oscillator. Our concept allows for high symbol rates and can be operated at an extremely low excess-noise level, as validated by experimental measurements., Comment: 12 pages, 6 figures, 1 table

Quantum key distribution using weak coherent states and homodyne detection is a promising candidate for practical quantum-cryptographic implementations due to its compatibility with existing telecom equipment and high detection efficiencies. However, despite the actual simplicity of the protocol, the security analysis of this method is rather involved compared to discrete-variable QKD. In this article we review the theoretical foundations of continuous-variable quantum key distribution (CV-QKD) with Gaussian modulation and rederive the essential relations from scratch in a pedagogical way. The aim of this paper is to be as comprehensive and self-contained as possible in order to be well intelligible even for readers with little pre-knowledge on the subject. Although the present article is a theoretical discussion of CV-QKD, its focus lies on practical implementations, taking into account various kinds of hardware imperfections and suggesting practical methods to perform the security analysis subsequent to the key exchange. Apart from a review of well known results, this manuscript presents a set of new original noise models which are helpful to get an estimate of how well a given set of hardware will perform in practice., Comment: 71 pages, 9 figures, 1 table

We propose a scheme for quantum key distribution (QKD) in a passive optical network (PON) based on differential phase shift (DPS) coding. A centralized station including all expensive components serves many users, making it suitable for a true multi-user network in a local environment with moderate distances on the order of a few kilometers. The emphasis lies on an asymmetric design for cost effective implementation of network end points., Comment: 10 pages, 6 figures

The ability to generate entangled photon-pairs over a broad wavelength range opens the door to the simultaneous distribution of entanglement to multiple users in a network by using centralized sources and flexible wavelength-division multiplexing schemes. Here we show the design of a metropolitan optical network consisting of tree-type access networks whereby entangled photon-pairs are distributed to any pair of users, independent of their location. The network is constructed employing commercial off-the-shelf components and uses the existing infrastructure, which allows for moderate deployment costs. We further develop a channel plan and a network-architecture design to provide a direct optical path between any pair of users, thus allowing classical and one-way quantum communication as well as entanglement distribution. This allows the simultaneous operation of multiple quantum information technologies. Finally, we present a more flexible backbone architecture that pushes away the load limitations of the original network design by extending its reach, number of users and capabilities., Comment: 26 pages, 12 figures