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Optics Letters

Monolithic 850 nm VCSEL array for quantum key distribution applications via the polarization-based BB84 and decoy-state protocol

Michael Zimmer, Katharina Dahler, Michael Jetter, Peter Michler

Published Aug 4, 2025• 2 citations
Publisher

Abstract

Quantum cryptography based on the application of single photons offers a secure communication based on the laws of physics. However, given the sophisticated and challenging implementation of true-single photon sources, the majority of implementations takes advantage of the decoy state protocol, which uses weak coherent pulses instead. For polarization-based protocols, a typical sender configuration includes one or multiple laser sources with external optical elements to realize four different polarization states and two intensity levels (signal and decoy). In this regard, the application of eight independent lasers with fixed polarization makes an external state preparation unnecessary and allows for a higher signal rate, while requiring otherwise identical optical properties of all laser devices such as wavelength and linewidth. Here, we demonstrate a solution on the basis of a compact, monolithic VCSEL array operating at room temperature. The array consists of eight VCSELs with integrated subwavelength gratings for polarization control. Orthogonal polarization suppression ratios (OPSRs) of up to 19.7 dB (19.1 dB) are reached for the rectilinear (diagonal) basis. By careful adjustment of the individual VCSEL's operating point, a wavelength matching of all VCSELs is achieved at an emission wavelength of 851.2 nm.

DOI
10.1364/ol.573598

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Quantum cryptography based on the application of single photons offers a secure communication based on the laws of physics. However, given the sophisticated and challenging implementation of true-single photon sources, the majority of implementations takes advantage of the decoy state protocol, which uses weak coherent pulses instead. For polarization-based protocols, a typical sender configuration includes one or multiple laser sources with external optical elements to realize four different polarization states and two intensity levels (signal and decoy). In this regard, the application of eight independent lasers with fixed polarization makes an external state preparation unnecessary and allows for a higher signal rate, while requiring otherwise identical optical properties of all laser devices such as wavelength and linewidth. Here, we demonstrate a solution on the basis of a compact, monolithic VCSEL array operating at room temperature. The array consists of eight VCSELs with integrated subwavelength gratings for polarization control. Orthogonal polarization suppression ratios (OPSRs) of up to 19.7 dB (19.1 dB) are reached for the rectilinear (diagonal) basis. By careful adjustment of the individual VCSEL's operating point, a wavelength matching of all VCSELs is achieved at an emission wavelength of 851.2 nm.
In recent years, the need for secure data communication has increased. Here, quantum key distribution (QKD) offers fundamental advantages over classical key distribution. Despite its high level of security, QKD comes a long with numerous challenges regarding the application of single photons. In this view, the decoy state protocol offers the possibility to implement QKD using classical light sources such as semiconductor lasers. In this contribution we present the newest results of our approach for the realization of a monolithic 850 nm vertical-cavity surface-emitting laser (VCSEL) array capable for QKD via the BB84 and decoy state protocol.
Quantum key distribution (QKD) offers the advantage of secure data communication based on the laws of physics. However, the application of single photons and hence true single photon sources comes along with sophisticated operation schemes and bulky device sizes, hindering their large-scale implementation. Offering a small device foot print and low power consumption, VCSELs represent an excellent candidate for a light source for satellite based QKD via the decoy state protocol. In this study we present recent results of an eight VCSEL array at an emission wavelength of 850 nm with integrated subwavelength polarization gratings for polarization control.
We report on a BB84 decoy-state quantum key distribution (QKD) system constructed using commercial off-the-shelf (COTS) components. Four 794 nm vertical-cavity surface-emitting lasers (VCSELs) are current-modulated at 10 MHz rate with three power levels to form a decoy state transmitter. The COTS VCSELs exhibit long term stability with high polarization extinction ratio, narrow band operation (sub-nanometer bandwidth), and wavelength tunability and stability suitable for constructing four indistinguishable qubit channels. A 780 nm, 10 MHz time-transfer channel is used for transferring the timing information along with a start and end marker for the qubit transfer period. Internally-developed transmitter laser drivers and receiver detectors are controlled and read out with COTS system-on-chip (SoC) boards. We obtain a nominal bit-error-rate (BER) of ~4% for the system. We also report on the development of a synchronous (100 MHz) single photon detector for increasing the repetition rate of our QKD system. This work shows promise for building a COTS-based, small size, weight, and power hardware for space applications.
Currently most quantum key distribution (QKD) experiments are focusing on efficient long-distance implementations. Yet the recent development of miniaturized photonic modules and integrated quantum optics circuits could open new perspectives toward secure short-distance communication for daily-life applications. Here, we present the design of a new integrated optics architecture with an effective size of 25 mm × 2 mm × 1 mm. Our objective is to obtain an ultraflat microoptics QKD add-on suitable for integration into handheld platforms such as smartphones. In this context, we evaluated the suitability of various optical subsystems. We tested an array of four vertical cavity surface emitting lasers (VCSEL) with highly similar emission properties capable of producing subnanosecond near-infrared pulses at 100-MHz repetition rate. As short pulses exhibit a low polarization degree, their polarization can be externally controlled by a micropolarizer array. The fabrication of such elements is quite straightforward using standard lithographic techniques and extinction ratios up to 29 dB have been measured. To guarantee spatial indistinguishability of the qubits, we investigate the option of using low-birefringence, single-mode waveguide array manufactured via femtosecond laser micromachining.
A novel integrated optical source capable of emitting faint pulses with different polarization states and with different intensity levels at 100 MHz has been developed. The source relies on a single laser diode followed by four semiconductor optical amplifiers and thin film polarizers, connected through a fiber network. The use of a single laser ensures high level of indistinguishability in time and spectrum of the pulses for the four different polarizations and three different levels of intensity. The applicability of the source is demonstrated in the lab through a free space quantum key distribution experiment which makes use of the decoy state BB84 protocol. We achieved a lower bound secure key rate of the order of 3.64 Mbps and a quantum bit error ratio as low as 1.14×10⁻² while the lower bound secure key rate became 187 bps for an equivalent attenuation of 35 dB. To our knowledge, this is the fastest polarization encoded QKD system which has been reported so far. The performance, reduced size, low power consumption and the fact that the components used can be space qualified make the source particularly suitable for secure satellite communication.
Signal state preparation in quantum key distribution schemes can be realized using either an active or a passive source. Passive sources might be valuable in some scenarios; for instance, in those experimental setups operating at high transmission rates, since no externally driven element is required. Typical passive transmitters involve parametric down-conversion. More recently, it has been shown that phase-randomized coherent pulses also allow passive generation of decoy states and Bennett–Brassard 1984 (BB84) polarization signals, though the combination of both setups in a single passive source is cumbersome. In this paper, we present a complete passive transmitter that prepares decoy-state BB84 signals using coherent light. Our method employs sum-frequency generation together with linear optical components and classical photodetectors. In the asymptotic limit of an infinite long experiment, the resulting secret key rate (per pulse) is comparable to the one delivered by an active decoy-state BB84 setup with an infinite number of decoy settings.