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Vertical-Cavity Surface-Emitting Lasers

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

Michael Zimmer, Moritz Birkhold, Michael Jetter, Peter Michler

Published Mar 12, 2024• 1 citation
Publisher

Abstract

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.

DOI
10.1117/12.3000679

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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 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.
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.
Quantum key distribution (QKD) theoretically offers information-theoretic security. The prevailing approach is the prepare-and-measure BB84 protocol, which implements QKD using conventional laser rather than single-photon source via the decoy-state method. However, side-channel attacks targeting sources severely threaten system security. Despite extensive efforts, including fully passive scheme, this vulnerability persists even with perfect single-photon source. Here, we propose a source-independent (SI) QKD protocol that resolves all known and unknown source-side attacks without pre-sending entanglement source. Aligning with advances in quantum light sources, our protocol simultaneously doubles the transmission distance while remaining robustness against imperfection of source. Theoretical analysis shows that non-classical light source provides practical security advantages unattainable with conventional laser.
The original proposal of quantum key distribution (QKD) was based on ideal single-photon sources, which, 40 years later, are still challenging to develop. Therefore, the development of decoy-state protocols using weak coherent states (WCS) from lasers set the frontier in terms of secure-key rates and distances. Here, we propose and experimentally demonstrate two simple-to-implement QKD protocols that allow practical far from ideal sub-Poissonian photon sources to outperform state-of-the-art WCS. By engineering the photon statistics of a biexciton-exciton cascade in room-temperature single-photon sources based on giant colloidal quantum dots coupled to nanoantennas, we show that either a truncated decoy-state protocol or a heralded-purification protocol can be employed to achieve a significantly increased performance in terms of the maximal allowed channel loss for secure-key creation, which can exceed even that of ideal WCS by more than 3 dB. We then experimentally emulate a BB84 QKD using such a quantum dot source, verifying the superiority of our protocols over the best possible BB84 WCS performance. These protocols can be utilized efficiently on a host of various quantum emitters having controllable photon statistics with a finite photon-number basis, offering a practical approach to QKD without the hindering requirements on the single-photon purity of the photon source.
To date, only active-decoy-state methods have been adopted in current measurement-device-independent (MDI) quantum key distribution (QKD) protocols, causing security loopholes when modulating multiple light intensities with practical devices. To solve this problem, the passive-decoy-state method will be a good choice. Until today practical passive-decoy-state MDI QKD is still missing. In this paper, based on a recently discussed structure of heralded single-photon sources, we propose a passive-decoy-state MDI QKD protocol. In present work, one does not need to modulate light sources into different intensities, but utilizes the built-in local detection events to passively generate different decoy states. Furthermore, through carrying out an appropriate mathematical method, we are able to give very precise estimations for single-photon-pair contributions. As a result, our protocol can exhibit distinct advantages compared with state-of-the-art MDI QKD schemes even when the finite-key-size effect is taken into account. Therefore, our protocol seems a promising candidate to enhance both the security and performance of MDI QKD in practical implementation.