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IEEE International Conference on Quantum Computing and Engineering (QCE)

Decoy-state quantum key distribution with direct modulated commercial off-the-shelf VCSEL lasers

Noel De La Cruz, Uttam Paudel, Pavel I. Ionov, Ethan H. Tucker, Andrew K. Mollner, Joseph D. Touch, Joseph Betser, Joshua Stoermer

Published Oct 1, 2020• 2 citations
Publisher

Abstract

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.

DOI
10.1109/qce49297.2020.00034

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To enable practical real-time quantum key distribution (QKD) over free-space links, such as for satellite-to-ground secure communications, the encoding rate of the system should be sufficiently high to distribute long keys (>106bits) in a short period of time (< 300 seconds) [1]. Here, we report an optical assembly and driving electronics designed to form a low-size}, weight and power (SWaP) QKD transmitter, which can operate at gigahertz clock rates. The QKD transmitter employs multiple vertical cavity surface emission lasers (VCSELs), which provide a low power and high polarization extinction ratio (PER) source of weak coherent pulses of light. These light pulses are combined using an intrinsically stable optical combining module utilising polarization-maintaining fiber. In the experiments presented we employ discrete variable QKD via the decoy-state BB84 protocol to achieve high secure key rates over large channel losses [2]. We characterise the source using a free-space polarization decoder over short free-space links within the laboratory and explore the performance of the system with commercially available single photon detectors. Fig. 1 Plot showing the effect of varying the channel loss over the free-space link. We show the measured quantum bit error rate (blue circles), sifting rate (green triangles) and secure key rate (red squares) alongside the rates simulated using the measured system parameters (solid lines). We characterise the system up to ~38 db of channel loss, revealing positive key rates at relatively high channel loss.
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.
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As quantum key distribution progresses into commercial products and services, there is growing interest in technological solutions that improve performance and/or reduce costs. We present an experimental demonstration of polarization-based decoy-state BB84 quantum key distribution protocol, at a clock frequency of 50 MHz, that relies on fewer resources at the receiver and increases robustness to side-channel attacks for both transmitter and receiver. A single-laser source and cascade of electro-optic modulators provide the four polarization states, while a photonic recombiner with time-multiplexed inputs enables detection with a single single-photon detector.
As quantum key distribution progresses into commercial products and services, there is growing interest in technological solutions that improve performance and/or reduce costs. We present an experimental demonstration of polarization-based decoy-state BB84 quantum key distribution protocol, at a clock frequency of 50 MHz, that relies on fewer resources at the receiver and increases robustness to side-channel attacks for both transmitter and receiver. A single-laser source and cascade of electro-optic modulators provide the four polarization states, while a photonic recombiner with time-multiplexed inputs enables detection with a single single-photon detector.
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.