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Quantum communications system with integrated photonic devices

J. E. Nordholt

Published Jan 23, 2023• 0 citations
OpenAlex

Abstract

Security is increased in quantum communication (QC) systems lacking a true single-photon laser source by encoding a transmitted optical signal with two or more decoy-states. A variable attenuator or amplitude modulator randomly imposes average photon values onto the optical signal based on data input and the predetermined decoy-states. By measuring and comparing photon distributions for a received QC signal, a single-photon transmittance is estimated. Fiber birefringence is compensated by applying polarization modulation. A transmitter can be configured to transmit in conjugate polarization bases whose states of polarization (SOPs) can be represented as equidistant points on a great circle on the Poincare sphere so that the received SOPs are mapped to equidistant points on a great circle and routed to corresponding detectors. Transmitters are implemented in quantum communication cards and can be assembled from micro-optical components, or transmitter components can be fabricated as part of a monolithic or hybrid chip-scale circuit.

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Quantum Computing, Communication, and Simulation•Mar 8, 2023•0 citations
Security is increased in quantum communication (QC) systems lacking a true single-photon laser source by encoding a transmitted optical signal with two or more decoy-states. A variable attenuator or amplitude modulator randomly imposes average photon values onto the optical signal based on data input and the predetermined decoy-states. By measuring and comparing photon distributions for a received QC signal, a single-photon transmittance is estimated. Fiber birefringence is compensated by applying polarization modulation. A transmitter can be configured to transmit in conjugate polarization bases whose states of polarization (SOPs) can be represented as equidistant points on a great circle on the Poincare sphere so that the received SOPs are mapped to equidistant points on a great circle and routed to corresponding detectors. Transmitters are implemented in quantum communication cards and can be assembled from micro-optical components, or transmitter components can be fabricated as part of a monolithic or hybrid chip-scale circuit.
Birefringence in optical fibers is compensated by applying polarization modulation at a receiver. Polarization modulation is applied so that a transmitted optical signal has states of polarization (SOPs) that are equally spaced on the Poincare sphere. Fiber birefringence encountered in propagation between a transmitter and a receiver rotates the great circle on the Poincare sphere that represents the polarization bases used for modulation. By adjusting received polarizations, polarization components of the received optical signal can be directed to corresponding detectors for decoding, regardless of the magnitude and orientation of the fiber birefringence. A transmitter can be configured to transmit in conjugate polarization bases whose SOPs can be represented as equidistant points on a great circle so that the received SOPs are mapped to equidistant points on a great circle and routed to corresponding detectors.
Due to the fast-paced advancements of the quantum computing field, current public key cryptographic protocols need to be rethought. One promising solution is Quantum Key Distribution (QKD), whose security is robust to any computational advancement. The security of QKD relies on the laws of quantum mechanics, ensuring that any eavesdropper attempting to access the data will be detected. In particular, polarization-encoding QKD systems allow to generate secret keys in two remote locations to encode/decode information, using the State of Polarization (SOP) of single photons. Although its security has been theoretically proven, its practical implementation poses some challenges, part of which are the main focus of this PhD thesis. One relevant process in QKD systems is the synchronization between the transmitter and receiver, namely, the frame synchronization. This way, in this thesis, a frame synchronization method is developed and implemented, showing its potential for the use in prepare-and-measure QKD systems. The developed method has been shown to be effective, even with a low number of photons reaching the receiver. Furthermore, its architecture allows it to be adjusted for different attenuation regimes of the QKD system. The ability to encode data on the physical properties of single photons, in this case, polarization, highly depends on the capacity to accurately generate the required polarization states on demand. This way, an automatic SOP generation method, comprising an optical polarization monitoring system and a dedicated algorithm was developed and implemented, capable of generating four SOPs. This method reached an average Quantum Bit Error Rate (QBER) of 0.75%, which is well below the security threshold. After the SOP generation, and their transmission through the quantum channel, polarization alignment algorithms are required to compensate for the random rotations induced by birefringence in the optical fiber. In this thesis, a non-iterative polarization compensation method was implemented. This method requires only three QBER estimations to re-align the polarization, reaching average QBERs of 1.8%. Additionally, multiple iterative methods were implemented for comparison purposes, with the results clearly showing a better efficiency of the deterministic method. Beyond fiber-based transmissions, free-space transmissions were analysed due to their ability to reach sites without installed fiber and due to its potential for satellite-based quantum communications. This thesis demonstrates the feasibility of polarization-encoding QKD over free-space channels, by subjecting quantum signals to atmospheric turbulence to study its impact. In that sense, polarization variations over a 1.8 km outdoor free-space channel were monitored, to understand the effect of atmospheric turbulence on polarization. These results have shown that SOP variations in free-space links are slow, allowing their compensation. Furthermore, it was identified that, under conditions tested, the Secret Key Rate (SKR) could reach values up to 8.7×10ˉ⁴ bits per second, allowing QKD over free space for distances up to 2.31 km.
We present here results of the Quantum Technology Flagship project UNIQORN in the area of integrated photonics for quantum communication applications. Three distinct integration platforms, namely indium phosphide based monolithic integration, polymer-based hybrid integration and the CMOS-compatible silicon platform, have been employed to manufacture components and sub-systems on chip for quantum communication devices. The choice of different platforms was made to exploit the best characteristics of each platform for the intended quantum communication device. The indium phosphide platform was employed to manufacture a transmitter chip for quantum key distribution featuring laser, modulators, and attenuators. The transmitter chip was evaluated in a QKD experiment achieving a secure rate of 1 kbit/s. The polymer platform was investigated for engineering non-classical light sources. Entangled and heralded single-photon sources, based on non-linear optics, were assembled on the polymer in a hybrid fashion together with waveguides and other passive micro-optical elements. A quantum random number generator, featuring a 70% randomness extraction efficiency, was also fabricated using the polymer integration technique. An array of 32 individual single-photon avalanche diodes, operating at room temperature and featuring an onboard coincidence logic, was coupled to the chip to demonstrate direct detection of photons on the polymer. Finally, a transimpedance amplifier based on gallium arsenide high electron mobility transistors was produced with an exceptional large electrical noise clearance of 28 dB at 100 MHz.
We report the development of an optical assembly and driving electronics for a low-SWaP polarization encoder design for use in satellite-to-ground quantum communication. The optical design multiplexes multiple lasers, which are selectively excited to produce a polarization encoded output. This implementation is intrinsically stable due to the use of only polarization maintaining fiber in the combining optics. The transmitter, provides a low-cost, low-power and high-speed platform to produce polarization encoded pulses. We use the transmitter to generate 4 polarization states with 2 intensity levels via multiplexing of 8 pulsed light sources. The module can generate the polarization states H, V, D and A, which correspond to polarization angles of 0, 90, 45 and -45 degrees respectively, forming two mutually unbiased bases. The transmitter is characterized via a polarization decoder over a free-space link within a laboratory setting. We characterize the source for varying optical channel loss which is introduced between the transmitter and receiver. The transmitter employs the T12 decoy-state BB84 protocol. We explore the performance of the system with commercially available single photon detectors for two clock rates of 500 MHz and 1 GHz. We find a similar secure key rate for both repetition rates, despite the expected 3 dB gain at 1 GHz. This is a result of detector jitter hindering the performance of the QKD system, resulting in a larger QBER when detection events leak into the adjacent time bins and ultimately reduces the secure key rate.