Precision synchronization is vital for robust long-distance quantum networking over fiber and free-space channels for which high-fidelity entanglement swapping between separate sources via an optical Bell state measurement requires temporal overlap of photonic qubits arriving from either source. This challenge is particularly distinct in satellite-based entanglement distribution in which relative motion, channel effects, and propagation delay must be addressed. This work presents a precision synchronization method for free space entanglement distribution, and reports on risk reduction testing in a quantum networking testbed at MIT Lincoln Laboratory. Primary consideration is for a dual-uplink architecture in which photons from entanglement sources at two ground locations interact in an optical Bell-state measurement implemented on a satellite in a low-earth orbit. The control approach uses independent entanglement sources at each ground location supplemented with a synchronization signal for feedback control from a timing discriminant measured at the spacecraft. The approach is being implemented in a laboratory testbed using 1-GHz repetition rate 1550-nm band entanglement sources generating ~10-MHz source entanglement rates with few-ps photon pulse lengths. The paper describes both fundamental architectural considerations and practical implementation details.
The Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time duplex laser communications between a lunar-orbiting satellite and ground stations on Earth with error-free downlink data rates up to 622 Mb/s utilizing an optical receiver based on photon-counting superconducting nanowires and operating near 1550 nm.
We report on the development and use of a high heralding-efficiency, single-mode-fiber coupled telecom-band source of entangled photons for quantum technology applications. The source development efforts consisted of theoretical and experimental efforts and we demonstrated a correlated-mode coupling efficiency of 97% ± 2%, the highest efficiency yet achieved for this type of system. We then incorporated these beneficial source development techniques in a Sagnac configured telecom-band entangled photon source that generates photon pairs entangled in both time/energy and polarization degrees of freedom. We made use of these highly desirable entangled states to investigate several promising quantum technologies.
Superconducting circuits comprising SNSPDs placed in parallel—superconducting nanowire avalanche photodetectors, or SNAPs—have previously been demonstrated to improve the output signal-to-noise ratio (SNR) by increasing the critical current. In this work, we employ a 2-SNAP superconducting circuit with narrow (40 nm) niobium nitride (NbN) nanowires to improve the system detection efficiency to near-IR photons while maintaining high SNR. Additionally, while previous 2-SNAP demonstrations have added external choke inductance to stabilize the avalanching photocurrent, we show that the external inductance can be entirely folded into the active area by cascading 2-SNAP devices in series to produce a greatly increased active area. We fabricated series-2-SNAP (s2-SNAP) circuits with a nanowire length of 20 μm with cascades of 2-SNAPs providing the choke inductance necessary for SNAP operation. We observed that (1) the detection efficiency saturated at high bias currents, and (2) the 40 nm 2-SNAP circuit critical current was approximately twice that for a 40 nm non-SNAP configuration.
We describe a number of methods that have been pursued to develop superconducting nanowire single-photon detectors (SNSPDs) with attractive overall performance, including three systems that operate with >70% system detection efficiency and high maximum counting rates at wavelengths near 1550 nm. The advantages and tradeoffs of various approaches to efficient optical coupling, electrical readout, and SNSPD design are described and contrasted. Optical interfaces to the detectors have been based on fiber coupling, either directly to the detector or through the substrate, using both single-mode and multimode fibers with different approaches to alignment. Recent advances in electrical interfaces have focused on the challenges of scalability and ensuring stable detector operation at high count rates. Prospects for further advances in these and other methods are also described, which may enable larger arrays and higher-performance SNSPD systems in the future. Finally, the use of some of these techniques to develop fully packaged SNSPD systems will be described and the performance available from these recently developed systems will be reviewed.
The Lunar Lasercom Ground Terminal (LLGT) is the primary ground terminal for NASA’s Lunar Laser
Communication Demonstration (LLCD), which demonstrated for the first time high-rate duplex laser
communication between Earth and satellite in orbit around the Moon. The LLGT employed a novel
architecture featuring an array of telescopes and employed several novel technologies including a custom PM
multimode fiber and high-performance cryogenic photon-counting detector arrays. An overview of the LLGT
is presented along with selected results from the recently concluded LLCD.
Sampling rates of high-performance electronic analog-to-digital converters (ADC) are fundamentally limited by the timing jitter of the electronic clock. This limit is overcome in photonic ADC's by taking advantage of the ultra-low timing jitter of femtosecond lasers. We have developed designs and strategies for a photonic ADC that is capable of 40 GSa/s at a resolution of 8 bits. This system requires a femtosecond laser with a repetition rate of 2 GHz and timing jitter less than 20 fs. In addition to a femtosecond laser this system calls for the integration of a number of photonic components including: a broadband modulator, optical filter banks, and photodetectors. Using silicon-on-insulator (SOI) as the platform we have fabricated these individual components. The silicon optical modulator is based on a Mach-Zehnder interferometer architecture and achieves a VπL of 2 Vcm. The filter banks comprise 40 second-order microring-resonator filters with a channel spacing of 80 GHz. For the photodetectors we are exploring ion-bombarded silicon waveguide detectors and germanium films epitaxially grown on silicon utilizing a process that minimizes the defect density.
Photonic Analog-to-Digital Conversion (ADC) has a long history. The premise is that the superior noise performance of
femtosecond lasers working at optical frequencies enables us to overcome the bottleneck set by jitter and bandwidth of
electronic systems and components. We discuss and demonstrate strategies and devices that enable the implementation
of photonic ADC systems with emerging electronic-photonic integrated circuits based on silicon photonics. Devices
include 2-GHz repetition rate low noise femtosecond fiber lasers, Si-Modulators with up to 20 GHz modulation speed,
20 channel SiN-filter banks, and Ge-photodetectors. Results towards a 40GSa/sec sampling system with 8bits resolution
are presented.
Silicon Geiger-mode avalanche photodiodes (Si GM-APDs) have desirable properties for an optical
photon-counting receiver, including high single-photon detection efficiency, low reset time, and low
timing jitter; however, they do not detect near-IR photons. In this work, we demonstrated a sensitive
photon-counting receiver in the near-IR by combining a wavelength converter consisting of a
periodically-poled lithium niobate (PPLN) waveguide and a commercial Si GM-APD detector. We
measured a receiver sensitivity from 1.4 to 3.5 incident photons/bit from 5.5 Mb/s to 22 Mb/s for a
single detector, and achieved a sensitivity of 4 photons/bit at 78 Mb/s using an emulated array of 25
detectors.
Advances in femtosecond lasers and laser stabilization have led to the development of sources of ultrafast optical pulse
trains that show jitter on the level of a few femtoseconds over tens of milliseconds and over seconds if referenced to
atomic frequency standards. These low jitter sources can be used to perform opto-electronic analog to digital conversion
that overcomes the bottleneck set by electronic jitter when using purely electronic sampling circuits and techniques.
Electronic Photonic Integrated Circuits (EPICs) may enable in the near future to integrate such an opto-electronic
analog-to-digital converters (ADCs) completely. This presentation will give an overview of integrated optical devices
such as low jitter lasers, electro-optical modulators, Si-based filter banks, and high-speed Si-photodetectors that are
compatible with standard CMOS processing and which are necessary for the implementation of EPIC-chips for advanced
opto-electronic ADCs.
We derive the Schawlow-Townes linewidth of mode-locked lasers, and discuss how the timing jitter affects the linewidth in passively/actively mode-locked laser diodes (MLLD's). The linewidth of longitudinal modes of MLLD's is much broader than the Schawlow-Townes linewidth due to the linewidth enhancement factor of the gain medium, and its characteristics depend on whether the laser is mode-locked passively or actively. For the case of active mode-locking, the detuning of the modulation frequency from the natural cavity frequency broadens the linewidth. In the experiment, we measure the linewidth of 10-GHz external cavity MLLD's and investigate the correlation with the noise characteristics of timing jitter and amplitude noise of the pulse train. The experimental results agree well with theoretical predictions.
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