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Super-Resolving Quantum Imaging and Detection

Periodic Reporting for period 1 - SURQUID (Super-Resolving Quantum Imaging and Detection)

Reporting period: 2020-10-01 to 2021-09-30

The goal of the SURQUID project is to realize a multi-scale quantum sensing system that enables light detection and ranging (Lidar) with super-resolution and super-sensitivity. Current remote sensing solutions underly fundamental limitations in their key performance metrics: the diffraction limit restricts the resolution with which remote objects can be imaged and the shot noise limit defines the best achievable sensitivity. Quantum sensing solutions provide novel concepts to go beyond these classical limits but pose daunting challenges due to their sensitivity to signal loss. The SURQIUO project aims at exploiting advanced quantum detection schemes, based on photonic integrated quantum homodyne detection (QHD), combined with non-classical illumination using entangled coherent states (ECS) to provide a resolving power that is sub-Rayleigh diffraction limited (super-resolution) and a sensitivity that is sub-shot-noise limited (super-sensitivity). The SURQUID system will thus enable position detection and surface profiling with an accuracy that is impossible to achieve with current (i.e. classical) technology, thus benefitting a wide range of remote sensing application in autonomous navigation, tracking of air- and space-borne objects, and 3D imaging.
Specific objectives of the project include the realization of a novel portable source of Entangled Coherent States, a highly integrated quantum homodynedetection system on a semiconductor chip and the demonstration of quantum imaging with super-resolution and super-sensitivity.
Work in the initial phase of the project concentrated on designing the Entangled Coherent State (ECS) source and developing a photonic circuit integrated quantum homodyne detection system.

A design for the source-modules that are required for generating squeezed vacuum and non-Gaussian states of light was developed. The modules are designed for both pulsed and (quasi-) continuous wave (cw) operation. The source will consist of a parametric generation stage that produces entangled photons at telecom wavelengths.In the case of cw operation a cavity-enhancedparametric down conversion process in an optical parametric oscillator (OPO) below threshold will be employed, while pulsed operation uses a single-pass configuration through an optical parametric amplifier (OPA). Key performance indicators of the respective source implementations have been estimated based on the performance and characteristics of similar sources and prior literature. A detailed list of components has been generated, and procurement has been initiated.The OPA and OPO generation stage are in fabrication and will be characterized in the next period of the project.
During the design phase of the source it became apparent that most schemes require photon number resolving capabilities. The consortium managed to realize waveguide-integrated superconducting nanowire single-photon detectors that allow for resolving up to 16 photons and a prototype chip is under construction to be supplied to the source team.

The development of the homodyne detection system focused on the design of a chip layout to be produced in a foundry process, the realization of efficient optical interfaces to such chip, the optimization of superconducting thin film deposition recipes and the improvement of critical performance parameters of superconducting nanowire single photon detectors. A silicon-on-insulator chip was designed that contains prototype devices, such as balanced directional couplers, phase modulators, arrayed waveguide gratings for custom wave-division multiplexing and photon-number resolving detection, to allow for testing their performance under cryogenic conditions. The design was submitted to the imdc foundry service and is currently in production. To allow for efficient coupling of optical signals returning from remote objects into photonic integrated circuits a novel 3D interface between optical fibers and nanophotonic waveguides was designed, fabricated and tested. Employing finite difference simulations, 3D direct laser writing and optical transmission measurements coupling losses below 1 dB could be demonstrated. The optimization of superconducting NbTiN thin films in a dedicated magnetron sputter deposition process is ongoing and forms the basis for the fabrication of nanowire single photon detectors. In preliminary work it was possible to combine waveguide-integrated single-photon detectors with 3D fiber-chip interfaces and demonstrate a system detection efficiency of >70%. Using advanced electronic readout and signal processing it was further possible to achieve a timing accuracy (jitter) of below 4 ps when operating in the multi-photon regime.
The SURQUID project achieved several innovations beyond the state-of-the-art during the initial phase of the project.
- The design of an entangled coherent state source improves over several aspects of related schemes for producing Schrödinger cat and Schrödinger Kitten states.The anticipated implementation as a portable device is completely novel and will provide key functionalities of quantum light for (remote) sensing applications, in lossy environments. Source construction is ongoing throughout the following period.
- Highly efficient fiber-chip interfaces with less than 1 dB insertion loss were realized by exploiting novel 3D manufacturing capabilities. Fabricated devices feature extremely broad bandwidth of several hundred nanometers and are fully cry-compatible. State-of-the-art couplers do not readily achieve such performance or require highly involved nanofabrication routines or large footprints. Future work will focus on fine-tuning design and fabrication parameters to reach ultimate performance. We further expect to benefit from new direct laser writing capabilities to be installed at a partner site in the upcoming project period.
- The electrical readout and signal processing of waveguide integrated superconducting nanowire single photon detectors was improved to yield higher timing accuracy. The resulting sub-4 ps jitter performance approaches best-in-class devices but combines timing accuracy with competitive detection efficiency, which state-of-the-art devices lack. This combination of timing accuracy and detection efficiency directly benefits the depth resolution in ranging applications. In the second project period time-of-flight measurements with a short pulse-width laser source will be used to confirm the performance in a practically relevant use case.
- Combining on-chip electrical circuits with photonic integrated circuits enabled the configuration of photon number resolving detector solutions. While the concept is state-of-the-art, the implementation with nanophotonic waveguides and cryogenic signal processing is novel and resulted in distinguishing up to 16 photons with unprecedented resolution. Future work will focus on theoretical studies on the trade off between signal loss and stochastic sampling of an optical input mode as well as implementing photon subtraction techniques for small photon numbers.
Packaged prototype chip for quantum detection.