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We demonstrate two-photon interference and polarization entanglement at 2090 nm, constituting a crucial leap towards free-space mid-infrared quantum communication systems in a spectral region with high atmospheric transparency and reduced solar background.

Superconducting Nanowire Single photon Detectors (SNSPDs) offer unparalleled performance for IR photon counting, combining close to unity quantum efficiency, low intrinsic noise and ultrafast timing jitter. The ability of SNSPDs to count photons in mid-IR band up to 7 um wavelength opens up new possibilities in quantum optics, laser ranging, free space Quantum Key Distribution (QKD) and astronomy. Here we report on development of mid-IR SNSPDs including device design, fabrication, optimisation of superconducting materials and characterisation. We present a characterisation setup covering 1.5 - 4.2 um spectral region based on tuneable optical parametric oscillator with picosecond long pulses. We then demonstrate the viability of mid infrared SNSPDs for a variety of applications and report the results from single photon light detection and ranging (LIDAR) experiment with 2.3 um photons. This work paves the way for future app in free space QKD, deep space communication and astronomy.

In this work we design, fabricate and characterize superconducting nanowire single photon detectors (SNSPDs) optimized for mid infrared operation. The mid infrared is of interest for free space applications due to lower solar background than at shorter wavelengths as well as low atmospheric absorption. We show a proof-of-principle LIDAR imaging experiment at 2.3µm showing the viability of using SNSPDs for a variety of applications in the mid infrared.

Our results show a viable route to free-space quantum-secured communication in an unexplored atmospheric transparency window., Quantum-enhanced optical systems operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. However, to date, sources of entangled photons have been realized mainly in the near-infrared 700- to 1550-nm spectral window. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.

A superconducting nanowire single photon detector system designed for 2.3µm wavelength deployed into a single photon light detection and ranging setup. This wavelength takes advantage of lower solar flux and less atmospheric absorption.

Singlet oxygen luminescence dosimetry (SOLD) is a highly promising direct monitoring method for photodynamic therapy (PDT) in the treatment of cancer. Early SOLD systems have been hampered by inefficient excitation, poor optical collection and immature infrared single photon detection technology. We report carefully engineered improvements addressing all of these deficiencies. We use a supercontinuum source with a tunable filter to precisely target the peak absorption wavelength of the chosen photosensitizer; we have designed a compact and versatile optical package for precise alignment; we have successfully employed state-of-the-art superconducting photon counting technologies. Through these improvements, we can achieve real-time histogram acquisition from a photosensitizer in solution test sample. This setup opens the pathway to physiological SOLD studies for PDT dosimetry.

In this work, we show a proof-of-principle benchtop single-photon light detection and ranging (LIDAR) depth imager at 2.3µm, utilizing superconducting nanowire single-photon detectors (SNSPDs). We fabricate and fiber-couple SNSPDs to exhibit enhanced photon counting performance in the mid-infrared. We present characterization results using an optical parametric oscillator source and deploy these detectors in a scanning LIDAR setup at 2.3µm wavelength. This demonstrates the viability of these detectors for future free-space photon counting applications in the mid-infrared where atmospheric absorption and background solar flux are low.

Superconducting nanowire single photon detectors (SNSPD) offer excellent performance for infrared single photon detection, combining high efficiency, low timing jitter, low dark count rates and high photon counting rates. Promising application areas for SNSPDs include quantum key distribution, space-to-ground communications and single photon remote sensing [1]. SNSPDs are typically made with ultrathin niobium nitride (NbN) films with thickness 4 nm and a superconducting transition temperature above 9 K. NbN offers high performance in the near infrared but their sensitivity drops at wavelengths beyond 2 um. There is growing interest in potential photon counting applications in the mid infrared domain (for example remote sensing of greenhouse gases in the atmosphere [2]). One way to overcome the wavelength limit in NbN SNSPDs is to use films with a lower superconducting energy gap [3]. Here we report on the study of SNSPDs fabricated with thin films of titanium nitride (TiN). We compare TiN films deposited by atomic layer deposition (ALD) and by magnetron sputtering. The TiN films range in thickness from 5 to 60 nm, with superconducting transition temperatures from ~1 K to 3.5 K. We have analyzed the films via transmission electron microscopy and variable angle spectroscopic ellipsometry. We characterize TiN SNSPDs performance from near to mid-infrared at wavelengths (1-4 um) with fast optical parametric oscillator (OPO) source. We compare the performance of TiN SNSPDs to devices based on other lower gap materials: MoSi, NbTiN, WSi. [1] Natarajan et al Superconductor Science and Technology 25 063001 (2012) [2] Abshire et al Laser Applications to Chemical, Security and Environmental Analysis, (Optical Society of America, 2008) paper LMA4 [3] Verma et al Applied Physics Letters 105 022602 (2014)

Superconducting single-photon detectors (SNSPDs or SSPDs) based on superconducting nanowires offer single photon sensitivity at telecom wavelengths coupled with free running operation, high timing resolution and low dark counts. Amorphous superconducting materials open the pathway to improved yield and integration in advanced photonic applications. We report simulation, fabrication and experimental validation of waveguide integrated molybdenum silicide (MoSi) SNSPDs for scalable integration in quantum photonic circuits.

Superconducting nanowire single-photon detectors (SNSPDs) play an important role in emerging optical quantum technologies. We report on advanced nanometric characterization of a high efficiency near infrared SNSPD design based on a low roughness Tantalum pentoxide (Ta2O5)/ silicon dioxide (SiO2) distributed Bragg reflector (DBR) cavity structure. We have performed high resolution transmission electron microscopy (TEM) analysis to verify the smoothness of the DBR. Optical reflectance measurements show excellent correspondence with DBR simulations. We have carried out precision nano-optical photoresponse mapping studies at 940 nm wavelength at T = 3.5 K, indicating excellent large area device uniformity (peak efficiency 55 % at 100 Hz dark count rate [DCR]) with a full width half maximum (FWHM) timing jitter of 60 ps. With manual fibre coupling with single mode fibre, we achieve a system detection efficiency (SDE) of 57.5% at 940 nm wavelength (100 Hz DCR) at T = 2.3 K and a low polarization dependence of 1.20 ± 0.03. For coupling with multimode fibre, we achieve SDE of 90% at 940 nm (200 Hz DCR) at T= 2.3 K. These SNSPD devices are promising candidates for use in quantum dot photoluminescence studies and optical quantum technology applications.

We demonstrate two-photon interference and polarization entanglement at 2090 nm, constituting a crucial leap towards free-space mid-infrared quantum communication systems in a spectral region with high atmospheric transparency and reduced solar background.