Spectrometers are used in a wide range of applications to acquire light spectra, the detectors are limiting elements as it is only possible to detect light efficiently with silicon detectors up to a wavelength of 1.100 nm. Beyond this range, there are no high-performance detectors for the infrared. In addition, single photon detection with high time resolution remains a challenge for any wavelength. 

The superconducting detectors that have been developed in our group at KTH offer the unique combination of high detection efficiency over the visible and near-infrared as well as very precise timing of every single photon detection event. Yet another advantage of our detectors is their very low noise level. These detectors are also very robust compared to the current state of the art commercially available detectors, this is particularly important for application such as Lidar where intense stray light may enter the system. While superconducting detectors are currently only available as single pixel detectors, our work at KTH has demonstrated the possibility to develop arrays of these detectors. 

We therefore aim in this project at combining a one dimensional array of superconducting single photon detectors composed of 32 pixels to acquire the exact detection time of every single photon making up a spectrum. 

We use a grating based spectrometer equipped with 3 gratings to allow the user to access different wavelength ranges and different resolution. We couple the output of the grating to an array of single photon detectors mounted in a custom cryostat with an application window. We then generate a list of arrival times for each pixel that can be analyzed to extract spectra, time resolved data and photon statistics.

By combining spectra and arrival times, our system will generate large amounts of raw data that will require post processing to extract the relevant data. We therefore include in our project an important development effort for the software to make it user friendly.

We identify several use cases where our instrument provides important advantages over existing systems: in Lidar we enable operation at the single photon level at several simultaneous frequencies, several wavelengths can be monitored simultaneously to measure CO2 concentrations as well as other gases. In quantum communication, we enable parallel communication channels to be used to boost the bandwidth available in an optical fiber. In microscopy, we enable the acquisition of spectra to simultaneously track multiple fluorescent markers over an unprecedented spectral range.

Silicon detectors offer high detection efficiency in the visible but poor time resolution. In the infrared, InGaAs detectors offer limited efficiency with high noise levels and equally poor time resolution.

Streak cameras are routinely used for time resolved measurements offer good time resolution but very low detection efficiencies and very high noise levels and can only operate in a limited spectral range.

An important property of light is its statistics: i.e. the correlations between different photons which is particularly important for quantum applications. To measure light statistics, single pixel detectors are used in a separate configurations from a spectrometer.

No instrument is currently able to perform all these measurements, we therefore propose to build a quantum spectrometer that will efficiently perform spectroscopy, time resolved as well as photon statistics measurements in one acqusition.

The quantum spectrometer we propose will have a wide range of applications. We originally came up with the concept while developing optical instruments operating at the single photon level such as the quantum microscope and Lidar where light pulses are used to build 3D images. Our quantum spectrometer will also be very useful as it will precisely time every single photon at different wavelengths. In quantum communication, our system allows for quantum wavelength division multiplexing. In spectroscopy we offer unprecedented detection efficiency in the infrared along with the possibility to extract time resolved data and light statistics.