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Superconducting detector module for ALPS including two double-stage SQUIDs and two Transition-Edge Sensor chips located within two white ceramic fiber ferrules.

The ALPS II detector must be able to measure single photons with very low energies (1.165eV ≙ 1064nm near-infrared wavelength), while they hit the sensor only at an expected rate of 1 photon per day (≙ 10-5 photons per second). This presents us with enormous challenges to get a low noise setup and extremely low background as well as high detection efficiency (> 80%) and to keep the system stable over long measurements times.

The ALPS collaboration has decided to test two detector systems and integrate them one after the other into ALPS II: The implementation of the heterodyne detection will start end of 2020. We will replace the detection system with a Transition-Edge Sensor (TES) end of 2021.

Heterodyne Detection

Heterodyne detection schemes are used in a wide variety of experiments all over the world in order to perform measurements of oscillating signals with a high precision.  This technique relies on creating an interference `beat-note’ between a known reference oscillator and the signal being measured, as is shown in Figure 1. The reference must be coherent in phase with the signal being measured over the entire duration of the measurement, in order for the system work properly. 

One of the detectors implemented in ALPS II will use heterodyne detection to measure the photons that are reconverted from the axion-like particles. This is done by injecting a laser, known as the Local Oscillator (LO), to the regeneration cavity. The axion field will produce an electromagnetic field in the RC with the same frequency as the field circulating in the PC. Inside the RC this field will optically mix with the LO leading to power modulation (the interference beat-note) at the frequency difference between the two fields. The power of the reconverted field can then be measured by demodulating the electrical signal from the photodetector that observes the interference beat-note.

By using a power of the LO that is high enough to ensure that the measurement will not be limited by technical noise, the system will achieve a signal to noise ratio equal to the square root of the number of photons measured in the regenerated field. Therefore, after a one million second measurement, we will be able to very accurately measure photon rates on the order of 10^-5 photons per second.

We cannot directly observe the frequency of the lasers in the experiment with conventional photodetectors since these frequencies are on the order of hundreds of THz. Optically mixing two signals at the frequencies nu and nu + f will create a sinusoidal signal in the power measure by a photodetector at the difference frequency of f. By measuring the signal at f we can observe how the relative phase changes between the optical fields. If that phase is very stable we can even uncover very tiny signals normally hidden in the noise of the lasers.

Transition-Edge Sensor

Transition-Edge Sensors (TES) are superconductive micro calorimeters measuring the temperature difference dT induced by the absorption of a photon. They exploit the rapid change of the resistance, the superconducting phase transition.

A tungsten Transition-Edge Sensor is currently under test, which is optimized for photons with a wavelength of 1064nm and developed by the National Institute of Standards and Technology (NIST, USA). It achieves large quantum efficiencies of more than 95%, good time and energy resolution and very small intrinsic dark count rates below 10-4 photons per second.

The positioning of ALPS sensors within their superconductive transition is induced through a thermal link to a bath at about 80 mK and through a constant bias voltage. Thus, the TES chip needs to be cooled, for which we use a cryogenic chamber. The readout of the TES is done using a Superconducting Quantum Interference Device (SQUID, electronics by Magnicon) whose integration on a module is developed by the Physikalisch-Technische Bundesanstalt (PTB, Germany). The signal is fed to the TES chips using single-mode optical fibers.

Left: Sketch of the experimental set-up in the laboratory in HERA West including laser preparation, cryostat including cold filter bench @40K and the sensitive TES module with SQUID electronic @30mK stage. Readout electronic, data acquisition (DAQ) and computer with fitting is done at room temperature.
Right: Photograph of the inside of the cryostat, showing the temperature stages and the TES module at the bottom.

Current activities to further improve the sensitivity of the detector are the optimization of the data analysis , the characterization of the detection efficiency and the reduction of noise.

With further experimental upgrades, a single photon source including calibrated reference measurement will soon be established to perform accurate detection efficiency measurements and optical filters will be tested at 40K to study black-body pileup events occurring at our sensor. For a better magnetic shielding and thermal conductivity we also design a new shielding can for the sensor.