Instrumentation & Measurement Magazine 24-5 - 70

being caught, while reaching almost 100% detection efficiency
would cancel that possibility. The other key parameter for
QKD experiments is the low dark count rate that permits the
transmission of a crypted message without covering the signal
with high noise, also after hundred kilometres of attenuation
in optical fiber [10]. This point becomes crucial at telecommunication
wavelength, where conventional detectors exhibit
a low signal to noise ratio due to the combined effect of high
dark current and low detection efficiency.
As mentioned, another field in which SNSPDs could open
Fig. 1. SEM image of a MoSi/Al SNSPD, 150 nm wide, 9 nm thick and 100 μm
long. Al layer thickness is 2 nm.
folded in a meander shape to cover a square or a circular surface
to increase the sensitive area. An example of a 5 μm × 5 μm
SNSPD is shown in Fig. 1.
In the operating condition, the SNSPD is cooled below
the critical temperature and a direct bias current (lower than
the critical current) is applied to the device. According to the
detection mechanism presented in [1], an incident photon generates
a hot spot that is a resistive region that locally breaks the
superconductivity. The presence of a hot spot forces the bias
current to flow at the edges of the resistive region, and if the
device is sufficiently thin and narrow, the current density into
the edges overcomes the critical value. The result is a resistive
belt across the nanostrip which produces a finite voltage pulse.
The current is then diverted outside the superconductor by
the electric circuit, the zero-resistance state is recovered, and
the finite voltage decreases in time exponentially. Once superconductivity
is established again, the device is ready to detect
another photon.
Applications
Due to their high-level specifications, especially to the high
detection efficiency in the infrared wavelength domain and
the low dark count rate, SNSPDs appear very suitable for
many applications, and they can also be considered a substitute
for InGaAs single photon detectors at telecommunication
wavelength.
Important SNSPD applications involve all of the fields
where the signal to be measured is expected to be very low,
such as dark matter detection [5] and space to ground communication
[6], or where high precision is required, such as for
Bell's inequalities validation, boson sampling and quantum
computing [7], [8]. Moreover, the use of SNSPDs can make a
significative change in the fields of quantum information and
remote sensing.
Indeed, it was demonstrated that SNSPDs can strongly
enhance the results of quantum key distribution (QKD). The
first positive contribution is given by the detectors' high efficiency;
indeed, the authors in [9] proved that losses due to the
detector permit the eavesdropper to violate the key without
70
the way to innovative measurements is atmosphere monitoring
with Light Detection and Ranging (Lidar) technique.
Indeed, as claimed in [11], the competitive signal to noise ratio
provided by SNSPD in the infrared wavelength domain would
result in three interesting advantages. The first one is the enhancement
of the current measurements of aerosols at 1064
nm, specially at larger distance where the signal is low and its
detection is disturbed by detector's noise. Secondly, it would
be possible to study the aerosols also at larger wavelengths
(i.e., 1550 nm) with the combined advantages of a reduction
of the molecular contribution to the signal due to the nitrogen
in the atmosphere and the use of eye-safe source which could
hence be used also in highly populated areas.
The last novelty introduced by the combined Lidar and
SNSPD technologies in the infrared domain would be the observation
of carbon dioxide, a molecule which requires control
in related environmental issues and can be observed just with
a Lidar operating at λ > 2 μm, where the other detectors are almost
blind.
Molybdenum Silicide/Aluminium
SNSPD
Many materials have been tested to optimize SNSPDs performances.
Indeed, depending on the superconducting material,
it is possible to change many parameters which affect detectors'
likelihood, such as the energy gap, the thermalization
time [12], [13], the coherence length, the critical temperature
[14] and current, and the normal state resistivity. As shown
in (1), some of the listed parameters determine the maximum
detectable wavelength, but also other characteristics of the detector
such as the maximum counting rate, the dark count rate
and the efficiency might depend on the material.
Niobium-based superconductors, especially NbN and
NbTiN, are the most used materials for SNSPD fabrication because
they exhibit relatively high critical temperature and do
not require expensive refrigerators. Moreover, with these materials,
it is possible to obtain the best performances in terms
of efficiency (which was pushed to 95% at 2.2 K and to 98% at
lower temperatures) with low dark counts rate [2], [15].
Despite that, the use of amorphous materials such as Molybdenum
Silicide (MoSi) is also investigated [16] as these
superconductors exhibit even lower energy gaps, which make
them preferable for far infrared applications. The main disadvantage
in the use of this material lays in the operating
temperature which must be lowered below 1 K to obtain competitive
performances. Another undesired feature of MoSi is
IEEE Instrumentation & Measurement Magazine
August 2021
Instrumentation & Measurement Magazine 24-5

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