Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Free space-coupled superconducting nanowire single photon detectors for
  infrared optical communications   This paper describes the construction of a cryostat and an optical system
with a free-space coupling efficiency of 56.5% +/- 3.4% to a superconducting
nanowire single-photon detector (SNSPD) for infrared quantum communication and
spectrum analysis. A 1K pot decreases the base temperature to T = 1.7 K from
the 2.9 K reached by the cold head cooled by a pulse-tube cryocooler. The
minimum spot size coupled to the detector chip was 6.6 +/- 0.11 {\mu}m starting
from a fiber source at wavelength, {\lambda} = 1.55 {\mu}m. We demonstrated
efficient photon counting on a detector with an 8 x 7.3 {\mu}m^2 area. We
measured a dark count rate of 95 +/- 3.35 kcps and a system detection
efficiency of 1.64% +/- 0.13%. We explain the key steps that are required to
further improve the coupling efficiency.

36
The categories of this paper are: cmp-lg
4
The categories of this paper are: cond-mat
24
The categories of this paper are: dg-ga
19
The categories of this paper are: eess
0
The categories of this paper are: hep-ph
5
The categories of this paper are: gr-qc
9
The categories of this paper are: nlin
7
The categories of this paper are: hep-th
8
The categories of this paper are: hep-ex
18
The categories of this paper are: econ
20
The categories of this paper are: acc-phys
30
The categories of this paper are: q-alg
22
The categories of this paper are: chao-dyn
1
The categories of this paper are: cs
34
The categories of this paper are: chem-ph
35
The categories of this paper are: mtrl-th