Why Shot Noise Does Not Generally Detect Pairing in Mesoscopic Superconducting Tunnel Junctions

The shot noise in tunneling experiments reflects the Poissonian nature of the tunneling process. The shot-noise power is proportional to both the magnitude of the current and the effective charge of the carrier. Shot-noise spectroscopy thus enables us, in principle, to determine the effective charge <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>q</a:mi></a:math> of the charge carriers of that tunnel. This can be used to detect electron pairing in superconductors: In the normal state, the noise corresponds to single electron tunneling (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mi>q</c:mi><c:mo>=</c:mo><c:mn>1</c:mn><c:mi>e</c:mi></c:mrow></c:math>), while in the paired state, the noise corresponds to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mi>q</e:mi><e:mo>=</e:mo><e:mn>2</e:mn><e:mi>e</e:mi></e:mrow></e:math>. Here, we use a newly developed amplifier to reveal that in typical mesoscopic superconducting junctions, the shot noise does not reflect the signatures of pairing and instead stays at a level corresponding to <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:mi>q</g:mi><g:mo>=</g:mo><g:mn>1</g:mn><g:mi>e</g:mi></g:mrow></g:math>. We show that transparency can control the shot noise, and this <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mi>q</i:mi><i:mo>=</i:mo><i:mn>1</i:mn><i:mi>e</i:mi></i:mrow></i:math> is due to the large number of tunneling channels with each having very low transparency. Our results indicate that in typical mesoscopic superconducting junctions, one should expect <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:mi>q</k:mi><k:mo>=</k:mo><k:mn>1</k:mn><k:mi>e</k:mi></k:mrow></k:math> noise and lead to design guidelines for junctions that allow the detection of electron pairing. Published by the American Physical Society 2024