Quantum testing can compare unknown photon states in a manner suitable for a validation check of a quantum digital signature, testing of the operation of a quantum gate, or detecting of entanglement. In this method the metal tube is plastically deformed and shortened by a predetermined amount greater than the predetermined EFL and is plastically deformed after shortening to provide a controlled elongation thereof so as to reach the predetermined excess fiber length. The steel has a minimum yield strength of at least 345 MPa and even over 621 MPa adding titanium as the lone microalloy element for strengthening, with elongation of 15% and more. Furthermore, the independent feet are provided with check retainer which serves to engage in the sockets and the retaining holes on the transmitting and receiving head. The contact layer is integral with and harder than the elastomeric lip, therefore minimizing frictional losses.