Integrated superconducting nanobridge fast readout electronics for single photon detector arrays

The project will develop a compact, cryogenic readout for superconducting single photon detector SSPD arrays and assess
their suitability for commercialisation. The readout will be based on a superconducting single flux quantum (SFQ) circuit
where an input signal is converted into a series of quantised pulses that can be used for digital signal processing. The
building block of the SFQ circuits will be loops containing superconducting nanobridges that act as weak links displaying
the Josephson effect.
SSPD arrays have many applications; as a fast, high efficiency, low noise detector for quantum key distribution, as a
component in quantum computing and for enhanced quantum imaging. The major hurdle to commercialisation of SSPD
array systems is the lack of suitable readout electronics able to process a large number of signals without significant heat
loading of the detector cold stage. SFQ readout offers a low power solution, enabling multiple pixels in a marketable,
mechanically cooled system. Nanobridge SFQ circuits for SSPD array readout are a new, undemonstrated alternative to
conventional tunnel junction SFQ circuits, with the advantage of predominantly single layer fabrication, smaller device size
and potential for integration on the same chip as the SSPD array.
The research that will be carried out University College London will involve the actual fabrication of the nanobridge SFQ
circuits. The nanobridge circuits will be fabricated using e-beam lithography and lift-off of superconducting thin films
deposited in a cleanroom facilty. Initial work will be be needed to determine the electrical parameters of nanobridges of
various dimensions so that a full circuit model can be developed and tested. A complete SFQ device design can then be
fabricated and tested and assessed by the project partners at NPL. In order to couple the fast pulse from an SSPD to the
SFQ an input coil circuit will be needed to amplify the current. We will design and fabricate suitable multiturn input coils with
appropriate terminating resistors. These will be tested by the project partners at NPL. We will then fabricate fully integrated
devices (input coil and SFQ devices) which will be tested and evaluated at NPL, initially with external electrical input
pulses, but eventually with real SSPD signals from devices supplied by project partners at Glasgow University.

The work will benefit:
1) Academic researchers in many UK universities and elsewhere working in the fields of Quantum Communications and
Information, as well as those developing applications in Quantum Imaging.
2) Commerical organisations developing Quantum Communication applications including Toshiba and BT in the UK.
3) Government and commerical organisations interested in developing securer quantum based communications.
4) Academic and industrial researchers interested in nanoscale fabrication of superconducting circuits and other high
speed applications of superconducting circuits for e.g. digital signal processing.
5) Companies who supply equipment needed to fabricate, operate and support future quantum devices - lithopgraphy,
optical components and cryogenic suppliers.
6) In the longer term the public through potentially securer (financial or personal) communications, and more sensitive
medical imaging technology amongst other applications.