I am an experimental physicist with expertise in nanofabrication and interests in research in superconductivity at the nanoscale. I am interested in investigating development of useful devices for quantum technologies. Recent technological advances have made usable technologies based on quantum effects like superposition and entanglement a realistic proposition for development. At present I am particularly interested in investigating quantum phase-slips in superconducting nanowires and investigating the potential to harness this phenomenon in devices for sensing and metrology quantum technologies.

I am currently funded by a 5-year EPSRC (Early Career) Fellowship (running to Sept. 2018) and a PDRA is assisting me in the research. I have previously been an EPSRC Postdoctoral Research Partner at the National Physical Laboratory (NPL) in Teddington, and gained additional funding for this work from UCL Enterprise.

Here is a brief explanation of the physics of quantum phase-slips: Superconductivity was an early-recognised example of a quantum phenomenon observable at macroscopic scales. Superconducting electrons form pairs and occupy a single quantum state characterised by a complex order parameter which has a phase which is the same across the superconductor (or varies in a coherent way, for example when the superconductor is carrying a current). For superconducting wires in the presence of fluctuations, there is a finite probability of an event in which a small volume of the wire fluctuates leading to the superconducting electrons on each side of this volume becoming decoupled briefly from those on the other side. For a single event like this, the change in the superconducting phase variable across the wire will change by 2π (a “phase slip”) and there will a brief voltage pulse across the wire. The probability of such events increases exponentially the smaller the wire cross-section. The effects of phase slips caused by thermal fluctuations were observed and studied in the 1970s, but recent advances in nanofabrication techniques have now enabled experimental studies of *quantum* phase-slips. These *quantum* fluctuations lead to some very interesting properties:

By contrast to the case for thermally activated phase-slip in which the phase slips simply lead to brief voltage pulses, Mooij and Nazarov [1] predicted that devices containing coherent quantum phase-slip (CQPS) junctions show behaviour which is the exact dual of that of Josephson junction devices. One example of the implications of this is that there is a dual to the Shapiro effect in Josephson junctions exposed to microwaves. In the Shapiro effect in Josephson junctions, steps appear in the IV characteristic at voltages proportional to the microwave frequency and this now provides the basis of the international voltage standard. In CQPS junctions in the presence of microwaves, steps will appear in the current through the device, at values proportional to the frequency of applied microwaves. This effect could be harnessed to develop a quantum current standard based on CQPS in superconducting nanowires.

In order to achieve such CQPS devices using superconducting nanowires, the lateral dimensions of the nanowires must be around 20nm (for NbSi or NbN), so that the quantum phase-slip rate is sufficiently large to localise the charge in the wire. (This means that, counterintuitively, the ‘superconducting' wire becomes a perfect electrical insulator for low applied voltages.) We are making use of LCN's Raith 150-TWO electron-beam lithography system to realise these nanowires and are now able to routinely fabricate 20nm-wide nanowires. We then carry out measurements at low temperatures on the wires at UCL using our 300mK He3 system, or one of LCN’s dilution fridges for measurements at lower temperatures. We are investigating devices for operation at both dc and at microwave frequencies.

In my past research, I have set up a number of systems for sensitive electrical measurements with cryogenic systems and developed expertise in focussed ion-beam systems, high-vacuum systems for deposition and etching as well as general cleanroom expertise. I have also carried out simulation work relating to various systems, from a thermal model related to my PhD experimental work (published in J. Appl. Phys.) to a model of thermally activated electrical switching processes in moderately damped Josephson junctions (published in Phys. Rev. B), a model for electrostatically induced bending in free-standing nanowires (published in Appl. Phys. Lett., 2013) and numerical simulations to try to better understand the thermal and electrical properties of QPS devices (published in Phys. Rev. B, 2013).

I have supervised several MSc student research projects and currently lecture on two courses, on advanced/quantum Josephson junctions and on LabVIEW, on the MRes in Delivering Quantum Technologies which is part of the CDT in Delivering Quantum Technologies hosted at UCL.

*Selected Publications*

•*Superconducting NbN nanowires and coherent quantum phase-slips in dc transport**, J.C. Fenton, J. Burnett, ***IEEE Trans. Appl. Supercond. 26 2200505 (2016)**.

•*Embedding NbN nanowires into quantum circuits with a neon focused ion beam**, J. Burnett, J. Sagar, P.A. Warburton, J.C. Fenton, ***IEEE Trans. Appl. Supercond. 26 1700104 (2016)**.

•*Compact chromium oxide thin film resistors for use in nanoscale quantum circuits**, C.R. Nash, J.C. Fenton, N.G.N. Constantino, P.A. Warburton, ***J. Appl. Phys. 116 224501 (2014)**.

•*NbSi nanowire quantum-phase-slip circuits: DC supercurrent blockade, microwave measurements and thermal analysis*, C.H. Webster, J.C. Fenton, T.T. Hongisto, S.P. Giblin, A.B. Zorin and P.A. Warburton, **Physical Review B 87 144510 (2013)**.

•*Ion-beam induced bending of freestanding amorphous nanowires: Importance of substrate material and charging*, A. Cui, J.C. Fenton, W. Li, Z. Liu, Q. Luo, T.H. Shen and C. Gu, **Applied Physics Letters 102 213112 (2013).**

•*Felling of individual freestanding nanoobjects using focused-ion-beam milling for investigations of structural and transport properties*, W. Li, J.C. Fenton, A. Cui, H. Wang, Y. Wang, C. Gu, D.W. McComb and P.A. Warburton, **Nanotechnology 23 105301 (2012)**.

• *Materials for superconducting nanowires for quantum phase-slip devices*, J.C. Fenton, C.H. Webster and P.A. Warburton, **J. Phys.: Conf. Series 286 012024 (2011)**.

• *Dissipative enhancement of the supercurrent in Tl2Ba2CaCu2O8 intrinsic Josephson junctions*, P.A. Warburton, S. Saleem, J.C. Fenton, M. Korsah and C.R.M. Grovenor, **Phys. Rev. Lett. 103 217002 (2009)**.

• *The radio-frequency impedance of individual intrinsic Josephson junctions*, J. Leiner, S. Saleem, J.C. Fenton, S. Speller, C.R.M. Grovenor and P.A. Warburton, **Appl. Phys. Lett. 95 252505 (2009)**.

• *Skewness variations of switching-current distributions in moderately damped Josephson junctions due to thermally induced multiple escape and retrapping*, J.C. Fenton and P.A. Warburton, **J. Phys.: Conf. Ser. 150 052052 (2009)**.

• *Monte Carlo simulations of thermal fluctuations in moderately damped Josephson junctions: Multiple escape and retrapping, switching- and return-current distributions and hysteresis*, J.C. Fenton and P.A. Warburton, **Phys. Rev. B 78 054526 (2008)**.

• *Switchable phase diffusion in intrinsic Josephson junction arrays*, J.C. Fenton, M. Korsah, C.R.M. Grovenor and P.A. Warburton, **Physica C 460 1407 (2007)**.

• *Josephson current suppression in three-dimensional focused-ion-beam fabricated sub-micron intrinsic junctions*, P.A. Warburton, J.C. Fenton, M. Korsah, C.R.M. Grovenor, **Superconductor Science & Technology 19 (5): S187-S190 (2006)**.

• *Heating in mesa structures*, J.C. Fenton and C.E. Gough, **J. Appl. Phys. 94 4665 (2003)**.

• *System for fast time-resolved measurements of c-axis quasiparticle conductivity in intrinsic Josephson junctions of 2212-BSCCO*, J.C. Fenton, P.J. Thomas, G. Yang and C.E. Gough, **Appl. Phys. Lett. 80 2535 (2002)**.

• *Time dependence of current-voltage measurements of c-axis quasiparticle conductivity in 2212-BSCCO mesa structures*, J.C. Fenton, G. Yang and C.E. Gough, **Physica C 388 341 (2003)**.

*References*

[1] J. E. Mooij, Yu.V. Nazarov, Nat. Phys. 2 169 (2006).