Deterministic atom placement by ion implantation: Few and single atom devices for quantum computer technology

Ion implanted phosphorus (³¹P) atoms in isotopically pure silicon devices have driven a sequence of discoveries reporting exceptionally long coherence times for the ³¹P nuclear spin quantum bit [1-4] with coherence times longer than 30 s [2].  To make the devices used for this work, a small number of ³¹P atoms are implanted into a nano-scale construction site on a ²⁸Si substrate that is later surrounded by nanocircuitry for programming and read out of the qubit.   Post-implant donor selection by tuning gate electrodes is used to select a single ³¹P donor atom for the experiments.  However the next step requires achieving the key milestone of qubit entanglement in an ordered array of ³¹P dopant atoms placed with nanometer precision.  Deterministic implantation of atoms into such arrays is possible by counting the ion-implantation-induced transient pulse of electron-hole pairs created in the substrate that incorporates suitable detector electrodes.  The detector electrodes produce a gate pulse signalling the implantation of single ions that have a random arrival time.  Achieving the required spatial precision is difficult owing to straggling and channelling effects.  This can be addressed by low energy (<10 keV) implantation of ³¹P ions, but in this regime the signal from the generation of  electron-hole pairs in the substrate is close to the noise threshold of present charge-sensitive electronics.  This method could also be extended for the deterministic placement of colour centres in diamond provided the implanted-ion to colour-centre conversation problem is solved.  This presentation shows how these challenges are being addressed [5] both within CQC2T and in parallel developments in the UK, EU and USA.  Within CQC2T our approach will take this technology to the next stage by building deterministic arrays of single atoms with the goal of 6 nm positioning precision in new architectures that could form the building blocks of a future CMOS quantum computer fabricated with the standard tools of the semiconductor industry.

[1] Bell's inequality violation with spins in silicon, JP Dehollain, et al., Nature Nanotechnology 11 p242 (2016)

[2] Storing quantum information for 30 seconds in a nanoelectronic device, JT Muhonen, et al. Nature Nanotechnology 9, p986 (2014)

[3] High-fidelity readout and control of a nuclear spin qubit in silicon, JJ Pla, et al., Nature 496, p334 (2013)

[4] A single-atom electron spin qubit in silicon, JJ Pla, et al., Nature 489, p541 (2012)

Single-shot readout of an electron spin in silicon, A Morello, et al., Nature 467, p687 (2010)

[5] Single atom devices by ion implantation, JA van Donkelaar, et al., J. Phys. Cond. Mat. 27, 154204 (2015)