A phosphorus (31P) donor in silicon is, almost literally, the equivalent of a hydrogen atom in vacuum. It possesses electron and nuclear spins 1/2 which act as natural qubits, and the host material can be isotopically purified to be almost perfectly free of other spin species, ensuring extraordinary coherence times.
I will present the current state-of-the-art in silicon quantum information technologies. Both the electron [1] and the nuclear [2] spin of a single 31P atom can be read out in single-shot [3] with high fidelity, through a nanoelectronic device compatible with standard semiconductor fabrication. High-frequency microwave [4] pulses can be used to prepare arbitrary quantum states of the spin qubits, with fidelity in excess of 99%. Our latest experiment on the 31P nucleus has established the record coherence time (35 seconds) for any single qubit in solid state [5], by making use of an isotopically enriched 28Si epilayer.
Finally, I will discuss current efforts to scale up the system to multi-qubit quantum logic operations. We have demonstrated on a single-atom device the long-sought “A-gate” electrical control of a spin in a continuous microwave field [6], which greatly facilitates addressing multiple qubits. We have observed the singlet/triplet states of a strongly-coupled donor pair [7], proposed a new scheme for entangling two-qubit logic gates [8] that does not require atomically precise placement of the 31P donors, and we are exploring cavity-mediated long-distance spin coupling.
These results show that silicon – the material underpinning the whole modern computing era – can be successfully adapted to host quantum information hardware.
[1] J. Pla et al., Nature 489, 541 (2012)
[2] J. Pla et al., Nature 496, 334 (2013)
[3] A. Morello et al., Nature 467, 687 (2010)
[4] J. Dehollain et al., Nanotechnology 24, 015202 (2013)
[5] J.T. Muhonen et al., Nature Nanotechnology 9, 986 (2014)
[6] A. Laucht et al., Science Advances 1, e1500022 (2015)
[7] J.P. Dehollain et al., Phys. Rev. Lett. 112, 236801 (2014)
[8] R. Kalra et al., Phys. Rev. X 4, 021044 (2014)
Centre for Quantum Computation & Communication Technology, School of Electrical Engineering & Telecommunications, UNSW Australia