Donor spin qubit in silicon

A phosphorus (31P) donor inside a silicon crystal has both an electron spin and a nuclear spin that can be used for quantum computation

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 any nuclear spin carrying isotopes, ensuring extraordinary coherence times. Silicon – the semiconductor underpinning the whole modern computing era – is also the perfect material for quantum information hardware [Kane1998].


Rabi oscillations of an electron spin qubit. The high contrast is evidence for the good initialization, control and readout fidelities.In our labs, we have demonstrated that both the electron [Pla2012] and the nuclear [Pla2013] spin of a single, ion-implanted31P atom can be read out in single-shot [Morello2010, VanDonkelaar2015] with high fidelity, through a nanoelectronic device compatible with standard semiconductor fabrication(ion-implantation done in collaboration with David N. Jamieson and Jeffrey C. McCallum from the University of Melbourne). High-frequency microwave [Dehollain2013] pulses can be used to prepare arbitrary quantum states of the spin qubits, with fidelity in excess of 99.9% [Muhonen2015, Dehollain2016].

Nuclear spin coherence time obtained from CPMG decay measurements with 1024 pairs of refocusing pulses. The measured coherence exceeds half minute. [Muhonen, Nature Nano 2014]

 Our experiment on the 31P nucleus has established the record coherence time (35 seconds) for any single qubit in the solid state [Muhonen2014], by making use of an isotopically enriched 28Si epilayer [Itoh2014](isotopically engineered substrates supplied by Kohei M. Itoh from Keio University).






Animation of the dressed qubit initialization, control and readout sequence. [Laucht, Nature Nano 2017]The exceptional quality of these qubits has allowed us to perform a variety of more complex quantum experiments. Recent results include the electrical control of a spin in a continuous microwave field (Kane’s A-gate) [Laucht2016], Histograms of Bell signal values obtained for the zero-quantum-coherence state (ZQC), showing a clear violation of Bell’s inequality. [Dehollain, Nature Nano 2016]the violation of Bell’s inequality in the electron-nuclear two-qubit system [Dehollain2016],the operation of a spin qubit in the noisy environment of a cryogen-free dilution refrigerator [Kalra, Rev Sci Instr 2016], using dressing techniques to create a new hybrid qubit with extended coherence times and alternative control mechanisms [Laucht2017, Laucht2016] a nuclear spin quantum memory [Freer2017], and weak measurements and quantum steering [Muhonen]. 


A positive voltage applied to a gate above donor pulls the electron wavefunction towards the interface and creates a large electric dipole. [Laucht, Science Advances 2016. Tosi, arXiv:1405.1231]

Currently, we are trying to improve these outstanding qubits even further. We are performing T1 measurements to investigate the low-B0-field limit of the electron spin lifetime, looking at the effects of co-tunneling and evanescent wave Johnson noise. We also want to improve our electron spin initialization fidelity using FPGA-based, real-time feedback during the initialization phase. Further experiments look at exchanged-coupled donor-donor pairs and the realization of conditional 2-qubit quantum gates [Kalra2014].

While donor spins in silicon make excellent single qubits, demonstrating scalability to more than a few qubits is still an outstanding challenge. Several proposals to integrate them into scalable architectures exist [Kane1998, Hollenberg2006, Tosi2017, Hill2015, Pica2016]. Our approach focuses on creating a large electric dipole by moving the electron wavefunction towards the interface and using electric dipole coupling to achieve qubit-qubit coupling.This approach has the advantages of long-distance coupling that is robust against uncertainties in donor placement, requiring low-power electric fields for gate operations, and offering coherence protection due to an intrinsic clock transition [Tosi2017]. We are currently carrying out experiments to demonstrate the building blocks of such a scalable architecture. This includes the observation of an electrically-driven spin transition. Next generation samples contain two qubit-units separated by 250 nanometers, and are designed to fully characterize the first high-fidelity 2-qubit gate in silicon that can be scaled up using available fabrication techniques.
Device scheme for coupling two distant flip-flop qubits via electric dipole-dipole interactions. Indicated are the dipole field lines Edip produced by the left qubit. [Tosi, arXiv:1405.1231]
Team members on this project


Jeffrey McCallum, University of Melbourne, donor implantation