The team has already improved the control of these qubits to an accuracy of above 99% and established the world record for how long quantum information can be stored in the solid state.
The method of qubit control demonstrated here fulfills all the practical requirements for a large-scale quantum computer, because control gates can be applied simultaneously to arbitrarily many qubits while requiring only one CW microwave source together with inexpensive multichannel baseband pulse generators.
Electron wave in a phosphorus atom, distorted by a local electric field.
Electric field dependence of electron and nuclear energy states. (A) False-colored scanning electron microscope image of a device similar to the one used in the experiment. Blue, microwave (MW) antenna; yellow, gates used to induce the SET charge sensor under the SiO2 insulator; pink, A-gates, comprising gates labeled Donor Fast (DF), Donor Slow (DS), and Top Gate AC (TGAC). These gates are used to tune the potential and electric field at the donor location. (B) Electron wavefunction of a donor under an electrostatic gate. A positive voltage applied to the gate attracts the electron toward the Si-SiO2 interface. For illustration purposes, the wavefunction distortion is largely exaggerated as compared to the actual effect taking place in the experiment. (C) Energy level diagram of the neutral e−-31P system. Gate-controlled distortion of the electron wavefunction modifies A and γe, shifting the ESR νe1 and νe2, and the NMR νn1 and νn2 transition frequencies.
“We demonstrated that a highly coherent qubit, like the spin of a single phosphorus atom in isotopically enriched silicon, can be controlled using electric fields, instead of using pulses of oscillating magnetic fields,” explained UNSW’s Dr Arne Laucht, post-doctoral researcher and lead author of the study.
Associate Professor Morello said the method works by distorting the shape of the electron cloud attached to the atom, using a very localized electric field.
“This distortion at the atomic level has the effect of modifying the frequency at which the electron responds.
“Therefore, we can selectively choose which qubit to operate. It’s a bit like selecting which radio station we tune to, by turning a simple knob. Here, the ‘knob’ is the voltage applied to a small electrode placed above the atom.”
The findings suggest that it would be possible to locally control individual qubits with electric fields in a large-scale quantum computer using only inexpensive voltage generators, rather than the expensive high-frequency microwave sources.
Moreover, this specific type of quantum bit can be manufactured using a similar technology to that employed for the production of everyday computers, drastically reducing the time and cost of development.
The device used in this experiment was fabricated at the NSW node of the Australian National Fabrication Facility, in collaboration with the group led by UNSW Scientia Professor Andrew Dzurak.
Key to the success of this electrical control method is the placement of the qubits inside a thin layer of specially purified silicon, containing only the silicon-28 isotope.
“This isotope is perfectly non-magnetic and, unlike those in naturally occurring silicon, does not disturb the quantum bit,” Associate Professor Morello said.
Large-scale quantum computers must be built upon quantum bits that are both highly coherent and locally controllable. We demonstrate the quantum control of the electron and the nuclear spin of a single 31P atom in silicon, using a continuous microwave magnetic field together with nanoscale electrostatic gates. The qubits are tuned into resonance with the microwave field by a local change in electric field, which induces a Stark shift of the qubit energies. This method, known as A-gate control, preserves the excellent coherence times and gate fidelities of isolated spins, and can be extended to arbitrarily many qubits without requiring multiple microwave sources.
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