Quantum computing and quantum chaos with single dopants in silicon
The modern information era is built upon nanometre-scale silicon electronic devices. The future of quantum technology may be built on silicon too, by using the quantum states of individual spins as carriers of quantum information.
The electron  and nuclear  spins of single dopant atoms in silicon constitute some of the most coherent quantum systems in the solid state, with coherence times exceeding 30 seconds  and quantum gate fidelities approaching 99.99%. Electron and nucleus can be entangled with each with enough fidelity to allow the violation of Bell’s inequality with record value S=2.70 .
The next challenge in silicon-based quantum computing is engineering the interaction between the spins and manufacturing large-scale arrays of qubits with controllable couplings and local addressability. For this goal, we have invented a new type of qubit, called “flip-flop” qubit, where information is encoded in the combined electron-nuclear states of a single 31P atom. Flip-flop qubits can be controlled electrically and coupled at long-distance (~200 nm) using switchable electric dipole interactions .
Beside the applications in quantum information processing, single dopants in silicon can be used to study fundamental questions such as the transition from quantum to classical dynamics, as controlled by the emergence of chaos. Heavier group-V donors, like Sb or Bi, possess large nuclear spins which, in the presence of strong quadrupole interactions and periodic driving, embody the quantum version of the chaotic “kicked-top” . Combined with the exceptionally long coherence of nuclear spins in silicon, this system will allow the observation of all the striking phenomena associated with quantum chaos, from enhanced decoherence to dynamical tunneling.
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