Quantum Spin Control

Any computer requires a suitable two-state system to encode and elaborate information. Classically, the two-state system is a field-effect transistor, which can be switched ON or OFF by a control voltage, and where the information is carried by a large number of electrical charges. Interestingly, the same physical object – an electron – which is involved in classical information processing, also carries the fundamental quantum bit (qubit) of information in its spin S = 1/2. It is therefore conceivable to exploit many of the advanced technologies developed for highly miniaturized silicon microchips and apply them to confine single electrons. A network of interacting and controllable spin qubits can act as a quantum computer, able to solve otherwise intractable computational problems. However, because of their magnetic and quantum-mechanical nature, the spin qubits must be controlled and measured using radically different techniques as compared to classical, transistor-based bits. The development of these novel techniques is the core mission of the QSC program.

The QSC program manager, Dr. Andrea Morello, has over 10 years of experience with ambitious experiments on quantum spin systems at ultra-low temperatures. The team includes 2 postdoctoral fellows, 2 highly skilled electronics and cryogenics technicians, and several talented graduate and undergraduate students. The research program has already gained solid international recognition with a series of ground-breaking results on the control and readout of single spins in silicon. These include:

Measurement of the Zeeman energy splitting of deliberately implanted single dopants, observed through quantum transport. Metal-oxide-semiconductor structures were developed ad hoc to introduce individual phosphorus dopants between electrostatically induced source-drain contacts and measure the energy splitting between spin-down and spin-up states, i.e. the two basic states of the spin qubit (Figure 1). We have also exploited the sharpness of such states to investigate the density of electron states in the induced charge reservoirs.

Single-shot readout of an electron spin in silicon. We have conceived and experimentally demonstrated a novel architecture to couple individual donors to a metal-oxide-semiconductor single-electron transistor (SET). Exploiting a clever spin-to-charge conversion method, our experiment was the first ever demonstration of single-shot measurement of the spin state of a single electron in silicon, obtained on timescales down to 3 μs with visibility better than 90% (Figure 2). We also observed the longest excited state lifetime of any electrically-detected single spin in solid state, T1 = 6 seconds.

Theory of spin decoherence in solid state. While the QSC program is primarily experimental, we actively collaborate with leading theory groups to gain deeper understanding of the spin decoherence mechanisms in realistic solid-state environments. A recent achievement was the perfect matching between bulk spin resonance results and a novel theory of spin decoherence arising from dipole-dipole interactions between background dopants, accounting for their interplay with 29Si nuclear spins.

The current efforts focus on the coherent control of the spin state, integrated with the single-shot readout we just demonstrated. We have designed, modeled and tested an advanced coplanar waveguide structure, which efficiently couples resonant microwave pulses to the donor spin (Figure 3). This complete qubit structure will allow the full quantum control of the spin qubit (Rabi oscillations). The presence of a nuclear spin I = 1/2 in each phosphorus donor open exciting possibilities for electron-nuclear entanglement and long-lived spin memory.

Further developments will aim at measuring and controlling the exchange interaction between pairs of spins, to demonstrate a fully functional 2-qubit quantum logic gate. In addition, we will seek the demonstration of spin transport across large distances in a silicon chip, by assessing the performance of a range of techniques, e.g. coherent transport by adiabatic passage, spin bus, and charge shuttling.