Gate-defined quantum dots store qubits in spin states of individual electrons or holes confined by potential landscapes that are shaped by voltages applied to electrodes. Semiconductor quantum dot platform has many favorable attributes for realizing scalable and individually addressable integrated quantum system owing to (1) spin’s long coherence time, (2) potential scalability including well-established fabrication technology, and (3) high degree of Hamiltonian parameter tunability. Depending on the degree of spin-charge mixing, elementary quantum operations of Loss-DiVincenzo (single electron, purely magnetic control), singlet-triplet (ST0, two-electron, electric control under magnetic field gradient), and various three electron spin qubits (all-electric control) have been demonstrated so far.
While we are interested in all of the qubit types above, DKim lab at SNU currently focuses on optimizing two-electron singlet-triplet qubits in both silicon and GaAs material system (see above for material choice). Despite added device complexity compared to single-electron spin qubits, that is, one qubit per two coupled quantum dots instead of one quantum dot, this type has many merits including fast electrical spin controls in the presence of magnetic field gradients, large extended sweet spot insensitive to external charge fluctuations, and favorable low operating frequency below 1 GHz enabling relatively easy integration of crowded control lines for multi-qubit operations. Below we describe details of selected sub-topics.