Quantum Dephasing in a Gated GaAs Triple Quantum Dot due to Nonergodic Noise.

We extract the phase coherence of a qubit defined by singlet and triplet electronic states in a gated GaAs triple quantum dot, measuring on time scales much shorter than the decorrelation time of the environmental noise. In this nonergodic regime, we observe that the coherence is boosted and several dephasing times emerge, depending on how the phase stability is extracted. We elucidate their mutual relations, and demonstrate that they reflect the noise short-time dynamics.

Critical Behavior of Alternately Pumped Nuclear Spins in Quantum Dots.

Nuclear spins in a spin-blocked quantum dot can be pumped and eventually polarized in either of two opposite directions that are selected by applying two different source-drain voltages. Applying a square pulse train as the source-drain voltage can continuously switch the pumping direction alternately. We propose and demonstrate a critical behavior in the polarization after alternate pumping, where the final polarization is sensitive to the initial polarization and pulse conditions. This sensitivity leads to stochastic behavior in the final polarization under nominally the same pumping conditions.

Two- and three-electron Pauli spin blockade in series-coupled triple quantum dots.

We investigate two- and three-electron spin blockade in three vertical quantum dots (QDs) coupled in series. Two-electron spin blockade is found in a region where sequential tunneling through all QDs is forbidden but tunneling involving virtual hopping through an empty QD is allowed. It is observed only for the hole cycle with a distinct bias threshold for access to the triplet state. Three-electron spin blockade involving the quadruplet state is observed for nonequibilium conditions where sequential tunneling is allowed and the triplet state is accessible. Our results shine light on the importance of the nonequibilium conditions to obtain sufficient population of triplet and quadruplet states necessary for spin blockade.

Aharonov-Bohm oscillations changed by indirect interdot tunneling via electrodes in parallel-coupled vertical double quantum dots.

Aharonov-Bohm (AB) oscillations are studied for a parallel-coupled vertical double quantum dot with a common source and drain electrode. We observe AB oscillations of current via a one-electron bonding state as the ground state and an antibonding state as the excited state. As the center gate voltage becomes more negative, the oscillation period is clearly halved for both the bonding and antibonding states, and the phase changes by half a period for the antibonding state. This result can be explained by a calculation that takes account of the indirect interdot coupling via the two electrodes.

A fast quantum interface between different spin qubit encodings.

Single-spin qubits in semiconductor quantum dots hold promise for universal quantum computation with demonstrations of a high single-qubit gate fidelity above 99.9% and two-qubit gates in conjunction with a long coherence time. However, initialization and readout of a qubit is orders of magnitude slower than control, which is detrimental for implementing measurement-based protocols such as error-correcting codes. In contrast, a singlet-triplet qubit, encoded in a two-spin subspace, has the virtue of fast readout with high fidelity. Here, we present a hybrid system which benefits from the different advantages of these two distinct spin-qubit implementations. A quantum interface between the two codes is realized by electrically tunable inter-qubit exchange coupling. We demonstrate a controlled-phase gate that acts within 5.5 ns, much faster than the measured dephasing time of 211 ns. The presented hybrid architecture will be useful to settle remaining key problems with building scalable spin-based quantum computers.

Coherent three-level mixing in an electronic quantum dot.

We observe magnetic-field-induced level mixing and quantum superposition phenomena between three approaching single-particle states in a quantum dot probed via the ground state of an adjacent quantum dot by single-electron resonant tunneling. The mixing is attributed to anisotropy and anharmonicity in realistic dot confining potentials. The pronounced anticrossing and transfer of strengths (both enhancement and suppression) between resonances can be understood with a simple coherent level mixing model. Superposition can lead to the formation of a dark state by complete cancellation of an otherwise strong resonance, an effect resembling coherent population trapping in a three-level-system of quantum and atom optics.