The qubit is the fundamental building block of a quantum computer. We fabricate a qubit in a silicon double-quantum dot with an integrated micromagnet in which the qubit basis states are the singlet state and the spin-zero triplet state of two electrons. Because of the micromagnet, the magnetic field difference ΔB between the two sides of the double dot is large enough to enable the achievement of coherent rotation of the qubit's Bloch vector around two different axes of the Bloch sphere. By measuring the decay of the quantum oscillations, the inhomogeneous spin coherence time T 2 * is determined. By measuring T 2 * at many different values of the exchange coupling J and at two different values of ΔB, we provide evidence that the micromagnet does not limit decoherence, with the dominant limits on T 2 * arising from charge noise and from coupling to nuclear spins.semiconductor spin qubit | quantum nanoelectronics F abricating qubits composed of electrons in semiconductor quantum dots is a promising approach for the development of a large-scale quantum computer because of the approach's potential for scalability and for integrability with classical electronics. Much recent progress has been made, and spin manipulation has been demonstrated in systems of two (1-5), three (6, 7), and four (8) quantum dots. A great deal of attention has focused on the singlet-triplet qubit in quantum dots (1, 2, 9-18), which consists of the S z = 0 subspace of two electrons, for which the basis can be chosen to be a singlet and a triplet state. Full two-axis control on the Bloch sphere is achieved by electrical gating in the presence of a magnetic field difference ΔB between the two dots. In previous experiments (2, 9-14), ΔB arises from coupling to nuclear spins in the material, and slow fluctuations in these nuclear fields lead to inhomogeneous decoherence times that, without special nuclear state preparation, typically are shorter than the period of the quantum oscillations. In III-V materials, ΔB is large, so fast oscillation periods of order 10 ns are achievable, but the inhomogeneous dephasing time is also ∼10 ns, so that oscillations from ΔB are overdamped, ending before a complete cycle is observed (2). The fluctuations of the nuclear spin bath can be mitigated to some extent (10), but inhomogeneous dephasing times in III-V materials are short enough that high-fidelity control is still very challenging. Coupling to nuclear spins in silicon is substantially weaker, leading to longer coherence times, but also smaller field differences and hence slower quantum oscillations (14,19).Here, we report the operation of a singlet-triplet qubit in which the magnetic field difference ΔB between the dots is imposed by an external micromagnet (20,21). Because the field from the micromagnet is stable in time, a large ΔB can be imposed without creating inhomogeneous dephasing. We present data demonstrating underdamped quantum oscillations, and, by investigating a variety of voltage configurations and two ΔB configurations, we show that the m...

Fast quantum oscillations of a charge qubit in a double quantum dot fabricated in a Si/SiGe heterostructure are demonstrated and characterized experimentally. The measured inhomogeneous dephasing time T * 2 ranges from 127 ps to 2.1 ns; it depends substantially on how the energy difference of the two qubit states varies with external voltages, consistent with a decoherence process that is dominated by detuning noise (charge noise that changes the asymmetry of the qubit's double-well potential). In the regime with the shortest T * 2 , applying a charge-echo pulse sequence increases the measured inhomogeneous decoherence time from 127 to 760 ps, demonstrating that low-frequency noise processes are an important dephasing mechanism.

We study the dynamics of a pulse-gated semiconductor double quantum dot qubit. In our experiments, the qubit coherence times are relatively long, but the visibility of the quantum oscillations is low. We show that these observations are consistent with a theory that incorporates decoherence arising from charge noise that gives rise to detuning fluctuations of the double dot. Because effects from charge noise are largest near the singlet-triplet avoided level crossing, the visibility of the oscillations are low when the singlet-triplet avoided level crossing occurs in the vicinity of the charge degeneracy point crossed during the manipulation, but there is only modest dephasing at the large detuning value at which the quantum phase accumulates. This theory agrees well with experimental data and predicts that the visibility can be increased greatly by appropriate tuning of the interdot tunneling rate.

Abstract. We report the fabrication and characterization of a gate-defined double quantum dot formed in a Si/SiGe nanomembrane. In the past, all gatedefined quantum dots in Si/SiGe heterostructures were formed on top of straingraded virtual substrates. The strain grading process necessarily introduces misfit dislocations into a heterostructure, and these defects introduce lateral strain inhomogeneities, mosaic tilt, and threading dislocations. The use of a SiGe nanomembrane as the virtual substrate enables the strain relaxation to be entirely elastic, eliminating the need for misfit dislocations. However, in this approach the formation of the heterostructure is more complicated, involving two separate epitaxial growth procedures separated by a wet-transfer process that results in a buried non-epitaxial interface 625 nm from the quantum dot. We demonstrate that in spite of this buried interface in close proximity to the device, a double quantum dot can be formed that is controllable enough to enable tuning of the inter-dot tunnel coupling, the identification of spin states, and the measurement of a singlet-to-triplet transition as a function of an applied magnetic field.