Sandra Foletti

Universal Quantum Control of Two-electron Spin Quantum Bits Using Dynamic Nuclear Polarization

One fundamental requirement for quantum computation is to carry out universal manipulations of quantum bits at rates much faster than the qubit’s rate of decoherence. Recently, fast gate operations have been demonstrated in logical spin qubits composed of two electron spins where the rapid exchange of the two electrons permits electrically controllable rotations around one axis of the qubit. However, universal control of the qubit requires arbitrary rotations around at least two axes. Here, we show that by subjecting each electron spin to a magnetic field of different magnitude, we achieve full quantum control of the two-electron logical spin qubit with nanosecond operation times. Using a single device, a magnetic-field gradient of several hundred millitesla is generated and sustained using dynamic nuclear polarization of the underlying Ga and As nuclei. Universal control of the two-electron qubit is then demonstrated using quantum state tomography. The presented technique provides the basis for single- and potentially multiple-qubit operations with gate times that approach the threshold required for quantum error correction. The spin state of two electrons in a double well is a promising qubit. Now, such qubits can be arbitrarily rotated around two different axes by applying a magnetic field of different magnitude to each electron. This can be done in nanoseconds, before the stored information is lost.

Enhancing the coherence of a spin qubit by operating it as a feedback loop that controls its nuclear spin bath.

In many realizations of electron spin qubits the dominant source of decoherence is the fluctuating nuclear spin bath of the host material. The slowness of this bath lends itself to a promising mitigation strategy where the nuclear spin bath is prepared in a narrowed state with suppressed fluctuations. Here, this approach is realized for a two-electron spin qubit in a GaAs double quantum dot and a nearly tenfold increase in the inhomogeneous dephasing time T₂* is demonstrated. Between subsequent measurements, the bath is prepared by using the qubit as a feedback loop that first measures its nuclear environment by coherent precession, and then polarizes it depending on the final state. This procedure results in a stable fixed point at a nonzero polarization gradient between the two dots, which enables fast universal qubit control.

Semiclassical model for the dephasing of a two-electron spin qubit coupled to a coherently evolving nuclear spin bath

We study electron spin decoherence in a two-electron double quantum dot due to the hyperfine interaction, under spin-echo conditions as studied in recent experiments. We develop a semiclassical model for the interaction between the electron and nuclear spins, in which the time-dependent Overhauser fields induced by the nuclear spins are treated as classical vector variables. Comparison of the model with experimentally obtained echo signals allows us to quantify the contributions to the nuclear spin evolution of various processes such as coherent Larmor precession and spin diffusion.

Characterization of S − T + transition dynamics via correlation measurements

Nuclear spins are an important source of dephasing for electron spin qubits in GaAs quantum dots. Most studies of their dynamics have focused on the relatively slow longitudinal polarization. We show, based on a semiclassical model and experimentally, that the dynamics of the transverse hyperfine field can be probed by correlating individual Landau-Zener sweeps across the S-T

Long coherence of electron spins coupled to a nuclear spin bath

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Dynamic nuclear polarization using a single pair of electrons

transition of a two-electron spin qubit. The relative Larmor precession of different nuclear spin species leads to oscillations in these correlations, whose decay arises from dephasing of the nuclei. In the presence of spin orbit coupling, oscillations with the absolute Larmor frequencies whose amplitude reflects the spin orbit coupling strength are expected.