High fidelity gates, dynamic nuclear polarization and spin-orbit interaction in GaAs two-electron spin qubits.
Final Report Abstract
Quantum computing promises an exponential speedup for certain computational task. One approach to realize the underlying quantum bits or qubits is to use the spin of electrons in semiconductor nanostructures. GaAs quantum dots are amongst the most advanced types of such spin qubits. While the material system has very favorable electronic properties, the hyperfine interaction of the electron with nuclear spins complicates high performance qubit operation. On the one hand, mitigation methods to suppress nuclear spin fluctuations are required. Second, advanced approaches to control the qubit are needed. This project addresses both of these needs for twoelectron spin qubits. We have realized gate operations for manipulating qubits and demonstrated a gate fidelity of 99 % that was limited by electrical noise rather than nuclear spin fluctuations. This fidelity is a key figure of merit for any qubit. For practical exploitation, the absolute minimum requirement is about 99%, and we expect that further improvement of our approach can reach the 99.9% range that would lead to an acceptable overhead for correcting errors. Our current results reflect the current state of the art for any GaAs base qubits and are competitive with most results obtained in Si, where the hyperfine interaction is less severe. We experimentally realized these gate operations using control pulses stemming from realistic simulations which were then fine tuned on the experiment to eliminate systematic errors (arising, e.g., from poorly calibrated control pulses and coupling to the qubit). The gate fidelity was obtained using randomized benchmarking, which has emerged as the standard characterization method. In addition, we find that leakage, a specific type of error that is a potential concern for our qubit in particular, is below 0.4 % (0.1 % in ongoing follow-up experiments). Future work will extend the approach to two-qubit gates. An important ingredient to reaching such high fidelities are methods based on dynamic nuclear polarization with feedback to suppress fluctuations of the nuclear spin, so called narrowing procedures. A second work package of the project investigated the effectiveness of these methods and their improvement. We have shown that the performance of the method employed so far is consistent with the measured rate of polarization and the fluctuation spectrum. We have analysed an alternative method based on electric dipole spin resonance (EDSR) that promises a much higher performance. We have furthermore made first steps to experimentally implement this method and showed that it works in principle, but found that its effectiveness in some cases lacks behind expectations. A possible explanation is the dominance of spin-orbit coupling as a competing mechanism. However, we also found evidence for the expected strong polarization for one specific procedure. A detailed understanding of these results and the improvement of the decoherence suppression procedure will require further work. In addition, we carried out some theoretical work that resulted in a quantitative model for earlier measurements of correlations of qubit transitions that are driven by the transverse component of nuclear spins.
Publications
- “High-Fidelity Single-Qubit Gates for Two-Electron Spin Qubits in GaAs”. PRL 113, 150501 (2014)
P. Cerfontaine, T. Botzem, D. P. DiVincenzo, and H. Bluhm
(See online at https://doi.org/10.1103/PhysRevLett.113.150501) - “Characterization of S-T+transition dynamics via correlation measurements”. Phys. Rev. B 92, 125402 (2015)
C. Dickel, S. Foletti, V. Umansky, and H. Bluhm
(See online at https://doi.org/10.1103/PhysRevB.92.125402) - “Narrowing of the Overhauser field distribution by feedback-enhanced dynamic nuclear polarization”. Phys. Rev. B 92, 195428 (2015)
S. Tenberg, R. P. G. McNeil, S. Rubbert, and H. Bluhm
(See online at https://doi.org/10.1103/PhysRevB.92.195428) - Feedback-tuned noise-resilient gates for encoded spin qubits
P. Cerfontaine, T. Botzem, S. S. Humpohl, D. Schuh, D. Bougeard, and H. Bluhm
- “Quadrupolar and anisotropy effects on dephasing in two-electron spin qubits in GaAs”. Nat. Commun. 7, 11170 (2016)
T. Botzem, R. P. G. McNeil, J.-M. Mol, D. Schuh, D. Bougeard, and H. Bluhm
(See online at https://doi.org/10.1038/ncomms11170)
