Navigating the 16-dimensional Hilbert space of a high-spin donor qudit with electric and magnetic fields

Efficient scaling and flexible control are key aspects of useful quantum computing hardware. Spins in semiconductors combine quantum information processing with electrons, holes or nuclei, control with electric or magnetic fields, and scalable coupling via exchange or dipole interaction. However, accessing large Hilbert space dimensions has remained challenging, due to the short-distance nature of the interactions. Here, we present an atom-based semiconductor platform where a 16-dimensional Hilbert space is built by the combined electron-nuclear states of a single antimony donor in silicon. We demonstrate the ability to navigate this large Hilbert space using both electric and magnetic fields, with gate fidelity exceeding 99.8% on the nuclear spin, and unveil fine details of the system Hamiltonian and its susceptibility to control and noise fields. These results establish high-spin donors as a rich platform for practical quantum information and to explore quantum foundations.Comment: 31 pages and 19 figures including Supplementary Material

Coherence and high fidelity control of two-electron spin qubits in GaAs quantum dots

Electron spin qubits confined in GaAs quantum dots are among the most established and well understood qubit systems. Long coherence times due to their weak interactionwith the environment and the electrical tunability of the semiconductor quantum dot have allowed GaAs-based spin qubits to play a central role in demonstrating the keyoperations of semiconductor spin qubits, such as initialization, read-out, universal control and two-qubit gates. Furthermore, spins confined in semiconductor nanostructuresprovide a solid-state approach to quantum computation which leverages current, well established semiconductor production technology for device fabrication and potential scalability.However, the interaction with nuclear spins in the GaAs host material complicates not only the preservation of qubit coherence, but also the precise control of the electronspins. As both these properties, the coherence time and the fidelity of gate operations, play a crucial role as prerequisites for quantum computing, the focus of this thesis areexperiments addressing these challenges on the basis of two-electron spin qubits. Interesting effects arise from the quadrupolar interaction of nuclear spins with electric field gradients. We show experimentally that quadrupolar broadening of the nuclear Larmor precession reduces electron spin coherence via faster decorrelation of transversenuclear fields. However, this effect disappears for appropriate field directions. Furthermore, we observe an additional modulation of coherence attributed to an anisotropicelectronic g-tensor. These results complete our understanding of dephasing in gated quantum dots and point to mitigation strategies. A key requirement for quantum computation are high-fidelity single qubit operations, which so far have not been demonstrated for encoded qubits in GaAs. Here, we realize such accurate operations by iteratively tuning of the all-electrical control pulses. Using randomized benchmarking, a well established characterization method, we find anaverage gate fidelity of F = (98.5 ± 0.1) % and determine the sum of gate leakage out of and back into the computational subspace to be L = (0.4 ± 0.1) %. These results demonstrate that high fidelity gates can be realized even in the presence of nuclear spins as existent in all III-V semiconductors.The potential of a feedback mechanism based on electric dipole spin resonance for narrowing the nuclear hyperfine field and its effectiveness for extending qubit coherencetime is investigated in a last experiment. Compared to a previously developed feedback mechanism, this polarization scheme promises higher and more stable pump rates andthe ability to set local magnetic fields in each quantum dot individuall

Quadrupolar and anisotropy effects on dephasing in two-electron spin qubits in GaAs

Understanding the decoherence of electron spins in semiconductors due to their interaction with nuclear spins is of fundamental interest as they realize the central spin model and of practical importance for using them as qubits. Interesting effects arise from the quadrupolar interaction of nuclear spins with electric field gradients, which have been shown to suppress diffusive nuclear spin dynamics and might thus enhance electron spin coherence. Here we show experimentally that for gate-defined GaAs quantum dots, quadrupolar broadening of the nuclear Larmor precession reduces electron spin coherence by causing faster decorrelation of transverse nuclear fields. However, this effect disappears for appropriate field directions. Furthermore, we observe an additional modulation of coherence attributed to an anisotropic electronic g-tensor. These results complete our understanding of dephasing in gated quantum dots and point to mitigation strategies. They may also help to unravel unexplained behaviour in self-assembled quantum dots and III-V nanowires

Measurement of Backaction from Electron Spins in a Gate-Defined GaAs Double Quantum dot Coupled to a Mesoscopic Nuclear Spin Bath

Decoherence of a quantum system arising from its interaction with an environment is a key concept for understanding the transition between the quantum and classical world as well as performance limitations in quantum technology applications. The effects of large, weakly coupled environments are often described as a classical, fluctuating field whose dynamics is unaffected by the qubit, whereas a fully quantum description still implies some back-action from the qubit on the environment. Here we show direct experimental evidence for such a back-action for an electron-spin-qubit in a GaAs quantum dot coupled to a mesoscopic environment of order $10^6$ nuclear spins. By means of a correlation measurement technique, we detect the back-action of a single qubit-environment interaction whose duration is comparable to the qubit's coherence time, even in such a large system. We repeatedly let the qubit interact with the spin bath and measure its state. Between such cycles, the qubit is reinitialized to different states. The correlations of the measurement outcomes are strongly affected by the intermediate qubit state, which reveals the action of a single electron spin on the nuclear spins.Comment: 6 pages, 2 figure

Closed-loop control of a GaAs-based singlet-triplet spin qubit with 99.5% gate fidelity and low leakage

Semiconductor spin qubits have recently seen major advances in coherence time and control fidelities, leading to a single-qubit performance that is on par with other leading qubit platforms. Most of this progress is based on microwave control of single spins in devices made of isotopically purified silicon. For controlling spins, the exchange interaction is an additional key ingredient which poses new challenges for high-fidelity control. Here, we demonstrate exchange-based single-qubit gates of two-electron spin qubits in GaAs double quantum dots. Using careful pulse optimization and closed-loop tuning, we achieve a randomized benchmarking fidelity of (99.50 +/- 0.04)% and a leakage rate of 0.13% out of the computational subspace. These results open new perspectives for microwave-free control of singlet-triplet qubits in GaAs and other materials

Tuning Methods for Semiconductor Spin Qubits

We present efficient methods to reliably characterize and tune gate-defined semiconductor spin qubits. Our methods are developed for double quantum dots in GaAs heterostructures, but they can easily be adapted to other quantum-dot-based qubit systems. These tuning procedures include the characterization of the interdot tunnel coupling, the tunnel coupling to the surrounding leads, and the identification of various fast initialization points for the operation of the qubit. Since semiconductor-based spin qubits are compatible with standard semiconductor process technology and hence promise good prospects of scalability, the challenge of efficiently tuning the dot’s parameters will only grow in the near future, once the multiqubit stage is reached. With the anticipation of being used as the basis for future automated tuning protocols, all measurements presented here are fast-to-execute and easy-to-analyze characterization methods. They result in quantitative measures of the relevant qubit parameters within a couple of seconds and require almost no human interference