Fully Tunable Hyperfine Interactions of Hole Spin Qubits in Si and Ge Quantum Dots

Spatially correlated classical and quantum noise in driven qubits

Correlated noise across multiple qubits poses a significant challenge for achieving scalable and fault-tolerant quantum processors. Despite recent experimental efforts to quantify this noise in various qubit architectures, a comprehensive understanding of its role in qubit dynamics remains elusive. Here, we present an analytical study of the dynamics of driven qubits under spatially correlated noise, including both Markovian and non-Markovian noise. Surprisingly, we find that by operating the qubit system at low temperatures, where correlated quantum noise plays an important role, significant long-lived entanglement between qubits can be generated. Importantly, this generation process can be controlled on-demand by turning the qubit driving on and off. On the other hand, we demonstrate that by operating the system at a higher temperature, the crosstalk between qubits induced by the correlated noise is unexpectedly suppressed. We finally reveal the impact of spatio-temporally correlated 1/f noise on the decoherence rate, and how its temporal correlations restore lost entanglement. Our findings provide critical insights into not only suppressing crosstalk between qubits caused by correlated noise but also in effectively leveraging such noise as a beneficial resource for controlled entanglement generation.

Control of Ge island coalescence for the formation of nanowires on silicon

Germanium nanowires could be the building blocks of hole-spin qubit quantum computers. Selective area epitaxy enables the direct integration of Ge nanowires on a silicon chip while controlling the device design, density, and scalability. For this to become a reality, it is essential to understand and control the initial stages of the epitaxy process. In this work, we highlight the importance of surface treatment in the reactor prior to growth to achieve high crystal quality and connected Ge nanowire structures. In particular, we demonstrate that exposure to AsH3 during the high-temperature treatment enhances lateral growth of initial Ge islands and promotes faster formation of continuous Ge nanowires in trenches. The Kolmogorov-Johnson-Mehl-Avrami crystallization model supports our explanation of Ge coalescence. These results provide critical insight into the selective epitaxy of horizontal Ge nanowires on lattice-mismatched Si substrates, which can be translated to other material systems.

Nuclear Spin-Depleted, Isotopically Enriched 70 Ge/28 Si70 Ge Quantum Wells

The p-symmetry of the hole wavefunction is associated with a weaker hyperfine interaction, which makes hole spin qubits attractive candidates to implement quantum processors. However, recent studies demonstrate that hole qubits are still very sensitive to nuclear spin bath, thus highlighting the need for nuclear spin-free germanium (Ge) qubits to suppress this decoherence channel. Herein, this work demonstrates the epitaxial growth of 73 Ge- and 29 Si-depleted, isotopically enriched 70 Ge/silicon-germanium (SiGe) quantum wells. The growth is achieved by reduced pressure chemical vapor deposition using isotopically purified monogermane 70 GeH4 and monosilane 28 SiH4 with an isotopic purity higher than 99.9% and 99.99%, respectively. The quantum wells consist of a series of 70 Ge/SiGe heterostructures grown on Si wafers. The isotopic purity is investigated using atom probe tomography (APT) following an analytical procedure addressing the discrepancies caused by the overlap of isotope peaks in mass spectra. The nuclear spin background is found to be sensitive to the growth conditions with the lowest concentration of 73 Ge and 29 Si is below 0.01% in the Ge well and SiGe barriers. The measured average distance between nuclear spins reaches 3-4 nm in 70 Ge/28 Si70 Ge, which is an order of magnitude larger than in natural Ge/SiGe heterostructures. The spread of the hole wavefunction and the residual nuclear spin background in APT voluminals comparable to the size of realistic quantum dots are also discussed.

Phase-Driving Hole Spin Qubits

The spin-orbit interaction in spin qubits enables spin-flip transitions, resulting in Rabi oscillations when an external microwave field is resonant with the qubit frequency. Here, we introduce an alternative driving mechanism mediated by the strong spin-orbit interactions in hole spin qubits, where a far-detuned oscillating field couples to the qubit phase. Phase-driving at radio frequencies, orders of magnitude slower than the microwave qubit frequency, induces highly nontrivial spin dynamics, violating the Rabi resonance condition. By using a qubit integrated in a silicon fin field-effect transistor, we demonstrate a controllable suppression of resonant Rabi oscillations and their revivals at tunable sidebands. These sidebands enable alternative qubit control schemes using global fields and local far-detuned pulses, facilitating the design of dense large-scale qubit architectures with local qubit addressability. Phase-driving also decouples Rabi oscillations from noise, an effect due to a gapped Floquet spectrum and can enable Floquet engineering high-fidelity gates in future quantum processors.

Charge-Noise-Induced Dephasing in Silicon Hole-Spin Qubits

We investigate, theoretically, charge-noise-induced spin dephasing of a hole confined in a quasi-two-dimensional silicon quantum dot. Central to our treatment is accounting for higher-order corrections to the Luttinger Hamiltonian. Using experimentally reported parameters, we find that the new terms give rise to sweet spots for the hole-spin dephasing, which are sensitive to device details: dot size and asymmetry, growth direction, and applied magnetic and electric fields. Furthermore, we estimate that the dephasing time at the sweet spots is boosted by several orders of magnitude, up to on the order of milliseconds.

Anomalous Zero-Field Splitting for Hole Spin Qubits in Si and Ge Quantum Dots

An anomalous energy splitting of spin triplet states at zero magnetic field has recently been measured in germanium quantum dots. This zero-field splitting could crucially alter the coupling between tunnel-coupled quantum dots, the basic building blocks of state-of-the-art spin-based quantum processors, with profound implications for semiconducting quantum computers. We develop an analytical model linking the zero-field splitting to the Rashba spin-orbit interaction that is cubic in momentum. Such interactions naturally emerge in hole nanostructures, where they can also be tuned by external electric fields, and we find them to be particularly large in silicon and germanium, resulting in a significant zero-field splitting in the μeV range. We confirm our analytical theory by numerical simulations of different quantum dots, also including other possible sources of zero-field splitting. Our findings are applicable to a broad range of current architectures encoding spin qubits and provide a deeper understanding of these materials, paving the way toward the next generation of semiconducting quantum processors.

Fully Tunable Longitudinal Spin-Photon Interactions in Si and Ge Quantum Dots

Spin qubits in silicon and germanium quantum dots are promising platforms for quantum computing, but entangling spin qubits over micrometer distances remains a critical challenge. Current prototypical architectures maximize transversal interactions between qubits and microwave resonators, where the spin state is flipped by nearly resonant photons. However, these interactions cause backaction on the qubit that yields unavoidable residual qubit-qubit couplings and significantly affects the gate fidelity. Strikingly, residual couplings vanish when spin-photon interactions are longitudinal and photons couple to the phase of the qubit. We show that large and tunable spin-photon interactions emerge naturally in state-of-the-art hole spin qubits and that they change from transversal to longitudinal depending on the magnetic field direction. We propose ways to electrically control and measure these interactions, as well as realistic protocols to implement fast high-fidelity two-qubit entangling gates. These protocols work also at high temperatures, paving the way toward the implementation of large-scale quantum processors.

Coherent Hole Transport in Selective Area Grown Ge Nanowire Networks

Holes in germanium nanowires have emerged as a realistic platform for quantum computing based on spin qubit logic. On top of the large spin-orbit coupling that allows fast qubit operation, nanowire geometry and orientation can be tuned to cancel out charge noise and hyperfine interaction. Here, we demonstrate a scalable approach to synthesize and organize Ge nanowires on silicon (100)-oriented substrates. Germanium nanowire networks are obtained by selectively growing on nanopatterned slits in a metalorganic vapor phase epitaxy system. Low-temperature electronic transport measurements are performed on nanowire Hall bar devices revealing high hole doping of ∼10¹⁸ cm^-3 and mean free path of ∼10 nm. Quantum diffusive transport phenomena, universal conductance fluctuations, and weak antilocalization are revealed through magneto transport measurements yielding a coherence and a spin-orbit length of the order of 100 and 10 nm, respectively.