17 research outputs found
Controlling spin-orbit interactions in silicon quantum dots using magnetic field direction
Silicon quantum dots are considered an excellent platform for spin qubits,
partly due to their weak spin-orbit interaction. However, the sharp interfaces
in the heterostructures induce a small but significant spin-orbit interaction
which degrade the performance of the qubits or, when understood and controlled,
could be used as a powerful resource. To understand how to control this
interaction we build a detailed profile of the spin-orbit interaction of a
silicon metal-oxide-semiconductor double quantum dot system. We probe the
derivative of the Stark shift, -factor and -factor difference for two
single-electron quantum dot qubits as a function of external magnetic field and
find that they are dominated by spin-orbit interactions originating from the
vector potential, consistent with recent theoretical predictions. Conversely,
by populating the double dot with two electrons we probe the mixing of singlet
and spin-polarized triplet states during electron tunneling, which we conclude
is dominated by momentum-term spin-orbit interactions that varies from 1.85 MHz
up to 27.5 MHz depending on the magnetic field orientation. Finally, we exploit
the tunability of the derivative of the Stark shift of one of the dots to
reduce its sensitivity to electric noise and observe an 80 % increase in
. We conclude that the tuning of the spin-orbit interaction will be
crucial for scalable quantum computing in silicon and that the optimal setting
will depend on the exact mode of qubit operations used
Three-carrier spin blockade and coupling in bilayer graphene double quantum dots
The spin degree of freedom is crucial for the understanding of any condensed
matter system. Knowledge of spin-mixing mechanisms is not only essential for
successful control and manipulation of spin-qubits, but also uncovers
fundamental properties of investigated devices and material. For
electrostatically-defined bilayer graphene quantum dots, in which recent
studies report spin-relaxation times T1 up to 50ms with strong magnetic field
dependence, we study spin-blockade phenomena at charge configuration
. We examine the dependence of the spin-blockade
leakage current on interdot tunnel coupling and on the magnitude and
orientation of externally applied magnetic field. In out-of-plane magnetic
field, the observed zero-field current peak could arise from finite-temperature
co-tunneling with the leads; though involvement of additional spin- and
valley-mixing mechanisms are necessary for explaining the persistent sharp side
peaks observed. In in-plane magnetic field, we observe a zero-field current
dip, attributed to the competition between the spin Zeeman effect and the
Kane-Mele spin-orbit interaction. Details of the line shape of this current dip
however, suggest additional underlying mechanisms are at play
Pauli Blockade of Tunable Two-Electron Spin and Valley States in Graphene Quantum Dots
Pauli blockade mechanisms -- whereby carrier transport through quantum dots
(QDs) is blocked due to selection rules even when energetically allowed -- are
of both fundamental and technological interest, as a direct manifestation of
the Pauli exclusion principle and as a key mechanism for manipulating and
reading out spin qubits. Pauli spin blockade is well established for systems
such as GaAs QDs, where the two-electron spin-singlet ground state is separated
from the three triplet states higher in energy. However, Pauli blockade physics
remains largely unexplored for systems in which the Hilbert space is expanded
due to additional degrees of freedom, such as the valley quantum numbers in
carbon-based materials or silicon. Here we report experiments on coupled
graphene double QDs in which the spin and valley states can be precisely
controlled. We demonstrate that gate and magnetic-field tuning allows switching
between a spin-triplet--valley-singlet ground state with charge occupancy
(2,0), where valley-blockade is observed, and a spin-singlet--valley-triplet
ground state, where spin blockade is shown. These results demonstrate how the
complex two-particle Hilbert space of graphene quantum dots can be unravelled
experimentally, with implications for future spin and valley qubits
Spectroscopy of a single-carrier bilayer graphene quantum dot from time-resolved charge detection
We measured the spectrum of a single-carrier bilayer graphene quantum dot as
a function of both parallel and perpendicular magnetic fields, using a
time-resolved charge detection technique that gives access to individual tunnel
events. Thanks to our unprecedented energy resolution of 4eV, we could
distinguish all four levels of the dot's first orbital, in particular in the
range of magnetic fields where the first and second excited states cross
(mT). We thereby experimentally establish, the hitherto
extrapolated, single-charge carrier spectrum picture and provide a new upper
bound for the inter-valley mixing, equal to our energy resolution
Consistency of high-fidelity two-qubit operations in silicon
The consistency of entangling operations between qubits is essential for the
performance of multi-qubit systems, and is a crucial factor in achieving
fault-tolerant quantum processors. Solid-state platforms are particularly
exposed to inconsistency due to the materials-induced variability of
performance between qubits and the instability of gate fidelities over time.
Here we quantify this consistency for spin qubits, tying it to its physical
origins, while demonstrating sustained and repeatable operation of two-qubit
gates with fidelities above 99% in the technologically important silicon
metal-oxide-semiconductor (SiMOS) quantum dot platform. We undertake a detailed
study of the stability of these operations by analysing errors and fidelities
in multiple devices through numerous trials and extended periods of operation.
Adopting three different characterisation methods, we measure entangling gate
fidelities ranging from 96.8% to 99.8%. Our analysis tools also identify
physical causes of qubit degradation and offer ways to maintain performance
within tolerance. Furthermore, we investigate the impact of qubit design,
feedback systems, and robust gates on implementing scalable, high-fidelity
control strategies. These results highlight both the capabilities and
challenges for the scaling up of spin-based qubits into full-scale quantum
processors
One- and two-qubit operations in Si-MOS quantum dots
Quantum computers have potential to solve hard problems that even the most advanced supercomputers are not capable of, such as prime factoring, database search, and quantum simulation. Quantum bits, or qubits, made from silicon quantum dots is one scalable approach that can be used to realize a quantum computer. Even though the essential ingredients for this platform have been demonstrated, certain physical phenomena that can influence quantum bit behaviour, for example, the valley- and spin-orbit coupling, require further understanding. In this thesis, we investigate harnessing the effect of spin and valley orbit couplings in silicon quantum dots. We propose a mechanism for a significant enhancement in the electrically-driven spin rotation frequency for silicon quantum dot qubits in the presence of a step at a hetero-interface. We calculate single qubit gate times of 170 ns for a quantum dot confined at a silicon/silicon-dioxide interface.To understand the origin of spin-orbit coupling, we experimentally probe the g-factor for two quantum dot qubits as a function of magnetic field and find that they are dominated by spin-orbit interactions originating from the vector potential. By populating the double dot we probe the mixing of singlet and spin-polarized triplet states, which we conclude is dominated by momentum-term in spin-orbit interactions. Finally, we exploit the Stark shift to reduce its sensitivity to electric noise and observe an 80% increase in the coherence time T_2^*. We proved that when understood and controlled, the small but significant spin-orbit interaction, can be used as a powerful resource. While a variety of qubit systems have shown high fidelities at the one-qubit level, superconductor technologies have been the only solid-state qubits manufactured via standard lithographic techniques which have demonstrated two-qubit fidelities near the fault-tolerant threshold. Here in silicon quantum dots, we demonstrate the single-qubit randomized benchmarking with an average Clifford gate fidelity of 99.96% and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7% and average Controlled-ROT fidelity of 98.0%.The enhanced understanding of valley-orbit and spin-orbit coupling, together with demonstrated high fidelity single- and two-qubit gates provides opportunities that are promising for the scalability of spin qubit systems
Electrically driven spin qubit based on valley mixing
The electrical control of single spin qubits based on semiconductor quantum dots is of great interest for scalable quantum computing since electric fields provide an alternative mechanism for qubit control compared with magnetic fields and can also be easier to produce. Here we outline the mechanism for a drastic enhancement in the electrically-driven spin rotation frequency for silicon quantum dot qubits in the presence of a step at a heterointerface. The enhancement is due to the strong coupling between the ground and excited states which occurs when the electron wave function overcomes the potential barrier induced by the interface step. We theoretically calculate single qubit gate times tπ of 170 ns for a quantum dot confined at a silicon/silicon-dioxide interface. The engineering of such steps could be used to achieve fast electrical rotation and entanglement of spin qubits despite the weak spin-orbit coupling in silicon.QCD/Veldhorst La