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Register a new userMapping the Chemical Potential Landscape of a Triple Quantum Dot
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We investigate the non-equilibrium charge dynamics of a triple quantum dot and demonstrate how electron transport through these systems can give rise to non-trivial tunnelling paths. Using a real-time charge sensing method we establish tunnelling pathways taken by particular electrons under well-defined electrostatic configurations. We show how these measurements map to the chemical potentials for different charge states across the system. We use a modified Hubbard Hamiltonian to describe the system dynamics and show that it reproduces all experimental observations.
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We present studies of thermal entanglement of a three-spin system in triangular symmetry. Spin correlations are described within an effective Heisenberg Hamiltonian, derived from the Hubbard Hamiltonian, with super-exchange couplings modulated by an effective electric field. Additionally a homogenous magnetic field is applied to completely break the degeneracy of the system. We show that entanglement is generated in the subspace of doublet states with different pairwise spin correlations for the ground and excited states. At low temperatures thermal mixing between the doublets with the same spin destroys entanglement, however one can observe its restoration at higher temperatures due to the mixing of the states with an opposite spin orientation or with quadruplets (unentangled states) always destroys entanglement. Pairwise entanglement is quantified using concurrence for which analytical formulae are derived in various thermal mixing scenarios. The electric field plays a specific role -- it breaks the symmetry of the system and changes spin correlations. Rotating the electric field can create maximally entangled qubit pairs together with a separate spin (monogamy) that survives in a relatively wide temperature range providing robust pairwise entanglement generation at elevated temperatures.
We propose an approach for achieving ground-state cooling of a nanomechanical resonator (NAMR) capacitively coupled to a triple quantum dot (TQD). This TQD is an electronic analog of a three-level atom in $Lambda$ configuration which allows an electron to enter it via lower-energy states and to exit only from a higher-energy state. By tuning the degeneracy of the two lower-energy states in the TQD, an electron can be trapped in a dark state caused by destructive quantum interference between the two tunneling pathways to the higher-energy state. Therefore, ground-state cooling of an NAMR can be achieved when electrons absorb readily and repeatedly energy quanta from the NAMR for excitations.
We report a method to derive the potential barrier profile shape in a dynamic quantum dot and show the loading statistics, and hence accuracy of electron transfer, depend significantly on the shape of the barrier. This method takes a further step towards tunable barrier shapes, which would greatly increase the accuracy of single electron sources, allowing the single electron current to be useful for quantum sensing, quantum information and metrology. We apply our method to the case of a tunable-barrier single-electron pump, an exemplary device that shows promise as a source of hot single electron wavepackets.
Spin qubits involving individual spins in single quantum dots or coupled spins in double quantum dots have emerged as potential building blocks for quantum information processing applications. It has been suggested that triple quantum dots may provide additional tools and functionalities. These include the encoding of information to either obtain protection from decoherence or to permit all-electrical operation, efficient spin busing across a quantum circuit, and to enable quantum error correction utilizing the three-spin Greenberger-Horn-Zeilinger quantum state. Towards these goals we demonstrate for the first time coherent manipulation between two interacting three-spin states. We employ the Landau-Zener-Stuckelberg approach for creating and manipulating coherent superpositions of quantum states. We confirm that we are able to maintain coherence when decreasing the exchange coupling of one spin with another while simultaneously increasing its coupling with the third. Such control of pairwise exchange is a requirement of most spin qubit architectures but has not been previously demonstrated.
Electrons confined in silicon quantum dots exhibit orbital, spin, and valley degrees of freedom. The valley degree of freedom originates from the bulk bandstructure of silicon, which has six degenerate electronic minima. The degeneracy can be lifted in silicon quantum wells due to strain and electronic confinement, but the valley splitting of the two lowest lying valleys is known to be sensitive to atomic-scale disorder. Large valley splittings are desirable to have a well-defined spin qubit. In addition, an understanding of the inter-valley tunnel coupling that couples different valleys in adjacent quantum dots is extremely important, as the resulting gaps in the energy level diagram may affect the fidelity of charge and spin transfer protocols in silicon quantum dot arrays. Here we use microwave spectroscopy to probe spatial variations in the valley splitting, and the intra- and inter-valley tunnel couplings ($t_{ij}$ and $t_{ij}$) that couple dots $i$ and $j$ in a triple quantum dot (TQD). We uncover large spatial variations in the ratio of inter-valley to intra-valley tunnel couplings $t_{12}/t_{12}=0.90$ and $t_{23}/t_{23}=0.56$. By tuning the interdot tunnel barrier we also show that $t_{ij}$ scales linearly with $t_{ij}$, as expected from theory. The results indicate strong interactions between different valley states on neighboring dots, which we attribute to local inhomogeneities in the silicon quantum well.
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