15 research outputs found
Two-electron spin correlations in precision placed donors in silicon
Substitutional donor atoms in silicon are promising qubits for quantum computation with extremely long relaxation and dephasing times demonstrated. One of the critical challenges of scaling these systems is determining inter-donor distances to achieve controllable wavefunction overlap while at the same time performing high fidelity spin readout on each qubit. Here we achieve such a device by means of scanning tunnelling microscopy lithography. We measure anti-correlated spin states between two donor-based spin qubits in silicon separated by 16 ± 1 nm. By utilising an asymmetric system with two phosphorus donors at one qubit site and one on the other (2P−1P), we demonstrate that the exchange interaction can be turned on and off via electrical control of two in-plane phosphorus doped detuning gates. We determine the tunnel coupling between the 2P−1P system to be 200 MHz and provide a roadmap for the observation of two-electron coherent exchange oscillations
Silicon-based spin and charge quantum computation
Silicon-based quantum-computer architectures have attracted attention because
of their promise for scalability and their potential for synergetically
utilizing the available resources associated with the existing Si technology
infrastructure. Electronic and nuclear spins of shallow donors (e.g.
phosphorus) in Si are ideal candidates for qubits in such proposals due to the
relatively long spin coherence times. For these spin qubits, donor electron
charge manipulation by external gates is a key ingredient for control and
read-out of single-qubit operations, while shallow donor exchange gates are
frequently invoked to perform two-qubit operations. More recently, charge
qubits based on tunnel coupling in P substitutional molecular ions in Si
have also been proposed. We discuss the feasibility of the building blocks
involved in shallow donor quantum computation in silicon, taking into account
the peculiarities of silicon electronic structure, in particular the six
degenerate states at the conduction band edge. We show that quantum
interference among these states does not significantly affect operations
involving a single donor, but leads to fast oscillations in electron exchange
coupling and on tunnel-coupling strength when the donor pair relative position
is changed on a lattice-parameter scale. These studies illustrate the
considerable potential as well as the tremendous challenges posed by donor spin
and charge as candidates for qubits in silicon.Comment: Review paper (invited) - to appear in Annals of the Brazilian Academy
of Science
Quantal and graded stimulation of B lymphocytes as alternative strategies for regulating adaptive immune responses
Lymphocytes undergo a typical response pattern following stimulation in vivo: they proliferate, differentiate to effector cells, cease dividing and predominantly die, leaving a small proportion of long-lived memory and effector cells. This pattern results from cell-intrinsic processes following activation and the influence of external regulation. Here we apply quantitative methods to study B-cell responses in vitro. Our results reveal that B cells stimulated through two Toll-like receptors (TLRs) require minimal external direction to undergo the basic pattern typical of immunity. Altering the stimulus strength regulates the outcome in a quantal manner by varying the number of cells that participate in the response. In contrast, the T-cell-dependent CD40 activation signal induces a response where division times and differentiation rates vary in relation to stimulus strength. These studies offer insight into how the adaptive antibody response may have evolved from simple autonomous response patterns to the highly regulable state that is now observed in mammals
Real-time tracking of cell cycle progression during CD8(+) effector and memory T-cell differentiation
The precise pathways of memory T-cell differentiation are incompletely understood. Here we exploit transgenic mice expressing fluorescent cell cycle indicators to longitudinally track the division dynamics of individual CD8(+) T cells. During influenza virus infection in vivo, naive T cells enter a CD62L(intermediate) state of fast proliferation, which continues for at least nine generations. At the peak of the anti-viral immune response, a subpopulation of these cells markedly reduces their cycling speed and acquires a CD62L(hi) central memory cell phenotype. Construction of T-cell family division trees in vitro reveals two patterns of proliferation dynamics. While cells initially divide rapidly with moderate stochastic variations of cycling times after each generation, a slow-cycling subpopulation displaying a CD62L(hi) memory phenotype appears after eight divisions. Phenotype and cell cycle duration are inherited by the progeny of slow cyclers. We propose that memory precursors cell-intrinsically modulate their proliferative activity to diversify differentiation pathways
Gate-induced quantum-confinement transition of a single dopant atom in a silicon FinFET
The ability to build structures with atomic precision is one of the defining features of nanotechnology. Achieving true atomic- level functionality, however, requires the ability to control the wavefunctions of individual atoms. Here, we investigate an approach that could enable just that. By collecting and analysing transport spectra of a single donor atom in the channel of a silicon FinFET, we present experimental evidence for the emergence of a new type of hybrid molecule system. Our experiments and simulations suggest that the transistor\u27s gate potential can be used to control the degree of hybridization of a single electron donor state between the nuclear potential of its donor atom and a nearby quantum well. Moreover, our theoretical analysis enables us to determine the species of donor (arsenic) implanted into each device as well as the degree of confinement imposed by the gate
Spin blockade and exchange in Coulomb-confined silicon double quantum dots
Electron spins confined to phosphorus donors in silicon are promising candidates as qubits(1) because of their long coherence times, exceeding seconds in isotopically purified bulk silicon(2). With the recent demonstrations of initialization, readout(3) and coherent manipulation(4) of individual donor electron spins, the next challenge towards the realization of a Si:P donor-based quantum computer is the demonstration of exchange coupling(1,5,6) in two tunnel-coupled phosphorus donors. Spin-to-charge conversion(3,7) via Pauli spin blockade(8,9), an essential ingredient for reading out individual spin states, is challenging in donor-based systems due to the inherently large donor charging energies (similar to 45 meV), requiring large electric fields (\u3e1 MV m(-1)) to transfer both electron spins onto the same donor(10). Here, in a carefully characterized double donor-dot device, we directly observe spin blockade of the first few electrons and measure the effective exchange interaction between electron spins in coupled Coulomb-confined systems