105 research outputs found
Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics
We review our recent efforts in building atom-scale quantum-dot cellular
automata circuits on a silicon surface. Our building block consists of silicon
dangling bond on a H-Si(001) surface, which has been shown to act as a quantum
dot. First the fabrication, experimental imaging, and charging character of the
dangling bond are discussed. We then show how precise assemblies of such dots
can be created to form artificial molecules. Such complex structures can be
used as systems with custom optical properties, circuit elements for
quantum-dot cellular automata, and quantum computing. Considerations on
macro-to-atom connections are discussed.Comment: 28 pages, 19 figure
Correction to probing the quantum states of a single atom transistor at microwave frequencies
Abstract not availableGiuseppe Carlo Tettamanzi, Samuel James Hile, Matthew Gregory House, Martin Fuechsle, Sven Rogge, and Michelle Y. Simmon
An addressable quantum dot qubit with fault-tolerant control fidelity
Exciting progress towards spin-based quantum computing has recently been made
with qubits realized using nitrogen-vacancy (N-V) centers in diamond and
phosphorus atoms in silicon, including the demonstration of long coherence
times made possible by the presence of spin-free isotopes of carbon and
silicon. However, despite promising single-atom nanotechnologies, there remain
substantial challenges in coupling such qubits and addressing them
individually. Conversely, lithographically defined quantum dots have an
exchange coupling that can be precisely engineered, but strong coupling to
noise has severely limited their dephasing times and control fidelities. Here
we combine the best aspects of both spin qubit schemes and demonstrate a
gate-addressable quantum dot qubit in isotopically engineered silicon with a
control fidelity of 99.6%, obtained via Clifford based randomized benchmarking
and consistent with that required for fault-tolerant quantum computing. This
qubit has orders of magnitude improved coherence times compared with other
quantum dot qubits, with T_2* = 120 mus and T_2 = 28 ms. By gate-voltage tuning
of the electron g*-factor, we can Stark shift the electron spin resonance (ESR)
frequency by more than 3000 times the 2.4 kHz ESR linewidth, providing a direct
path to large-scale arrays of addressable high-fidelity qubits that are
compatible with existing manufacturing technologies
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
Molecule-by-Molecule Writing Using a Focused Electron Beam
The resolution of lithography techniques needs to be extended beyond their current limits to continue the trend of miniaturization and enable new applications. But what is the ultimate spatial resolution? It is known that single atoms can be imaged with a highly focused electron beam. Can single atoms also be written with an electron beam? We verify this with focused electron-beam-induced deposition (FEBID), a direct-write technique that has the current record for the smallest feature written by (electron) optical lithography. We show that the deposition of an organometallic precursor on graphene can be followed molecule-by-molecule with FEBID. The results show that mechanisms that are inherent to the process inhibit a further increase in control over the process. Hence, our results present the resolution limit of (electron) optical lithography techniques. The writing of isolated, subnanometer features with nanometer precision can be used, for instance, for the local modification of graphene and for catalysis.</p
Determining the Electronic Confinement of a Subsurface Metallic State
Dopant profiles in semiconductors are important for understanding nanoscale electronics. Highly conductive and extremely confined phosphorus doping profiles in silicon, known as Si:P δ-layers, are of particular interest for quantum computer applications, yet a quantitative measure of their electronic profile has been lacking. Using resonantly enhanced photoemission spectroscopy, we reveal the real-space breadth of the Si:P δ-layer occupied states and gain a rare view into the nature of the confined orbitals. We find that the occupied valley-split states of the δ-layer, the so-called 1Γ and 2Γ, are exceptionally confined with an electronic profile of a mere 0.40 to 0.52 nm at full width at half-maximum, a result that is in excellent agreement with density functional theory calculations. Furthermore, the bulk-like Si 3pz orbital from which the occupied states are derived is sufficiently confined to lose most of its pz-like character, explaining the strikingly large valley splitting observed for the 1Γ and 2Γ states
Quantum simulation of the Hubbard model with dopant atoms in silicon
In quantum simulation, many-body phenomena are probed in controllable quantum
systems. Recently, simulation of Bose-Hubbard Hamiltonians using cold atoms
revealed previously hidden local correlations. However, fermionic many-body
Hubbard phenomena such as unconventional superconductivity and spin liquids are
more difficult to simulate using cold atoms. To date the required single-site
measurements and cooling remain problematic, while only ensemble measurements
have been achieved. Here we simulate a two-site Hubbard Hamiltonian at low
effective temperatures with single-site resolution using subsurface dopants in
silicon. We measure quasiparticle tunneling maps of spin-resolved states with
atomic resolution, finding interference processes from which the entanglement
entropy and Hubbard interactions are quantified. Entanglement, determined by
spin and orbital degrees of freedom, increases with increasing covalent bond
length. We find separation-tunable Hubbard interaction strengths that are
suitable for simulating strongly correlated phenomena in larger arrays of
dopants, establishing dopants as a platform for quantum simulation of the
Hubbard model.Comment: 6 pages, 5 figures. Supplementary: 13 pages, 7 figures. New version
with some additional discussion, accepted in Nature Communication
Quantum dot spectroscopy using a single phosphorus donor
Using a deterministic single P donor placed with atomic precision accuracy next to a nanoscale silicon quantum dot, we present a way to analyze the energy spectrum of small quantum dots in silicon by tunnel-coupled transport measurements. The energy-level structure of the quantum dot is observed as resonance features within the transport bias triangles when the donor chemical potential is aligned with states within the quantum dot as confirmed by a numeric rate equation solver SIMON. This technique allows us to independently extract the quantum dot level structure irrespective of the density of states in the leads. Such a method is useful for the investigation of silicon quantum dots in the few-electron regime where the level structure is governed by an intricate interplay between the spin- and the valley-orbit degrees of freedom
- …