71 research outputs found
A surface code quantum computer in silicon
The exceptionally long quantum coherence times of phosphorus donor nuclear spin qubits in silicon, coupled with the proven scalability of silicon-based nano-electronics, make them attractive candidates for large-scale quantum computing. However, the high threshold of topological quantum error correction can only be captured in a two-dimensional array of qubits operating synchronously and in parallel—posing formidable fabrication and control challenges. We present an architecture that addresses these problems through a novel shared-control paradigm that is particularly suited to the natural uniformity of the phosphorus donor nuclear spin qubit states and electronic confinement. The architecture comprises a two-dimensional lattice of donor qubits sandwiched between two vertically separated control layers forming a mutually perpendicular crisscross gate array. Shared-control lines facilitate loading/unloading of single electrons to specific donors, thereby activating multiple qubits in parallel across the array on which the required operations for surface code quantum error correction are carried out by global spin control. The complexities of independent qubit control, wave function engineering, and ad hoc quantum interconnects are explicitly avoided. With many of the basic elements of fabrication and control based on demonstrated techniques and with simulated quantum operation below the surface code error threshold, the architecture represents a new pathway for large-scale quantum information processing in silicon and potentially in other qubit systems where uniformity can be exploited
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
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
Spectroscopy of a Deterministic Single-Donor Device in Silicon
We present a single electron transistor (SET) based on an individual phosphorus dopant atom in an epitaxial silicon environment. Using scanning tunneling microscope (STM) hydrogen lithography, the single impurity is deterministically placed with a spatial accuracy of ±1 lattice site within a donor-based transport device. Low temperature transport measurements confirm the presence of the single donor and show that the donor charge state can be precisely controlled via gate voltages. We observe a charging energy that is remarkably similar to the value expected for isolated P donors in bulk silicon, which is in sharp contrast to previous experiments on single-dopant transport devices. We show that atomistic modeling can fully capture the effects of the highly-doped transport electrodes on the electronic states of the donor, thus highlighting the high level of control over the electrostatic device properties afforded by a deterministic single donor architecture. Our fabrication method therefore opens the door for the realization of a scalable donor-based qubit architecture in silicon
STM patterned nanowire measurements using photolithographically defined implants in Si(100)
Valley Splitting in a Silicon Quantum Device Platform
By suppressing an undesirable surface Umklapp process, it is possible to resolve the two most occupied states (1Г and 2 Г) in a buried two-dimensional electron gas (2DEG) in silicon. The 2DEG exists because of an atomically sharp profile of phosphorus dopants which have been formed beneath the Si(001) surface (a δ -layer). The energy separation, or valley splitting, of the two most occupied bands has critical implications for the properties of δ -layer derived devices, yet until now, has not been directly measurable. Density functional theory (DFT) allows the 2DEG band structure to be calculated, but without experimental verification the size of the valley splitting has been unclear. Using a combination of direct spectroscopic measurements and DFT we show that the measured band structure is in good qualitative agreement with calculations and reveal a valley splitting of 132 ± 5 meV. We also report the effective mass and occupation of the 2DEG states and compare the dispersions and Fermi surface with DFT
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