Suddhasatta Mahapatra

A single-atom transistor

Over the past decade we have developed a radical new strategy for the fabrication of atomic-scale devices in silicon [1]. Using this process we have demonstrated few electron, single crystal quantum dots [2], conducting nanoscale wires with widths down to ~1.5nm [3] and most recently a single atom transistor [4]. We will present atomic-scale images and electronic characteristics of these atomically precise devices and demonstrate the impact of strong vertical and lateral confinement on electron transport. We will also discuss the opportunities ahead for atomic-scale quantum computing architectures and some of the challenges to achieving truly atomically precise devices in all three spatial dimensions.

Ohm’s Law Survives to the Atomic Scale

Wiring Up Silicon Surfaces One of the challenges in downsizing electronic circuits is maintaining low resistivity of wires, because shrinking their diameter to near atomic dimensions increases interface effects and can decrease the effectiveness of dopants. Weber et al. (p. 64; see the Perspective by Ferry) created nanowires on a silicon surface with the deposition of phosphorus atoms through decomposition of PH3 with a scanning tunneling microscope tip. A brief thermal annealing embedded these nanowires, which varied from 1.5 to 11 nanometers in width, into the silicon surface. Their resistivity was independent of width, and their current-carrying capability was comparable to that of thicker copper interconnects. Nanowires created by embedding phosphorus atoms within silicon exhibit a low, diameter-independent resistivity. As silicon electronics approaches the atomic scale, interconnects and circuitry become comparable in size to the active device components. Maintaining low electrical resistivity at this scale is challenging because of the presence of confining surfaces and interfaces. We report on the fabrication of wires in silicon—only one atom tall and four atoms wide—with exceptionally low resistivity (~0.3 milliohm-centimeters) and the current-carrying capabilities of copper. By embedding phosphorus atoms within a silicon crystal with an average spacing of less than 1 nanometer, we achieved a diameter-independent resistivity, which demonstrates ohmic scaling to the atomic limit. Atomistic tight-binding calculations confirm the metallicity of these atomic-scale wires, which pave the way for single-atom device architectures for both classical and quantum information processing.

Spectroscopy of few-electron single-crystal silicon quantum dots.

A defining feature of modern CMOS devices and almost all quantum semiconductor devices is the use of many different materials. For example, although electrical conduction often occurs in single-crystal semiconductors, gates are frequently made of metals and dielectrics are commonly amorphous. Such devices have demonstrated remarkable improvements in performance over recent decades, but the heterogeneous nature of these devices can lead to defects at the interfaces between the different materials, which is a disadvantage for applications in spintronics and quantum information processing. Here we report the fabrication of a few-electron quantum dot in single-crystal silicon that does not contain any heterogeneous interfaces. The quantum dot is defined by atomically abrupt changes in the density of phosphorus dopant atoms, and the resulting confinement produces novel effects associated with energy splitting between the conduction band valleys. These single-crystal devices offer the opportunity to study how very sharp, atomic-scale confinement--which will become increasingly important for both classical and quantum devices--influences the operation and performance of devices.

Spin blockade and exchange in Coulomb-confined silicon double quantum dots

Electron spins confined to phosphorus donors in silicon are promising candidates as qubits because of their long coherence times, exceeding seconds in isotopically purified bulk silicon. With the recent demonstrations of initialization, readout and coherent manipulation 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 in two tunnel-coupled phosphorus donors. Spin-to-charge conversion via Pauli spin blockade, an essential ingredient for reading out individual spin states, is challenging in donor-based systems due to the inherently large donor charging energies (∼45 meV), requiring large electric fields (>1 MV m(-1)) to transfer both electron spins onto the same donor. 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.

Spin readout and addressability of phosphorus-donor clusters in silicon

The spin states of an electron bound to a single phosphorus donor in silicon show remarkably long coherence and relaxation times, which makes them promising building blocks for the realization of a solid-state quantum computer. Here we demonstrate, by high-fidelity (93%) electrical spin readout, that a long relaxation time T1 of about 2 s, at B=1.2 T and T≈100 mK, is also characteristic of electronic spin states associated with a cluster of few phosphorus donors, suggesting their suitability as hosts for spin qubits. Owing to the difference in the hyperfine coupling, electronic spin transitions of such clusters can be sufficiently distinct from those of a single phosphorus donor. Our atomistic tight-binding calculations reveal that when neighbouring qubits are hosted by a single phosphorus atom and a cluster of two phosphorus donors, the difference in their electron spin resonance frequencies allows qubit rotations with error rates ≈10(-4). These results provide a new approach to achieving individual qubit addressability.

CdSe quantum dot microdisk laser

Laser emission from optically pumped CdSe quantum dots embedded in a ZnSe∕ZnMgSSe microdisk was observed at low temperatures. Laser thresholds below 20μW and spontaneous emission coupling β-factors exceeding 0.8 were determined. For different modes of the same microdisk cavity the laser threshold increases with higher mode energy, which the authors associate to a better coupling of larger quantum dots with the optical mode. Also the linewidth and energy shift of lasing modes as a function of pump power were analyzed.

Charge sensing of precisely positioned p donors in Si.

Real-time sensing of (spin-dependent) single-electron tunneling is fundamental to electrical readout of qubit states in spin quantum computing. Here, we demonstrate the feasibility of detecting such single-electron tunneling events using an atomically planar charge sensing layout, which can be readily integrated in scalable quantum computing architectures with phosphorus-donor-based spin qubits in silicon (Si:P). Using scanning tunneling microscopy (STM) lithography on a Si(001) surface, we patterned a single-electron transistor (SET), both tunnel and electrostatically coupled to a coplanar ultrasmall quantum dot, the latter consisting of approximately four P donors. Charge transitions of the quantum dot could be detected both in time-averaged and single-shot current response of the SET. Single electron tunneling between the quantum dot and the SET island on a time-scale (τ ∼ ms) two-orders-of-magnitude faster than the spin-lattice relaxation time of a P donor in Si makes this device geometry suitable for projective readout of Si:P spin qubits. Crucial to scalability is the ability to reproducibly achieve sufficient electron tunnel rates and charge sensitivity of the SET. The inherent atomic-scale control of STM lithography bodes extremely well to precisely optimize both of these parameters.

Circular-to-Linear and Linear-to-Circular Conversion of Optical Polarization by Semiconductor Quantum Dots

We report circular-to-linear and linear-to-circular conversion of optical polarization by semiconductor quantum dots. The polarization conversion occurs under continuous wave excitation in absence of any magnetic field. The effect originates from quantum interference of linearly and circularly polarized photon states, induced by the natural anisotropic shape of the self assembled dots. The behavior can be qualitatively explained in terms of a pseudospin formalism.

Engineering Independent Electrostatic Control of Atomic-Scale (∼4 nm) Silicon Double Quantum Dots

Scalable quantum computing architectures with electronic spin qubits hosted by arrays of single phosphorus donors in silicon require local electric and magnetic field control of individual qubits separated by ∼10 nm. This daunting task not only requires atomic-scale accuracy of single P donor positioning to control interqubit exchange interaction but also demands precision alignment of control electrodes with careful device design at these small length scales to minimize cross capacitive coupling. Here we demonstrate independent electrostatic control of two Si:P quantum dots, each consisting of ∼15 P donors, in an optimized device design fabricated by scanning tunneling microscope (STM)-based lithography. Despite the atomic-scale dimensions of the quantum dots and control electrodes reducing overall capacitive coupling, the electrostatic behavior of the device shows an excellent match to results of a priori capacitance calculations. These calculations highlight the importance of the interdot angle in achieving independent control at these length-scales. This combination of predictive electrostatic modeling and the atomic-scale fabrication accuracy of STM-lithography, provides a powerful tool for scaling multidonor dots to the single donor limit.

Whispering gallery modes in high quality ZnSe/ZnMgSSe microdisks with CdSe quantum dots studied at room temperature

The authors observed whispering gallery modes with quality factors exceeding 2000 in ZnSe∕ZnMgSSe microdisks with self-assembled CdSe quantum dots at room temperature. The microdisks were realized by electron beam lithography and reactive ion etching and mounted on a glass plate. The far field emission of the microdisks exhibits a maximum in the direction parallel to the disk plane and couples efficiently into the glass plate. The microdisk spectra were analyzed in terms of mode separation for disk diameters ranging from 1.5to3.0μm. Transversal magnetic and transversal electric modes were identified.