Atomic precision fabrication of quantum devices down to the single atom regime

The ability to control matter at the atomic scale and build devices with atomic precision is central to nanotechnology. The scanning tunnelling microscope can manipulate individual atoms and molecules on surfaces, but the manipulation of silicon to make atomic-scale logic circuits has been hampered by the covalent nature of its bonds. Resist-based strategies have allowed the formation of atomic-scale structures on silicon surfaces, but the fabrication of working devices-such as transistors with extremely short gate lengths, spin-based quantum computers and solitary dopant optoelectronic devices-requires the ability to position individual atoms in a silicon crystal with atomic precision. Here, we use a combination of scanning tunnelling microscopy and hydrogen-resist lithography to demonstrate a single-atom transistor in which an individual phosphorus dopant atom has been deterministically placed within an epitaxial silicon device architecture with a spatial accuracy of one lattice site. The transistor operates at liquid helium temperatures, and millikelvin electron transport measurements confirm the presence of discrete quantum levels in the energy spectrum of the phosphorus atom. We find a charging energy that is close to the bulk value, previously only observed by optical spectroscopy.

We present experimental techniques for using individual neutral atoms as qubits : trapping and moving single atoms, encoding qubits on hyperfine states, and entangling them. Special emphasis is given to recent techniques using Rydberg blockade.

We demonstrate nondestructive (lossless) fluorescent state detection of individual neutral atom qubits trapped in an optical lattice. The hyperfine state of the atom is measured with a 95% accuracy and an atom loss rate of 1%. Individual atoms are initialized and detected over 100 times before being lost from the trap, representing a 100-fold improvement in data collection rates over previous experiments. Microwave Rabi oscillations are observed with repeated measurements of one and the same single atom.

Determining the atomic structure of internal interfaces in materials and devices is critical to understanding their functional properties. Interfacial doping is one promising technique for controlling interfacial properties at the atomic scale, but it is still a major challenge to directly characterize individual dopant atoms within buried crystalline interfaces. Here, we demonstrate atomic-scale plan-view observation of a buried crystalline interface (an yttrium-doped alumina high-angle grain boundary) using aberration-corrected Z-contrast scanning transmission electron microscopy. The focused electron beam transmitted through the off-axis crystals clearly highlights the individual yttrium atoms located on the monoatomic layer interface plane. Not only is their unique two-dimensional ordered positioning directly revealed with atomic precision, but local disordering at the single-atom level, which has never been detected by the conventional approaches, is also uncovered. The ability to directly probe individual atoms within buried interface structures adds new dimensions to the atomic-scale characterization of internal interfaces and other defect structures in many advanced materials and devices.

Modern research in optical physics has achieved quantum control of strong interactions between a single atom and one photon within the setting of cavity quantum electrodynamics (cQED). However, to move beyond current proof-of-principle experiments involving one or two conventional optical cavities to more complex scalable systems that employ N >> 1 microscopic resonators requires the localization of individual atoms on distance scales < 100 nm from a resonator's surface. In this regime an atom can be strongly coupled to a single intracavity photon while at the same time experiencing significant radiative interactions with the dielectric boundaries of the resonator. Here, we report an initial step into this new regime of cQED by way of real-time detection and high-bandwidth feedback to select and monitor single Cesium atoms localized ~100 nm from the surface of a micro-toroidal optical resonator. We employ strong radiative interactions of atom and cavity field to probe atomic motion through the evanescent field of the resonator. Direct temporal and spectral measurements reveal both the significant role of Casimir-Polder attraction and the manifestly quantum nature of the atom-cavity dynamics. Our work sets the stage for trapping atoms near micro- and nano-scopic optical resonators for applications in quantum information science, including the creation of scalable quantum networks composed of many atom-cavity systems that coherently interact via coherent exchanges of single photons.

We report on the novel optimization method to realize highly uniform microtrap arrays for single atom trapping with a spatial light modulator (SLM). This method consists of two iterative feedback loops with the measurements of both diffracted light intensities and in-trap fluorescence intensities from each microtrap. By applying this method to the single 87Rb atom trapping, we can reduce the variance of trap depths from 20.8% to 1.7% for 4 × 4 square arrays and less than 4% for various arrays with up to 62 sites. The detection error of individual single atoms is also reduced from 1.7% to 0.0054% on average.

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing dynamical behaviour can arise because the particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report direct, time-resolved observations of the correlated tunnelling of two interacting ultracold atoms through a barrier in a double-well potential. For the regime in which the interactions between the atoms are weak and tunnel coupling dominates, individual atoms can tunnel independently, similar to the case of a normal Josephson junction. However, when strong repulsive interactions are present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second-order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime in which a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Such second-order tunnelling events, which are the dominating dynamical effect in the strongly interacting regime, have not been previously observed with ultracold atoms. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of proposals for realizing quantum magnetism.

We demonstrate full quantum state control of two species of single atoms using optical tweezers and assemble the atoms into a molecule. Our demonstration includes 3D ground-state cooling of a single atom (Cs) in an optical tweezer, transport by several microns with minimal heating, and merging with a single Na atom. Subsequently, both atoms occupy the simultaneous motional ground state with 61(4)\% probability. This realizes a sample of exactly two co-trapped atoms near the phase-space-density limit of one, and allows for efficient stimulated-Raman transfer of a pair of atoms into a molecular bound state of the triplet electronic ground potential $a^3\Sigma^+$. The results are key steps toward coherent creation of single ultracold molecules, for future exploration of quantum simulation and quantum information processing.

A single atom is the prototypical quantum system, and a natural candidate for a quantum bit, or qubit--the elementary unit of a quantum computer. Atoms have been successfully used to store and process quantum information in electromagnetic traps, as well as in diamond through the use of the nitrogen-vacancy-centre point defect. Solid-state electrical devices possess great potential to scale up such demonstrations from few-qubit control to larger-scale quantum processors. Coherent control of spin qubits has been achieved in lithographically defined double quantum dots in both GaAs (refs 3-5) and Si (ref. 6). However, it is a formidable challenge to combine the electrical measurement capabilities of engineered nanostructures with the benefits inherent in atomic spin qubits. Here we demonstrate the coherent manipulation of an individual electron spin qubit bound to a phosphorus donor atom in natural silicon, measured electrically via single-shot read-out. We use electron spin resonance to drive Rabi oscillations, and a Hahn echo pulse sequence reveals a spin coherence time exceeding 200 µs. This time should be even longer in isotopically enriched (28)Si samples. Combined with a device architecture that is compatible with modern integrated circuit technology, the electron spin of a single phosphorus atom in silicon should be an excellent platform on which to build a scalable quantum computer.

Summary form only given. Over the past years, ultracold quantum gases have offered remarkable opportunities to investigate quantum matter through new probing techniques and in previously unexplored regimes. They can in fact be seen as one of the first realizations of Richard Feynman's visionary concept of a quantum simulator offering far-reaching possibilities to engineer new phases of matter.In my talk, I will give an overview over some of the recent scientific highlights in the field. For example, I will explain how atoms can be imaged in optical lattices with single-site and single-atom resolution thereby allowing one to obtain single snapshots of a quantum many-body systems, where every individual atom of the system can be detected. Such high-resolution imaging has enabled our group to directly probe quantum fluctuations and reveal hidden order in a many-body setting. Having such high resolution at hand, it has also been possible to address and coherently control individual atoms, thereby allowing one to realize single atom impurities that can be tracked in a time resolved way during their propagation through a complex correlated quantum system. By making use of adiabatic state evolution in quantum mechanics or laser induced hopping in optical lattices, it has now also become possible to realize artificial magnetic fields for neutral particles. The field strengths that can be achieved for such systems can by far surpass even the strongest fields from the most powerful magnets on earth and allow one to probe quantum matter under the most extreme conditions.

Single Hf atoms inside the ultrathin SiO2 interlayer between a HfO2 dielectric film and the Si substrate: How do they modify the interface?

Current status of the research on the use of single neutral atoms, captured in optical traps, as a quantum computer's qubits, is briefly reviewed. The specifics of qubits based on cold atoms and t quantum logical operations via dipole interaction by short-time laser excitation in a Rydberg state are discussed. The latest experimental results of the observation of dipole interaction between two Rydberg atoms and of atoms' entangled states in a ground state are given.

Introduction When a crystal such as quartz or feldspar is bombarded by a stream of high-energy electrons in a scanning electron microscope or other suitable instrument, photons (“particles” of light) are emitted, a phenomenon called cathodoluminescence (CL). Much is known about the origin of CL in artificial crystals because of their economic importance in the manufacture of television screens, computer monitors, and the like. Although less is known about the origin of CL in naturally occurring minerals, the fundamental causes of CL emissions are moderately well understood. To visualize the factors responsible for CL emissions, it is useful to consider atoms in crystals in terms of the band theory of solids. Energy states of electrons in crystals depend upon whether the electrons are bound in particular atoms (inner-shell electrons) or are delocalized. Delocalized electrons are electrons that are not associated with individual atoms or identifiable chemical bonds, but are shared collectively by all the constituent atoms or ions of a substance. Delocalized electrons have wave functions (in quantum mechanics, a complex function of time and position) that in effect traverse the entire crystal. Individual atoms have discrete energy states that are associated with the orbits of shells of electrons in the atom. When atoms are spaced far apart, as in a gas, they have very little influence upon each other. By contrast, atoms within a solid, such as quartz or feldspar, have a marked effect upon other atoms in the crystal (atomic orbitals combine to form molecular orbitals).

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Many important structural problems in biology can be approached if particular components in an assembly can be labeled with a reagent that is detectable in an electron microscope. The development of the high resolution STEM, able to image single heavy atoms, raised the possibility of using individual atoms as labels. However, the imaging of single atoms requires very high dosages which are damaging to the specimen. To avoid this the undecagold cluster compound was introduced by Bartlett et al. (1978).