% IMPORTANT: The following is UTF-8 encoded.  This means that in the presence
% of non-ASCII characters, it will not work with BibTeX 0.99 or older.
% Instead, you should use an up-to-date BibTeX implementation like “bibtex8” or
% “biber”.

@ARTICLE{Schreiber:861562,
      author       = {Schreiber, Lars R. and Bluhm, Hendrik},
      title        = {{T}oward a silicon-based quantum computer},
      journal      = {Science},
      volume       = {359},
      number       = {6374},
      issn         = {0036-8075},
      address      = {Cambridge, Mass.},
      publisher    = {Moses King},
      reportid     = {FZJ-2019-02013},
      pages        = {393 - 394},
      year         = {2018},
      abstract     = {Quantum computing could enable exponential speedups for
                      certain classes of problems by exploiting superposition and
                      entanglement in the manipulation of quantum bits (qubits).
                      The leading quantum systems that can be used include trapped
                      ions, superconducting qubits, and spins in semiconductors.
                      The latter are considered particularly promising for scaling
                      to very large numbers of qubits. On page 439 of this issue,
                      Zajac et al. (1) demonstrate a quantum operation involving
                      two qubits in silicon (Si), which is a major step for the
                      field of semiconductor qubits. Together with
                      easier-to-achieve manipulation of single qubits, these
                      operations represent the basic steps of any quantum
                      algorithm.The coupling between the two qubits is achieved
                      through the so-called exchange interaction, which results
                      from coupling of the two electrons through a tunnel barrier.
                      This barrier can be controlled by changing the voltage on
                      the central gate. The authors further use microwave
                      excitation to implement the desired operation. In an
                      external magnetic field, spins that are not aligned with the
                      field precess around it like an asymmetrically suspended
                      top. If an excitation (a microwave signal applied to one of
                      the gates, which translates to a magnetic microwave field in
                      the inhomogeneous stray field of a micromagnet) has the same
                      frequency as the precession, it is possible to rotate the
                      spin direction, for example, from parallel to antiparallel
                      with the field.This technique, commonly used to control
                      individual qubits, enables two-qubit operation via the
                      dependence of the precession rate of one spin on the state
                      of the other because of the exchange interaction. Whether
                      single-qubit or two-qubit operation is executed can be
                      selected by the choice of tunnel coupling and microwave
                      frequency. For the two-qubit operation, they are set such
                      that if the so-called control spin is aligned with the
                      external field, the other so-called target spin is on
                      resonance with the microwave signal (and off resonance if
                      anti-aligned). Whether the target spin is inverted depends
                      on the state of the control spin, so this system functions
                      as a controlled NOT (CNOT) gate.Zajac et al. used a device
                      consisting of a layer of Si that was strained by being grown
                      between two layers of SiGe (Si/SiGe), which confine
                      electrons to the Si layer. Additional lateral confinement
                      was provided by electrostatic gates fabricated on top of the
                      structure, which were arranged such that two electrons can
                      be captured (see the figure). The spin of each of those
                      electrons encoded one qubit. Similar qubits have been
                      realized in GaAs/AlGaAs heterostructures, but in that
                      material system, the interaction with unavoidable nuclear
                      spins is a major complication that impedes highly accurate
                      qubit operation (2). Only $4.7\%$ of the nuclei carry spin
                      in Si with a natural composition, and that fraction can be
                      further reduced with isotopic purification. This approach
                      has recently led to record-setting coherence times over
                      which quantum states could be preserved (3). However, the
                      controlled confinement of single electrons in Si has been a
                      major challenge because of disorder in the material. Petta
                      and co-workers (4) made important progress on sample quality
                      and design, culminating in an array of nine quantum dots,
                      which could in principle host nine qubits.},
      cin          = {PGI-11},
      ddc          = {500},
      cid          = {I:(DE-Juel1)PGI-11-20170113},
      pnm          = {144 - Controlling Collective States (POF3-144)},
      pid          = {G:(DE-HGF)POF3-144},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:29371457},
      UT           = {WOS:000423283200029},
      doi          = {10.1126/science.aar6209},
      url          = {https://juser.fz-juelich.de/record/861562},
}