  Quantum computation requires qubits that can be coupled and realized in a
scalable manner, together with universal and high-fidelity one- and two-qubit
logic gates \cite{DiVincenzo2000, Loss1998}. Strong effort across several
fields have led to an impressive array of qubit realizations, including trapped
ions \cite{Brown2011}, superconducting circuits \cite{Barends2014}, single
photons\cite{Kok2007}, single defects or atoms in diamond \cite{Waldherr2014,
Dolde2014} and silicon \cite{Muhonen2014}, and semiconductor quantum dots
\cite{Veldhorst2014}, all with single qubit fidelities exceeding the stringent
thresholds required for fault-tolerant quantum computing \cite{Fowler2012}.
Despite this, high-fidelity two-qubit gates in the solid-state that can be
manufactured using standard lithographic techniques have so far been limited to
superconducting qubits \cite{Barends2014}, as semiconductor systems have
suffered from difficulties in coupling qubits and dephasing \cite{Nowack2011,
Brunner2011, Shulman2012}. Here, we show that these issues can be eliminated
altogether using single spins in isotopically enriched silicon\cite{Itoh2014}
by demonstrating single- and two-qubit operations in a quantum dot system using
the exchange interaction, as envisaged in the original Loss-DiVincenzo proposal
\cite{Loss1998}. We realize CNOT gates via either controlled rotation (CROT) or
controlled phase (CZ) operations combined with single-qubit operations. Direct
gate-voltage control provides single-qubit addressability, together with a
switchable exchange interaction that is employed in the two-qubit CZ gate. The
speed of the two-qubit CZ operations is controlled electrically via the
detuning energy and we find that over 100 two-qubit gates can be performed
within a two-qubit coherence time of 8 \textmu s, thereby satisfying the
criteria required for scalable quantum computation.
