  Measurement of quantum systems inevitably involves disturbance in various
forms. Within the limits imposed by quantum mechanics, however, one can design
an "ideal" projective measurement that does not introduce a back action on the
measured observable, known as a quantum nondemolition (QND) measurement. Here
we demonstrate an all-electrical QND measurement of a single electron spin in a
gate-defined quantum dot via an exchange-coupled ancilla qubit. The ancilla
qubit, encoded in the singlet-triplet two-electron subspace, is entangled with
the single spin and subsequently read out in a single shot projective
measurement at a rate two orders of magnitude faster than the spin relaxation.
The QND nature of the measurement protocol is evidenced by observing a
monotonic increase of the readout fidelity over one hundred repetitive
measurements against arbitrary input states. We extract information from the
measurement record using the method of optimal inference, which is tolerant to
the presence of the relaxation and dephasing. The QND measurement allows us to
observe spontaneous spin flips (quantum jumps) in an isolated system with small
disturbance. Combined with the high-fidelity control of spin qubits, these
results pave the way for various measurement-based quantum state manipulations
including quantum error correction protocols.
