We observe the quantum Zeno effect — where the act of measurement slows the rate of quantum state transitions — in a superconducting qubit using linear circuit quantum electrodynamicsreadout and a near-quantum-limited following amplifier. Under simultaneous strong measurement and qubit drive, the qubit undergoes a series of quantum jumps between states. These jumps are visible in the experimental measurement record and are analyzed using maximum likelihood estimation to determine qubit transition rates. The observed rates agree with both analytical predictions and numerical simulations. The analysis methods are suitable for processing general noisy random telegraph signals
We observe measurement-induced qubit state mixing in a transmon qubit
dispersively coupled to a planar readout cavity. Our results indicate that
dephasing noise at the qubit-readoutdetuning frequency is up-converted by
readout photons to cause spurious qubit state transitions, thus limiting the
nondemolition character of the readout. Furthermore, we use the qubit
transition rate as a tool to extract an equivalent flux noise spectral density
at f ~ 1 GHz and find agreement with values extrapolated from a $1/f^alpha$
fit to the measured flux noise spectral density below 1 Hz.
The act of measurement bridges the quantum and classical worlds by projecting
a superposition of possible states into a single, albeit probabilistic,
outcome. The time-scale of this„instantaneous“ process can be stretched using
weak measurements so that it takes the form of a gradual random walk towards a
final state. Remarkably, the interim measurement record is sufficient to
continuously track and steer the quantum state using feedback. We monitor the
dynamics of a resonantly driven quantum two-level system — a superconducting
quantum bit –using a near-noiseless parametric amplifier. The high-fidelity
measurement output is used to actively stabilize the phase of Rabi
oscillations, enabling them to persist indefinitely. This new functionality
shows promise for fighting decoherence and defines a path for continuous
quantum error correction.
We demonstrate high-fidelity, quantum nondemolition, single-shot readout of a
superconducting flux qubit in which the pointer state distributions can be
resolved to below one part in1000. In the weak excitation regime, continuous
measurement permits the use of heralding to ensure initialization to a fiducial
state, such as the ground state. This procedure boosts readout fidelity to
93.9% by suppressing errors due to spurious thermal population. Furthermore,
heralding potentially enables a simple, fast qubit reset protocol without
changing the system parameters to induce Purcell relaxation.