77 research outputs found
Single-spin magnetometry with multi-pulse sensing sequences
We experimentally demonstrate single-spin magnetometry with multi-pulse
sensing sequences. The use of multi-pulse sequences can greatly increase the
sensing time per measurement shot, resulting in enhanced ac magnetic field
sensitivity. We theoretically derive and experimentally verify the optimal
number of sensing cycles, for which the effects of decoherence and increased
sensing time are balanced. We perform these experiments for oscillating
magnetic fields with fixed phase as well as for fields with random phase.
Finally, by varying the phase and frequency of the ac magnetic field, we
measure the full frequency-filtering characteristics of different multi-pulse
schemes and discuss their use in magnetometry applications.Comment: 4 pages, 4 figures. Final versio
Initialization by measurement of a two-qubit superconducting circuit
We demonstrate initialization by joint measurement of two transmon qubits in
3D circuit quantum electrodynamics. Homodyne detection of cavity transmission
is enhanced by Josephson parametric amplification to discriminate the two-qubit
ground state from single-qubit excitations non-destructively and with 98.1%
fidelity. Measurement and postselection of a steady-state mixture with 4.7%
residual excitation per qubit achieve 98.8% fidelity to the ground state, thus
outperforming passive initialization.Comment: 5 pages, 4 figures, and Supplementary Information (7 figures, 1
table
Heralded state preparation in a superconducting qubit
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 in 1000. 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.Comment: 5 pages, 5 figure
Bootstrap tomography of high-precision pulses for quantum control
Long-time dynamical decoupling and quantum control of qubits require
high-precision control pulses. Full characterization (quantum tomography) of
imperfect pulses presents a bootstrap problem: tomography requires initial
states of a qubit which can not be prepared without imperfect pulses. We
present a protocol for pulse error analysis, specifically tailored for a wide
range of the single solid-state electron spins. Using a single electron spin of
a nitrogen-vacancy (NV) center in diamond, we experimentally verify the
correctness of the protocol, and demonstrate its usefulness for quantum control
tasks
Partial-measurement back-action and non-classical weak values in a superconducting circuit
We realize indirect partial measurement of a transmon qubit in circuit
quantum electrodynamics by interaction with an ancilla qubit and projective
ancilla measurement with a dedicated readout resonator. Accurate control of the
interaction and ancilla measurement basis allows tailoring the measurement
strength and operator. The tradeoff between measurement strength and qubit
back-action is characterized through the distortion of a qubit Rabi oscillation
imposed by ancilla measurement in different bases. Combining partial and
projective qubit measurements, we provide the solid-state demonstration of the
correspondence between a non-classical weak value and the violation of a
Leggett-Garg inequality.Comment: 5 pages, 4 figures, and Supplementary Information (8 figures
Reversing quantum trajectories with analog feedback
We demonstrate the active suppression of transmon qubit dephasing induced by
dispersive measurement, using parametric amplification and analog feedback. By
real-time processing of the homodyne record, the feedback controller reverts
the stochastic quantum phase kick imparted by the measurement on the qubit. The
feedback operation matches a model of quantum trajectories with measurement
efficiency , consistent with the result obtained by
postselection. We overcome the bandwidth limitations of the amplification chain
by numerically optimizing the signal processing in the feedback loop and
provide a theoretical model explaining the optimization result.Comment: 5 pages, 4 figures, and Supplementary Information (7 figures
Feedback control of a solid-state qubit using high-fidelity projective measurement
We demonstrate feedback control of a superconducting transmon qubit using
discrete, projective measurement and conditional coherent driving. Feedback
realizes a fast and deterministic qubit reset to a target state with 2.4% error
averaged over input superposition states, and cooling of the transmon from 16%
spurious excitation to 3%. This closed-loop qubit control is necessary for
measurement-based protocols such as quantum error correction and teleportation.Comment: 5 pages, 4 figures, and Supplementary Information (5 figures
Detecting bit-flip errors in a logical qubit using stabilizer measurements
Quantum data is susceptible to decoherence induced by the environment and to
errors in the hardware processing it. A future fault-tolerant quantum computer
will use quantum error correction (QEC) to actively protect against both. In
the smallest QEC codes, the information in one logical qubit is encoded in a
two-dimensional subspace of a larger Hilbert space of multiple physical qubits.
For each code, a set of non-demolition multi-qubit measurements, termed
stabilizers, can discretize and signal physical qubit errors without collapsing
the encoded information. Experimental demonstrations of QEC to date, using
nuclear magnetic resonance, trapped ions, photons, superconducting qubits, and
NV centers in diamond, have circumvented stabilizers at the cost of decoding at
the end of a QEC cycle. This decoding leaves the quantum information vulnerable
to physical qubit errors until re-encoding, violating a basic requirement for
fault tolerance. Using a five-qubit superconducting processor, we realize the
two parity measurements comprising the stabilizers of the three-qubit
repetition code protecting one logical qubit from physical bit-flip errors. We
construct these stabilizers as parallelized indirect measurements using
ancillary qubits, and evidence their non-demolition character by generating
three-qubit entanglement from superposition states. We demonstrate
stabilizer-based quantum error detection (QED) by subjecting a logical qubit to
coherent and incoherent bit-flip errors on its constituent physical qubits.
While increased physical qubit coherence times and shorter QED blocks are
required to actively safeguard quantum information, this demonstration is a
critical step toward larger codes based on multiple parity measurements.Comment: 6 pages, 4 figures, 10 supplementary figure
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