5 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
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
Spin dynamics in the optical cycle of single nitrogen-vacancy centres in diamond
We investigate spin-dependent decay and intersystem crossing in the optical
cycle of single negatively-charged nitrogen-vacancy (NV) centres in diamond. We
use spin control and pulsed optical excitation to extract both the
spin-resolved lifetimes of the excited states and the degree of
optically-induced spin polarization. By optically exciting the centre with a
series of picosecond pulses, we determine the spin-flip probabilities per
optical cycle, as well as the spin-dependent probability for intersystem
crossing. This information, together with the indepedently measured decay rate
of singlet population provides a full description of spin dynamics in the
optical cycle of NV centres. The temperature dependence of the singlet
population decay rate provides information on the number of singlet states
involved in the optical cycle.Comment: 11 pages, 5 figure
Comparison of dynamical decoupling protocols for a nitrogen-vacancy center in diamond
We perform a detailed theoretical-experimental study of the dynamical
decoupling (DD) of the nitrogen-vacancy (NV) center in diamond. We investigate
the DD sequences applied to suppress the dephasing of the electron spin of the
NV center induced by the coupling to a spin bath composed of the substitutional
nitrogen atoms. The decoupling efficiency of various DD schemes is studied,
including both periodic and periodic pulse sequences. For ideal control pulses,
we find that the DD protocols with the Carr-Purcell-Meiboom-Gill (CPMG) timing
of the pulses provides best performance. We show that, as the number of control
pulses increases, the decoupling fidelity scaling differs qualitatively from
the predictions of the Magnus expansion, and explain the origin of this
difference. In particular, more advanced symmetrized or concatenated protocols
do not improve the DD performance. Next, we investigate the impact of the
systematic instrumental pulse errors in different periodic and aperiodic pulse
sequences. The DD protocols with the single-axis control do not preserve all
spin components in the presence of the pulse errors, and the two-axis control
is needed. We demonstrate that the two-axis control sequence with the CPMG
timing is very robust with respect to the pulse errors. The impact of the pulse
errors can be diminished further by symmetrizing this protocol. For all
protocols studied here, we present a detailed account of the pulse error
parameters which make strongest impact on the DD performance. In conclusion, we
give specific recommendations about choosing the decoupling protocol for the
system under investigation.Comment: 16 pages, 11 figure
Aperiodic dynamical decoupling sequences in presence of pulse errors
Dynamical decoupling (DD) is a promising tool for preserving the quantum
states of qubits. However, small imperfections in the control pulses can
seriously affect the fidelity of decoupling, and qualitatively change the
evolution of the controlled system at long times. Using both analytical and
numerical tools, we theoretically investigate the effect of the pulse errors
accumulation for two aperiodic DD sequences, the Uhrig's DD UDD) protocol [G.
S. Uhrig, Phys. Rev. Lett. {\bf 98}, 100504 (2007)], and the Quadratic DD (QDD)
protocol [J. R. West, B. H. Fong and D. A. Lidar, Phys. Rev. Lett {\bf 104},
130501 (2010)]. We consider the implementation of these sequences using the
electron spins of phosphorus donors in silicon, where DD sequences are applied
to suppress dephasing of the donor spins. The dependence of the decoupling
fidelity on different initial states of the spins is the focus of our study. We
investigate in detail the initial drop in the DD fidelity, and its long-term
saturation. We also demonstrate that by applying the control pulses along
different directions, the performance of QDD protocols can be noticeably
improved, and explain the reason of such an improvement. Our results can be
useful for future implementations of the aperiodic decoupling protocols, and
for better understanding of the impact of errors on quantum control of spins.Comment: updated reference