207 research outputs found
Anisotropic Stark Effect and Electric-Field Noise Suppression for Phosphorus Donor Qubits in Silicon
We report the use of novel, capacitively terminated coplanar waveguide (CPW)
resonators to measure the quadratic Stark shift of phosphorus donor qubits in
Si. We confirm that valley repopulation leads to an anisotropic spin-orbit
Stark shift depending on electric and magnetic field orientations relative to
the Si crystal. By measuring the linear Stark effect, we estimate the effective
electric field due to strain in our samples. We show that in the presence of
this strain, electric-field sources of decoherence can be non-negligible. Using
our measured values for the Stark shift, we predict magnetic fields for which
the spin-orbit Stark effect cancels the hyperfine Stark effect, suppressing
decoherence from electric-field noise. We discuss the limitations of these
noise-suppression points due to random distributions of strain and propose a
method for overcoming them
Addressing spin transitions on 209Bi donors in silicon using circularly-polarized microwaves
Over the past decade donor spin qubits in isotopically enriched Si
have been intensely studied due to their exceptionally long coherence times.
More recently bismuth donor electron spins have become popular because Bi has a
large nuclear spin which gives rise to clock transitions (first-order
insensitive to magnetic field noise). At every clock transition there are two
nearly degenerate transitions between four distinct states which can be used as
a pair of qubits. Here it is experimentally demonstrated that these transitions
are excited by microwaves of opposite helicity such that they can be
selectively driven by varying microwave polarization. This work uses a
combination of a superconducting coplanar waveguide (CPW) microresonator and a
dielectric resonator to flexibly generate arbitrary elliptical polarizations
while retaining the high sensitivity of the CPW
Spin relaxation and coherence times for electrons at the Si/SiO2 interface
While electron spins in silicon heterostructures make attractive qubits,
little is known about the coherence of electrons at the Si/SiO2 interface. We
report spin relaxation (T1) and coherence (T2) times for mobile electrons and
natural quantum dots at a 28Si/SiO2 interface. Mobile electrons have short T1
and T2 of 0.3 us at 5 K. In line with predictions, confining electrons and
cooling increases T1 to 0.8 ms at 350 mK. In contrast, T2 for quantum dots is
around 10 us at 350 mK, increasing to 30 us when the dot density is reduced by
a factor of two. The quantum dot T2 is shorter than T1, indicating that T2 is
not controlled by T1 at 350 mK but is instead limited by an extrinsic
mechanism. The evidence suggests that this extrinsic mechanism is an exchange
interaction between electrons in neighboring dots.Comment: Extended with more experiments and rewritten. 6 pages, 5 figures, to
be submitted to Phys. Rev.
Spin Coherence and N ESEEM Effects of Nitrogen-Vacancy Centers in Diamond with X-band Pulsed ESR
Pulsed ESR experiments are reported for ensembles of negatively-charged
nitrogen-vacancy centers (NV) in diamonds at X-band magnetic fields
(280-400 mT) and low temperatures (2-70 K). The NV centers in synthetic
type IIb diamonds (nitrogen impurity concentration ~ppm) are prepared with
bulk concentrations of cm to cm
by high-energy electron irradiation and subsequent annealing. We find that a
proper post-radiation anneal (1000C for 60 mins) is critically
important to repair the radiation damage and to recover long electron spin
coherence times for NVs. After the annealing, spin coherence times of T~ms at 5~K are achieved, being only limited by C nuclear spectral
diffusion in natural abundance diamonds. At X-band magnetic fields, strong
electron spin echo envelope modulation (ESEEM) is observed originating from the
central N nucleus. The ESEEM spectral analysis allows for accurate
determination of the N nuclear hypefine and quadrupole tensors. In
addition, the ESEEM effects from two proximal C sites (second-nearest
neighbor and fourth-nearest neighbor) are resolved and the respective C
hyperfine coupling constants are extracted.Comment: 10 pages, 5 figure
Superconducting coplanar waveguide resonators for low temperature pulsed electron spin resonance spectroscopy
We discuss the design and implementation of thin film superconducting
coplanar waveguide micro- resonators for pulsed ESR experiments. The
performance of the resonators with P doped Si epilayer samples is compared to
waveguide resonators under equivalent conditions. The high achievable filling
factor even for small sized samples and the relatively high Q-factor result in
a sensitivity that is superior to that of conventional waveguide resonators, in
particular to spins close to the sample surface. The peak microwave power is on
the order of a few microwatts, which is compatible with measurements at ultra
low temperatures. We also discuss the effect of the nonuniform microwave
magnetic field on the Hahn echo power dependence
Electron Paramagnetic Resonance of Boron Acceptors in Isotopically Purified Silicon
The electron paramagnetic resonance (EPR) linewidths of B acceptors in Si are
found to reduce dramatically in isotopically purified 28Si single crystals.
Moreover, extremely narrow substructures in the EPR spectra are visible
corresponding to either an enhancement or a reduction of the absorbed microwave
on resonance. The origin of the substructures is attributed to a combination of
simultaneous double excitation and spin relaxation in the four level spin
system of the acceptors. A spin population model is developed which
qualitatively describes the experimental results.Comment: 4 pages, 3 figure
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