239 research outputs found
Optical detection of magnetic resonance
The combination of magnetic resonance with laser spectroscopy provides some interesting options for increasing the sensitivity and information content of magnetic resonance. This review covers the basic physics behind the relevant processes, such as angular momentum conservation during absorption and emission. This can be used to enhance the polarization of the spin system by orders of magnitude compared to thermal polarisation as well as for detection with sensitivities down to the level of individual spins. These fundamental principles have been used in many different fields. This review summarises some of the examples in different physical system, including atomic and molecular systems, dielectric solids composed of rare earth and transition metal ions and semiconductors
Efficient quantum gates for individual nuclear spin qubits by indirect control
Hybrid quantum registers, such as electron-nuclear spin systems, have emerged
as promising hardware for implementing quantum information and computing
protocols in scalable systems. Nevertheless, the coherent control of such
systems still faces challenges. Particularly, the lower gyromagnetic ratios of
the nuclear spins cause them to respond slowly to control fields, resulting in
gate times that are generally longer than the coherence time of the electron
spin. Here, we demonstrate a scheme for circumventing this problem by indirect
control: We apply a small number of short pulses only to the electron spin and
let the full system undergo free evolution under the hyperfine coupling between
the pulses. Using this scheme, we realize robust quantum gates in an
electron-nuclear spin system, including a Hadamard gate on the nuclear spin and
a controlled-NOT gate with the nuclear spin as the target qubit. The durations
of these gates are shorter than the electron spin coherence time, and thus
additional operations to extend the system coherence time are not needed. Our
demonstration serves as a proof of concept for achieving efficient coherent
control of electron-nuclear spin systems, such as NV centers in diamond. Our
scheme is still applicable when the nuclear spins are only weakly coupled to
the electron spin.Comment: Supplementary material added; Accepted for publication in PR
Optimal pulse spacing for dynamical decoupling in the presence of a purely-dephasing spin-bath
Maintaining quantum coherence is a crucial requirement for quantum
computation; hence protecting quantum systems against their irreversible
corruption due to environmental noise is an important open problem. Dynamical
decoupling (DD) is an effective method for reducing decoherence with a low
control overhead. It also plays an important role in quantum metrology, where
for instance it is employed in multiparameter estimation. While a sequence of
equidistant control pulses (CPMG) has been ubiquitously used for decoupling,
Uhrig recently proposed that a non-equidistant pulse sequence (UDD) may enhance
DD performance, especially for systems where the spectral density of the
environment has a sharp frequency cutoff. On the other hand, equidistant
sequences outperform UDD for soft cutoffs. The relative advantage provided by
UDD for intermediate regimes is not clear. In this paper, we analyze the
relative DD performance in this regime experimentally, using solid-state
nuclear magnetic resonance. Our system-qubits are 13C nuclear spins and the
environment consists of a 1H nuclear spin-bath whose spectral density is close
to a normal (Gaussian) distribution. We find that in the presence of such a
bath, the CPMG sequence outperforms the UDD sequence. An analogy between
dynamical decoupling and interference effects in optics provides an intuitive
explanation as to why the CPMG sequence performs superior to any
non-equidistant DD sequence in the presence of this kind of environmental
noise.Comment: To be published in Phys. Rev. A. 15 pages, 16 figures. Presentation
of the work was improved. One Figure and some Refs. were adde
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