Precision sensing, and in particular high precision magnetometry, is a central goal of research into quantum technologies. For magnetometers, often trade-offs exist between sensitivity, spatial resolution, and frequency range. The precision, and thus
the sensitivity of magnetometry, scales as $1/sqrt {T_2}$ with the phase coherence time, $T_2$, of the sensing system playing the role of a key determinant. Adapting a dynamical decoupling scheme that allows for extending $T_2$ by orders of magnitude and merging it with a magnetic sensing protocol, we achieve a measurement sensitivity even for high frequency fields close to the standard quantum limit. Using a single atomic ion as a sensor, we experimentally attain a sensitivity of $4.6$ pT $/sqrt{Hz}$ for an alternating-current magnetic field near 14 MHz. Based on the principle demonstrated here, this unprecedented sensitivity combined with spatial resolution in the nanometer range and tunability from direct-current to the gigahertz range could be used for magnetic imaging in as of yet inaccessible parameter regimes.
We investigate the performance of inhomogeneously broadened spin ensembles as quantum memories under continuous dynamical decoupling. The role of the continuous driving field is two-fold: first, it decouples individual spins from magnetic noise; seco
nd and more important, it suppresses and reshapes the spectral inhomogeneity of spin ensembles. We show that a continuous driving field, which itself may also be inhomogeneous over the ensemble, can enhance the decay of the tails of the inhomogeneous broadening distribution considerably. This fact enables a spin ensemble based quantum memory to exploit the effect of cavity protection and achieve a much longer storage time. In particular, for a spin ensemble with a Lorentzian spectral distribution, our calculations demonstrate that continuous dynamical decoupling has the potential to improve its storage time by orders of magnitude for the state-of-art experimental parameters.
The loss of coherence is one of the main obstacles for the implementation of quantum information processing. The efficiency of dynamical decoupling schemes, which have been introduced to address this problem, is limited itself by the fluctuations in
the driving fields which will themselves introduce noise. We address this challenge by introducing the concept of concatenated continuous dynamical decoupling, which can overcome not only external magnetic noise but also noise due to fluctuations in driving fields. We show theoretically that this approach can achieve relaxation limited coherence times, and demonstrate experimentally that already the most basic implementation of this concept yields an order of magnitude improvement of the decoherence time for the electron spin of nitrogen vacancy centers in diamond. The proposed scheme can be applied to a wide variety of other physical systems including, trapped atoms and ions, quantum dots, and may be combined with other quantum technologies challenges such as quantum sensing and quantum information processing.
Recently, a framework was established to systematically construct novel universal resource states for measurement-based quantum computation using techniques involving finitely correlated states. With these methods, universal states were found which a
re in certain ways much less entangled than the original cluster state model, and it was hence believed that with this approach many of the extremal entanglement features of the cluster states could be relaxed. The new resources were constructed as computationally universal states--i.e. they allow one to efficiently reproduce the classical output of each quantum computation--whereas the cluster states are universal in a stronger sense since they are universal state preparators. Here we show that the new resources are universal state preparators after all, and must therefore exhibit a whole class of extremal entanglement features, similar to the cluster states.
We construct a linear optics measurement process to determine the entanglement measure, named emph{I-concurrence}, of a set of $4 times 4$ dimensional two-photon entangled pure states produced in the optical parametric down conversion process. In our
experiment, an emph{equivalent} symmetric projection for the two-fold copy of single subsystem (presented by L. Aolita and F. Mintert, Phys. Rev. Lett. textbf{97}, 050501 (2006)) can be realized by observing the one-side two-photon coincidence without any triggering detection on the other subsystem. Here, for the first time, we realize the measurement for entanglement contained in bi-photon pure states by taking advantage of the indistinguishability and the bunching effect of photons. Our method can determine the emph{I-concurrence} of generic high dimensional bipartite pure states produced in parametric down conversion process.
