14 research outputs found
Quantum Discord for Investigating Quantum Correlations without Entanglement in Solids
Quantum systems unfold diversified correlations which have no classical
counterparts. These quantum correlations have various different facets. Quantum
entanglement, as the most well known measure of quantum correlations, plays
essential roles in quantum information processing. However, it has recently
been pointed out that quantum entanglement cannot describe all the
nonclassicality in the correlations. Thus the study of quantum correlations in
separable states attracts widely attentions. Herein, we experimentally
investigate the quantum correlations of separable thermal states in terms of
quantum discord. The sudden change of quantum discord is observed, which
captures ambiguously the critical point associated with the behavior of
Hamiltonian. Our results display the potential applications of quantum
correlations in studying the fundamental properties of quantum system, such as
quantum criticality of non-zero temperature.Comment: 4 pages, 4 figure
Experimental Time-Optimal Universal Control of Spin Qubits in Solids
Quantum control of systems plays an important role in modern science and technology. The ultimate goal of quantum control is to achieve high-fidelity universal control in a time-optimal way. Although high-fidelity universal control has been reported in various quantum systems, experimental implementation of time-optimal universal control remains elusive. Here, we report the experimental realization of time-optimal universal control of spin qubits in diamond. By generalizing a recent method for solving quantum brachistochrone equations [X. Wang et al., Phys. Rev. Lett. 114, 170501 (2015)], we obtained accurate minimum-time protocols for multiple qubits with fixed qubit interactions and a constrained control field. Single- and two-qubit time-optimal gates are experimentally implemented with fidelities of 99% obtained via quantum process tomography. Our work provides a time-optimal route to achieve accurate quantum control and unlocks new capabilities for the emerging field of time-optimal control in general quantum systems.National Basic Research Program of China (973 Program) (Grants 2013CB921800 and 2016YFB0501603)National Natural Science Foundation (China) (Grants 11227901, 31470835, and 11275183)Chinese Academy of Sciences. Strategic Priority Research Program (Grant XDB01030400)National Science Foundation (U.S.) (Project CCF-1350397
Dopant-assisted stabilization of negatively charged single nitrogen-vacancy centers in phosphorus-doped diamond at low temperatures
Charge state instabilities have been a bottleneck for the implementation of
solid-state spin systems and pose a major challenge to the development of
spin-based quantum technologies. Here we investigate the stabilization of
negatively charged nitrogen-vacancy (NV) centers in phosphorus-doped
diamond at liquid helium temperatures. Photoionization of phosphorous donors in
conjunction with charge diffusion at the nanoscale enhances NV to NV
conversion and stabilizes the NV charge state without the need for an
additional repump laser. The phosphorus-assisted stabilization is explored and
confirmed both with experiments and our theoretical model. Stable
photoluminescence-excitation spectra are obtained for NV centers created
during the growth. The fluorescence is continuously recorded under resonant
excitation to real-time monitor the charge state and the ionization and
recombination rates are extracted from time traces. We find a linear laser
power dependence of the recombination rate as opposed to the conventional
quadratic dependence, which is attributed to the photo-ionization of phosphorus
atoms.Comment: 8 pages, 4 figure