A fundamental axiom of quantum mechanics requires the Hamiltonians to be Hermitian which guarantees real eigen-energies and probability conservation. However, a class of non-Hermitian Hamiltonians with Parity-Time (PT ) symmetry can still display entirely real spectra [1]. The Hermiticity requirement may be replaced by PT symmetry to develop an alternative formulation of quantum mechanics [2, 3]. A series of experiments have been carried out with classical systems including optics [4], electronics [5][6][7], microwaves[8], mechanics [9] and acoustics [10][11][12]. However, there are few experiments to investigate PT symmetric physics in quantum systems. Here we report the first observation of the PT symmetry breaking in a single spin system. We have developed a novel method to dilate a general PT symmetric Hamiltonian into a Hermitian one, which can be realized in a practical quantum system. Then the state evolutions under PT symmetric Hamiltonians, which range from PT symmetric unbroken to broken regions, have been experimentally observed with a single nitrogen-vacancy (NV) center in diamond. Due to the universality of the dilation method, our result opens a door for further exploiting and understanding the physical properties of PT symmetric Hamiltonian in quantum systems. arXiv:1812.05226v1 [quant-ph]

Rare-earth-doped crystals are excellent hardware for quantum storage of photons. Additional functionality of these materials is added by their waveguiding properties allowing for on-chip photonic networks. However, detection and coherent properties of rare-earth single-spin qubits have not been demonstrated so far. Here we present experimental results on high-fidelity optical initialization, effcient coherent manipulation and optical readout of a single-electron spin of Ce 3 þ ion in a yttrium aluminium garnet crystal. Under dynamic decoupling, spin coherence lifetime reaches T 2 ¼ 2 ms and is almost limited by the measured spin-lattice relaxation time T 1 ¼ 4.5 ms. Strong hyperfine coupling to aluminium nuclear spins suggests that cerium electron spins can be exploited as an interface between photons and long-lived nuclear spin memory. Combined with high brightness of Ce 3 þ emission and a possibility of creating photonic circuits out of the host material, this makes cerium spins an interesting option for integrated quantum photonics.

Although first principles methods are gaining interest, the crystal field model is at present the only practicable model to analyze and simulate the energy level structures of lanthanide ions (Ln(3+)) in crystal hosts at the accuracy level of approximately 10 cm(-1). Three criteria are suggested to assess the use of energy parameters, especially crystal field parameters, from the crystal field parametrization of 4f(N) energy level data sets for the entire lanthanide ion (Ln(3+)) series, except Pm(3+). Systematic analyses have been performed upon the most complete energy level data sets available for Ln(3+) situated at sites of high symmetry in crystals of Cs(2)NaLnCl(6). This presents a stringent test for theory because the number of energy parameters is considerably reduced, and the data sets are representative and fairly complete. The results from these data set fittings are shown to comply with the three criteria put forward. First, the fittings of data sets are accurate, and a predictive capability has been employed to calculate the energy levels of Pm(3+) and to elucidate and list all of the potentially luminescent levels of Ln(3+) in the hexachloroelpasolite hosts. Second, the systematic and smooth variations of parameter values over the lanthanide series have been described by simple equations and rationalized. Third, a physical insight of the crystal field parameter variation across this series of elements has been achieved by utilizing a simple semiquantitative model considering the distributions of the 4f radial wave functions at the edge of the Ln(3+) ions, where the ligand orbitals extend. The parameter trends for an individual Ln(3+) ion have been shown to be consistent also for the Cs(2)NaLnF(6) host lattice, and predictions of the individual crystal field parameter values are made.

The measurement of the microwave field is crucial for many developments in microwave technology and related applications. However, measuring microwave fields with high sensitivity and spatial resolution under ambient conditions remains elusive. In this work, we propose and experimentally demonstrate a scheme to measure both the strength and orientation of the microwave magnetic field by utilizing the quantum coherent dynamics of nitrogen vacancy centres in diamond. An angular resolution of 5.7 mrad and a sensitivity of 1.0 μT Hz−1/2 are achieved at a microwave frequency of 2.6000 GHz, and the microwave magnetic field vectors generated by a copper wire are precisely reconstructed. The solid-state microwave magnetometry with high resolution and wide frequency range that can work under ambient conditions proposed here enables unique potential applications over other state-of-art microwave magnetometry.

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In order to achieve reliable quantum-information processing results, we need to protect quantum gates along with the qubits from decoherence. Here we demonstrate experimentally on a nitrogen-vacancy system that by using a continuous-wave dynamical decoupling method, we might not only prolong the coherence time by about 20 times but also protect the quantum gates for the duration of the controlling time. This protocol shares the merits of retaining the superiority of prolonging the coherence time and at the same time easily combining with quantum logic tasks. This method can be useful in tasks where the duration of quantum controlling exceeds far beyond the dephasing time.

Yttrium orthorborate YBO 3 containing 0.5 or 5 atom % of Eu 3+ has been prepared by the spray-pyrolysis method. Low-temperature, high-resolution emission spectroscopy has been used to probe the local site symmetries of Eu 3+ ions accommodated on the Y 3+ sites in YBO 3 :Eu 3+ (0.5 atom %) and hence to determine the crystal structure of the title compound. The use of point group selection rules enabled a consistent spectral interpretation by envisaging distinct C i and C 1 symmetry Eu 3+ sites, in accordance with the neutron diffraction study of Lin et al. (Chem. Mater. 2004, 16, 2418. The assignment of features in the 10 K visible absorption spectrum of YBO 3 :Eu 3+ (5 atom %) is also consistent with the two-site model, but it was found that the site occupation by Eu 3+ differs from the expected ratio 2C 1 :1C i . The fitting of 31 crystal-field levels of the C 1 site has been made with some approximations by employing 7 parameters. In addition, the emission spectrum of YBO 3 :Eu 3+ containing a trace amount of Y 3 BO 6 has been compared with those of YBO 3 :Eu 3+ and Y 3 BO 6 : Eu 3+ , which serves as a reference to elucidate the concern on the red/orange color ratio of YBO 3 :Eu 3+ emission.