Topological semimetals have recently attracted extensive research interests as host materials to condensed matter physics counterparts of Dirac and Weyl fermions originally proposed in high energy physics. Although Lorentz invariance is required in high energy physics, it is not necessarily obeyed in condensed matter physics, and thus Lorentz-violating type-II Weyl/Dirac fermions could be realized in topological semimetals. The recent realization of type-II Weyl fermions raises the question whether their spin-degenerate counterpart—type-II Dirac fermions—can be experimentally realized too. Here, we report the experimental evidence of type-II Dirac fermions in bulk stoichiometric PtTe2 single crystal. Angle-resolved photoemission spectroscopy measurements and first-principles calculations reveal a pair of strongly tilted Dirac cones along the Γ-A direction, confirming PtTe2 as a type-II Dirac semimetal. Our results provide opportunities for investigating novel quantum phenomena (e.g., anisotropic magneto-transport) and topological phase transition.

SmB 6 , a well-known Kondo insulator, exhibits a transport anomaly at low temperature. This anomaly is usually attributed to states within the hybridization gap. Recent theoretical work and transport measurements suggest that these in-gap states could be ascribed to topological surface states, which would make SmB 6 the first realization of topological Kondo insulator. Here by performing angle-resolved photoemission spectroscopy experiments, we directly observe several dispersive states within the hybridization gap of SmB 6 . These states show negligible k z dependence, which indicates their surface origin. Furthermore, we perform photoemission circular dichroism experiments, which suggest that the in-gap states possess chirality of the orbital angular momentum. These states vanish simultaneously with the hybridization gap at around 150 K. Together, these observations suggest the possible topological origin of the in-gap states.

By combining scanning tunneling microscopy and spectroscopy, angle-resolved photoemission spectroscopy, and density functional theory band calculations, we directly observe and resolve the one-dimensional edge states of single bilayer (BL) Bi(111) islands on clean Bi(2)Te(3) and Bi(111)-covered Bi(2)Te(3) substrates. The edge states are localized in the vicinity of step edges having an ∼2  nm wide spatial distribution in real space and reside in the energy gap of the Bi(111) BL. Our results demonstrate the existence of nontrivial topological edge states of single Bi(111) bilayer as a two-dimensional topological insulator.

Magnetic topological insulators (MTIs) offer a combination of topologically nontrivial characteristics and magnetic order and show promise in terms of potentially interesting physical phenomena such as the quantum anomalous Hall (QAH) effect and topological axion insulating states. However, the understanding of their properties and potential applications have been limited due to a lack of suitable candidates for MTIs. Here, we grow two-dimensional single crystals of Mn(SbxBi(1-x))2Te4 bulk and exfoliate them into thin flakes in order to search for intrinsic MTIs. We perform angle-resolved photoemission spectroscopy, low-temperature transport measurements, and first-principles calculations to investigate the band structure, transport properties, and magnetism of this family of materials, as well as the evolution of their topological properties. We find that there exists an optimized MTI zone in the Mn(SbxBi(1-x))2Te4 phase diagram, which could possibly host a high-temperature QAH phase, offering a promising avenue for new device applications.

The dynamics of two variants of quantum Fisher information under decoherence are investigated from a geometrical point of view. We first derive the explicit formulas of these two quantities for a single qubit in terms of the Bloch vector. Moreover, we obtain analytical results for them under three different decoherence channels, which are expressed as affine transformation matrices. Using the hierarchy equation method, we numerically study the dynamics of both the two information in a dissipative model and compare the numerical results with the analytical ones obtained by applying the rotating-wave approximation. We further express the two information quantities in terms of the Bloch vector for a qudit, by expanding the density matrix and Hermitian operators in a common set of generators of the Lie algebra $\mathfrak{su}(d)$. By calculating the dynamical quantum Fisher information, we find that the collisional dephasing significantly diminishes the precision of phase parameter with the Ramsey interferometry.Comment: 16 pages, 4 figure

Quantum entanglement emerges naturally in interacting quantum systems and plays a central role in quantum information processing [1][2][3][4] . But the generation of entanglement does not require direct interactions: single-photon detection in spin-flip Raman scattering projects two distant spins onto a maximally entangled state, provided that it is impossible to determine the source of the detected photon 5 . Here, we demonstrate such heralded quantum entanglement [6][7][8][9] of two quantum-dot hole spins separated by 5 m using single-photon interference. Thanks to fast spin initialization in 10 ns, hole-spin coherence lasting ∼40 ns and e cient photon extraction from dots [10][11][12] embedded in leaky microcavity structures, we generate 2,300 entangled spin pairs per second, which represents a 1,000-fold improvement as compared to previous experiments 13. The delayed two-photon interference scheme we developed allows the e cient verification of quantum correlations. Combined with schemes for transferring quantum information to a long-lived memory qubit 14 , fast entanglement generation could impact quantum repeater architectures.In contrast to previous experiments demonstrating electron spin photon entanglement [10][11][12] , our experiments are based on heavyhole pseudo-spins in self-assembled quantum dots (QD) that have been shown to exhibit long coherence times [15][16][17][18] . Figure 1a depicts our experimental set-up, incorporating two QDs separated by 5 m that are resonantly driven by weak 3.2 ns-long pulses from a Ti:Sapphire laser, termed the entanglement laser. Additional diode laser pulses ensure that each QD is optically charged with a single excess heavy hole and that the hole pseudo-spin is prepared in the requisite state. The QDs are embedded in distributed Bragg reflector (DBR) structures 19 which, together with a ZnO solid immersion lens, allow efficient (∼20%) collection of the generated resonance fluorescence. Figure 1b shows the relevant energy-level diagram as well as the allowed optical transitions for single-hole charged QDs when an external magnetic field (B x ) is applied perpendicular to the growth direction (Voigt geometry; refs 20,21). The initial states of the optical transitions in the single-hole charged regime are metastable states identified by the orientation of the heavy-hole pseudo-spin, with |⇑ (|⇓ ) denoting +3/2 (−3/2) hole angular momentum projection. The presence of B x = 0 yields a finite splitting of the pseudo-spin states due to heavy-light hole mixing 22 . Spontaneous emission of a V (H) polarized photon at frequency ω blue (ω diag1 ) from the trion state |T b at rate Γ /2 brings the QD back into the |⇓ (|⇑ ) state. Owing to these selection rules, addressing any of the four allowed transitions with a single laser will efficiently transfer the spin population into the opposite ground state within 10 ns (see Supplementary information). As the intensity of the entanglement laser is chosen to be well below saturation, the ensuing optical transitions lead to either V-...

Valleytronics is an appealing alternative to conventional charge-based electronics which aims at encoding data in the valley degree of freedom, i.e. the information over which extreme of the conduction or valence band carriers are occupying. The ability to create and control valleycurrents in solid state devices could therefore enable new paradigms for information processing. Transition metal dichalcogenides (TMDCs) are a promising platform for valleytronics, due to the presence of two inequivalent valleys with spin-valley locking 1 and a direct band gap 2,3 , which allows optical initialization and readout of the valley-state 4,5. Recent progresses on the control of interlayer excitons in these materials 6-8 could offer an effective way to realize optoelectronic devices based on the valley degree of freedom. Here, we show the generation and transport over mesoscopic distances of valley-polarized excitons in a device based on a type-II TMDC heterostructure. Engineering of the interlayer coupling results in enhanced diffusion of valleypolarized excitons, which can be controlled and switched electrically. Furthermore, using electrostatic traps, we can increase exciton concentration by an order of magnitude, reaching densities in the order of 10 12 cm-2 , opening the route to achieving a coherent quantum state of valley-polarized excitons via Bose-Einstein condensation. Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:

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