Dielectric loss from two-level states is shown to be a dominant decoherence source in superconducting quantum bits. Depending on the qubit design, dielectric loss from insulating materials or the tunnel junction can lead to short coherence times. We show that a variety of microwave and qubit measurements are well modeled by loss from resonant absorption of two-level defects. Our results demonstrate that this loss can be significantly reduced by using better dielectrics and fabricating junctions of small area . With a redesigned phase qubit employing low-loss dielectrics, the energy relaxation rate has been improved by a factor of 20, opening up the possibility of multiqubit gates and algorithms.

Plasmons are quantized collective oscillations of electrons and have been observed in metals and doped semiconductors. The plasmons of ordinary, massive electrons have been the basic ingredients of research in plasmonics and in optical metamaterials for a long time. However, plasmons of massless Dirac electrons have only recently been observed in graphene, a purely two-dimensional electron system. Their properties are promising for novel tunable plasmonic metamaterials in the terahertz and mid-infrared frequency range. Dirac fermions also occur in the two-dimensional electron gas that forms at the surface of topological insulators as a result of the strong spin-orbit interaction existing in the insulating bulk phase. One may therefore look for their collective excitations using infrared spectroscopy. Here we report the first experimental evidence of plasmonic excitations in a topological insulator (Bi2Se3). The material was prepared in thin micro-ribbon arrays of different widths W and periods 2W to select suitable values of the plasmon wavevector k. The linewidth of the plasmon was found to remain nearly constant at temperatures between 6 K and 300 K, as expected when exciting topological carriers. Moreover, by changing W and measuring the plasmon frequency in the terahertz range versus k we show, without using any fitting parameter, that the dispersion curve agrees quantitatively with that predicted for Dirac plasmons.

With high quality topological insulator Bi(2)Se(3) thin films, we report thickness-independent transport properties over wide thickness ranges. Conductance remained nominally constant as the sample thickness changed from 256 to ∼8  QL (where QL refers to quintuple layer, 1  QL≈1  nm). Two surface channels of very different behaviors were identified. The sheet carrier density of one channel remained constant at ∼3.0×10(13)  cm(-2) down to 2 QL, while the other, which exhibited quantum oscillations, remained constant at ∼8×10(12)  cm(-2) only down to ∼8  QL. The weak antilocalization parameters also exhibited similar thickness independence. These two channels are most consistent with the topological surface states and the surface accumulation layers, respectively.

One of the many challenges of building a scalable quantum computer is single-shot measurement of all the quantum bits (qubits). We have used simultaneous single-shot measurement of coupled Josephson phase qubits to directly probe interaction of the qubits in the time domain. The concept of measurement crosstalk is introduced, and we show that its effects are minimized by careful adjustment of the timing of the measurements. We observe the antiphase oscillation of the two-qubit 01 and 10 states, consistent with quantum mechanical entanglement of these states, thereby opening the possibility for full characterization of multiqubit gates and elementary quantum algorithms.

Topological insulators (TIs) are newly discovered states of matter with robust metallic surface states protected by the topological properties of the bulk wavefunctions [1][2][3][4][5][6]. A quantum phase transition (QPT) from a TI to a conventional insulator and a change in topological class can only occur when the bulk band gap closes [3]. In this work, we have utilized time-domain terahertz spectroscopy (TDTS) to investigate the low frequency conductance in (Bi 1−x In x ) 2 Se 3 as we tune through this transition by indium substitution. Above certain substitution levels we observe a collapse in the transport lifetime that indicates the destruction of the topological phase. We associate this effect with the threshold where states from opposite surfaces hybridize. The substitution level of the threshold is thickness dependent and only asymptotically approaches the bulk limit x ≈ 0.06 where a maximum in the midinfrared absorption is exhibited. This absorption can be identified with the bulk band gap closing and a change in topological class. The correlation length associated with the QPT appears as the evanescent length of the surface states. The observation of the thickness-dependent collapse of the transport lifetime shows the unusual role that finite size effects play in this topological QPT.The topological character of TIs is determined by the nature of their valence-band wave functions, which can be quantified by 4 Z 2 invariants. Fu and Kane have shown that for inversion symmetric crystals it is possible to evaluate these invariants directly with knowledge of the parity of Bloch wave functions for the occupied electronic states at high symmetry points in the Brillouin zone [10]. Although their argument is formulated for inversion symmetric systems, a material's topological classification does not require inversion or translation symmetry. Therefore the expectation is that the alloying of known TIs with lighter elements by reducing spin-orbit coupling or the tuning of lattice constant can cause the bulk band gap ∆ to close and invert at a quantum critical point where the topological class changes (See cartoon * Electronic address: npa@pha.jhu.edu Fig. 1a). This has been investigated in the thalliumbased ternary chalcogenide alloy TlBi(S 1−x Se x ) 2 [7-9], but thus far only with photoemission (Supplementary Information (SI) section B). Although signatures of topological surface state (TSS) conduction have been found in Bi 2 Se 3 [11-14], a demonstration that the surface transport changes dramatically when the band gap closes and the bulk changes topological class [15] would be strong evidence for the topological nature of these materials and is still lacking. In this regard, it was pointed out recently that indium (In) substitutes for bismuth to form a solid solution in Bi 2 Se 3 and that the non-topological end member In 2 Se 3 of the (Bi 1−x In x ) 2 Se 3 series shares the common rhombohedral D 5 3d structure with Bi 2 Se 3 [6]. In Ref.[6] a topological to trivial transition was observed in a range x ∼ 0.0...