R. T. Brierley

Large isotropic negative thermal expansion above a structural quantum phase transition

Perovskite structured materials contain myriad tunable ordered phases of electronic and magnetic origin with proven technological importance and strong promise for a variety of energy solutions. An always-contributing influence beneath these cooperative and competing interactions is the lattice, whose physics may be obscured in complex perovskites by the many coupled degrees of freedom, which makes these systems interesting. Here, we report signatures of an approach to a quantum phase transition very near the ground state of the nonmagnetic, ionic insulating, simple cubic perovskite material

Creation of entangled states in coupled quantum dots via adiabatic rapid passage

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Adiabatic state preparation of interacting two-level systems

, and show that its physical properties are strongly effected as much as 100 K above the putative transition. Spatial and temporal correlations in the high-symmetry cubic phase determined using energy- and momentum-resolved inelastic x-ray scattering as well as x-ray diffraction reveal that soft mode, central peak, and thermal expansion phenomena are all strongly influenced by the transition.

Fluxonium-Based Artificial Molecule with a Tunable Magnetic Moment

Quantum state preparation through external control is fundamental to established methods in quantum information processing and in studies of dynamics. In this respect, excitons in semiconductor quantum dots are of particular interest, since their coupling to light allows them to be driven into a specified state using the coherent interaction with a tuned optical field, such as an external laser pulse. We propose a protocol, based on adiabatic rapid passage, for the creation of entangled states in an ensemble of pairwise coupled two-level systems, such as an ensemble of coupled quantum dots. We show by quantitative analysis using realistic parameters for semiconductor quantum dots that this method is feasible where other approaches are unavailable. Furthermore, this scheme can be generically transferred to some other physical systems, including circuit QED, nuclear and electron spins in solid-state environments, and photonic coupled cavities.

Amplitude-Mode Dynamics of Polariton Condensates

We consider performing adiabatic rapid passage (ARP) using frequency-swept driving pulses to excite a collection of interacting two-level systems. Such a model arises in a wide range of many-body quantum systems, such as cavity QED or quantum dots, where a nonlinear component couples to light. We analyze the one-dimensional case using the Jordan-Wigner transformation, as well as the mean-field limit where the system is described by a Lipkin-Meshkov-Glick Hamiltonian. These limits provide complementary insights into the behavior of many-body systems under ARP, suggesting our results are generally applicable. We demonstrate that ARP can be used for state preparation in the presence of interactions, and identify the dependence of the required pulse shapes on the interaction strength. In general, interactions increase the pulse bandwidth required for successful state transfer, introducing new restrictions on the pulse forms required.

Implementing and Characterizing Precise Multiqubit Measurements

Engineered quantum systems allow us to observe phenomena that are not easily accessible naturally. The LEGO-like nature of superconducting circuits makes them particularly suited for building and coupling artificial atoms. Here, we introduce an artificial molecule, composed of two strongly coupled fluxonium atoms, which possesses a tunable magnetic moment. Using an applied external flux, one can tune the molecule between two regimes: one in which the ground-excited state manifold has a magnetic dipole moment and one in which the ground-excited state manifold has only a magnetic quadrupole moment. By varying the applied external flux, we find the coherence of the molecule to be limited by local flux noise. The ability to engineer and control artificial molecules paves the way for building more complex circuits for protected qubits and quantum simulation.

New Class of Quantum Error-Correcting Codes for a Bosonic Mode

We study the stability of collective amplitude excitations in nonequilibrium polariton condensates. These excitations correspond to renormalized upper polaritons and to the collective amplitude modes of atomic gases and superconductors. They would be present following a quantum quench or could be created directly by resonant excitation. We show that uniform amplitude excitations are unstable to the production of excitations at finite wave vectors, leading to the formation of density-modulated phases. The physical processes causing the instabilities can be understood by analogy to optical parametric oscillators and the atomic Bose supernova.

Accelerating dark-matter axion searches with quantum measurement technology

Multiqubit measurements will play a vital role in quantum information processing. A new experiment constructs complex measurements on three superconducting qubits and develops important tools toward characterizing them.