Quantum spin mixing in a binary mixture of spin-1 atomic condensates

We study quantum spin mixing in a binary mixture of spin-1 condensates including coherent interspecies mixing process, using the familiar spinor condensates of $^{87}$Rb and $^{23}$Na atoms in the ground lower hyperfine F=1 manifolds as prototype examples. Within the single spatial mode approximation for each of the two spinor condensates, the mixing dynamics reduce to that of three coupled nonlinear pendulums with clear physical interpretations. Using suitably prepared initial states, it is possible to determine the interspecies singlet-pairing as well as spin-exchange interactions from the subsequent mixing dynamics.Comment: 6 pages, 3 figure

Higher order symmetry-protected topological states for interacting bosons and fermions

Higher-order topological insulators have a modified bulk-boundary correspondence compared to other topological phases: instead of gapless edge or surface states, they have gapped edges and surfaces, but protected modes at corners or hinges. Here, we explore symmetry protected topological phases in strongly interacting many-body systems with this generalized bulk-boundary correspondence. We introduce several exactly solvable bosonic lattice models as candidates for interacting higher order symmetry protected topological (HOSPT) phases protected by spatial symmetries, and develop a topological field theory that captures the non-trivial nature of the gapless corner and hinge modes. We show how, for rotational symmetry, this field theory leads to a natural relationship between HOSPT phases and conventional SPT phases with an enlarged internal symmetry group. We also explore the connection between bosonic and fermionic HOSPT phases in the presence of strong interactions, and comment on the implications of this connection for the classification of interacting fermionic HOSPT phases. Finally, we explore how gauging internal symmetries of these phases leads to topological orders characterized by nontrivial braiding statistics between topological vortex excitations and geometrical defects related to the spatial symmetry.Comment: 24 pages, 15 figures. updated referenc

Switchable coupling between charge and flux qubits

We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective inter-qubit coupling controlled by the external magnetic flux. This switchable inter-qubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.Comment: 5 pages, 1 figur

Optical selection rules and phase-dependent adiabatic state control in a superconducting quantum circuit

We analyze the optical selection rules of the microwave-assisted transitions in a flux qubit superconducting quantum circuit (SQC). We show that the parities of the states relevant to the superconducting phase in the SQC are well-defined when the external magnetic flux $\Phi_{e}=\Phi_{0}/2$, then the selection rules are same as the ones for the electric-dipole transitions in usual atoms. When $\Phi_{e}\neq \Phi_{0}/2$, the symmetry of the potential of the artificial "atom'' is broken, a so-called $\Delta$-type "cyclic" three-level atom is formed, where one- and two-photon processes can coexist. We study how the population of these three states can be selectively transferred by adiabatically controlling the electromagnetic field pulses. Different from $\Lambda$-type atoms, the adiabatic population transfer in our three-level $\Delta$-atom can be controlled not only by the amplitudes but also by the phases of the pulses

A qubit strongly-coupled to a resonant cavity: asymmetry of the spontaneous emission spectrum beyond the rotating wave approximation

We investigate the spontaneous emission spectrum of a qubit in a lossy resonant cavity. We use neither the rotating-wave approximation nor the Markov approximation. The qubit-cavity coupling strength is varied from weak, to strong, even to lower bound of the ultra-strong. For the weak-coupling case, the spontaneous emission spectrum of the qubit is a single peak, with its location depending on the spectral density of the qubit environment. Increasing the qubit-cavity coupling increases the asymmetry (the positions about the qubit energy spacing and heights of the two peaks) of the two spontaneous emission peaks (which are related to the vacuum Rabi splitting) more. Explicitly, for a qubit in a low-frequency intrinsic bath, the height asymmetry of the splitting peaks becomes larger, when the qubit-cavity coupling strength is increased. However, for a qubit in an Ohmic bath, the height asymmetry of the spectral peaks is inverted from the same case of the low-frequency bath, when the qubit is strongly coupled to the cavity. Increasing the qubit-cavity coupling to the lower bound of the ultra-strong regime, the height asymmetry of the left and right peak heights are inverted, which is consistent with the same case of low-frequency bath, only relatively weak. Therefore, our results explicitly show how the height asymmetry in the spontaneous emission spectrum peaks depends not only on the qubit-cavity coupling, but also on the type of intrinsic noise experienced by the qubit.Comment: 10pages, 5 figure