Spin squeezing: transforming one-axis-twisting into two-axis-twisting

Squeezed spin states possess unique quantum correlation or entanglement that are of significant promises for advancing quantum information processing and quantum metrology. In recent back to back publications [C. Gross \textit{et al, Nature} \textbf{464}, 1165 (2010) and Max F. Riedel \textit{et al, Nature} \textbf{464}, 1170 (2010)], reduced spin fluctuations are observed leading to spin squeezing at -8.2dB and -2.5dB respectively in two-component atomic condensates exhibiting one-axis-twisting interactions (OAT). The noise reduction limit for the OAT interaction scales as $\propto 1/{N^{2/3}}$, which for a condensate with $N\sim 10^3$ atoms, is about 100 times below standard quantum limit. We present a scheme using repeated Rabi pulses capable of transforming the OAT spin squeezing into the two-axis-twisting type, leading to Heisenberg limited noise reduction $\propto 1/N$, or an extra 10-fold improvement for $N\sim 10^3$.Comment: 4 pages, 3 figure

Continuous vortex pumping into a spinor condensate with magnetic fields

We study the mechanisms and the limits of pumping vorticity into a spinor condensate through manipulations of magnetic (B-) fields. We discover a fundamental connection between the geometrical properties of the magnetic fields and the quantized circulation of magnetically trapped atoms, a result which generalizes several recent experimental and theoretical studies. The optimal procedures are devised that are capable of continuously increasing or decreasing a condensate's vorticity by repeating certain two step B-field manipulation protocols. We carry out detailed numerical simulations that support the claim that our protocols are highly efficient, stable, and robust against small imperfections of all types. Our protocols can be implemented experimentally within current technologies.Comment: 9 pages, 6 figure

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

Probing tiny motions of nanomechanical resonators: classical or quantum mechanical?

We propose a spectroscopic approach to probe tiny vibrations of a nanomechanical resonator (NAMR), which may reveal classical or quantum behavior depending on the decoherence-inducing environment. Our proposal is based on the detection of the voltage-fluctuation spectrum in a superconducting transmission line resonator (TLR), which is {\it indirectly} coupled to the NAMR via a controllable Josephson qubit acting as a quantum transducer. The classical (quantum mechanical) vibrations of the NAMR induce symmetric (asymmetric) Stark shifts of the qubit levels, which can be measured by the voltage fluctuations in the TLR. Thus, the motion of the NAMR, including if it is quantum mechanical or not, could be probed by detecting the voltage-fluctuation spectrum of the TLR.Comment: 4 pages, 3 figures. to appear in Physical Review Letter

Generation and control of Greenberger-Horne-Zeilinger entanglement in superconducting circuits

Going beyond the entanglement of microscopic objects (such as photons, spins, and ions), here we propose an efficient approach to produce and control the quantum entanglement of three macroscopic coupled superconducting qubits. By conditionally rotating, one by one, selected Josephson charge qubits, we show that their Greenberger-Horne-Zeilinger (GHZ) entangled states can be deterministically generated. The existence of GHZ correlations between these qubits could be experimentally demonstrated by effective single-qubit operations followed by high-fidelity single-shot readouts. The possibility of using the prepared GHZ correlations to test the macroscopic conflict between the noncommutativity of quantum mechanics and the commutativity of classical physics is also discussed.Comment: 4 Pages with 1 figure. to appear in Physical Review Letter

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

Quantum computation with Josephson-qubits by using a current-biased information bus

We propose an effective scheme for manipulating quantum information stored in a superconducting nanocircuit. The Josephson qubits are coupled via their separate interactions with an information bus, a large current-biased Josephson junction treated as an oscillator with adjustable frequency. The bus is sequentially coupled to only one qubit at a time. Distant Josephson qubits without any direct interaction can be indirectly coupled with each other by independently interacting with the bus sequentially, via exciting/de-exciting vibrational quanta in the bus. This is a superconducting analog of the successful ion trap experiments on quantum computing. Our approach differs from previous schemes that simultaneously coupled two qubits to the bus, as opposed to their sequential coupling considered here. The significant quantum logic gates can be realized by using these tunable and selective couplings. The decoherence properties of the proposed quantum system are analyzed within the Bloch-Redfield formalism. Numerical estimations of certain important experimental parameters are provided.Comment: 13 pages with 2 figures. submitte

The high strain compression of micro- and nano-sized random irregular honeycombs

This paper investigates the effects of cell wall thickness, initial stress/strain, and cell regularity on the high strain compressive responses of micro- and nano-sized low density random irregular honeycombs. The strain gradient effects at the micrometer scale, and the surface elasticity and initial stress effects at the nanometer scale are incorporated into the dominant deformation mechanisms in finite element simulations. It is found that the dimensionless compressive stress strain relation strongly depends on the thickness of the cell walls at the micron scale, and at the nano-meter scale, this relation is not only size-dependent, but are also tunable and controllable over a large range. It is also found that under high strain compression, the Poisson's ratios of micro- and nano-sized low density random irregular honeycombs strongly depend on the cell regularity, but are almost independent of the cell wall thickness and the amplitudes of the initial stress or strain

Strongly enhanced light-matter interaction in a hybrid photonic-plasmonic resonator

We propose a hybrid photonic-plasmonic resonant structure which consists of a metal nanoparticle (MNP) and a whispering gallery mode (WGM) microcavity. It is found that the hybrid mode enables a strong interaction between the light and matter, and the single-atom cooperativity is enhanced by more than two orders of magnitude compared to that in a bare WGM microcavity. This remarkable improvement originates from two aspects: (1) the MNP offers a highly enhanced local field in the vicinity of an emitter, and (2), surprisingly, the high-\textit{Q} property of WGMs can be maintained in the presence of the MNP. Thus the present system has great advantages over a single microcavity or a single MNP, and holds great potential in quantum optics, nonlinear optics and highly sensitive biosening.Comment: 5 pages, 4 figure