How to Measure the Quantum State of Collective Atomic Spin Excitation

The spin state of an atomic ensemble can be viewed as two bosonic modes, i.e., a quantum signal mode and a $c$-numbered ``local oscillator'' mode when large numbers of spin-1/2 atoms are spin-polarized along a certain axis and collectively manipulated within the vicinity of the axis. We present a concrete procedure which determines the spin-excitation-number distribution, i.e., the diagonal elements of the density matrix in the Dicke basis for the collective spin state. By seeing the collective spin state as a statistical mixture of the inherently-entangled Dicke states, the physical picture of its multi-particle entanglement is made clear.Comment: 6 pages, to appear in Phys. Rev.

Quantum levitation by left-handed metamaterials

Left-handed metamaterials make perfect lenses that image classical electromagnetic fields with significantly higher resolution than the diffraction limit. Here we consider the quantum physics of such devices. We show that the Casimir force of two conducting plates may turn from attraction to repulsion if a perfect lens is sandwiched between them. For optical left-handed metamaterials this repulsive force of the quantum vacuum may levitate ultra-thin mirrors

Optimal cloning of mixed Gaussian states

We construct the optimal 1 to 2 cloning transformation for the family of displaced thermal equilibrium states of a harmonic oscillator, with a fixed and known temperature. The transformation is Gaussian and it is optimal with respect to the figure of merit based on the joint output state and norm distance. The proof of the result is based on the equivalence between the optimal cloning problem and that of optimal amplification of Gaussian states which is then reduced to an optimization problem for diagonal states of a quantum oscillator. A key concept in finding the optimum is that of stochastic ordering which plays a similar role in the purely classical problem of Gaussian cloning. The result is then extended to the case of n to m cloning of mixed Gaussian states.Comment: 8 pages, 1 figure; proof of general form of covariant amplifiers adde

Quantum optical effective-medium theory for loss-compensated metamaterials

A central aim in metamaterial research is to engineer sub-wavelength unit cells that give rise to desired effective-medium properties and parameters, such as a negative refractive index. Ideally one can disregard the details of the unit cell and employ the effective description instead. A popular strategy to compensate for the inevitable losses in metallic components of metamaterials is to add optical gain material. Here we study the quantum optics of such loss-compensated metamaterials at frequencies for which effective parameters can be unambiguously determined. We demonstrate that the usual effective parameters are insufficient to describe the propagation of quantum states of light. Furthermore, we propose a quantum-optical effective-medium theory instead and show that it correctly predicts the properties of the light emerging from loss-compensated metamaterials.Comment: 6 pages, 3 figures. Accepted for Physical Review Letter

Comment on "Quantum Friction - Fact or Fiction?"

If quantum friction existed [J.B. Pendry, New J. Phys. 12, 033028 (2010)] an unlimited amount of useful energy could be extracted from the quantum vacuum and Lifshitz theory would fail. Both are unlikely to be true.Comment: Comment on J.B. Pendry, New J. Phys. 12, 033028 (2010

The Minor Fall, the Major Lift? College Students Do Not Report Listening to Mood-Congruent Music

Music has become an integral part of daily life in Western culture. Individuals use music for various purposes including emotion regulation, and each individual has different tendencies and preferences for how they use music. Previous research indicates that people are likely to listen to mood-congruent music and that personality characteristics--specifically those of the Big 5 personality inventory-- may predict music preference and how people choose to use music for emotion regulation. To further address these questions, we assessed personality and music usage in a sample of undergraduate students. We predicted that affect-related traits like Neuroticism and Extraversion would predict both general emotional tone of music choice as well as some specific music use habits (e.g., using music to regulate emotion). In keeping with expectations, Neurotic individuals were more likely to listen to sad music in general, but contrary to previous findings, we found that the majority of participants, regardless of personality, indicated a preference for mood-incongruent music. Implications for future research are discussed

Multipartite minimum uncertainty products

In our previous work we have found a lower bound for the multipartite uncertainty product of the position and momentum observables over all separable states. In this work we are trying to minimize this uncertainty product over a broader class of states to find the fundamental limits imposed by nature on the observable quantites. We show that it is necessary to consider pure states only and find the infimum of the uncertainty product over a special class of pure states (states with spherically symmetric wave functions). It is shown that this infimum is not attained. We also explicitly construct a parametrized family of states that approaches the infimum by varying the parameter. Since the constructed states beat the lower bound for separable states, they are entangled. We thus show that there is a gap that separates the values of a simple measurable quantity for separable states from entangled ones and we also try to find the size of this gap.Comment: 18 pages, 5 figure

Measuring Polynomial Invariants of Multi-Party Quantum States

We present networks for directly estimating the polynomial invariants of multi-party quantum states under local transformations. The structure of these networks is closely related to the structure of the invariants themselves and this lends a physical interpretation to these otherwise abstract mathematical quantities. Specifically, our networks estimate the invariants under local unitary (LU) transformations and under stochastic local operations and classical communication (SLOCC). Our networks can estimate the LU invariants for multi-party states, where each party can have a Hilbert space of arbitrary dimension and the SLOCC invariants for multi-qubit states. We analyze the statistical efficiency of our networks compared to methods based on estimating the state coefficients and calculating the invariants.Comment: 8 pages, 4 figures, RevTex4, v2 references update

Wigner distributions for non Abelian finite groups of odd order

Wigner distributions for quantum mechanical systems whose configuration space is a finite group of odd order are defined so that they correctly reproduce the marginals and have desirable transformation properties under left and right translations. While for the Abelian case we recover known results, though from a different perspective, for the non Abelian case, our results appear to be new.Comment: Latex, 9 pages, text restructured and some new material adde