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R. J. Lewis-Swan

Publications and source records attributed to R. J. Lewis-Swan.

25 records · Page 2Linked to original sources
arXiv2017

Exploring adiabatic quantum dynamics of the Dicke model in a trapped ion quantum simulator

We use a self-assembled two-dimensional Coulomb crystal of $\sim 70$ ions in the presence of an external transverse field to engineer a quantum simulator of the Dicke Hamiltonian. This Hamiltonian has spin and bosonic degrees of freedom which are encoded by two hyperfine states in each ion and the center of mass motional mode of the crystal, respectively. The Dicke model features a quantum critical point separating two distinct phases: the superradiant (ferromagnetic) and normal (paramagnetic) phases. We experimentally explore protocols that aim to adiabatically prepare the superradiant ground state, a spin-boson cat state with macroscopic phonon occupation, which is well-suited for enhanced metrology and quantum information processing. We start in the normal phase, with all spins aligned along a large transverse field and ramp down the field across the critical point following various protocols. We measure the spin observables, both experimentally and in our simulations to characterize the state of the system at the end of the ramp. We find that under current operating conditions an optimally designed ramp is not sufficient to achieve significant fidelity with the superradiant ground state. However, our theoretical investigation shows that slight modifications of experimental parameters, together with modest reductions in decoherence rates and thermal noise can increase the cat-state fidelity to $\sim 75\%$ for $N \sim 20$ spins. Our results open a path for the use of large ensembles of trapped ions as powerful quantum sensors and quantum computers.

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arXiv2016

Correspondence between stochastic Wigner trajectories and individual experimental runs

We examine the interpretation of individual phase-space trajectories of the Wigner function as corresponding to possible outcomes of single experimental trials. To this end, we investigate the relation between the true (measured) particle number distribution $P_n$ for a single-mode state and that obtained by discretely binning the individual stochastic realisations of squared mode amplitudes $|α|^2$ of the sampled Wigner distribution $W(α)$, which we denote via $\tilde{P}_n$. We provide an operational definition of $\tilde{P}_n$ in terms of the underlying Wigner function, which allows us to explicitly calculate the overlap between the two number distributions and hence quantify the statistical distance between them. We find that there is indeed a close quantitative correspondence between $P_n$ and $\tilde{P}_n$ for a wide range of states, justifying the broadly accepted view that, for highly occupied modes, individual stochastic realisations of Wigner trajectories should approximately correspond to outcomes of single experiments. However, we also find counterexamples for which high mode occupation may not be sufficient for such an interpretation, we find instead that a more relevant and sufficient requirement is the smoothness and broadness of the Wigner function $W(α)$ for the state of interest relative to the scale of oscillations of the Wigner functions for the relevant Fock states.

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arXiv2016

Approximate particle number distribution from direct stochastic sampling of the Wigner function

We consider the Wigner quasi-probability distribution function of a single mode of an electromagnetic or matter-wave field to address the question of whether a direct stochastic sampling and binning of the absolute square of the complex field amplitude can yield a distribution function $\tilde{P}_n$ that closely approximates the true particle number probability distribution $P_n$ of the underlying quantum state. By providing an operational definition of the binned distribution $\tilde{P}_n$ in terms of the Wigner function, we explicitly calculate the overlap between $\tilde{P}_n$ and ${P}_n$ and hence quantify the statistical distance between the two distributions. We find that there is indeed a close quantitative correspondence between $\tilde{P}_n$ and $P_n$ for a wide range of quantum states that have smooth and broad Wigner function relative to the scale of oscillations of the Wigner function for the relevant Fock state. However, we also find counterexamples, including states with high mode occupation, for which $\tilde{P}_n$ does not closely approximate $P_n$.

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arXiv2015

Proposal for a motional-state Bell inequality test with ultracold atoms

We propose and theoretically simulate an experiment for demonstrating a motional-state Bell inequality violation for pairs of momentum-entangled atoms produced in Bose-Einstein condensate collisions. The proposal is based on realizing an atom-optics analog of the Rarity-Tapster optical scheme: it uses laser-induced Bragg pulses to implement two-particle interferometry on the underlying Bell-state for two pairs of atomic scattering modes with equal but opposite momenta. The collision dynamics and the sequence of Bragg pulses are simulated using the stochastic Bogoliubov approach in the positive-P representation. We predict values of the Clauser-Horne-Shimony-Holt (CHSH) parameter up to S~2.5 for experimentally realistic parameter regimes, showing a strong violation of the CSHS-Bell inequality bounded classically by S<2.

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arXiv2014

Proposal for demonstrating the Hong-Ou-Mandel effect with matter waves

The Hong-Ou-Mandel (HOM) effect is a striking demonstration of destructive quantum interference between pairs of indistinguishable bosons, realised so far only with massless photons. Here we propose an experiment which can realise this effect in the matter-wave regime using pair-correlated atoms produced via a collision of two Bose-Einstein condensates and subjected to two laser induced Bragg pulses. We formulate a novel measurement protocol appropriate for the multimode matter-wave field, which---unlike the typical two-mode optical case---bypasses the need for repeated measurements under different displacement settings of the beam-splitter, thus dramatically reducing the number of experimental runs required to map out the interference visibility. The protocol can be utilised in related matter-wave schemes; here we focus on condensate collisions and by simulating the entire experiment we predict a HOM-dip visibility of ~69%. By being larger than 50%, such a visibility highlights strong quantum correlations between the atoms and paves the way for a possible demonstration of a Bell inequality violation with massive particles in a related Rarity-Tapster setup.

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arXiv2013

Sensitivity to thermal noise of atomic Einstein-Podolsky-Rosen entanglement

We examine the prospect of demonstrating Einstein-Podolsky-Rosen (EPR) entanglement for massive particles using spin-changing collisions in a spinor Bose-Einstein condensate. Such a demonstration has recently been attempted by Gross et al. [Nature 480, 219 (2011)] using a condensate of Rb-87 atoms trapped in an optical lattice potential. For the condensate initially prepared in the (F,m_{F})=(2,0) hyperfine state, with no population in the m_{F}=+-1 states, we predict a significant suppression of the product of inferred quadrature variances below the Heisenberg uncertainty limit, implying strong EPR entanglement. However, such EPR entanglement is lost when the collisions are initiated in the presence of a small (currently undetectable) thermal population n_{th} in the m_{F}=+-1 states. For condensates containing 150 to 200 atoms, we predict an upper bound of n_{th}~1 that can be tolerated in this experiment before EPR entanglement is lost.

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