arXiv subjects

Klaus Molmer

Publications and source records attributed to Klaus Molmer.

At least 55 records · Page 3Linked to original sources

arXiv2007

Photon number states generated from a continuous-wave light source

Conditional preparation of photon number states from a continuous-wave nondegenerate optical parametric oscillator is investigated. We derive the phase space Wigner function for the output state conditioned on photo detection events that are not necessarily simultaneous, and we maximize its overlap with the desired photon number state by choosing the optimal temporal output state mode function. We present a detailed numerical analysis for the case of two-photon state generation from a parametric oscillator driven with an arbitrary intensity below threshold, and in the low intensity limit, we present a formalism that yields the optimal output state mode function and fidelity for higher photon number states.

quant-ph↗

arXiv2007

Tunnelling couplings in discrete lattices, single particle band structure and eigenstates of interacting atom pairs

By adjusting the tunnelling couplings over longer than nearest neighbor distances it is possible in discrete lattice models to reproduce the properties of the lowest energy band of a real, continuous periodic potential. We propose to include such terms in problems with interacting particles and we show that they have significant consequences for scattering and bound states of atom pairs in periodic potentials.

cond-mat.other↗

arXiv2007

Fidelity of quantum operations

We present a derivation and numerous applications of a compact explicit formula for the average fidelity of a quantum operation on a finite dimensional quantum system. The formula can be applied to averages over particularly relevant subspaces; it is easily generalized to multi-component systems, and as a special result, we show that when the same completely positive trace-preserving map is applied to a large number of qubits with one-bit fidelity F close to unity, the average fidelity of the operation on the full K-bit register scales as $F^{3K/2}$.

quant-ph↗

arXiv2007

Geometric phase gates based on stimulated Raman adiabatic passage in tripod systems

We consider stimulated Raman adiabatic passage (STIRAP) processes in tripod systems and show how to generate purely geometric phase changes of the quantum states involved. The geometric phases are controlled by three laser fields where pulse shapes, relative field strength and phases can be controlled. We present a robust set of universal gates for quantum computing based on these geometric phases: a one-qubit phase gate, a Hadamard gate and a two-qubit phase gate.

quant-ph↗

arXiv2007

Trapping of light pulses in ensembles of stationary Lambda atoms

We present a detailed theoretical description of the generation of stationary light pulses by standing wave electromagnetically induced transparency in media comprised of stationary atoms. We show that, contrary to thermal gas media, the achievable storage times are limited only by the ground state dephasing rate of the atoms, making such media ideally suited for nonlinear optical interactions between stored pulses. Furthermore, we find significant quantitative and qualitative differences between the two types of media, which are important for quantum information processing schemes involving stationary light pulses.

quant-ph↗

arXiv2006

Stationary light pulses in ultra cold atomic gasses

We present a theoretical treatment of electromagnetically induced transparency and light storage using standing wave coupling fields in a medium comprised of stationary atoms, such as an ultra cold atomic gas or a solid state medium. We show that it is possible to create stationary pulses of light which have a qualitatively different behavior than in the case of a thermal gas medium, offering greater potential for quantum information processing applications.

quant-ph↗

arXiv2006

Polarization squeezing by optical Faraday rotation

We show that it is possible to generate continuous-wave fields and pulses of polarization squeezed light by sending classical, linearly polarized laser light twice through an atomic sample which causes an optical Faraday rotation of the field polarization. We characterize the performance of the process, and we show that an appreciable degree of squeezing can be obtained under realistic physical assumptions.

quant-ph↗

arXiv2006

Estimation of fluctuating magnetic fields by an atomic magnetometer

We present a theoretical analysis of the ability of atomic magnetometers to estimate a fluctuating magnetic field. Our analysis makes use of a Gaussian state description of the atoms and the probing field, and it presents the estimator of the field and a measure of its uncertainty which coincides in the appropriate limit with the achievements for a static field. We show by simulations that the estimator for the current value of the field systematically lags behind the actual value of the field, and we suggest a more complete theory, where measurement results at any time are used to update and improve both the estimate of the current value and the estimate of past values of the B-field.

quant-ph↗

arXiv2006

Entanglement between remote continuous variable quantum systems: effects of transmission loss

We study the effects of losses on the entanglement created between two separate atomic gases by optical probing and homodyne detection of the transmitted light. The system is well-described in the Gaussian state formulation. Analytical results quantifying the degree of entanglement between the two gases are derived and compared with the entanglement in a pair of light pulses generated by an EPR source. For low (high) transmission losses the highest degree of entanglement is obtained by probing with squeezed (antisqueezed) light. In an asymmetric setup where light is only sent one way through the atomic samples, we find that the logarithmic negativity of entanglement attains a constant value $-\log_2(N)$ with $N=1/3$ irrespectively of the loss along the transmission line.

quant-ph↗

arXiv2006

Fidelities for transformations of unknown quantum states

We present a general theoretical formalism to compute the fidelity of transformations of unknown quantum states. We then focus on the case of Gaussian transformations of continuous variable quantum systems, where, for the case of a Gaussian distribution of displaced coherent states, the theory is readily tractable by a covariance matrix formalism. We present analytical results for recently implemented teleportation and memory storage protocols for continuous variables.

quant-ph↗

arXiv2006

Non-Gaussian states from continuous-wave Gaussian light sources

We present a general analysis of the state obtained by subjecting the output from a continuous-wave (cw) Gaussian field to non-Gaussian measurements. The generic multimode state of cw Gaussian fields is characterized by an infinite dimensional covariance matrix involving the noise correlations of the source. Our theory extracts the information relevant for detection within specific temporal output modes from these correlation functions . The formalism is applied to schemes for production of non-classical light states from a squeezed beam of light.

quant-ph↗

arXiv2005

Quantum state tomography of molecular rotation

We show how the rotational quantum state of a linear or symmetric top rotor can be reconstructed from finite time observations of the polar angular distribution under certain conditions. The presented tomographic method can reconstruct the complete rotational quantum state in many non-adiabatic alignment experiments. Our analysis applies for measurement data available with existing measurement techniques.

quant-ph↗

arXiv2005

Tomographic reconstruction of quantum states in N spatial dimensions

Most quantum tomographic methods can only be used for one-dimensional problems. We show how to infer the quantum state of a non-relativistic N-dimensional harmonic oscillator system by simple inverse Radon transforms. The procedure is equally applicable to finding the joint quantum state of several distinguishable particles in different harmonic oscillator potentials. A requirement of the procedure is that the angular frequencies of the N harmonic potentials are incommensurable. We discuss what kind of information can be found if the requirement of incommensurability is not fulfilled and also under what conditions the state can be reconstructed from finite time measurements. As a further example of quantum state reconstruction in N dimensions we consider the two related cases of an N-dimensional free particle with periodic boundary conditions and a particle in an N-dimensional box, where we find a similar condition of incommensurability and finite recurrence time for the one-dimensional system.

quant-ph↗

arXiv2005

Molecule Formation in Optical Lattice Wells by Resonantly Modulated Magnetic Fields

We present a theoretical model for formation of molecules in an optical lattice well where a resonant coupling of atomic and molecular states is provided by small oscillations of a magnetic field in the vicinity of a Feshbach resonance. As opposed to an adiabatic sweep over the full resonance, this provides a coherent coupling with a frequency that can be tuned to meet resonance conditions in the system. The effective Rabi frequencies for this coupling are calculated and simulations show perfect Rabi oscillations. Robust production of molecules with an adiabatic sweep of the modulation frequency is demonstrated. For very large oscillation amplitudes, the Rabi oscillations are distorted but still effective and fast association is possible.

cond-mat.other↗

arXiv2005

Gaussian State Description of Squeezed Light

We present a Gaussian state description of squeezed light generated in an optical parametric oscillator. Using the Gaussian state description we describe the dynamics of the system conditioned on homodyne detection on the output field. Our theory shows that the output field is squeezed only if observed for long enough times or by a detector with finite bandwidth. As an application of the present approach we consider the use of finite bandwidth squeezed light together with a sample of spin-polarized atoms to estimate a magnetic field.

quant-ph↗

arXiv2004

Magnetometry with entangled atomic samples

We present a theory for the estimation of a scalar or a vector magnetic field by its influence on an ensemble of trapped spin polarized atoms. The atoms interact off-resonantly with a continuous laser field, and the measurement of the polarization rotation of the probe light, induced by the dispersive atom-light coupling, leads to spin-squeezing of the atomic sample which enables an estimate of the magnetic field which is more precise than that expected from standard counting statistics. For polarized light and polarized atoms, a description of the non-classical components of the collective spin angular momentum for the atoms and the collective Stokes vectors of the light-field in terms of effective gaussian position and momentum variables is practically exact. The gaussian formalism describes the dynamics of the system very effectively and accounts explicitly for the back-action on the atoms due to measurement and for the estimate of the magnetic field. Multi-component magnetic fields are estimated by the measurement of suitably chosen atomic observables and precision and efficiency is gained by dividing the atomic gas in two or more samples which are entangled by the dispersive atom-light interaction.

quant-ph↗

arXiv2004

Tomographic reconstruction of quantum correlations in excited Bose-Einstein condensates

We propose to use quantum tomography to characterize the state of a perturbed Bose-Einstein condensate. We assume knowledge of the number of particles in the zero-wave number mode and of density distributions in space at different times, and we treat the condensate in the Bogoliubov approximation. For states that can be treated with the Gross-Pitaevskii equation, we find that the reconstructed density operator gives excellent predictions of the second moments of the atomic creation- and annihilation operators, including the one-body density matrix. Additional inclusion of the momentum distribution at one point of time enables somewhat reliable predictions to be made for the second moments for mixed states, making it possible to distinguish between coherent and thermal perturbations of the condensate. Finally, we find that with observation of the zero-wave number mode's anomalous second moment the reconstructed density operator gives reliable predictions of the second moments of locally amplitude squeezed states.

cond-mat.other↗

arXiv2004

The field inside a random distribution of parallel dipoles

We determine the probability distribution for the field inside a random uniform distribution of electric or magnetic dipoles. For parallel dipoles, simulations and an analytical derivation show that although the average contribution from any spherical shell around the probe position vanishes, the Levy stable distribution of the field is symmetric around a non-vanishing field amplitude. In addition we show how omission of contributions from a small volume around the probe leads to a field distribution with a vanishing mean, which, in the limit of vanishing excluded volume, converges to the shifted distribution.

quant-ph↗