The Heisenberg limit of quantum measurement where the measurement precision scales as N-1 with the number of atoms N can be achieved by introducing quantum correlations between the atoms. In the present work, we show how to reach Heisenberg scaling by implementing a new Ramsey measurement scheme for cold-atom metrological devices. The proposed protocol consists of a sequence of one-axis twisting pulses and total collective spin rotations. It results in the creation of atomic Schrödinger-cat states, a superposition of two coherent spin states. Analyzing the Fisher information, we discuss the main features of the states and their interferometric precision.
We present an analysis of the robustness of existing analytic schemes for the implementation of an atomic fountain interferometer, and develop concepts for improving this robustness through the use of optimal control theory. For an interferometer operating in the Raman regime, we consider an implementation that manipulates the atomic momentum states with a series of Rabi pulses, and analyze how robust the population dynamics are with respect to variations in the effective pulse amplitude seen by the atoms in the atomic clouds, and variations in the initial velocity of the atoms relative to the rest frame. We then show that using rapid adiabatic passage to implement momentum transfer can significantly improve this robustness. Finally, we formulate the most general control conditions for an atomic fountain interferometer and design a functional that can be used for an ensemble optimization over the robustness landscape. We show preliminary results of optimizing the system using Krotov's method, suggesting that optimal control may be able to significantly enhance the robustness of atom interferometers.
We present a multiplexed quantum repeater protocol based on an ensemble of laser-cooled and trapped rubidium atoms inside an optical ring cavity. We have already demonstrated strong collective coupling in such a system and have constructed a multiplexing apparatus based on a two-dimensional acousto-optical deflector. Here, we show how this system could enable a multiplexed quantum repeater using collective excitations with non-trivial spatial phase profiles (spinwaves). Calculated entanglement generation rates over long distances reveal that such a multiplexed ensemble-cavity platform is a promising route towards long distance quantum entanglement and networking.
The nonlocal correlations between quantum states in an entangled system are essential to many quantum communications applications. A basic quantum operation, which permits the distribution of entanglement between two initially uncorrelated systems, is entanglement swapping. Here we present a rigorous formulation of entanglement swapping of any two partially mixed two-qubit states without limiting ourselves to any specific type of state or noise. Further, for two important classes of the input states, Bell diagonal and pure states, we describe how the concurrence of the final state is related to the concurrence of the initial states. First, we consider Bell diagonal states, and find bounds on the concurrence of the final state in terms of the concurrences of the initial states. These bounds are important for communications applications because polarization mode dispersion in fibers produces Bell diagonal states up to a local unitary rotation. Second, we show that swapping pure states occasionally results in a state of higher concurrence than either of the initial states. In addition, we find that two pure states are most likely to be capable of swapping to a state of increased concurrence when the two initial states have similar concurrences. Our analysis offers a completely general framework for investigating the behavior of any pair of two-qubit states when used for entanglement swapping.
A technique to drive stimulated Raman transition between spin and/or momentum states of ultracold 87Rb
atoms confined on an atomchip trap is discussed. We present our experimental and theoretical approach to an
all-optical manipulation of the atoms using an optical frequency comb emitted by a modelocked ultrafast pulsed
laser.
We demonstrate the adiabatic passage based method to maximize CARS coherence and present numerical results
of the roof scheme implementation without making an assumption of adiabatic elimination of detuned excited
electronic states. Also we analyze influence of fast decoherence in the molecular samples on the results of proposed
scheme. It is shown that the adiabatic method allows achieving chemical sensitivity with high resolution and
can be used to obtain CARS signal with efficiently suppressed background in molecular systems with coherence
times of several hundred of femtoseconds.
Coherent control of quantum dynamics by phase-manipulation of the driving fields, has long been established as an essential tool for state-selective preparation of systems. On the other hand, the basic manipulations of qubits in most physical realizations of quantum-computation devices use Rabi pulses that operate on the Bloch sphere, particularly for weakly-coupled, slow-decoherent systems. In this work we analyze the role of phase-control and phase-dependence of Rabi pulses that prepare Bell states in a system of distinguishable qubits interacting in a harmonic trap. We show that the population dynamics and the properties of the entanglement exhibit a strong dependence to the relative phase. For coherent phonon distributions, collapse and full revival of entanglement occur, while for thermal distributions, except for a few "protected" phases, decoherence with partial revivals are observed.
The free induction decay (FID) after saturation by a laser radiation pulse of finite duration is studied for systems with spectral diffusion. The exact solutions of the FID signal shape have been obtained in the framework of telegraph noise model as well as model of noncorrelated spectral exchange. These solutions take into account the finite duration of saturating field and are valid at arbitrary value of the spectral exchange rate and the amplitude of the coherent field. A selfconsistent explanation of the field dependence of the FID rate found by Szabo and Muramoto is obtained under the slow noncorrelated spectral diffusion.
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