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Register a new userFundamental limit of phase coherence in two-component Bose-Einstein condensates
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We experimentally and theoretically study phase coherence in two-component Bose-Einstein condensates of $^{87}{rm Rb}$ atoms on an atom chip. Using Ramsey interferometry we measure the temporal decay of coherence between the $|F=1,m_{F}=-1rangle$ and $|F=2,m_{F}=+1rangle$ hyperfine ground states. We observe that the coherence is limited by random collisional phase shifts due to the stochastic nature of atom loss. The mechanism is confirmed quantitatively by a quantum trajectory method based on a master equation which takes into account collisional interactions, atom number fluctuations, and losses in the system. This decoherence process can be slowed down by reducing the density of the condensate. Our findings are relevant for experiments on quantum metrology and many-particle entanglement with Bose-Einstein condensates and the development of chip-based atomic clocks.
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Under the Thomas-Fermi approximation, a relatively much simpler analytical solutions of the coupled Gross-Pitaevskii equations for the two-species BEC have been derived. Additionally, a model for the asymmetric states has been proposed, and the competition between the symmetric and asymmetric states has been evaluated. The whole parameter-space is divided into zones, each supports a specific phase, namely, the symmetric miscible phase, the symmetric immiscible phase, or the asymmetric phase. Based on the division the phase-diagrams against any set of parameters can be plotted. Thereby, the effects of these parameters can be visualized. There are three critical values in the inter-species interaction $% V_{AB} $ and one in the ratio of particle numbers $N_{A}/N_{B}$. They govern the transitions between the phases. Two cases, (i) the repulsive $V_{AB}$ matches the repulsive $% V_{A}+V_{B}$, and (ii) the attractive $V_{AB}$ nearly cancels the effect of the repulsive $V_{A}+V_{B}$ have been particularly taken into account. The former leads to a complete separation of the two kinds of atoms , while the latter lead to a collapse. Finally, based on an equation derived in the paper, a convenient experimental approach is proposed to determine the ratio of particle numbers .
Atom interferometers covering macroscopic domains of space-time are a spectacular manifestation of the wave nature of matter. Due to their unique coherence properties, Bose-Einstein condensates are ideal sources for an atom interferometer in extended free fall. In this paper we report on the realization of an asymmetric Mach-Zehnder interferometer operated with a Bose-Einstein condensate in microgravity. The resulting interference pattern is similar to the one in the far-field of a double-slit and shows a linear scaling with the time the wave packets expand. We employ delta-kick cooling in order to enhance the signal and extend our atom interferometer. Our experiments demonstrate the high potential of interferometers operated with quantum gases for probing the fundamental concepts of quantum mechanics and general relativity.
We experimentally investigate the dynamic instability of Bose-Einstein condensates in an optical ring resonator that is asymmetrically pumped in both directions. We find that, beyond a critical resonator-pump detuning, the system becomes stable regardless of the pump strength. Phase diagrams and quenching curves are presented and described by numerical simulations. We discuss a physical explanation based on a geometric interpretation of the underlying nonlinear equations of motion.
We demonstrate detection of a weak alternate-current magnetic field by application of the spin echo technique to F = 2 Bose-Einstein condensates. A magnetic field sensitivity of 12 pT/Hz^1/2 is attained with the atom number of 5*10^3 at spatial resolution of 99 mu m^2. Our observations indicate magnetic field fluctuations synchronous with the power supply line frequency. We show that this noise is greatly suppressed by application of a reverse phase magnetic field. Our technique is useful in order to create a stable magnetic field environment, which is an important requirement for atomic experiments which require a weak bias magnetic field.
The authors previously considered a method solving optimization problems by using a system of interconnected network of two component Bose-Einstein condensates (Byrnes, Yan, Yamamoto New J. Phys. 13, 113025 (2011)). The use of bosonic particles was found to give a reduced time proportional to the number of bosons N for solving Ising model Hamiltonians by taking advantage of enhanced bosonic cooling rates. In this paper we consider the same system in terms of neural networks. We find that up to the accelerated cooling of the bosons the previously proposed system is equivalent to a stochastic continuous Hopfield network. This makes it clear that the BEC network is a physical realization of a simulated annealing algorithm, with an additional speedup due to bosonic enhancement. We discuss the BEC network in terms of typical neural network tasks such as learning and pattern recognition and find that the latter process may be accelerated by a factor of N.
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