Dynamics stabilization and transport coherency in a rocking ratchet for cold atoms

Cold atoms in optical lattices have emerged as an ideal system to investigate the ratchet effect, as demonstrated by several recent experiments. In this work we analyze theoretically two aspects of ac driven transport in cold atoms ratchets. We first address the issue of whether, and to which extent, an ac driven ratchet for cold atoms can operate as a motor. We thus study theoretically a dissipative motor for cold atoms, as obtained by adding a load to a 1D non-adiabatically driven rocking ratchet. We demonstrate that a current can be generated also in the presence of a load, e.g. the ratchet device can operate as a motor. Correspondingly, we determine the stall force for the motor, which characterizes the range of loads over which the device can operate as a motor, and the differential mobility, which characterizes the response to a change in the magnitude of the load. Second, we compare our results for the transport in an ac driven ratchet device with the transport in a dc driven system. We observe a peculiar phenomenon: the bi-harmonic ac force stabilizes the dynamics, allowing the generation of uniform directed motion over a range of momentum much larger than what is possible with a dc bias. We explain such a stabilization of the dynamics by observing that a non-adiabatic ac drive broadens the effective cooling momentum range, and forces the atom trajectories to cover such a region. Thus the system can dissipate energy and maintain a steady-state energy balance. Our results show that in the case of a finite-range velocity-dependent friction, a ratchet device may offer the possibility of controlling the particle motion over a broader range of momentum with respect to a purely biased system, although this is at the cost of a reduced coherency

Current reversals in a rocking ratchet: dynamical vs symmetry-breaking mechanisms

Directed transport in ratchets is determined by symmetry-breaking in a system out of equilibrium. A hallmark of rocking ratchets is current reversals: an increase in the rocking force changes the direction of the current. In this work for a bi-harmonically driven spatially symmetric rocking ratchet we show that a class of current reversal is precisely determined by symmetry-breaking, thus creating a link between dynamical and symmetry-breaking mechanisms

Realisation and Characterisation the BEC for 87Rb Atoms

تكاثف بوز- انشتاين هي حالة من حالات المادة يتم خلقها عن طريق تبريد ذرات غاز لدرجة حرارية فائقة الأنخفاض وبكثافة منخفضة جداً. تم توليد تكاثف بوز- انشتاين الى ذرات 87Rb عن طريق المصيدة المغناطيسية بترتيب يطلق عليه Quadrupole –Ioffe Configuration  الذي يتكون من مجال مغناطيسي رباعي يتولد عن رتيب زوج من الملفات بترتيب يسمى Anti-Helmholtz coils  . كما يستخدم سلك منفرد اضافي يولد مجال عمودي على المجال السابق يطلق عليه Offset coil  والغرض منه هو كبح انتقالات ماجورانا Majorana flip-flop  . اول خطوات العمل هو تبريد وقنص الذرات باستخدام الليزر في المصيدة المغناطيسية الضوئية Magneto-Optical Trap  وصولاً الى عدد ذرات بحدود  ذرة بزمن 3 ثواني. بعد ذلك يتم تبريد الذرات في ميكانيكية Optical Molasses  للوصول الى درجة حرارة أقل وتحقيقاً لكثافة ذرية عالية. الخطوة التالية هوتحميل الذرات الى المصيدة المغناطيسية بزمن عمر قدره    وهو كافي لتبريد الذرات لدرجة حرارية اقل لعملية التبخير Evaporative cooling  ومن ثم انتاج ال BEC  . يتم التبريد بالتبخير باستخدام ترددات راديوية للوصول الى انتقال الطور الى BEC  . تم قياس درجة حرارة الذرات باستخدام تقنية زمن الطيران Time Of Flight  وان سرعة تمدد الغيمة الذرية قيست بمقدار     والتي تودي الى قياس درجة الحرارة بمقدار  and   وهي مقاربة الى درجة حرارة الارتداد Recoil temperature للربيديوم.The Bose-Einstein condensate (BEC) is created in a magnetic trap in the Quadrupole-Ioffe configuration (QUIC). This kind of trap combines an anti-Helmholtz quadrupole field with an offset field produced by a single coil perpendicular to the quadrupole field axis to suppress Majorana transitions. In the quadrupole trap evaporative cooling is performed by using radio frequency, reaching the phase transition to a BEC in the QUIC trap. By using Time of Flight (TOF) technique, the expansion velocity is measured with  and  which lead to temperature of  and  It is roughly around the recoil temperature. &nbsp

Current reversals in a rocking ratchet: the frequency domain

Motivated by recent work [D. Cubero et al., Phys. Rev. E 82, 041116 (2010)], we examine the mechanisms which determine current reversals in rocking ratchets as observed by varying the frequency of the drive. We found that a class of these current reversals in the frequency domain are precisely determined by dissipation-induced symmetry breaking. Our experimental and theoretical work thus extends and generalizes the previously identified relationship between dynamical and symmetry-breaking mechanisms in the generation of current reversals

Vibrational mechanics in an optical lattice: controlling transport via potential renormalization

We demonstrate theoretically and experimentally the phenomenon of vibrational resonance in a periodic potential, using cold atoms in an optical lattice as a model system. A high-frequency (HF) drive, with frequency much larger than any characteristic frequency of the system, is applied by phase-modulating one of the lattice beams. We show that the HF drive leads to the renormalization of the potential. We used transport measurements as a probe of the potential renormalization. The very same experiments also demonstrate that transport can be controlled by the HF drive via potential renormalization.Comment: Phys. Rev. Lett., in pres

Cold atom realizations of Brownian motors

Brownian motors are devices which "rectify" Brownian motion, i.e. they can generate a current of particles out of unbiased fluctuations. Brownian motors are important for the understanding of molecular motors, and are also promising for the realization of new nanolelectronic devices. Among the different systems that can be used to study Brownian motors, cold atoms in optical lattices are quite an unusual one: there is no thermal bath and both the potential and the fluctuations are determined by laser fields. In this article recent experimental implementations of Brownian motors using cold atoms in optical lattices are reviewed

Temporal build-up of electromagnetically induced transparency and absorption resonances in degenerate two-level transitions

The temporal evolution of electromagnetically induced transparency (EIT) and absorption (EIA) coherence resonances in pump-probe spectroscopy of degenerate two-level atomic transition is studied for light intensities below saturation. Analytical expression for the transient absorption spectra are given for simple model systems and a model for the calculation of the time dependent response of realistic atomic transitions, where the Zeeman degeneracy is fully accounted for, is presented. EIT and EIA resonances have a similar (opposite sign) time dependent lineshape, however, the EIA evolution is slower and thus narrower lines are observed for long interaction time. Qualitative agreement with the theoretical predictions is obtained for the transient probe absorption on the $^{85}Rb$ $D_{2}$ line in an atomic beam experiment.Comment: 10 pages, 9 figures. Submitted to Phys. Rev.