We report on the implementation of degenerate Raman sideband cooling of 40 K atoms. The scheme in... more We report on the implementation of degenerate Raman sideband cooling of 40 K atoms. The scheme incorporates a 3D optical lattice, which confines the atoms and drives the Raman transitions. The optical cooling cycle is closed by two optical pumping beams. The wavelength of the laser beams forming the lattice is close to the D2 atomic transition, while the optical pumping is operated near the D1 transition. With this cooling method, we achieve temperature of ∼1µK of a cloud with ∼10 7 atoms. This corresponds to a phase space density of ≥10 -3 . Moreover, the fermionic ensemble is spin polarized to conditions which are favorable for subsequent evaporative cooling. We study the dependence of the cooling scheme on several parameters, including the applied magnetic field, the detuning, duration, and intensity profile of the optical pumping beams. Adding this optical cooling stage to current Fermi gas experiments can improve the final conditions and increase the data rate.
We present sub-wavelength periodic spin textures of spatially varying Raman Rabi frequency create... more We present sub-wavelength periodic spin textures of spatially varying Raman Rabi frequency created by evanescent-wave optical lattice. Our results open the door to the study of spin excitation dynamics in ultracold gases.
We propose a measurement scheme enabling reconstruction of the amplitude and phase of wavefunctio... more We propose a measurement scheme enabling reconstruction of the amplitude and phase of wavefunctions in Bose-Einstein condensates from their momentum power-spectrum. Our method reduces ambiguities and allows the reconstruction of arrays of vortices.
Background: The high momentum distribution of atoms in two spin-state ultra-cold atomic gases wit... more Background: The high momentum distribution of atoms in two spin-state ultra-cold atomic gases with strong short-range interactions between atoms with different spins, which can be described using Tan's contact, are dominated by short range pairs of different fermions and decreases as k −4. In atomic nuclei the momentum distribution of nucleons above the Fermi momentum (k > kF ≈ 250 Mev/c) is also dominated by short range correlated different-fermion (neutron-proton) pairs. Purpose: Compare high-momentum unlike-fermion momentum distributions in atomic and nuclear systems. Methods: We show that, for k > kF MeV/c, nuclear momentum distributions are proportional to that of the deuteron. We then examine the deuteron momentum distributions derived from a wide variety of modern nucleon-nucleon potentials that are consistent with N N-scattering data. Results: The high momentum tail of the deuteron momentum distribution, and hence of the nuclear momentum distributions appears to decrease as k −4. This behavior is shown to arise from the effects of the tensor part of the nucleon-nucleon potential. In addition, when the dimensionless interaction strength for the atomic system is chosen to be similar to that of atomic nuclei, the probability for finding a short range different-fermion pair in both systems is the same. Conclusions: Although nuclei do not satisfy all of the conditions for Tan's contact, the observed similarity of the magnitude and k −4 shape of nuclear and atomic momentum distributions is remarkable because these systems differ by about 20 orders of magnitude in density. This similarity may lead to a greater understanding of nuclei and the density dependence of nuclear systems.
An optimal dynamical decoupling of a quantum system coupled to a noisy environment must take into... more An optimal dynamical decoupling of a quantum system coupled to a noisy environment must take into account also the imperfections of the control pulses. We present a new formalism which describes, in a closed-form expression, the evolution of the system, including the spectral function of both the environment and control noise. We show that by measuring these spectral functions, our expression can be used to optimize the decoupling pulse sequence. We demonstrate this approach with an ensemble of optically trapped ultra-cold Rubidium atoms, and use quantum process tomography to identify the effect of the environment and control noise. Our approach is applicable and important for any realistic implementation of quantum information processing.
Bulletin of the American Physical Society, Jun 11, 2015
Submitted for the DAMOP15 Meeting of The American Physical Society The spectral function of the n... more Submitted for the DAMOP15 Meeting of The American Physical Society The spectral function of the normal phase in the BCS-BEC crossover TARA DRAKE, JILA and University of Colorado, YOAV SAGI, Technion, Israel, RABIN PAUDEL, ROMAN CHAPURIN, JILA and the University of Colorado, DEBORAH JIN, NIST and the University of Colorado and JILA-The nature of the normal phase of an ultracold Fermi gas in the BCS-BEC crossover regime is an interesting and unresolved question. While the many-body ground state remains a condensate of paired fermions, the normal state must evolve from a Fermi liquid to a Bose gas of molecules as interactions increase. We explore how this occurs with measurements of the distribution of single-particle energies and momenta in a nearly homogeneous gas above Tc. We find that the data fit well to a two-part function that includes a peak corresponding to fermionic quasiparticles and an "incoherent background" that can be modeled using the dispersion of thermal molecules. The data show signatures of both Fermi liquid behavior and pairing. As the strength of interactions is increased, the quasiparticle spectral weight vanishes, which signals the breakdown of a Fermi liquid description.
An ensemble of two level quantum systems coupled to a fluctuating external environment is a commo... more An ensemble of two level quantum systems coupled to a fluctuating external environment is a common paradigm in many fields of study. This coupling leads to decoherence that limits the usefulness of these systems, e.g. as qubits in quantum computation systems. Application of external pulses can reduce the decoherence by utilizing symmetry properties of the coupling Hamiltonian to average out
As was already mentioned, cold atomic ensembles can be used as an interface between matter and ph... more As was already mentioned, cold atomic ensembles can be used as an interface between matter and photonic qubits in quantum networks, and in recent years vast experimental advances in this direction have been reported [35, 53–57]. The effect of collisional fluctuations was analyzed in the previous chapters and the decoherence it induces is well understood. Though fluctuations at low frequencies can be overcome by a single population inverting pulse - the celebrated coherence echo technique [58, 59], as the collision rate increases this is no longer possible due to higher frequency components. Dynamical decoupling (DD) theories generalize this technique to multi-pulse sequences by harnessing symmetry properties of the coupling Hamiltonian [60, 61, 62, 7, 8]. Though DD was demonstrated in several experiments [63–68], its use with atomic ensembles remains unexplored to date. In this chapter we study experimentally DD in a dense cold atomic ensemble and report on a substantial suppression of collisional decoherence.
We show that the spectrum of an ensemble of two-level systems can be broadened through 'resetting... more We show that the spectrum of an ensemble of two-level systems can be broadened through 'resetting' discrete fluctuations, in contrast to the well-known motional-narrowing effect. We establish that the condition for the onset of motional broadening is that the ensemble frequency distribution has heavy tails with a diverging first moment. We find that the asymptotic motional-broadened lineshape is a Lorentzian, and derive an expression for its width. We explain why motional broadening persists up to some fluctuation rate, even when there is a physical upper cutoff to the frequency distribution.
We used Caliban [30] to manage all experiments in a reproducible environment in Google Cloud's AI... more We used Caliban [30] to manage all experiments in a reproducible environment in Google Cloud's AI Platform. In all experiments, we used 16-bit operations on V100 GPUs except Batch Normalization which was kept at 32-bit. A.1 Architectures The ResNet18 architecture used in our experiments is the same its original version introduced in He et al. [13] for training ImageNet dataset except that the first convolution layer has kernel size 3 by 3 and it is not followed by a max-pooling layer. This changes are made to adjust the architecture for smaller image sizes used in our experiments. 3-FC architecture is a fully connected network with two hidden layers having the same number of hidden units with ReLU activation and Batch Normalization. Finally, please see Tables 3 and 4 for details of D-CONV, S-CONV and their locally and fully connected counterparts. Layer Description #param D-CONV D-LOCAL D-FC conv1 CONV3x3(↵, 1) 9 ⇥ 3↵ 9 ⇥ 3s 2 ↵ 3s 4 ↵ conv2 CONV3x3(2↵, 2) 9 ⇥ 2 1 ↵ 2 9 ⇥ s 2 ↵ 2 /2 s 4 ↵ 2 /2 conv3 CONV3x3(2↵, 1) 9 ⇥ 2 2 ↵ 2 9 ⇥ s 2 ↵ 2 /4 s 4 ↵ 2 /4 conv4 CONV3x3(4↵, 2) 9 ⇥ 2 3 ↵ 2 9 ⇥ s 2 ↵ 2 /8 s 4 ↵ 2 /8 conv5 CONV3x3(4↵, 1) 9 ⇥ 2 4 ↵ 2 9 ⇥ s 2 ↵ 2 /16 s 4 ↵ 2 /16 conv6 CONV3x3(8↵, 2) 9 ⇥ 2 5 ↵ 2 9 ⇥ s 2 ↵ 2 /32 s 4 ↵ 2 /32 conv7 CONV3x3(8↵, 1) 9 ⇥ 2 6 ↵ 2 9 ⇥ s 2 ↵ 2 /64 s 4 ↵ 2 /64 conv8 CONV3x3(16↵, 2) 9 ⇥ 2 7 ↵ 2 9 ⇥ s 2 ↵ 2 /128 s 4 ↵ 2 /128 fc1 FC(64↵)
As already explained, inhomogeneities in \(\delta \) over the ensemble lead to dephasing of a sto... more As already explained, inhomogeneities in \(\delta \) over the ensemble lead to dephasing of a stored coherence. Intriguingly, fluctuations in \(\delta \) can prolong the coherence time—a phenomenon called motional narrowing. Historically, motional narrowing was first observed in liquid NMR, where a large reduction in the width of spectral lines was observed in comparison to solid NMR due to the thermal motion of the nuclei. Later the effect was also reported in other fields such as molecular physics, semiconductor microcavities and quantum dots. In this chapter we study motional narrowing due to elastic collisions in a dense atomic ensemble. In contrast to previous experiments, our apparatus enables precise and independent control over the thermodynamic parameters of the system. Owing to this, we are able to quantitatively analyze the dependance of the narrowed linewidth on the fluctuations rate and strength, and demonstrate an inverse linear dependence on the former and quadratic dependence on the latter. We also find that the narrowed linewidth exhibits universal scaling with the atomic phase space density.
Collisional Narrowing and Dynamical Decoupling in a Dense Ensemble of Cold Atoms, 2012
In this chapter we aim to exactly solve for the spectrum (and the decay of coherence) of an ensem... more In this chapter we aim to exactly solve for the spectrum (and the decay of coherence) of an ensemble with an arbitrary detuning distribution \(P_0(\delta )\). Historically, the spectrum of a TLS ensemble was first studied by Kubo who considered an ensemble of harmonic oscillators with randomly varying resonant frequencies, and found an analytic expression for the ensemble spectrum for a frequency which is a Gaussian process. In the prevalent case, where the frequency autocorrelation function decays exponentially in time, the Kubo solution gives a generalized Gumbel function, as defined in Eq. (4.4).
We study experimentally the anomalous diffusion of atoms in one dimension. The ultra-cold atoms c... more We study experimentally the anomalous diffusion of atoms in one dimension. The ultra-cold atoms continuously scatter photons from a lattice which is in a configuration identical to the one used in the well-known Sisyphus cooling scheme. This produces a steady-state atomic velocity distribution which is a power law, with an exponent that depends on the lattice depth [1]. We image the atomic density distribution after a varying waiting time. The width of the atomic cloud exhibits a power law time dependence, and we extract its characteristic exponent for various lattice depths. We also show that the density distribution at different times is self-similar with the same characteristic exponent, in accordance with the predictions of a fractional diffusion equation [2]. [4pt] [1] P. Douglas, S. Bergamini, and F. Renzoni, Phys. Rev. Lett. 96, 110601 (2006). [2] R. Metzler and J. Klafter, Physics Reports 339, 1 (2000).
The inherent density inhomogeneity of a trapped gas can complicate interpretation of experiments ... more The inherent density inhomogeneity of a trapped gas can complicate interpretation of experiments and can wash out sharp features. This is especially important for a Fermi gas, where interaction effects as well as the local Fermi energy, or Fermi momentum, depend on the density. We report on experiments that use optical pumping with shaped light beams to spatially select the center part of a trapped gas for probing. This technique is compatible with momentum resolved measurements. For a weakly interacting Fermi gas of ^40K atoms, we present measurements of the momentum distribution that reveal for the first time a sharp Fermi surface. We then apply this technique to a strongly interacting Fermi gas at the Feshbach resonance, where we measured the temperature dependence of the Tan's contact locally in the trapped gas.
Superfluidity in strongly correlated matter is still an open question after many years of researc... more Superfluidity in strongly correlated matter is still an open question after many years of research. High Tc superconductors, for example, are known to exhibit a pseudo-gap phase above the critical temperature - a phase in which pairing exists in spite of the absence of long range order. An ultra-cold strongly interacting Fermi gas realizes a clean and controllable model system for studying these questions. It was shown recently that the normal state of a strongly interacting Fermi gas has a spectral behavior typical to a pseudo-gap phase [1]. One of the problems, however, which complicates the interpretation of these experiments is the inherent density inhomogeneity of the gas which arises due to the harmonic confinement. Here we present a technique to overcome this difficulty by spatially selecting only part of the cloud for interrogation while still retaining momentum resolution. We present measurements of the momentum distribution of a degenerate gas of 40K atoms revealing for th...
Cold atoms trapped in a conservative potential can be used for quantum information processing, wh... more Cold atoms trapped in a conservative potential can be used for quantum information processing, where their high density can be employed to achieve high efficiency in quantum operations. The high atomic density, on the other hand, alters the atomic coherence dynamics and leads to a surprising prolongation of the coherence times, a phenomenon we call collisional narrowing in analog to the well known motional narrowing effect in NMR. Here we present our theoretical and experimental study of collisional narrowing in optically trapped atoms. We show that elastic collisions reduce the spread of the atomic phase distribution and modify its time evolution from ballistic to diffusive. We perform Ramsey experiments on cold ^87Rb atoms trapped in a far-off-resonance laser, and observe a decrease of the dephasing rate for an increasing collision rate. We also show that the new emerging dephasing timescale depends only on the atomic phase space density of the ensemble.
2007 Quantum Electronics and Laser Science Conference, 2007
ABSTRACT By releasing ultra-cold atoms from a small red detuned Gaussian trap to an optical wedge... more ABSTRACT By releasing ultra-cold atoms from a small red detuned Gaussian trap to an optical wedge billiard we reduce the energy broadening of the atoms and perform spatial selection on the initial occupied phase space.
We report on the implementation of degenerate Raman sideband cooling of 40 K atoms. The scheme in... more We report on the implementation of degenerate Raman sideband cooling of 40 K atoms. The scheme incorporates a 3D optical lattice, which confines the atoms and drives the Raman transitions. The optical cooling cycle is closed by two optical pumping beams. The wavelength of the laser beams forming the lattice is close to the D2 atomic transition, while the optical pumping is operated near the D1 transition. With this cooling method, we achieve temperature of ∼1µK of a cloud with ∼10 7 atoms. This corresponds to a phase space density of ≥10 -3 . Moreover, the fermionic ensemble is spin polarized to conditions which are favorable for subsequent evaporative cooling. We study the dependence of the cooling scheme on several parameters, including the applied magnetic field, the detuning, duration, and intensity profile of the optical pumping beams. Adding this optical cooling stage to current Fermi gas experiments can improve the final conditions and increase the data rate.
We present sub-wavelength periodic spin textures of spatially varying Raman Rabi frequency create... more We present sub-wavelength periodic spin textures of spatially varying Raman Rabi frequency created by evanescent-wave optical lattice. Our results open the door to the study of spin excitation dynamics in ultracold gases.
We propose a measurement scheme enabling reconstruction of the amplitude and phase of wavefunctio... more We propose a measurement scheme enabling reconstruction of the amplitude and phase of wavefunctions in Bose-Einstein condensates from their momentum power-spectrum. Our method reduces ambiguities and allows the reconstruction of arrays of vortices.
Background: The high momentum distribution of atoms in two spin-state ultra-cold atomic gases wit... more Background: The high momentum distribution of atoms in two spin-state ultra-cold atomic gases with strong short-range interactions between atoms with different spins, which can be described using Tan's contact, are dominated by short range pairs of different fermions and decreases as k −4. In atomic nuclei the momentum distribution of nucleons above the Fermi momentum (k > kF ≈ 250 Mev/c) is also dominated by short range correlated different-fermion (neutron-proton) pairs. Purpose: Compare high-momentum unlike-fermion momentum distributions in atomic and nuclear systems. Methods: We show that, for k > kF MeV/c, nuclear momentum distributions are proportional to that of the deuteron. We then examine the deuteron momentum distributions derived from a wide variety of modern nucleon-nucleon potentials that are consistent with N N-scattering data. Results: The high momentum tail of the deuteron momentum distribution, and hence of the nuclear momentum distributions appears to decrease as k −4. This behavior is shown to arise from the effects of the tensor part of the nucleon-nucleon potential. In addition, when the dimensionless interaction strength for the atomic system is chosen to be similar to that of atomic nuclei, the probability for finding a short range different-fermion pair in both systems is the same. Conclusions: Although nuclei do not satisfy all of the conditions for Tan's contact, the observed similarity of the magnitude and k −4 shape of nuclear and atomic momentum distributions is remarkable because these systems differ by about 20 orders of magnitude in density. This similarity may lead to a greater understanding of nuclei and the density dependence of nuclear systems.
An optimal dynamical decoupling of a quantum system coupled to a noisy environment must take into... more An optimal dynamical decoupling of a quantum system coupled to a noisy environment must take into account also the imperfections of the control pulses. We present a new formalism which describes, in a closed-form expression, the evolution of the system, including the spectral function of both the environment and control noise. We show that by measuring these spectral functions, our expression can be used to optimize the decoupling pulse sequence. We demonstrate this approach with an ensemble of optically trapped ultra-cold Rubidium atoms, and use quantum process tomography to identify the effect of the environment and control noise. Our approach is applicable and important for any realistic implementation of quantum information processing.
Bulletin of the American Physical Society, Jun 11, 2015
Submitted for the DAMOP15 Meeting of The American Physical Society The spectral function of the n... more Submitted for the DAMOP15 Meeting of The American Physical Society The spectral function of the normal phase in the BCS-BEC crossover TARA DRAKE, JILA and University of Colorado, YOAV SAGI, Technion, Israel, RABIN PAUDEL, ROMAN CHAPURIN, JILA and the University of Colorado, DEBORAH JIN, NIST and the University of Colorado and JILA-The nature of the normal phase of an ultracold Fermi gas in the BCS-BEC crossover regime is an interesting and unresolved question. While the many-body ground state remains a condensate of paired fermions, the normal state must evolve from a Fermi liquid to a Bose gas of molecules as interactions increase. We explore how this occurs with measurements of the distribution of single-particle energies and momenta in a nearly homogeneous gas above Tc. We find that the data fit well to a two-part function that includes a peak corresponding to fermionic quasiparticles and an "incoherent background" that can be modeled using the dispersion of thermal molecules. The data show signatures of both Fermi liquid behavior and pairing. As the strength of interactions is increased, the quasiparticle spectral weight vanishes, which signals the breakdown of a Fermi liquid description.
An ensemble of two level quantum systems coupled to a fluctuating external environment is a commo... more An ensemble of two level quantum systems coupled to a fluctuating external environment is a common paradigm in many fields of study. This coupling leads to decoherence that limits the usefulness of these systems, e.g. as qubits in quantum computation systems. Application of external pulses can reduce the decoherence by utilizing symmetry properties of the coupling Hamiltonian to average out
As was already mentioned, cold atomic ensembles can be used as an interface between matter and ph... more As was already mentioned, cold atomic ensembles can be used as an interface between matter and photonic qubits in quantum networks, and in recent years vast experimental advances in this direction have been reported [35, 53–57]. The effect of collisional fluctuations was analyzed in the previous chapters and the decoherence it induces is well understood. Though fluctuations at low frequencies can be overcome by a single population inverting pulse - the celebrated coherence echo technique [58, 59], as the collision rate increases this is no longer possible due to higher frequency components. Dynamical decoupling (DD) theories generalize this technique to multi-pulse sequences by harnessing symmetry properties of the coupling Hamiltonian [60, 61, 62, 7, 8]. Though DD was demonstrated in several experiments [63–68], its use with atomic ensembles remains unexplored to date. In this chapter we study experimentally DD in a dense cold atomic ensemble and report on a substantial suppression of collisional decoherence.
We show that the spectrum of an ensemble of two-level systems can be broadened through 'resetting... more We show that the spectrum of an ensemble of two-level systems can be broadened through 'resetting' discrete fluctuations, in contrast to the well-known motional-narrowing effect. We establish that the condition for the onset of motional broadening is that the ensemble frequency distribution has heavy tails with a diverging first moment. We find that the asymptotic motional-broadened lineshape is a Lorentzian, and derive an expression for its width. We explain why motional broadening persists up to some fluctuation rate, even when there is a physical upper cutoff to the frequency distribution.
We used Caliban [30] to manage all experiments in a reproducible environment in Google Cloud's AI... more We used Caliban [30] to manage all experiments in a reproducible environment in Google Cloud's AI Platform. In all experiments, we used 16-bit operations on V100 GPUs except Batch Normalization which was kept at 32-bit. A.1 Architectures The ResNet18 architecture used in our experiments is the same its original version introduced in He et al. [13] for training ImageNet dataset except that the first convolution layer has kernel size 3 by 3 and it is not followed by a max-pooling layer. This changes are made to adjust the architecture for smaller image sizes used in our experiments. 3-FC architecture is a fully connected network with two hidden layers having the same number of hidden units with ReLU activation and Batch Normalization. Finally, please see Tables 3 and 4 for details of D-CONV, S-CONV and their locally and fully connected counterparts. Layer Description #param D-CONV D-LOCAL D-FC conv1 CONV3x3(↵, 1) 9 ⇥ 3↵ 9 ⇥ 3s 2 ↵ 3s 4 ↵ conv2 CONV3x3(2↵, 2) 9 ⇥ 2 1 ↵ 2 9 ⇥ s 2 ↵ 2 /2 s 4 ↵ 2 /2 conv3 CONV3x3(2↵, 1) 9 ⇥ 2 2 ↵ 2 9 ⇥ s 2 ↵ 2 /4 s 4 ↵ 2 /4 conv4 CONV3x3(4↵, 2) 9 ⇥ 2 3 ↵ 2 9 ⇥ s 2 ↵ 2 /8 s 4 ↵ 2 /8 conv5 CONV3x3(4↵, 1) 9 ⇥ 2 4 ↵ 2 9 ⇥ s 2 ↵ 2 /16 s 4 ↵ 2 /16 conv6 CONV3x3(8↵, 2) 9 ⇥ 2 5 ↵ 2 9 ⇥ s 2 ↵ 2 /32 s 4 ↵ 2 /32 conv7 CONV3x3(8↵, 1) 9 ⇥ 2 6 ↵ 2 9 ⇥ s 2 ↵ 2 /64 s 4 ↵ 2 /64 conv8 CONV3x3(16↵, 2) 9 ⇥ 2 7 ↵ 2 9 ⇥ s 2 ↵ 2 /128 s 4 ↵ 2 /128 fc1 FC(64↵)
As already explained, inhomogeneities in \(\delta \) over the ensemble lead to dephasing of a sto... more As already explained, inhomogeneities in \(\delta \) over the ensemble lead to dephasing of a stored coherence. Intriguingly, fluctuations in \(\delta \) can prolong the coherence time—a phenomenon called motional narrowing. Historically, motional narrowing was first observed in liquid NMR, where a large reduction in the width of spectral lines was observed in comparison to solid NMR due to the thermal motion of the nuclei. Later the effect was also reported in other fields such as molecular physics, semiconductor microcavities and quantum dots. In this chapter we study motional narrowing due to elastic collisions in a dense atomic ensemble. In contrast to previous experiments, our apparatus enables precise and independent control over the thermodynamic parameters of the system. Owing to this, we are able to quantitatively analyze the dependance of the narrowed linewidth on the fluctuations rate and strength, and demonstrate an inverse linear dependence on the former and quadratic dependence on the latter. We also find that the narrowed linewidth exhibits universal scaling with the atomic phase space density.
Collisional Narrowing and Dynamical Decoupling in a Dense Ensemble of Cold Atoms, 2012
In this chapter we aim to exactly solve for the spectrum (and the decay of coherence) of an ensem... more In this chapter we aim to exactly solve for the spectrum (and the decay of coherence) of an ensemble with an arbitrary detuning distribution \(P_0(\delta )\). Historically, the spectrum of a TLS ensemble was first studied by Kubo who considered an ensemble of harmonic oscillators with randomly varying resonant frequencies, and found an analytic expression for the ensemble spectrum for a frequency which is a Gaussian process. In the prevalent case, where the frequency autocorrelation function decays exponentially in time, the Kubo solution gives a generalized Gumbel function, as defined in Eq. (4.4).
We study experimentally the anomalous diffusion of atoms in one dimension. The ultra-cold atoms c... more We study experimentally the anomalous diffusion of atoms in one dimension. The ultra-cold atoms continuously scatter photons from a lattice which is in a configuration identical to the one used in the well-known Sisyphus cooling scheme. This produces a steady-state atomic velocity distribution which is a power law, with an exponent that depends on the lattice depth [1]. We image the atomic density distribution after a varying waiting time. The width of the atomic cloud exhibits a power law time dependence, and we extract its characteristic exponent for various lattice depths. We also show that the density distribution at different times is self-similar with the same characteristic exponent, in accordance with the predictions of a fractional diffusion equation [2]. [4pt] [1] P. Douglas, S. Bergamini, and F. Renzoni, Phys. Rev. Lett. 96, 110601 (2006). [2] R. Metzler and J. Klafter, Physics Reports 339, 1 (2000).
The inherent density inhomogeneity of a trapped gas can complicate interpretation of experiments ... more The inherent density inhomogeneity of a trapped gas can complicate interpretation of experiments and can wash out sharp features. This is especially important for a Fermi gas, where interaction effects as well as the local Fermi energy, or Fermi momentum, depend on the density. We report on experiments that use optical pumping with shaped light beams to spatially select the center part of a trapped gas for probing. This technique is compatible with momentum resolved measurements. For a weakly interacting Fermi gas of ^40K atoms, we present measurements of the momentum distribution that reveal for the first time a sharp Fermi surface. We then apply this technique to a strongly interacting Fermi gas at the Feshbach resonance, where we measured the temperature dependence of the Tan's contact locally in the trapped gas.
Superfluidity in strongly correlated matter is still an open question after many years of researc... more Superfluidity in strongly correlated matter is still an open question after many years of research. High Tc superconductors, for example, are known to exhibit a pseudo-gap phase above the critical temperature - a phase in which pairing exists in spite of the absence of long range order. An ultra-cold strongly interacting Fermi gas realizes a clean and controllable model system for studying these questions. It was shown recently that the normal state of a strongly interacting Fermi gas has a spectral behavior typical to a pseudo-gap phase [1]. One of the problems, however, which complicates the interpretation of these experiments is the inherent density inhomogeneity of the gas which arises due to the harmonic confinement. Here we present a technique to overcome this difficulty by spatially selecting only part of the cloud for interrogation while still retaining momentum resolution. We present measurements of the momentum distribution of a degenerate gas of 40K atoms revealing for th...
Cold atoms trapped in a conservative potential can be used for quantum information processing, wh... more Cold atoms trapped in a conservative potential can be used for quantum information processing, where their high density can be employed to achieve high efficiency in quantum operations. The high atomic density, on the other hand, alters the atomic coherence dynamics and leads to a surprising prolongation of the coherence times, a phenomenon we call collisional narrowing in analog to the well known motional narrowing effect in NMR. Here we present our theoretical and experimental study of collisional narrowing in optically trapped atoms. We show that elastic collisions reduce the spread of the atomic phase distribution and modify its time evolution from ballistic to diffusive. We perform Ramsey experiments on cold ^87Rb atoms trapped in a far-off-resonance laser, and observe a decrease of the dephasing rate for an increasing collision rate. We also show that the new emerging dephasing timescale depends only on the atomic phase space density of the ensemble.
2007 Quantum Electronics and Laser Science Conference, 2007
ABSTRACT By releasing ultra-cold atoms from a small red detuned Gaussian trap to an optical wedge... more ABSTRACT By releasing ultra-cold atoms from a small red detuned Gaussian trap to an optical wedge billiard we reduce the energy broadening of the atoms and perform spatial selection on the initial occupied phase space.
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Papers by Yoav Sagi