Novel steady state of a microtubule assembly in a confined geometry

We study the steady state of an assembly of microtubules in a confined volume, analogous to the situation inside a cell where the cell boundary forms a natural barrier to growth. We show that the dynamical equations for growing and shrinking microtubules predict the existence of two steady states, with either exponentially decaying or exponentially increasing distribution of microtubule lengths. We identify the regimes in parameter space corresponding to these steady states. In the latter case, the apparent catastrophe frequency near the boundary was found to be significantly larger than that in the interior. Both the exponential distribution of lengths and the increase in the catastrophe frequency near the cell margin is in excellent agreement with recent experimental observations.Comment: 8 pages, submitted to Phys. Rev.

Physical implementations of quantum absorption refrigerators

Absorption refrigerators are autonomous thermal machines that harness the spontaneous flow of heat from a hot bath into the environment in order to perform cooling. Here we discuss quantum realizations of absorption refrigerators in two different settings: namely, cavity and circuit quantum electrodynamics. We first provide a unified description of these machines in terms of the concept of virtual temperature. Next, we describe the two different physical setups in detail and compare their properties and performance. We conclude with an outlook on future work and open questions in this field of research.Comment: Patrick P. Potts was formerly known as Patrick P. Hofe

Probing the dynamic structure factor of a neutral Fermi superfluid along the BCS-BEC crossover using atomic impurity qubits

We study an impurity atom trapped by an anharmonic potential, immersed within a cold atomic Fermi gas with attractive interactions that realizes the crossover from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Bose-Einstein condensate (BEC). Considering the qubit comprising the lowest two vibrational energy eigenstates of the impurity, we demonstrate that its dynamics probes the equilibrium density fluctuations encoded in the dynamic structure factor of the superfluid. Observing the impurity's evolution is thus shown to facilitate nondestructive measurements of the superfluid order parameter and the contact between collective and single-particle excitation spectra. Our setup constitutes a novel model of an open quantum system interacting with a thermal reservoir, the latter supporting both bosonic and fermionic excitations that are also coupled to each other.Comment: Updated to final author version. 9+7 pages, 18 figure

Minimal Absorption Measurements

We show that it is not possible to discriminate two close transparencies without a certain number of photons being absorbed. We extend this to the discrimination of patterns of transparency (images).Comment: 11 pages (latex

Thermometry by correlated dephasing of impurities in a 1D Fermi gas

We theoretically investigate the pure dephasing dynamics of two static impurity qubits embedded within a common environment of ultracold fermionic atoms, which are confined to one spatial dimension. Our goal is to understand how bath-mediated interactions between impurities affect their performance as nonequilibrium quantum thermometers. By solving the dynamics exactly using a functional determinant approach, we show that the impurities become correlated via retarded interactions of the Ruderman-Kittel-Kasuya-Yosida type. Moreover, we demonstrate that these correlations can provide a metrological advantage, enhancing the sensitivity of the two-qubit thermometer beyond that of two independent impurities. This enhancement is most prominent in the limit of low temperature and weak collisional coupling between the impurities and the gas. We show that this precision advantage can be exploited using standard Ramsey interferometry, with no need to prepare correlated initial states nor to individually manipulate or measure the impurities. We also quantitatively assess the impact of ignoring these correlations when constructing a temperature estimate, finding that acceptable precision can still be achieved from a simplified model of independent impurities. Our results demonstrate the rich nonequilibrium physics of impurities dephasing in a common Fermi gas, and may help to provide better temperature estimates at ultralow temperatures.Comment: v1: 16 pages, 6 figures. Comments and feedback welcome as always v2: included temperature dependent decoherence and added reference

Mitotic spindle assembly by two different pathways in vitro

Abstract. We have used Xenopus egg extracts to study spindle morphogenesis in a cell-free system and have identified two pathways of spindle assembly in vitro using methods of fluorescent analogue cytochemistry. When demembranated sperm nuclei are added to egg extracts arrested in a mitotic state, individual nuclei direct the assembly of polarized microtubule arrays, which we term half-spindles; half-spindles then fuse pairwise to form bipolar spindles. In contrast, when sperm nuclei are added to extracts that are induced to enter interphase and arrested in the following mitosis, a single sperm nucleus can direct the assembly of a complete spindle. We find that microtubule arrays in vitro are strongly biased towards chromatin, but this does not depend on specific kinetochore-microtubul