227 research outputs found
Distinguishability of hyperentangled Bell state by linear evolution and local projective measurement
Measuring an entangled state of two particles is crucial to many quantum
communication protocols. Yet Bell state distinguishability using a finite
apparatus obeying linear evolution and local measurement is theoretically
limited. We extend known bounds for Bell-state distinguishability in one and
two variables to the general case of entanglement in two-state variables.
We show that at most classes out of hyper-Bell states can be
distinguished with one copy of the input state. With two copies, complete
distinguishability is possible. We present optimal schemes in each case.Comment: 5 pages, 2 figure
The potential energy of a K Fermi gas in the BCS-BEC crossover
We present a measurement of the potential energy of an ultracold trapped gas
of K atoms in the BCS-BEC crossover and investigate the temperature
dependence of this energy at a wide Feshbach resonance, where the gas is in the
unitarity limit. In particular, we study the ratio of the potential energy in
the region of the unitarity limit to that of a non-interacting gas, and in the
T=0 limit we extract the universal many-body parameter . We find ; this value is consistent with previous measurements
using Li atoms and also with recent theory and Monte Carlo calculations.
This result demonstrates the universality of ultracold Fermi gases in the
strongly interacting regime
Evolution of the Normal State of a Strongly Interacting Fermi Gas from a Pseudogap Phase to a Molecular Bose Gas
Wave-vector resolved radio frequency (rf) spectroscopy data for an ultracold
trapped Fermi gas are reported for several couplings at Tc, and extensively
analyzed in terms of a pairing-fluctuation theory. We map the evolution of a
strongly interacting Fermi gas from the pseudogap phase into a fully gapped
molecular Bose gas as a function of the interaction strength, which is marked
by a rapid disappearance of a remnant Fermi surface in the single-particle
dispersion. We also show that our theory of a pseudogap phase is consistent
with a recent experimental observation as well as with Quantum Monte Carlo data
of thermodynamic quantities of a unitary Fermi gas above Tc.Comment: 9 pages, 9 figures. Substantially revised version (to appear in Phys.
Rev. Lett.
The assembly and alignment of the 4MOST Wide Field Corrector
The 4-metre Multi-Object Spectroscopic Telescope (4MOST) is a fibre-fed multi-object spectrograph for the VISTA telescope at the ESO Paranal Observatory in Chile. The goal of the 4MOST project is to create a general-purpose and highly efficient spectroscopic survey facility for astronomers in the 4MOST consortium and the ESO community. The instrument itself will record 2436 simultaneous spectra over a ∼4.2 square degree field of view and consists of an optical Wide-Field Corrector (WFC), a fibre positioner system based on a tilting spine design, and three spectrographs giving both high and low spectral dispersion. The WFC comprises of 6 lenses grouped into 4 elements, 2 of which are cemented doublets that act as an atmospheric dispersion corrector (ADC). The first lens element is 0.9m in diameter whilst the diameter of the other elements is 0.65m. For the instrument to meet its science goals, each lens needs to be aligned to ∼50µm – a major challenge. This is achieved using contact metrology methods supplemented by pencil beam laser probes. In particular, a novel off-axis laser beam system has been implemented to test the optics’ alignment before and after shipment. This paper details the alignment and assembly methods and presents the latest results on the achieved lens positioning and projected performance of the WF
Observation of a pairing pseudogap in a two-dimensional Fermi gas
Pairing of fermions is ubiquitous in nature and it is responsible for a large
variety of fascinating phenomena like superconductivity, superfluidity of
He, the anomalous rotation of neutron stars, and the BEC-BCS crossover in
strongly interacting Fermi gases. When confined to two dimensions, interacting
many-body systems bear even more subtle effects, many of which lack
understanding at a fundamental level. Most striking is the, yet unexplained,
effect of high-temperature superconductivity in cuprates, which is intimately
related to the two-dimensional geometry of the crystal structure. In
particular, the questions how many-body pairing is established at high
temperature and whether it precedes superconductivity are crucial to be
answered. Here, we report on the observation of pairing in a harmonically
trapped two-dimensional atomic Fermi gas in the regime of strong coupling. We
perform momentum-resolved photoemission spectroscopy, analogous to ARPES in the
solid state, to measure the spectral function of the gas and we detect a
many-body pairing gap above the superfluid transition temperature. Our
observations mark a significant step in the emulation of layered
two-dimensional strongly correlated superconductors using ultracold atomic
gases
Observation of pseudogap behavior in a strongly interacting Fermi gas
Ultracold atomic Fermi gases present an opportunity to study strongly
interacting Fermi systems in a controlled and uncomplicated setting. The
ability to tune attractive interactions has led to the discovery of
superfluidity in these systems with an extremely high transition temperature,
near T/T_F = 0.2. This superfluidity is the electrically neutral analog of
superconductivity; however, superfluidity in atomic Fermi gases occurs in the
limit of strong interactions and defies a conventional BCS description. For
these strong interactions, it is predicted that the onset of pairing and
superfluidity can occur at different temperatures. This gives rise to a
pseudogap region where, for a range of temperatures, the system retains some of
the characteristics of the superfluid phase, such as a BCS-like dispersion and
a partially gapped density of states, but does not exhibit superfluidity. By
making two independent measurements: the direct observation of pair
condensation in momentum space and a measurement of the single-particle
spectral function using an analog to photoemission spectroscopy, we directly
probe the pseudogap phase. Our measurements reveal a BCS-like dispersion with
back-bending near the Fermi wave vector k_F that persists well above the
transition temperature for pair condensation
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