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The 2dF (Two‐degree Field) facility at the prime focus of the Anglo‐Australian Telescope provides multiple‐object spectroscopy over a 2° field of view. Up to 400 target fibres can be independently positioned by a complex robot. Two spectrographs provide spectra with resolutions of between 500 and 2000, over wavelength ranges of 440 and 110 nm respectively. The 2dF facility began routine observations in 1997. 2dF was designed primarily for galaxy redshift surveys and has a number of innovative features. The large corrector lens incorporates an atmospheric dispersion compensator, essential for wide wavelength coverage with small‐diameter fibres. The instrument has two full sets of fibres on separate field plates, so that re‐configuring can be done in parallel with observing. The robot positioner places one fibre every 6 s, to a precision of 0.3 arcsec (20 μm) over the full field. All components of 2dF, including the spectrographs, are mounted on a 5‐m diameter telescope top end ring for ease of handling and to keep the optical fibres short in order to maximize UV throughput. There is a pipeline data reduction system which allows each data set to be fully analysed while the next field is being observed. 2dF has achieved its initial astronomical goals. The redshift surveys obtain spectra at the rate of 2500 galaxies per night, yielding a total of about 200 000 objects in the first four years. Typically a B=19 galaxy gives a spectrum with a signal‐to‐noise ratio of better than 10 per pixel in less than an hour; redshifts are derived for about 95 per cent of all galaxies, with 99 per cent reliability or better. Total system throughput is about 5 per cent. The failure rate of the positioner and fibre system is about 1:10 000 moves or once every few nights, and recovery time is usually short. In this paper we provide the historical background to the 2dF facility, the design philosophy, a full technical description and a summary of the performance of the instrument. We also briefly review its scientific applications and possible future developments.

A weak continuous quantum measurement of an atomic spin ensemble can be implemented via Faraday rotation of an off-resonance probe beam, and may be used to create and probe nonclassical spin states and dynamics. We show that the probe light shift leads to nonlinearity in the spin dynamics and limits the useful Faraday measurement window. Removing the nonlinearity allows a non-perturbing measurement on the much longer timescale set by decoherence. The nonlinear spin Hamiltonian is of interest for studies of quantum chaos and real-time quantum state estimation.PACS numbers: 42.50. Ct, 03.65.Ta The process of quantum measurement involves a fundamental tradeoff between information gain and disturbance. In a projective measurement, this backaction is strong enough to collapse the state of the system and disrupt its coherent evolution. In more realistic scenarios, the system is weakly coupled to a probe, which is then measured to gain small amounts of information at the cost of modest perturbation. Continuous versions of this weak measurement scheme are of particular interest in the context of real-time feedback control and the creation and probing of non-classical states and dynamics [1]. Generally, the coupling of a probe to a single quantum system is so weak that the signal carrying information about the system becomes masked by probe noise. The signal-to-noise ratio of the measurement can be improved by coupling the probe to an ensemble of identically prepared systems, while at the same time the backaction on individual ensemble members can be kept low. Of course the many-body system is now described by a collective quantum state, and when the measurement strength is sufficient to resolve the quantum fluctuations associated with a collective observable, backaction will be induced on the collective state and the uncertainty of the measured value can be squeezed [2]. The creation of such quantum correlation has applications in precision measurement and quantum information processing. [3] In this letter we use the linear Faraday effect to probe the spins in an ensemble of laser cooled Cs atoms. [4,5,6] Our setup employs a probe beam tuned near the D 2 transition at 852 nm, whose linear polarization is rotated by an angle proportional to the net spin component along the propagation axis. Measuring the rotation with a shotnoise limited polarimeter provides a weak measurement of the ensemble averaged spin in real time. If the sample is optically thick on resonance, the atom-probe coupling becomes strong enough to allow the collective spin to be measured with resolution below the quantum uncertainty of a many-body spin-coherent state, making it possible to generate quantum correlations within the ensemble. In the limit of large probe detuning, Faraday rotation has been employed as a quantum non-demolition (QND) measurement of the collective spin, and much interest has been focused on its ability to generate spin squeezed states [7,8], to perform sub-shot noise magnetometry [9] and to entangle separated spin ensembles....

We demonstrate a fast, robust and non-destructive protocol for quantum state estimation based on continuous weak measurement in the presence of a controlled dynamical evolution. Our experiment uses optically probed atomic spins as a testbed, and successfully reconstructs a range of trial states with fidelities of ∼ 90%. The procedure holds promise as a practical diagnostic tool for the study of complex quantum dynamics, the testing of quantum hardware, and as a starting point for new types of quantum feedback control.PACS numbers: 03.65. Wj, 03.65.Ta, 32.80.Qk Fast, accurate and robust quantum state estimation (tomography) is important for the study of complex quantum systems and dynamics, and promises to be an essential tool in the design and testing of hardware for quantum information processing [1]. Previous demonstrations range from optical [2] to atomic [3] and molecular systems [4], but with few exceptions these procedures have proven too cumbersome to be of use as practical laboratory tools. The procedure of quantum state estimation is usually formulated in terms of strong measurements of an informationally complete set of observables. Each such measurement erases the original quantum state, so the ensemble must be reprepared and the measurement apparatus reconfigured at each step. Here we demonstrate a general approach based instead on continuous weak measurement [5]. Using a weak measurement spreads quantum backaction across the ensemble and dilutes it to the point where it does not significantly affect any individual member. In the absence of backaction the quantum state remains largely intact, subject only to minimal damage from errors in the external drive fields and coupling to the environment. This allows us to estimate the state in a single interrogation of the ensemble, based on the measurement of a fixed observable O and a carefully designed system evolution.In the Heisenberg picture the chosen dynamics leads to a time-dependent observable, O → O(t), and the measurement history can be made informationally complete if the system is controllable, i. e. the dynamics can generate any unitary in SU (d) where d is the dimensionality of Hilbert space. With a non-destructive measurement and near-reversible dynamics, the entire ensemble remains available at the end of the estimation procedure, in a known quantum state that can be restored close to its initial form if desired. In principle, this means that the knowledge gained can be used as a basis for further action, for example real-time feedback control [6] or error correction [1]. From a practical viewpoint our procedure is highly efficient: the ∼ 1ms interrogation time is limited only by the control and measurement bandwidths, and data analysis is performed offline. It is also robust, in the sense that imperfections in the experiment can be included in the analysis if known, or estimated along with the state if they fluctuate in real time.The quantum system used in our laboratory implementation is the total spin-angular momentum (electron plus nu...