Low-Noise Amplification of a Continuous Variable Quantum State

We present an experimental realization of a low-noise, phase-insensitive optical amplifier using a four-wave mixing interaction in hot Rb vapor. Performance near the quantum limit for a range of amplifier gains, including near unity, can be achieved. Such low-noise amplifiers are essential for so-called quantum cloning machines and are useful in quantum information protocols. We demonstrate that amplification and ``cloning'' of one half of a two-mode squeezed state is possible while preserving entanglement.Comment: To appear in Physical Review Letters July 3rd. 4 pages, 4 figure

Cooling a nanomechanical resonator with quantum back-action

Quantum mechanics demands that the act of measurement must affect the measured object. When a linear amplifier is used to continuously monitor the position of an object, the Heisenberg uncertainty relationship requires that the object be driven by force impulses, called back-action. Here we measure the back-action of a superconducting single-electron transistor (SSET) on a radiofrequency nanomechanical resonator. The conductance of the SSET, which is capacitively coupled to the resonator, provides a sensitive probe of the latter's position;back-action effects manifest themselves as an effective thermal bath, the properties of which depend sensitively on SSET bias conditions. Surprisingly, when the SSET is biased near a transport resonance, we observe cooling of the nanomechanical mode from 550mK to 300mK-- an effect that is analogous to laser cooling in atomic physics. Our measurements have implications for nanomechanical readout of quantum information devices and the limits of ultrasensitive force microscopy (such as single-nuclear-spin magnetic resonance force microscopy). Furthermore, we anticipate the use of these backaction effects to prepare ultracold and quantum states of mechanical structures, which would not be accessible with existing technology.Comment: 28 pages, 7 figures; accepted for publication in Natur

Two-dimensional transport and transfer of a single atomic qubit in optical tweezers

Quantum computers have the capability of out-performing their classical counterparts for certain computational problems1. Several scalable quantum-computing architectures have been proposed. An attractive architecture is a large set of physically independent qubits arranged in three spatial regions where (1) the initialized qubits are stored in a register, (2) two qubits are brought together to realize a gate and (3) the readout of the qubits is carried out2, 3. For a neutral-atom-based architecture, a natural way to connect these regions is to use optical tweezers to move qubits within the system. In this letter we demonstrate the coherent transport of a qubit, encoded on an atom trapped in a submicrometre tweezer, over a distance typical of the separation between atoms in an array of optical traps4, 5, 6. Furthermore, we transfer a qubit between two tweezers, and show that this manipulation also preserves the coherence of the qubit

Signatures of a dissipative phase transition in photon correlation measurements

This work was supported by the Swiss National Science Foundation (SNSF) through the National Centre of Competence in Research - Quantum Science and Technology (NCCR QSIT). A.S., C.S., and S.H. acknowledge support by the State of Bavaria and the DFG within the Project Schn1376/3-1.Understanding and characterizing phase transitions in driven-dissipative systems constitutes a new frontier for many-body physics[1-8]. A generic feature of dissipative phase transitions is a vanishing gap in the Liouvillian spectrum [9], which leads to long-lived deviations from the steady state as the system is driven towards the transition. Here, we show that photon correlation measurements can be used to characterize the corresponding critical slowing down of non-equilibrium dynamics. We focus on the extensively studied phenomenon of optical bistability in GaAs cavity polaritons [10,11], which can be described as a first-order dissipative phase transition [12-14]. Increasing the excitation strength towards the bistable range results in an increasing photon-bunching signal along with a decay time that is prolonged by more than nine orders of magnitude as compared with that of single polaritons. In the limit of strong polariton interactions leading to pronounced quantum fluctuations, the mean-field bistability threshold is washed out. Nevertheless, the functional form with which the Liouvillian gap closes as the thermodynamic limit is approached provides a signature of the emerging dissipative phase transition. Our results establish photon correlation measurements as an invaluable tool for studying dynamical properties of dissipative phase transitions without requiring phase-sensitive interferometric measurements.PostprintPeer reviewe

Associations of depression and depressive symptoms with preeclampsia: results from a Peruvian case-control study

<p>Abstract</p> <p>Background</p> <p>Preeclampsia involves endothelial dysfunction, platelet dysfunction/activation and sympathetic over-activity similar to cardiovascular disorders (CVD). Depression, an independent risk factor for progression of CVD, was found to be associated with an increased risk of preeclampsia among Finnish women. We examined the relation between depression/depressive symptoms and preeclampsia risk among Peruvian women.</p> <p>Methods</p> <p>The study included 339 preeclamptic cases and 337 normotensive controls. Depression and depressive symptoms during pregnancy were assessed using the Patient Health Questionnaire (PHQ-9). Odds ratios (OR) and 95% confidence intervals (CI) were estimated from logistic regression models.</p> <p>Results</p> <p>The prevalence of moderate depression was 11.5% among cases and 5.3% among controls. The corresponding figures for moderate-severe depression were 3.5% for cases and 2.1% for controls. Compared with non-depressed women, those with moderate depression had a 2.3-fold increased risk of preeclampsia (95% CI: 1.2–4.4), while moderate-severe depression was associated with a 3.2-fold (95% CI: 1.1–9.6) increased risk of preeclampsia. Associations of each of the 9-items of the PHQ-9 depression screening module with preeclampsia risk were also observed.</p> <p>Conclusion</p> <p>Our findings are consistent with the only other published report on this topic. Collectively, available data support recent calls for expanded efforts to study and address depression among pregnant women.</p

Spinor Bose-Einstein-condensate phase-sensitive amplifier for SU(1,1) interferometry

The SU(1,1) interferometer was originally conceived as a Mach-Zehnder interferometer with the beam splitters replaced by parametric amplifiers. The parametric amplifiers produce states with correlations that result in enhanced phase sensitivity. F=1 spinor Bose-Einstein condensates (BECs) can serve as the parametric amplifiers for an atomic version of such an interferometer by collisionally producing entangled pairs of |F=1,m=±1) atoms. We simulate the effect of single- and double-sided seeding of the inputs to the amplifier using the truncated-Wigner approximation. We find that single-sided seeding degrades the performance of the interferometer exactly at the phase the unseeded interferometer should operate the best. Double-sided seeding results in a phase-sensitive amplifier, where the maximal sensitivity is a function of the phase relationship between the input states of the amplifier. In both single- and double-sided seeding we find there exists an optimal phase shift that achieves sensitivity beyond the standard quantum limit. Experimentally, we demonstrate a spinor phase-sensitive amplifier using a BEC of Na23 in an optical dipole trap. This configuration could be used as an input to such an interferometer. We are able to control the initial phase of the double-seeded amplifier and demonstrate sensitivity to initial population fractions as small as 0.1%