Low-loss transmission and sensitive recovery of weak radio-frequency and microwave signals is a ubiquitous challenge, crucial in radio astronomy, medical imaging, navigation, and classical and quantum communication. Efficient up-conversion of radio-frequency signals to an optical carrier would enable their transmission through optical fibres instead of through copper wires, drastically reducing losses, and would give access to the set of established quantum optical techniques that are routinely used in quantum-limited signal detection. Research in cavity optomechanics has shown that nanomechanical oscillators can couple strongly to either microwave or optical fields. Here we demonstrate a room-temperature optoelectromechanical transducer with both these functionalities, following a recent proposal using a high-quality nanomembrane. A voltage bias of less than 10 V is sufficient to induce strong coupling between the voltage fluctuations in a radio-frequency resonance circuit and the membrane's displacement, which is simultaneously coupled to light reflected off its surface. The radio-frequency signals are detected as an optical phase shift with quantum-limited sensitivity. The corresponding half-wave voltage is in the microvolt range, orders of magnitude less than that of standard optical modulators. The noise of the transducer--beyond the measured 800 pV Hz-1/2 Johnson noise of the resonant circuit--consists of the quantum noise of light and thermal fluctuations of the membrane, dominating the noise floor in potential applications in radio astronomy and nuclear magnetic imaging. Each of these contributions is inferred to be 60 pV Hz-1/2 when balanced by choosing an electromechanical cooperativity of ~150 with an optical power of 1 mW. The noise temperature of the membrane is divided by the cooperativity. For the highest observed cooperativity of 6,800, this leads to a projected noise temperature of 40 mK and a sensitivity limit of 5 pV Hz-1/2. Our approach to all-optical, ultralow-noise detection of classical electronic signals sets the stage for coherent up-conversion of low-frequency quantum signals to the optical domain.

Quantum memories : a review based on the European integrated project "Qubit Applications (QAP)"

Squeezing of quantum fluctuations by means of entanglement is a well-recognized goal in the field of quantum information science and precision measurements. In particular, squeezing the fluctuations via entanglement between 2-level atoms can improve the precision of sensing, clocks, metrology, and spectroscopy. Here, we demonstrate 3.4 dB of metrologically relevant squeezing and entanglement for 10 5 cold caesium atoms via a quantum nondemolition (QND) measurement on the atom clock levels. We show that there is an optimal degree of decoherence induced by the quantum measurement which maximizes the generated entanglement. A 2-color QND scheme used in this paper is shown to have a number of advantages for entanglement generation as compared with a single-color QND measurement. N A for the case of independent atoms also referred to as a coherent spin state (CSS). The CSS minimizes the Heisenberg uncertainty product so that, e.g., (δJ z ) 2 (δJ x ) 2 = 1 4| J y | 2 where J y is the expectation value of the spin projection operator. At the expense of an increase in (δJ x ) 2 , it is possible to reduce (δJ z ) 2 (or vice versa) below the projection noise limit while keeping their product constant. This constitutes an example of a spin squeezed state (SSS), for which the atoms need to be correlated. This correlation is ensured to be nonclassical ifwhere ξ defines the squeezing parameter. Under this condition, the atoms are entangled (3) and the prepared state improves the signal-to-noise ratio in spectroscopical and metrological applications (1). Systems of 2 to 3 ions have successfully been used to demonstrate spectroscopic performance with reduced quantum noise and entanglement (4, 5). The situation is somewhat different with macroscopic atomic ensembles where spin squeezing has been an active area of research in the past decade (6-13). To our knowledge, no results reporting ξ < 1 via interatomic entanglement in such ensembles have been reported so far, with a very recent exception of the paper (14) where entanglement in an external motional degree of freedom of 2 · 10 3 atoms via interactions in a Bose-Einstein condensate is demonstrated. Spin Squeezing by Quantum Nondemolition (QND) MeasurementsIn this article, we report on the generation of an SSS fulfilling Eq. 1 in an ensemble of ≈10 5 atoms via a QND measurement (7, 15-17) of J z . We show how to take advantage of the entanglement in this mesoscopic system by using Ramsey spectroscopy (1)-one of the methods of choice for precision measurements of time and frequency (18) (Fig. 1A). The figure presents the evolution of the pseudospin J whose tip is traveling over the Bloch sphere. The Ramsey method allows using the atomic ensemble as a sensor for external fields where the perturbation of the energy difference between the levels ΔE ↑↓ is measured, or as a clock where the frequency of an oscillator is locked to the transition frequency between the two states Ω = ΔE ↑↓ / . Fig. 1 B illustrates how a suitable SSS can improve the precision of the Ramsey measurement pr...

We produce a 600-ns pulse of 1.86-dB squeezed vacuum at 795 nm in an optical parametric amplifier and store it in a rubidium vapor cell for 1 mus using electromagnetically induced transparency. The recovered pulse, analyzed using time-domain homodyne tomography, exhibits up to 0.21+/-0.04 dB of squeezing. We identify the factors leading to the degradation of squeezing and investigate the phase evolution of the atomic coherence during the storage interval.