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Quantum Electronics and Laser Science Conference

Frequency tunable source of entangled optical fields

C. Schori, Brian Julsgaard, Jens Lykke Sørensen, Eugene Simon Polzik

Published Jun 25, 2003• 0 citations
Publisher

Abstract

Summary form only given. We have reported the construction of a tunable source of nonclassical light along with experimental data demonstrating the entanglement between signal and idler fields separated by 800 MHz. The entanglement corresponds to a noise reduction of 4 dB beyond the SQL. We have already used the tunable squeezed output of this source to probe spin polarised cesium atoms. Replacing the degenerate squeezed output with the nondegenerate EPR-correlated signal and idler fields makes the source suitable for future experiments with quantum memory read-out via atom-to-light teleportation.

DOI
10.1109/qels.2002.1031388

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Summary form only given. We have reported the construction of a tunable source of nonclassical light along with experimental data demonstrating the entanglement between signal and idler fields separated by 800 MHz. The entanglement corresponds to a noise reduction of 4 dB beyond the SQL. We have already used the tunable squeezed output of this source to probe spin polarised cesium atoms. Replacing the degenerate squeezed output with the nondegenerate EPR-correlated signal and idler fields makes the source suitable for future experiments with quantum memory read-out via atom-to-light teleportation.
We report the observation of nonclassical quantum correlations of continuous light variables from an altogether different type of source. It is a frequency nondegenerate optical parametric oscillator below threshold, where signal and idler fields are separated by 740 MHz corresponding to two free spectrum ranges of the parametric oscillator cavity. The degree of entanglement observed, $\ensuremath{-}3.8\mathrm{dB},$ is the highest to date for a narrow-band tunable source suitable for atomic quantum memory and other applications in atomic physics. Finally we use the latter to visualize the Einstein-Podolsky-Rosen paradox.
A frequency tunable source of squeezed light has been developed for application to a variety of problems in atomic physics. The basic experimental arrangement consists of a subthreshold optical parametric oscillator (OPO) that is pumped by the frequency-doubled output of a titanium-sapphire laser. Squeezed light with continuous tunability of 2 GHz has been generated with a directly observed nonclassical noise reduction of 5.0 dB relative to the vacuumstate limit in a balanced homodyne detector (5.2 dB when corrected for detector noise). Although our source is potentially broadly tunable over the noncritical phase-matching bandwidth of potassium niobate, current research has concentrated on operation at 852 nm for excitation of the D2 line of atomic cesium. Relative to the usual quantum limit associated with vacuumstate fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in FM saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB after correction for detector noise.
A frequency-tunable source of squeezed light would allow investigations of the atom-field interaction in a regime where fundamental new phenomena have been predicted.1 There are also opportunities for spectroscopy with sensitivity beyond the shot-noise limit. Toward these objectives, in our experiment we employ a single-frequency Ti-sapphire laser with tuning optimized around 852 nm (cesium resonance transition). The infrared output of the laser is frequency doubled in an external buildup cavity with a KNbO3 crystal as the nonlinear element. Efficient doubling provides 0.65 W of tunable blue output for 1.35 W of infrared input. This blue output is then used to pump an optical parametric oscillator (OPO) containing a second KNbO3 crystal for downconversion back to the infrared. The high power of the blue pump that we have achieved and the large nonlinearity of KNbO3 allows us to employ an OPO that is resonant only for the subharmonic infrared light. This design facilitates tuning of the OPO compared with the case in which the blue pump and the infrared are both resonant. The doubling cavity, OPO, and laser itself are locked relative to a stable reference cavity by an rf sideband technique, thus providing the possibility for tunability of the system with a single knob.
Ultimate limits for the sensing of fields and forces are set by the quantum noise of a sensor1–3. Entanglement allows for suppression of such noise and for achieving sensitivity beyond standard quantum limits4–7. Applicability of quantum optical sensing is often restricted by fixed wavelengths of available photonic quantum sources. Another ubiquitous limitation is associated with challenges of achieving quantum-noise-limited sensitivity in the acoustic noise frequency range relevant for several applications. Here we demonstrate a tool for broadband quantum sensing by performing quantum state processing that can be applied to a wide range of the optical spectrum and by suppressing quantum noise over an octave in the acoustic frequency range. An atomic spin ensemble is strongly coupled to one of the frequency-tunable beams of an Einstein–Podolsky–Rosen (EPR) source of light. The other EPR beam of light, entangled with the first one, is tuned to a disparate wavelength. Engineering the spin ensemble to act as a negative-mass or positive-mass oscillator, we demonstrate frequency-dependent quantum noise reduction for measurements at the disparate wavelength. The tunability of the spin ensemble enables targeting quantum noise in a variety of systems with dynamics ranging from kHz to MHz. As an example of broadband quantum noise reduction in the acoustic frequency range, we analyse the applicability of our approach to gravitational-wave detectors (GWDs). Other possible applications include continuous-variable quantum repeaters and distributed quantum sensing. A hybrid quantum network combining entangled light with an atomic spin ensemble engineered to act as a negative-mass oscillator enables frequency-dependent quantum noise reduction for measurements in the acoustic noise frequency range relevant for gravitational-wave detection.
We report the realization of a classically phase-locked source of quantum twin beams. Theoretical work by Reid and Drummond, and more recently by van Loock and Braunstein, predicts that such a source is adequate for creating bipartite and multipartite continuous-variable entanglement. The source is a type-II optical parametric oscillator (OPO) above threshold. Its exceptional frequency and intensity stability is derived from careful design and from three servo loops. The OPO can stably emit on the frequency-degenerate mode, pumped a few percent above the threshold. We observe a preliminary number-difference squeezing of 5.5 dB (6.4 dB inferred) at 200 kHz. In order to enable future EPR measurements, the signal and idler fields must also be classically phase-locked: we measure a signal-idler-frequency-difference linewidth smaller than 300 Hz for our OPO.
Parametric downconversion in media has, so far, proved to be the most effective way of generating squeezed light. The development of frequency tunable sources of such non-classical light opens up novel possibilities in atomic physics and spectroscopy. A brief overview of the results in this area of quantum optics is given, and a new experiment aimed on subshot-noise measurement of atomic orientation is described. We report a preliminary 5 dB quantum noise reduction observed directly around the 917 nm Cs line. We introduce a figure of merit for the medium utilized for squeezed light generation in an optical parametric oscillator and evaluate some available nonlinear media from this perspective.