Strong low-frequency quantum correlations from a four-wave-mixing amplifier

Abstract

We show that a simple scheme based on nondegenerate four-wave mixing in a hot atomic vapor behaves like a near-perfect phase-insensitive optical amplifier, which can generate bright twin beams with a measured quantum noise reduction in the intensity difference of more than 8 dB, close to the best optical parametric amplifiers and oscillators. The absence of a cavity makes the system immune to external perturbations, and the strong quantum noise reduction is observed over a large frequency range.PACS numbers: 42.50. Gy, 42.50.Dv Two-mode squeezed beams have become a valuable source of entanglement for quantum communications and quantum information processing [1]. These applications bring specifi requirements on the squeezed light sources. For instance, for squeeze light to be used as a quantum information carrier interacting with material system, as in an atomic quantum memory, the light field must be resonant with an atomic transition and spectrally narrow to ensure an efficient coupling between light and matter. In recent years, attention has also been brought to the problem of the manipulation of cold atomic samples with non-classical fields in order to produce non-classical matter waves [2,3,4]. In this case, the slow atomic dynamics also requires squeezing at low frequencies.The standard technique for generating nonclassical light fields is by parametric down-conversion in a crystal, with an optical parametric oscillator or an optical parametric amplifier [5,6]. While very large amounts of quantum noise reduction have been achieved in this way [7,8], controlling the frequency and the linewidth of the light remains a challenge. Only recently have sources based on periodically-poled nonlinear crystals been developed at 795 nm to couple to the Rb D1 atomic line [9,10]. On the other hand, stimulated four-wave mixing (4WM) naturally generates narrow-band light close to an atomic resonance, but its development as an efficient source of squeezed light has been hindered by fundamental limitations such as spontaneous emission. At the end of the 1990s, nondegenerate 4WM in a double-lambda scheme was identified as a possible workaround for these limitations, as described in Ref.[11] and references therein. It was not until recently that such a scheme was implemented in continuous mode in an efficient way in both the low [12,13,14] and the high [15,16] intensity regimes, where it was shown to generate twin beams where quantum correlations are not masked by competing effects.The double-lambda scheme gives rise to complex atomic dynamics and propagation properties, such as slow-light effects [17]. In this Letter, we show that in spite of this complexity, the quantum properties of the scheme can be accurately described as the combination of a perfect amplifier and a partial absorber. This model allows us to optimize the quantum noise reduction in the intensity difference of the bright twin beams and to isolate the limiting factors of this reduction. It also helps to identify regions of the parameter space where the syst...

Squeezing bandwidth controllable twin beam light and phase sensitive nonlinear interferometer based on atomic ensembles

“…Recently, low-frequency squeezing was found to be interesting for electromagnetically-induced transparency-based quantum information protocols and other applications at atomic transition wavelengths [16]. Our work [17] is derived from this motivation and inspired by previous work such as the creation of beams with a low-frequency quantum correlation based on FWM in a hot rubidium vapor [18]. We have generated an intensity difference squeezed light source at frequencies as low as 1.5 kHz which is so far the lowest frequency at which squeezing has been observed in an atomic system.…”

Observation of spatial quantum correlations in the macroscopic regime

“…When working with higher gains, the contribution of this noise is minimized. This has made it possible to obtain ∼9 dB of squeezing in the time domain with this source [6,10]. Besides these factors, the main source of excess noise is due to scattered pump photons, which can be minimized by using an atomic line filter [37].…”

“…First, the photon pairs generated by FWM have narrow bandwidths (in the MHz regime, even when working with hot atoms) [11,28,29], therefore they are useful for atom-light interaction-based quantum protocols [33]. Second, the FWM process offers large gains even in a single pass configuration unlike SPDC [6,34]. As a result, FWM can produce bright quantum correlated beams of light without a cavity [7].…”

“…Orthogonally linearly polarized pump and probe beams with 1/e 2 waist diameters of 4.5 mm and 0.2 mm respectively, interact at an angle of ∼ 0.5 • at the center of a 12 mm long 85 Rb vapor cell heated to 113 • C. As a result of the FWM in the double-Λ configuration, shown in the inset of Fig. 1, two pump photons are absorbed and quantum correlated probe and conjugate photons are generated [6]. After the cell, most of the pump beam is filtered with a polarization filter.…”

“…Dv, 42.50.Ar, Under certain conditions, the quantum fluctuations in beams of light can be reduced below the shot noise limit (SNL) not only in the temporal domain, but also in the transverse spatial degree of freedom [1]. To date, most of the attention has focused on the study of quantum noise reduction, or squeezing, in the temporal domain [2][3][4][5][6][7][8][9][10][11][12]. Nevertheless, many areas in quantum optics, specifically quantum metrology and quantum imaging, could greatly benefit from the study of the quantum correlations directly in the spatial domain [13][14][15].…”