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The quantum noise in photocurrent fluctuations usually gives incomplete information about the quantum state of spectral sideband modes of bright light beams involved in the detection. Each frequency component of the noise spectrum corresponds to two sideband modes symmetrically located around the bright optical field. In the case of the usual homodyne detection, it limits the ability to recover discriminated information of each mode involved. We theoretically show that complete reconstruction of the two-mode quantum state can be obtained by using phase-locked (coherent) resonator detection, even for non-Gaussian states. We experimentally demonstrate the technique by measuring a two-mode displaced coherent state.

Broadband single photons are usually considered not to couple efficiently to atomic gases because of the large mismatch in bandwidth. Contrary to this intuitive picture, here we demonstrate that the interaction of ultrashort single photons with a dense resonant atomic sample deeply modifies the temporal shape of their wavepacket mode without degrading their non-classical character, and effectively generates zero-area single-photon pulses. This is a clear signature of strong transient coupling between single broadband (THz-level) light quanta and atoms, with intriguing fundamental implications and possible new applications to the storage of quantum information., 5 pages, 4 figures

Zero area single photons are generated through the resonant interaction of a parametric-down-conversion single photon with a dense atomic ensemble. This indicates a new pathway for developing a quantum memory for broadband single photons.

Gaussian quantum states hold special importance in the continuous variable (CV) regime. In quantum information science, the understanding and characterization of central resources such as entanglement may strongly rely on the knowledge of the Gaussian or non-Gaussian character of the quantum state. However, the quantum measurement associated with the spectral photocurrent of light modes consists of a mixture of quadrature observables. Within the framework of two recent papers [Phys. Rev. A 88, 052113 (2013) and Phys. Rev. Lett. 111, 200402 (2013)], we address here how the statistics of the spectral photocurrent relates to the character of the Wigner function describing those modes. We show that a Gaussian state can be misidentified as non-Gaussian and vice-versa, a conclusion that forces the adoption of tacit \textit{a priori} assumptions to perform quantum state reconstruction. We experimentally analyze the light beams generated by the optical parametric oscillator (OPO) operating above threshold to show that the data strongly supports the generation of Gaussian states of the field, validating the use of necessary and sufficient criteria to characterize entanglement in this system.

We demonstrate that the interaction of ultrashort single photons with a dense resonant atomic sample deeply modifies the temporal shape of their wavepacket mode without degrading their non-classical character, and effectively generates zero-area single-photon pulses.

In the spectral domain, the quantum state of the three beams emitted by an above-threshold optical parametric oscillator encompasses six modes. We use resonator detection to characterize hexapartite entanglement.

Spectral homodyne detection, a widely used technique for measuring quantum properties of light beams, cannot retrieve all the information needed to reconstruct the quantum state of spectral field modes. We show that full quantum state reconstruction can be achieved with the alternative measurement technique of resonator detection. We experimentally demonstrate this difference by engineering a quantum state with features that go undetected by homodyne detection but are clearly revealed by resonator detection., Comment: 4 pages, 4 figures

We revisit the problem of quantum state reconstruction of light beams from the photocurrent quantum noise. As is well-known, but often overlooked, two longitudinal field modes contribute to each spectral component of the photocurrent (sideband modes). We show that spectral homodyne detection is intrinsically incapable of providing all the information needed for the full reconstruction of the two-mode spectral quantum state. Such a limitation is overcome by the technique of resonator detection. A detailed theoretical description and comparison of both methods is presented, as well as an experiment to measure the six-mode quantum state of pump-signal-idler beams of an optical parametric oscillator above the oscillation threshold., Comment: 16 pages, 10 figures

We demonstrated the direct generation of multipartite continuous-variable entanglement and investigated its robustness against losses. This led to the development of experimental capabilities to fully characterize a six-mode quantum optical state.

A quantum state of light is the excitation of a particular spatiotemporal mode of the electromagnetic field. A precise control of the mode structure is therefore essential for processing, detecting, and using photonic states in novel quantum technologies. Here we demonstrate an adaptive scheme, combining techniques from the fields of ultrafast coherent control and quantum optics, for probing the arbitrary complex spectrotemporal profile of an ultrashort quantum light pulse. The ability to access the modal structure of a quantum light state could boost the capacity of current quantum information protocols.

We experimentally investigate quantum entanglement among three beams of light produced by the optical parametric oscillator. We aim to understand how entanglement is distributed among the light beams and associated optical sideband field modes.

Interferometric techniques, combined with electronic signal processing, have provided powerful tools for the precise reconstruction of quantum states of the field. Nevertheless, in most cases the completeness of the measurement relies in strong assumptions about its symmetry. In the present work, I will show how the use of optical cavities as a tool for state reconstruction can provide a complete description of the state, relaxing a priori assumptions and revealing a broad distribution of entanglement among sidebands of different optical beams, as in the case of those generated by an optical parametric oscillator.

We describe a new method, combining techniques from the fields of ultrafast and quantum optics, for gaining full access to the spectral and temporal information encoded in the wavepacket mode of single, ultrashort, photons.

The possibility to generate multipartite entanglement in the laboratory allows further investigations of the structure of quantum correlations among many parties. We employ the optical parametric oscillator to directly produce three entangled bright beams of light and study their properties. In this tripartite continuous system, we observe a mixture of behaviours common to discrete systems pertaining to both the GHZ and the W classes of tripartite entangled states.

We investigate the impact of decoherence and static disorder on the dynamics of quantum particles moving in a periodic lattice. Our experiment relies on the photonic implementation of a one-dimensional quantum walk. The pure quantum evolution is characterized by a ballistic spread of a photon's wave packet along 28 steps. By applying controlled time-dependent operations we simulate three different environmental influences on the system, resulting in a fast ballistic spread, a diffusive classical walk, and the first Anderson localization in a discrete quantum walk architecture.

We describe the direct generation of tripartite three-color entanglement from a single optical parametric oscillator. The robustness of bipartite and tripartite entanglement against losses is also experimentally investigated.

We report on the existence of a novel kind of squeezing in photonic crystal fibers which is conceptually intermediate between the four-wave mixing induced squeezing, in which all the participant waves are monochromatic waves, and the self-phase modulation induced squeezing for a single pulse in a coherent state. This hybrid squeezing occurs when an arbitrary short soliton emits quasi-monochromatic resonant radiation near a zero group velocity dispersion point of the fiber. Photons around the resonant frequency become strongly correlated due to the presence of the classical soliton, and a reduction of the quantum noise below the shot noise level is predicted., Comment: 5 pages, 2 figures

We experimentally investigate the mode characteristics of multimode radiation fields propagating through frequency dependent Gaussian channels. After manipulating the twin beams emitted from a conventional parametric down-conversion source via spectral filtering, we study the changes in their mode characteristics, utilizing the joint normalized correlation functions. While filtering reduces the number of spectral modes, it also leads to an apparent mode mismatch, which destroys the perfect photon-number correlation between the twin beams, and influences the mode properties of heralded states., Comment: 3 pages, 2 figures

Entanglement in bipartite continuous-variable systems is investigated in the presence of partial losses, such as those introduced by a realistic quantum communication channel, e.g. by propagation in an optical fiber. We find that entanglement can vanish completely for partial losses, in a situa- tion reminiscent of so-called entanglement sudden death. Even states with extreme squeezing may become separable after propagation in lossy channels. Having in mind the potential applications of such entangled light beams to optical communications, we investigate the conditions under which entanglement can survive for all partial losses. Different loss scenarios are examined and we derive criteria to test the robustness of entangled states. These criteria are necessary and sufficient for Gaussian states. Our study provides a framework to investigate the robustness of continuous-variable entanglement in more complex multipartite systems., Comment: Phys. Rev. A (in press)

Subtle quantum properties offer exciting new prospects in optical communications. Quantum entanglement enables the secure exchange of cryptographic keys and the distribution of quantum information by teleportation. Entangled bright beams of light attract increasing interest for such tasks, since they enable the employment of well-established classical communications techniques. However, quantum resources are fragile and undergo decoherence by interaction with the environment. The unavoidable losses in the communication channel can lead to a complete destruction of useful quantum properties -- the so-called "entanglement sudden death". We investigate the precise conditions under which this phenomenon takes place for the simplest case of two light beams and demonstrate how to produce states which are robust against losses. Our study sheds new light on the intriguing properties of quantum entanglement and how they may be tamed for future applications., Comment: To be published - Nature Photonics

Direct generation of tripartite entanglement between pump, signal, and idler beams of an above-threshold optical parametric oscillator was implemented. All three bright beams of light have different wavelengths. Partial-loss disentanglement was also observed.

Broadband multimode squeezers constitute a powerful quantum resource with promising potential for different applications in quantum information technologies such as information coding in quantum communication networks or quantum simulations in higher dimensional systems. However, the characterization of a large array of squeezers that coexist in a single spatial mode is challenging. In this paper we address this problem and propose a straightforward method to determine the number of squeezers and their respective squeezing strengths by using broadband multimode correlation function measurements. These measurements employ the large detection windows of state of the art avalanche photodiodes to simultaneously probe the full Hilbert space of the generated state, which enables us to benchmark the squeezed states. Moreover, due to the structure of correlation functions, our measurements are not affected by losses. This is a significant advantage, since detectors with low efficiencies are sufficient. Our approach is less costly than full state tomography methods relying on multimode homodyne detection which build on much more demanding measurement and analysis tools and appear to be impractical for large Hilbert spaces., Comment: 21 pages, 9 figures

Parametric down-conversion (PDC) offers the possibility to control the fabrication of non-Gaussian states such as Fock states. However, in conventional PDC sources energy and momentum conservation introduce strict frequency and photon number correlations, which impact the fidelity of the prepared state. In our work we optimize the preparation of single-photon Fock states from the emission of waveguided PDC via spectral filtering. We study the effect of correlations via photon number resolving detection and quantum interference. Our measurements show how the reduction of mixedness due to filtering can be evaluated. Interfering the prepared photon with a coherent state we establish an experimentally measured fidelity of the produced target state of 78%., 15 pages, 10 Figures, published version

We experimentally investigate quadrature correlations between pump, signal, and idler fields in an above-threshold optical parametric oscillator. We observe new quantum correlations among the pump and signal or idler beams, as well as among the pump and a combined quadrature of signal and idler beams. A further investigation of unforeseen classical noise observed in this system is presented, which hinders the observation of the recently predicted tripartite entanglement. In spite of this noise, current results approach the limit required to demonstrate three-color entanglement., Comment: 10 pages, 5 figures, submitted to Opt. Express

Recent experiments with Optical Parametric Oscillators have shown [1–3] that an unpredicted extra noise on the phase of the fields reduces the level of squeezing or quantum correlations. In some cases, it can hinder the attempts to observe entanglement in those systems [4]. It is probably the main reason why signal and idler entanglement above threshold remained unobserved by almost 20 years after its prediction [5].

In systems described by continuous variables (CV), multipartite entanglement can be produced by combination of squeezed states of electromagnetic fields in beam splitters [1]. Entanglement in this case is limited to the same frequency for all parties involved, owing to its interferometric generation. So far, experimental entanglement between more than two systems of continuous variables has been restricted to this method. In order to generate multi-wavelength entanglement, nonlinear processes are required to couple different modes of the field, e.g. the bipartite entanglement demonstration in ref. [2]. We presently demonstrate the nonlinear system of the smallest order for direct tripartite entanglement production. As described in [3], an Optical Parametric Oscillator operating above threshold produces an inseparable state of pump, signal and idler modes. Previous attempts [4] had been frustrated by the presence of an extra noise source, which we have shown to be generated by photon scattering of light due to thermal phonons of the crystal. We have achieved the reduction of this noise by cooling the KTP crystal of our OPO (described in ref. [4]) down to −10° C. Reflected pump field, signal and idler fields are measured after reflection by empty cavities (enabling the measurement of amplitude or phase quadrature) using efficient photodiodes, allowing the complete measurement of the covariance matrix. The photocurrent component at 21 MHz, used in the analysis, was registered with the help of an analog-to-digital card and a computer.

We study the ability to measure the non-Gaussian character of the single photon Fock states via direct measurements of the photon number statistics of displaced quantum states.

We present the first robust implementation of a coined quantum walk over five steps using only passive optical elements. By employing a fiber network loop we keep the amount of required resources constant as the walker's position Hilbert space is increased. We observed a non-Gaussian distribution of the walker's final position, thus characterizing a faster spread of the photon wave-packet in comparison to the classical random walk. The walk is realized for many different coin settings and initial states, which opens the way for the implementation of a quantum walk-based search algorithm., Comment: 4 pages, 4 figures

In the nonlinear interaction of intense beams with a crystal inside an optical cavity, phonon scattering of the central carrier into sidebands acts as a source of extra phase noise for experiments in quantum optics.

We show theoretically and experimentally that scattered light by thermal phonons inside a second-order nonlinear crystal is the source of additional phase noise observed in optical parametric oscillators. This additional phase noise reduces the quantum correlations and has hitherto hindered the direct production of multipartite entanglement in a single nonlinear optical system. We cooled the nonlinear crystal and observed a reduction in the extra noise. Our treatment of this noise can be successfully applied to different systems in the literature.

Investigamos as propriedades quânticas conjuntas dos três feixes intensos de luz produzidos por um oscilador paramétrico ótico (OPO) operando acima do limiar, denominados bombeio, sinal e complementar. Analisamos experimentalmente as flutuações quânticas de intensidade e fase destes feixes, e como se correlacionam. Observamos pela primeira vez a presença de correlações quânticas entre as fases dos três campos. O ruído observado na soma das fases de sinal e complementar, Vq12=1,28(3)>1, passa a apresentar compressão de ruído após ser corrigido pelas flutuações de fase do feixe de bombeio, resultando em (Vq12)\' =0,84(3)2 e V2=2,09(7)>2. Com relação ao segundo critério de separabilidade, positividade sob transposição parcial, obtivemos os seguintes autovalores simpléticos a partir da matriz de covariância medida: A0=0,94(8) e A1=0,36(6)1, presents squeezing when corrected by the phase fluctuations of the pump beam, resulting in (Vq12)\'=0,84(3)2, and V2=2,09(7)>2. For the second criterion, the positivity under partial transpose, we have obtained the following symplectic eigenvalues calculated from the measured covariance matrix: A0=0,94(8) and A1=0,36(6)

The optimal discrimination of non-orthogonal quantum states with minimum error probability is a fundamental task in quantum measurement theory as well as an important primitive in optical communication. In this work, we propose and experimentally realize a new and simple quantum measurement strategy capable of discriminating two coherent states with smaller error probabilities than can be obtained using the standard measurement devices; the Kennedy receiver and the homodyne receiver., 5 pages, 3 figures

We show that scalable multipartite entanglement among light fields may be generated by optical parametric oscillators (OPO). The tripartite entanglement existent among the three bright beams produced by a single OPO -- pump, signal, and idler -- is scalable to a system of many OPOs by pumping them in cascade with the same optical field. This latter serves as an entanglement distributor. The special case of two OPOs is studied, as it is shown that the resulting five bright beams share genuine multipartite entanglement. In addition, the structure of entanglement distribution among the fields can be manipulated to some degree by tuning the incident pump power. The scalability to many fields is straightforward, allowing an alternative implementation of a multipartite quantum information network with continuous variables.

We review the measurement of entanglement in the above-threshold optical parametric oscillator, an experimental challenge that lasted 17 years. The challenge was in part technical, owing to the difficulty of measuring phase noise, but also related to unexpected noise features that remain unexplained. We describe the experimental strategies used to measure phase noise and to circumvent the "extra" noise. The theoretical prediction of a higher order of entanglement is presented, as well as measurements of bright three-color quantum correlations.

This paper presents an experimental demonstration of three-color quantum correlations between the bright pump, signal, and idler beams produced by an optical parametric oscillator (OPO). This paper shows that the degree of entanglement between the twin beams can be increased by using information from the pump beam.

Quantum correlations between bright pump, signal, and idler beams produced by an optical parametric oscillator, all with different frequencies, are experimentally demonstrated. We show that the degree of entanglement between signal and idler fields is improved by using information of pump fluctuations. This is the first observation of three-color optical quantum correlations., Comment: 3 pages, 3 figures

Diode and Ti:Sapphire lasers were used to analyze the laser noise influence on an EIT resonance. In the first case phase-to-amplitude noise conversion dominates while atomic dipole fluctuations emerge in the latter.

We characterize a polarization-entangled photon source based on femtosecond pulsed parametric downconversion in a periodically poled chi((2)) medium. Employing a Sagnac interferometer, we prepared different quantum states aiming at future investigations in quantum information and on the foundations of quantum mechanics. In this work we demonstrate the controlled generation of unbalanced entangled states, rather than the usual maximally entangled ones, by tuning the polarization of the system's pumping field. (C) 2015 Optical Society of America

In this work, first measurements that indicate entanglement between non-degenerate signal and idler beams above threshold are presented. These curves are obtained synchronously varying analysis cavities' lengths around their resonances.

We describe several recent experimental studies in the above-threshold optical parametric oscillator (OPO). Seventeen years after the original prediction, entanglement of the twin beams was experimentally demonstrated in 2005. It was followed by the prediction that the OPO should directly produce tripartite pump-signal-idler entangled beams. We also present experimental quantum correlations among these three fields.

We investigate entanglement in the above-threshold Optical Parametric Oscillator, both theoretically and experimentally, and discuss its potential applications to quantum information. The fluctuations measured in the subtraction of signal and idler amplitude quadratures are $\Delta^2 \hat p_-=0.50(1)$, or $-3.01(9)$ dB, and in the sum of phase quadratures are $\Delta^2 \hatq_+=0.73(1)$, or $-1.37(6)$ dB. A detailed experimental study of the noise behavior as a function of pump power is presented, and discrepancies with theory are discussed., Comment: 9 pages, 6 figs. Important reference for readers of quant-ph/0610197. J. Opt. Soc. Am. B, Feature Issue on Optical Quantum-Information Science, doc. ID 70938 (posted 5 September 2006, in press)

We produced and experimentally demonstrated quantum correlations between bright pump, signal, and idler beams in an optical parametric oscillator, all with different frequencies, Our group was the first to observe three-color optical quantum correlations.

We generated bright entangled beams in an optical parametric oscillator, operating above threshold. Tripartite pump-signal-idler entanglement, all with different frequencies, was also predicted and three-color quantum correlations already measured.

Quantum communication (QC) protocols rely on the fast, efficient and robust distribution of quantum states over large distances. One main limiting factor in all quantum protocols is the capacity of the applied channel. Here we present a method to boost the available bit rates by encoding the information on multiple orthogonal broadband spectral modes.

We developed a technique, based on evolutionary algorithms, for adaptively imprinting the mode structure of a quantum light state in a classical field. We probe the shape of the quantum state, including coherences among modes.