This work is supported by the ERA-NET CHIST-ERA (&#8216;QScale&#8217; project) and by the ERC starting grant &#8216;HybridNet&#8217;. F. Barbosa acknowledges the support from CNR and FAPESP, and K. Huang the support from the Foundation for the Author of National Excellent Doctoral Dissertation of China (PY2012004) and the China Scholarship Council. C. Fabre and J. Laurat are members of the Institut Universitaire de France.

1. Optical Parametric Oscillator
<ol>
	<li>Build a 4&#160;cm long semimonolithic linear cavity (for improved mechanical stability and reduced intracavity losses). The input mirror is directly coated on one face of the nonlinear crystal.</li>
	<li>Choose an input coupler reflection of 95% for the pump at 532 nm and high-reflection for the signal and idler at 1,064 nm. Inversely, choose the output coupler to be highly reflective for the pump and of transmittance T=10% for the infrared. The free spectral range of the OPO ...

<ol>
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The authors declare that they have no competing financial interests.

The conditional preparation technique presented here is always an interplay between the initial bipartite resource and the measurement performed by the heralding detector. These two components strongly influence the quantum properties of the generated state.
First, the purity of the prepared states strongly depends on the one of the initial resource, thus a &#8216;good&#8217; OPO is required. What is a &#8216;good&#8217; OPO? It is a device for which the escape efficiency &#951; is close to un...

The ability to engineer the quantum state of traveling optical fields is a central&#160;requirement for quantum information science and technology1, including quantum communication, computing and metrology. Here, we discuss the preparation and characterization of some specific quantum states using as a primary resource the light emitted by continuous-wave optical parametric oscillators3,4 operated below threshold. Specifically, two systems will be considered &#8211; a type-II phase-matched OPO and a type-I&#160;OPO &#8211; enabling respectively the reliable generation of heralded single-photons and of optical coherent state superpositions (CSS), i.e. states of the form |&#945;&#62; - |-&#945;&#62;. These states are important resources for the implementation of a variety of quantum information protocols, ranging from linear optical quantum computation6 to optical hybrid protocols5,7. Significantly, the method presented here permits obtaining a low admixture of vacuum and the emission into a well-controlled spatiotemporal mode.
Generally speaking, quantum states can be classified as Gaussian states and non-Gaussian states according to the shape of the quasi-probability distribution in phase space called the Wigner function W(x, p)8. For non-Gaussian states, the Wigner function can take negative values, a strong signature of non-classicality. Single-photon or coherent state superpositions are indeed non-Gaussian states.
An efficient procedure for generating such states is known as the conditional preparation technique, where an initial Gaussian resource is combined with a so-called non-Gaussian measurement such as photon counting9,10,11,12,13. This general scheme, probabilistic but heralded, is sketched on Figure 1a.
<img alt="Figure 1" fo:content-width="5in" fo:src="/files/ftp_upload/51224/51224fig1highres.jpg" src="/files/ftp_upload/51224/51224fig1.jpg" /> 
Figure 1.&#160;(a) Conceptual scheme of the conditional preparation technique. (b) Conditional preparation of single-photon state from orthogonally-polarized photon pairs (type-II OPO) separated on a polarizing beam splitter. (c) Conditional preparation of a coherent state superposition by subtracting a single-photon from a squeezed vacuum state (type-I OPO).
By measuring one mode of a bipartite entangled state, the other mode is projected into a state that will depend on this measurement and on the initial entangled resource12,13.
What are the required resource and heralding detector needed to generate the aforementioned states? Single-photon states can be generated using twin beams, i.e. photon-number correlated beams. The detection of a single-photon on one mode then heralds the generation of a single-photon on the other mode9,10,14,15. A frequency-degenerate type-II OPO16,17,18,19 is indeed a well-suited source for this purpose. Signal and idler photons are photon-number correlated and emitted with orthogonal polarizations. Detecting a single-photon on one polarization mode projects the other one into a single-photon state, as shown in Figure 1b.
Concerning coherent state superpositions, they can be generated by subtracting a single-photon from a squeezed vacuum state20 obtained either by pulsed single-pass parametric down-conversion11,21 or by a type-I OPO22,23. The subtraction is performed by tapping a small fraction of the light on a beam-splitter and detecting a single-photon in this mode (Figure 1c). A squeezed vacuum is a superposition of even photon-number states, thus subtracting a single-photon leads to a superposition of odd photon-number states, which has a high fidelity with a linear superposition of two coherent states of equal and small amplitude. For this reason, the name &#8216;Schr&#246;dinger kitten&#8217; has sometimes been given to this state.
The general procedure for generating these states is thus similar, but differs by the primary light source. Filtering of the heralding path and detection techniques are the same whatever the type of OPO used. The present series of protocols detail how to generate these two non-Gaussian states from continuous-wave optical parametric oscillators and how to characterize them with high efficiency.

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	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Pump laser</td>
			<td style="width:71px;">Innolight</td>
			<td style="width:119px;">Diabolo</td>
			<td style="width:217px;">Dual output, IR and 532 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">KTP and PPKTP crystal</td>
			<td style="width:71px;">Raicol</td>
			<td style="width:119px;"></td>
			<td>Available from other vendors</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Interferential filters</td>
			<td style="width:71px;">Barr associates</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">High efficiency photodiodes</td>
			<td style="width:71px;">Fermionics</td>
			<td style="width:119px;"></td>
			<td>Quantum efficiency above 97%</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Oscilloscope&#160;</td>
			<td style="width:71px;">Lecroy</td>
			<td style="width:119px;">Wave runner 610 Zi</td>
			<td>Used for data acquisition</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Spectrum analyser</td>
			<td style="width:71px;">Agilent</td>
			<td style="width:119px;">N9000A</td>
			<td>Available from other vendors</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Faraday rotator</td>
			<td style="width:71px;">Qioptic</td>
			<td style="width:119px;">FR-1060-5SC</td>
			<td>Available from other vendors</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PZT</td>
			<td style="width:71px;">PI</td>
			<td style="width:119px;">P-016.00H</td>
			<td>Available from other vendors</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Superconducting single-photon detectors</td>
			<td style="width:71px;">Scontel</td>
			<td style="width:119px;">SSPD</td>
			<td>low dark counts</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Optical switch</td>
			<td style="width:71px;">Thorlabs</td>
			<td style="width:119px;">OSW12-980E</td>
			<td>Available from other vendors</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;"></td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
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</table>

quantum state engineering of light with continuous-wave optical parametric oscillators

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states are indeed the resources for implementing various protocols, ranging from enhanced metrology to quantum communication and computing. A variety of devices can be used to generate non-classical states, such as single emitters, light-matter interfaces or non-linear systems3. We focus here on the use of a continuous-wave optical parametric oscillator3,4. This system is based on a non-linear &#967;2 crystal inserted inside an optical cavity and it is now well-known as a very efficient source of non-classical light, such as single-mode or two-mode squeezed vacuum depending on the crystal phase matching. 
Squeezed vacuum is a Gaussian state as its quadrature distributions follow a Gaussian statistics. However, it has been shown that number of protocols require non-Gaussian states5. Generating directly such states is a difficult task and would require strong &#967;3 non-linearities. Another procedure, probabilistic but heralded, consists in using a measurement-induced non-linearity via a conditional preparation technique operated on Gaussian states. Here, we detail this generation protocol for two non-Gaussian states, the single-photon state and a superposition of coherent states, using two differently phase-matched parametric oscillators as primary resources. This technique enables achievement of a high fidelity with the targeted state and generation of the state in a well-controlled spatiotemporal mode.

For the type-II OPO and the generation of high-fidelity single photon state: 
The tomographic reconstruction of the heralded state is shown in Figure 2, where the diagonal elements of the reconstructed density matrix and the corresponding Wigner function are displayed. Without any loss corrections, the heralded state exhibits a single-photon component as high as 78%. By taking into account the overall detection losses (15%), the state reaches a fidelity of 91% with a single-photon state. The two-pho...

Watch this Scientific Journal Video about Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators at JoVE.com

Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

We describe the reliable generation of non-Gaussian states of traveling optical fields, including single-photon states and coherent state superpositions, using a conditional preparation method operated on the non-classical light emitted by optical parametric oscillators. Type-I and type-II phase-matched oscillators are considered and common procedures, such as the required frequency filtering or the high-efficiency quantum state characterization by homodyning, are detailed.

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states ...

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14.3K Views.  Université Pierre et Marie Curie, Ecole Normale Supérieure, CNRS. We describe the reliable generation of non-Gaussian states of traveling optical fields, including single-photon states and coherent state superpositions, using a conditional preparation method operated on the non-classical light emitted by optical parametric oscillators. Type-I and type-II phase-matched oscillators are considered and common procedures, such as the required frequency filtering or the high-efficiency quantum state characterization by homodyning, are detaile...