The quantum state of a single photon stands amongst the most fundamental and intriguing manifestations of quantum physics [1]. At the same time single photons and pairs of single photons are important building blocks in the fields of linear optical based quantum computation [2] and quantum repeater infrastructure [3] . These fields possess enormous potential [4] and much scientific and technological progress has been made in developing individual components, like quantum memories and photon sources using various physical implementations [5][6][7][8][9][10][11]. However, further progress suffers from the lack of compatibility between these different components. Ultimately, one aims for a versatile source of single photons and photon pairs in order to overcome this hurdle of incompatibility. Such a photon source should allow for tuning of the spectral properties (wide wavelength range and narrow bandwidth) to address different implementations while retaining high efficiency. In addition, it should be able to bridge different wavelength regimes to make implementations compatible. Here we introduce and experimentally demonstrate such a versatile single photon and photon pair source based on the physics of whispering gallery resonators. A diskshaped, monolithic and intrinsically stable resonator is made of lithium niobate and supports a cavity-assisted triply-resonant spontaneous parametric down-conversion process. Measurements show that photon pairs are efficiently generated in two highly tunable resonator modes. We verify wavelength tuning over 100 nm between both modes with a controllable bandwidth between 7.2 and 13 MHz. Heralding of single photons yields anti-bunching with g (2) (0) < 0.2. This compact source provides unprecedented possibilities to couple to different physical quantum systems and makes it ideal for the implementation of quantum repeaters and optical quantum information processing.It is known that in a nonlinear medium a photon can spontaneously decay into a pair of photons, usually called signal and idler. This process, referred to as spontaneous parametric down-conversion (SPDC), preserves the energy and momentum of the parent photon. The resulting pair of photons posses the ability to bridge different wavelength ranges. At the same time detecting one photon of this pair unambiguously heralds the presence of a single photon. In principle, the process of SPDC has a very high bandwidth. By assisting it with a high quality factor (high-Q) resonator, the desired narrow bandwidth of a few MHz for the individual photons can be ensured [12]. A thorough description of this resonator-assisted SPDC leads to a two-mode EPR entangled state [13] and has successfully been used to generate heralded single photons [14]. Recently, resonator-assisted SPDC has led to a substantial progress towards an efficient narrow-band source [15]. However, the wavelength and bandwidth tunability remained a major challenge.We overcome this problem by using an optical whispering gallery mode resonator (WGMR). These resonators a...

Quantum systems such as, for example, photons, atoms, or Bose-Einstein condensates, prepared in complex states where entanglement between distinct degrees of freedom is present, may display several intriguing features. In this Letter we introduce the concept of such complex quantum states for intense beams of light by exploiting the properties of cylindrically polarized modes. We show that already in a classical picture the spatial and polarization field variables of these modes cannot be factorized. Theoretically it is proven that by quadrature squeezing cylindrically polarized modes one generates entanglement between these two different degrees of freedom. Experimentally we demonstrate amplitude squeezing of an azimuthally polarized mode by exploiting the nonlinear Kerr effect in a specially tailored photonic crystal fiber. These results display that such novel continuous-variable entangled systems can, in principle, be realized.

We experimentally demonstrate a protocol for entanglement distribution by a separable quantum system. In our experiment, two spatially separated modes of an electromagnetic field get entangled by local operations, classical communication, and transmission of a correlated but separable mode between them. This highlights the utility of quantum correlations beyond entanglement for the establishment of a fundamental quantum information resource and verifies that its distribution by a dual classical and separable quantum communication is possible. [6,7]. Furthermore, it extends our abilities to process information. Here, entanglement is used as a resource which needs to be shared between remote parties. However, entanglement is not the only manifestation of quantum correlations. Notably, separable quantum states can also be used as a shared resource for quantum communication. The experiment presented in this Letter highlights the quantumness of correlations in separable mixed states and the role of classical information in quantum communication by demonstrating entanglement distribution using merely a separable ancilla mode.The role of entanglement in quantum information is nowadays vividly demonstrated in a number of experiments. A pair of entangled quantum systems shared by two observers enables us to teleport [8] quantum states between them with a fidelity beyond the boundary set by classical physics. Concatenated teleportations [9] can further span entanglement over large distances [10] which can be subsequently used for secure communication [11]. An a priori shared entanglement also allows us to double the rate at which information can be sent through a quantum channel [12] or one can fuse bipartite entanglement into larger entangled cluster states that are "hardware" for quantum computing [13]. * contributed equally to this workThe common feature of all entangling methods used so far is that entanglement is either produced by some global operation on the systems that are to be entangled or it results from a direct transmission of entanglement (possibly mediated by a third system) between the systems. Even entanglement swapping [9,14], capable of establishing entanglement between the systems that do not have a common past, is not an exception to the rule because also here entanglement is directly transmitted between the participants.However, quantum mechanics admits conceptually different means of establishing entanglement which are free of transmission of entanglement. Remarkably, the creation of entanglement between two observers can be disassembled into local operations and the communication of a separable quantum system between them [15]. The impossibility of entanglement creation by LOCC is not violated because communication of a quantum system is involved. The corresponding protocol exists only in a mixed-state scenario and obviously utilizes fewer quantum resources in comparison with the previous cases because communication of only a discordant [16][17][18] separable quantum system is required.In this Le...