We report a proof-of-principle experimental demonstration of quantum lithography. Utilizing the entangled nature of a twophoton state, the experimental results have bettered the classical diffraction limit by a factor of two. This is a quantum mechanical two-photon phenomenon but not a violation of the uncertainty principle.

We perform a reconstruction of the polarization sector of the density matrix of an intense polarization squeezed beam starting from a complete set of Stokes measurements. By using an appropriate quasidistribution, we map this onto the Poincaré space providing a full quantum mechanical characterization of the measured polarization state.

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...

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Two types of interference were observed using two-photon spontaneous parametric radiation from two nonlinear interaction regions. Two experimental setups analogous to the Young and Mach-Zehnder interferometers were used. An interesting feature of the two-photon Young interference is the opposite conditions for its observation by two different methods: by measuring intensity of light at a single frequency and by measuring correlation of intensities at two conjugated frequencies ͑method of coincidences͒. Two-photon Mach-Zehnder interference resembles the Ramsey method of separated fields, which is used in beam spectroscopy. A simple macroscopic quantum model agrees well with the experimental results and enables their interpretation in terms of ''biphotons'' carrying information about the pump phase. ͓S1050-2947͑97͒03210-1͔