Correlated single photon pairs are the product of a non-linear process called Spontaneous Parametric Down Conversion (SPDC). Traditional SPDC produces correlated pairs broadened to several nanometers. A volume Bragg grating can be used to extract a narrow bandwidth from an otherwise broad spectrum beam. We apply this emerging technology to our non-degenerate and otherwise broadband SPDC single photon source. We compare the source properties for the single-pass and multi-pass cases by embedding the volume Bragg grating in a cavity. The SPDC wavelengths are selected to be suitable for both long distance transmission in a fiber and for quantum memory.

We propose entanglement microscope based on the principle of quantum metrology. Entanglement microscope is differential interference contrast microscope using NOON state. We report the experimental demonstration of entanglement microscope by using 2 photon entangled state, and confirm the improvement of signal to ratio compared to the classical microscope. It is expected that this research can be applied to non-invasive measurement for biological samples.

Quantum Processing through a Manifold of Dark States Santosh Kumar and Deepak Kumar School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110067, India We consider a network of N single-mode electromagnetic cavities connected by optical fibres. Each cavity contains a single three-level atom which can be addressed by a control laser. We find that under certain conditions, the system possesses a large number of degenerate dark states. By manipulating intensities and phases of control lasers on the cavities, one can pass adiabatically among these dark states. The network operates as a N -level quantum system in which one can generate computationally useful states by protocols of external controls. We obtain numerical results to demonstrate operations like the transport of states and generation of Fourier-like states.

We present an experimental implementation of spectral properties engineering on biphoton light, emitted via ultrafast type II spontaneous parametric down conversion (SPDC), based on the shaping of the pump pulse spectrum and propagation of the emitted correlated photons through dispersive media [1, 2]. Spectral properties of a biphoton state are fully characterized by the two-photon spectral amplitude (TPSA) [3-7]. Exploiting the group velocity dispersion (GVD) induced by the passage of optical fields through dispersive media, an energy to time two dimensional Fourier transform of the TPSA is operated [8-10]: this returns a technique to reconstruct TPSA through a temporal measurement among the delay between the laser pulse emission (trigger) and the detection times of the two correlated photons. Exploiting this kind of measurement it is possible to deeply resolve the interference pattern in the shape of TPSA. In this research we report on the conditions under which subtle structure on TPSA spectra can be deliberately engineered via modulation of the pump beam spectrum. 1) W. P. Grice and I. A. Walmsley, "Spectral information and distinguishability in type-II down- conversion with a broadband pump", Phys. Rev. A 56, 1627 (1997). 2) T. E. Keller and M. H. Rubin, "Theory of two-photon entanglement for spontaneous parametric down-conversion driven by a narrow pump pulse", Phys. Rev. A 56, 1534-1541 (1997). 3) M. V. Fedorov, M. A. Efremov, A. E. Kazakov, K. W. Chan, C. K. Law, and J. H. Eberly, "Packet narrowing and quantum entanglement in photoionization and photodissociation", Phys. Rev. A 69, 052117 (2004). 4) Yu. M. Mikhailova, P. A. Volkov, and M. V. Fedorov, "Biphoton wave packets in parametric down-conversion: Spectral and temporal structure and degree of entanglement", Phys. Rev. A 78, 062327 (2008). 5) P. A. Volkov, Yu. M. Mikhailova, and M. V. Fedorov, "Spectral Entanglement in Parametric Down-Conversion with Nondegenerate Frequencies", Advanced Science Letters 2, 511 (2009). 6) G. Brida, V. Caricato, M. V. Fedorov, M. Genovese, M. Gramegna, and S. P. Kulik, "Characterization of spectral entanglement of spontaneous parametric-down conversion biphotons in femtosecond pulsed regime", EPL 87, 64003 (2009). 7) P. J. Mosley, J. S. Lundeen, B. J. Smith, P. Wasylczyk, A. B. U'Ren, C. Silberhorn, and I. A. Walmsley, "Heralded Generation of Ultrafast Single Photons in Pure Quantum States", Phys. Rev. Lett. 100, 133601 (2008). 8) S. Y. Baek, O. Kwon, and Y.-H. Kim, "Nonlocal dispersion control of a single-photon waveform", Phys. Rev. A 78, 013816 (2008). 9) M. Avenhaus, A. Eckstein, P. J. Mosley, and C. Silberhorn, "Fiber-assisted single-photon spectrograph", Opt. Lett., 34, 2873 (2009). 10) G. Brida, V. Caricato, M. V. Chekhova, M. Genovese, M. Gramegna, and T. Sh. Iskhakov, "Testing ultrafast twophoton spectral amplitudes via optical fibres", Optics Express Vol. 18, Iss. 12, pp. 12915-12922 (2010).

By using a novel detection scheme of "phase-selection", thermal light ghost images of both single slit and double slit were observed with ∼100% visibility, which means an EPR type nonlocal position-position determination. The observed 100% contrast image-forming correlation is the result of two-photon interference. The nonlocal interference nature of thermal light then allowed us to revisit the historical challenge of Popper's thought experiment, which concerns the uncertainty principle of quantum mechanics. Realizing Popper's experiment, apparently, the measurement showed (equation). However, it does not mean a violation of the uncertainty principle. Although the experiment was done with thermal light, once again, we have shown, (equation), i.e., the measurement of two photons is different from that of one. This talk we first show that a high visibility ghost image could be achieved by using thermal light when we apply a novel detection scheme with manipulating phase-selection. The two different yet indistinguishable alternative ways of triggering a join-detection event by two independent photons, i.e. the correlation GAB⁽²⁾ can be extracted from the G⁽²⁾ measurement. Where for a thermal light ghost image, the transverse normalized second-order correlation function is given by (equation) and (equation) Here we created new virtual logic circuits (Pos-Neg identifiers), which are realized by programming on the TCSPC circuit, to process the photodetection events. (equation) where A and B represents two photon detectors in the object beam and reference beam. These four coincident counts are of one "shot". The accumulations of "shot by shot" finally yield the four correlations, RPP=∑j RjPP , RPN=∑j RjPN, RNP=∑j RjNP, and RNN=∑j RjNN. The mutual-correlation is then retrieved from Rcorr- Ranti= RPP + RNN - RPN - RNP. Furthermore, by using this new scheme we can investigate the Popper's experiment with thermal light, comparing the result with using entangled photons, we draw a conclusion that even though the ghost imaging is done by thermal light, the two-photon interference rules the joint detection measurement as well as in the entangled photon case, and it is essentially different than the single photon interference.

We have recently demonstrated how the "nature" of optical interference can be exploited to solve complex problems, as factoring large integers [1]. This works inspired us to take advantage of multi-photon interference based on n correlated measurements in order to perform at the same time an exponential number of computations in an exponentially large "multi-mode" space [2]. By exploiting correlated measurements we are able to simulate entanglement-type correlations for quantum computing purposes, as demonstrated in the case of second order interference with a thermal source [3]. We analyze the "computational power" of n-photon quantum interference based on generalized SH-HOM and Franson interferometers. Our work paves the way to multi-photon quantum computing networks where the output can be measured by correlated measurements in the polarization base. [1] V. Tamma et al. Phys. Rev. A Rap. Com. 83, 020304 (2011) [2] V. Tamma and Y. Shih, Computational power of multi-photon interference and correlated measurements, to be submitted [3] H. Chen et al., New Journal of Physics 13 (2011) 083018

It is well known that the classical schemes for microscopy and lithography are restricted by the diffraction limit. For example, the precision with which a pattern could be etched in interference lithography is limited by the wavelength of the light. In this talk we will discuss schemes to overcome this limit in optical lithography and microscopy schemes.

Experimental realization of superresolution imaging using quantum optics has been elusive despite significant interest to this subject. We experimentally demonstrate quantum imaging beyond diffraction limit in fluorescence microscopy using a technique relying on photon antibunching in non-resonant fluorescence. Antibunching microscopy enables superresolution imaging of regular fluorophores at room temperature, which makes quantum imaging relevant to life science microscopy.

We wish to report here the first successful experimental demonstration of ghost imaging using the sun, a natural thermal source, as a light source. Most importantly, the sunlight does not have speckles and the sun is also a near field source. When the speckle-to-speckle correlation theory fails to interpret this experimental result, the quantum theory of near-field thermal light ghost imaging explains this experimental result very convincingly and also clarifies the physics of near-field thermal light ghost imaging from the fundamental level. From the application point of view, sunlight-based ghost imaging is very important and useful in distant imaging applications.

It is claimed in the many papers that a trace distance: $d$ guarantees the universal composition security in quantum key distribution (QKD) like BB84 protocol. In this introduction paper, at first, it is explicitly explained what is the main misconception in the claim of the unconditional security for QKD theory. In general terms, the cause of the misunderstanding on the security claim is the Lemma in the paper of Renner. It suggests that the generation of the perfect random key is assured by the probability $ (1-d)$, and its failure probability is $d$. Thus, it concludes that the generated key provides the perfect random key sequence when the protocol is success. So the QKD provides perfect secrecy to the one time pad. This is the reason for the composition claim. However, the quantity of the trace distance (or variational distance) is not the probability for such an event. If $d $ is not small enough, always the generated key sequence is not uniform. Now one needs the reconstruction of the evaluation of the trace distance if one wants to use it. One should first go back to the indistinguishability theory in the computational complexity based, and to clarify the meaning of the value of the variational distance. In addition, the same analysis for the information theoretic case is necessary. The recent serial papers by H.P.Yuen have given the answer on such questions. In this paper, we show more concise description of Yuen's theory, and clarify that the upper bound theories for the trace distance by Tomamichel et al and Hayashi et al are constructed by the wrong reasoning of Renner and it is unsuitable as the security analysis. Finally, we introduce a new macroscopic quantum communication to replace Q-bit QKD.

Optical communication at many bits/photon is possible with bandwidth expansion, and at many bits/sec-Hz with bandwidth-efficient signaling. Previous work has shown that 10 bits/photon capacity at 5 bits/sec-Hz over a vacuum-propagation channel requires 189 low-loss spatial modes at Holevo capacity, and 4500 modes at the capacity limit for on-off keying modulation with a direct-detection receiver. These results, however, neglect the presence of atmospheric turbulence. In this paper we present lower bounds on the ergodic capacity of the turbulent channel for transmitters that use M-element spatial mode sets composed of either focused beams, Hermite-Gaussian modes, or Laguerre-Gaussian modes.

Inspired by recent demonstrations for OAM-based single-photon communication, we construct the density operator, based on OAM eigenkets, and propose the quantum channel model suitable for study of the quantum communication over atmospheric turbulence channels and corresponding quantum error correction. The quantum channel model is derived from OAM eigenkets' transition probabilities. By using the Holevo-Schumacher-Westmoreland (HSW) theorem, we adopt the proposed model to determine the OAM quantum channel capacity in the presence of atmospheric turbulence.

We present a scheme to commute the information of a quantum state (qubit C) from place A to another B (Alice and Bob), which is in a mixed or pure state. The communication is achieved via a shared two-qubit quantum channel between A and B. The quantum correlation of the quantum channel is weaker than the entanglement. When Alice makes a Bell measurement on the unknown qubit (C) together with Alice's qubit (A), the Bob's qubit will collapse to a state (B′). When the correlation matrix of the two-qubit state of the quantum channel is of full-rank (rank 3), complete information of the quantum state of qubit C is stored in the Bob's state (B′). Here the non-zero quantum correlation is necessary for the channel, but the entanglement for the quantum channel is not necessary required.

Optical coherent states can be interpreted as d-dimensional quantum systems, or qudits of even superposition of pseudo-number states. Cross-Kerr nonlinear interaction can generate the maximal entanglements of pseudo-phase and pseudo-number states from two opticl coherent states. Extended network of these entangled coherent states is a qudit cluster state and can be used as qudit communication network for d-dimensional teleportation or multi-user quantum cryptographic network.

The quantum illusionist: a game with correlations between Gaussian states A.Meda,M. Genovese,G.Brida, I.P.Degiovanni, S.Olivares, M.G.A.Paris Optical correlations at the quantum level represent a resource for the development of technologies overcoming the limits of the classical physics, with very promising opportunities for future widespread applications [1, 2]. These results, besides paving the way to new technologies, also permit deeper insights in quantum world. Here we will show, both theoretically and experimentally an innovative scheme that exploit correlations between Gaussian beams of light: the optical illusionist game. In the game, an “illusionist” ask to the public to measure correlations between two uncorrelated light beams, excited in the same Gaussian state [3], mixed in a beam splitter (BS). Both in the presence and in the absence of the BS, no correlations arise between them. However, the illusionist can identify the presence of the BS, by means of a correlation measurement, when the public is asked to insert the BS behind the illusionist’s back. Here we unveil the trick and the physics that explain these counterintuitive correlations of quantized light. References [1] Bouwmeester, D., et al. The Physics of Quantum Information: Quantum Cryptography,Quantum Teleportation,Quantum Computation (2000). [2] Genovese,M. Research on hidden variable theories: A review of recent progresses. Phys.Rep.413,319-396 (2005). [3] Ferraro, A.et al. Gaussian States in Quantum Information (2005)

Quantum key distribution (QKD) provides new measures of information security. However, as the range is limited, satellite based systems are desirable for global scale applications. As a preparatory experiment we demonstrate a BB84 key exchange from an airplane moving at 80m/s over a distance of 20km to an optical ground station featuring a 40cm telescope. For that we integrated a QKD transmitter and receiver with an existing system for free space optical laser communication. Observing sifted key rates of 145Bits/s with a quantum bit error rate of 4.8%, this experiment becomes a precursor for satellite based and thus global scale QKD.

Satellites offer the means for extending quantum communication and quantum key distribution towards global distances. We will outline the proposed QEYSSAT mission proposal, which involves a quantum receiver onboard a satellite with quantum signals sent from the ground towards the satellite. We will present our recent studies on the techniques that will help enable to perform quantum links from ground to space. This includes experimental results on demonstrating ultra-high loss quantum transmission, as well as studying the effect of the fluctuating optical link on quantum signals and how these fluctuations can actually be exploited to improve the link performance.

Two-photon interference is a fundamental phenomenon in quantum mechanics and stands at the base of numerous experimental observations. Here another manifestation of this phenomenon is described. A way of measuring this new manifestation is discussed.

We present a versatile dispersion-based technique to control the spatial characteristics of photon pairs produced via spontaneous parametric down-conversion in multimode nonlinear waveguides. This technique has been employed in a source of spatially pure near-infrared photon pairs based on the type-II process realized in a multimode periodically poled potassium titanyl phosphate (PPKTP) waveguide. Diffractive measurements of the beam quality factors in the heralded regime have confirmed that the photons are indeed generated in well-defined transverse modes. The generated photon pairs have been used to prepare polarization entanglement in the Shih-Alley configuration, shown to violate Bell's inequalities without spatial filtering.

A new-type of Franson interferometer with thermal light has been created to prepare $N$-photon qubits. We has also applied a novel detection scheme with manipulating phase-selection, with which we can observe Frason-type correlations from the two-photon qubits with visibilities more than 71\%. The success of two-photon qubits offers a perspective for producing a large number of N-photon qubits for practical quantum computing.

Nitrogen-vacancy (NV) color centers in diamond have demonstrated all the key properties needed to build room temperature quantum computers, including single shot readout and long range entanglement. However these properties also enable NV diamond to be used to probe spins in biological molecules with more sensitivity and spatial resolution than is possible with other sensors. In this talk, the uses of NV diamond for sub-diffraction imaging, and for the detection of spins outside of the diamond lattice will be reviewed.

In this work, we report about the experimental observation of an ultra-narrow temporal correlation (6 fs) of twin beams produced by a type I BBO and detected by the inverse process of sum-frequency generation. The ultra narrow localization occurs thanks to the huge spectral bandwidth involved (≥600nm) and the preservation of the phase-conjugation of the twin beams in the set-up. By adding a spatial shift in the near field between the phase-conjugated twin beams, the temporal correlation measurements give hints of the presence of the "arms" featured by the biphotonic amplitude.

The partial transpose by which a subsystem's quantum state is solely transposed is of unique importance in quantum information processing from both fundamental and practical point of view. In this work, we present a practical scheme to realize a physical approximation to the partial transpose using local measurements on individual quantum systems and classical communication. We then report its linear optical realization and show that the scheme works with no dependence on local basis of given quantum states. A proof-of-principle demonstration of entanglement detection using the physical approximation of the partial transpose is also reported [Lim et al., Phys. Rev. Lett. 107, 160401 (2011)].

We will report the first experimental demonstration of adaptive quantum estimation (AQE): optimally estimating a quantum state or a value of the parameter that specifies the quantum state. The angle of a half wave plate that initializes the linear polarization of input photons is estimated using AQE. The statistical analysis of these results verifies the optimality of AQE. It is expected that AQE will provide a useful methodology in the broad area of quantum information processing, communication, and metrology. We will also introduce our recent progress on optical quantum circuit, and solid single photon source using tapered nano-optical fiber.

We discuss the ultimate quantum limits in compressed sensing, a new technique which allows for accurate reconstruction of so-called "sparse" signals even when the sampling rate is far below the Nyquist limit.

Fluorescent emitters in nanodiamonds are considered to be a valuable resource for emerging fields such as quantum communication, quantum photonics, nanoscopy and biological imaging. In this paper we report a wide range of narrow bandwidth ( about 1.5 nm) spectral emission lines of different single defects (nickel, nickel-silicon and silicon) at room temperature, from 700 to 900 nm in the same sample. The centres were created by chemical vapour deposition of nanocrystalline diamond. The great variety of emitters produced in one sample could facilitate the realization of test-bed for quantum optics experiments, as well as could allow building efficient devices for optical interconnection, quantum information processing and sensitive metrology.

NASA is currently developing the capability to operate optical communications links for deep space missions. This has spurred interest in adding optical ranging capabilities to a deep-space terminal. In this paper, we investigate the precision achievable with two optical ranging systems in the low-flux signal-return regime typical of a deep space link. The first uses an intensity-modulated single-photon counting system, and is the scheme currently proposed for use in future deep-space missions. The second uses a coherent frequency-modulated continuous wave system. We compare the systems and draw comparisons to coherent microwave systems currently used by NASA.

We demonstrate the efficient heralded generation of high purity narrow-bandwidth single photons using four-wave mixing in a hot atomic vapour. Employing optical homodyne tomography, we fully reconstruct the density matrix of the generated photon and observe a Wigner function reaching a value of zero without correcting for inefficiencies. The narrow bandwidth of the photon produced is accompanied by a high generation rate yielding a high spectral brightness. The source is therefore compatible with atomic-based quantum memories as well as other applications in light-atom interfacing. This work paves the way to quantum state engineering of collective atomic excitations.

Upconversion of 1.3-micron photons and detection using silicon avalanche photodiodes (Si APDs) can produce high photon detection efficiencies (PDEs) with low dark count rates. We demonstrate a novel two-channel device based on a phase-modulated, periodically poled LiNbO3 waveguide that mixes 1302-nm signal photons with two pump beams at 1556 and 1571 nm. Both channels showed high PDEs with very low dark counts. Using wavelength- to time-division multiplexing in this dual-channel device, we produced clock rates that exceed the timing-jitter-limited rates of a system based on one Si APD. Higher clock rates are of interest for improved quantum communication systems.

Interferometry with quantum gases in microgravity can be exploited to probe fundamental concepts of quantum mechanics and general relativity. Interestingly, matter-wave interferometry offers itself as a possible test of the universality of free fall. We describe the fundamental aspects of quantum interference with Bose-Einstein Condensates evolving over time in an asymmetric Mach-Zehnder-type interferometer. Such aspects have been demonstrated in a recent experiment performed within the QUANTUS collaboration [1]. [1] H. Muetinga and the QUANTUS Team (http://www.zarm.uni-bremen.de/space-science/fundamental-physics/projects/quantus.html), to be submitted