One-way quantum channels play a fundamental role in the security of the communication between two distant parties, in particular within the frame of "quantum key distribution". Nevertheless quite recently it has been introduced the possibility of using two-way quantum channels for the same purpose. Although the first attempts in this direction did not feature any particular advantage with respect to the one-way counterpart some recent results obtained by our group suggest that this new class of protocols provides higher thresholds of security.

We present a review of our recent studies concerning remotely prepared entangled bits (ebits) made of a single photon coherently delocalized between two well-separated temporal modes (or time bins). The preparation scheme represents a remotely tunable source for tailoring arbitrary ebits, whether maximally or non-maximally entangled, which is highly desirable for applications in quantum information technology. The remotely prepared ebit is analyzed by performing both single-mode and two-mode homodyne tomography with the ultra-fast balanced homodyne detection scheme recently developed in our lab. Beside the non-classical behavior typical of single-photon Fock states (negative values around the origin), the reconstructed two-mode Wigner function is found to be characterized by an intriguing phase and by correlations between the two distant time bins sharing the single photon.

Single-photon detectors are of great importance in spectroscopy, laser ranging, astronomy and wherever weak light detection is required, so a precise knowledge of their quantum efficiency is essential, but their calibration is by no means trivial. Traditional methods need a black body radiation source or a highly attenuated laser beam as a reference standard, or an already calibrated reference detector. It is well known that absolute measurement of the quantum efficiency, absolute in the sense that the calibration is independent of any radiometric standard or the properties of another detector, can be performed using entangled twin photons generated in optical spontaneous parametric down-conversion together with a reference detector. We have performed the first proof-of-principle experiment to determine the quantum efficiency by which no second detector or reference standard is required. This absolute self-calibration method is also based on the time correlation of photons emitted in parametric down-conversion, but with the introduction of a suitable time delay between the arrival times of the twin photons. Furthermore, we describe a scheme for absolute self-calibration of the quantum efficiency at different wavelengths by means of a pulsed source of nondegenerate down-converted photons. This method should have even greater potential for applications.

We previously demonstrated a high speed, point to point, quantum key distribution (QKD) system with polarization coding over a fiber link, in which the resulting cryptographic keys were used for one-time pad encryption of real time video signals. In this work, we extend the technology to a three-node active QKD network - one Alice and two Bobs. A QKD network allows multiple users to generate and share secure quantum keys. In comparison with a passive QKD network, nodes in an active network can actively select a destination as a communication partner and therefore, its sifted-key rate can remain at a speed almost as high as that in the point-to-point QKD. We demonstrate our three-node QKD network in the context of a QKD secured real-time video surveillance system. In principle, the technologies for the three-node network are extendable to multi-node networks easily. In this paper, we report our experiments, including the techniques for timing alignment and polarization recovery during switching, and discuss the network architecture and its expandability to multi-node networks.

A source of single photons allows secure quantum key distribution, in addition, to being a critical resource for linear optics quantum computing. We describe our progress on deterministically creating single photons from spontaneous parametric downconversion, an extension of the Pittman, Jacobs and Franson scheme [Phys. Rev A, v66, 042303 (2002)]. Their idea was to conditionally prepare single photons by measuring one member of a spontaneously emitted photon pair and storing the remaining conditionally prepared photon until a predetermined time, when it would be "deterministically" released from storage. Our approach attempts to improve upon this by recycling the pump pulse in order to decrease the possibility of multiple-pair generation, while maintaining a high probability of producing a single pair. Many of the challenges we discuss are central to other quantum information technologies, including the need for low-loss optical storage, switching and detection, and fast feed-forward control.

In a series of articles concerned with quantum stream cipher by Y-00 protocol on this conference, we have claimed that the quantum stream cipher has a pretty good security against several concrete attacks. On the other hand, it has been pointed out that one can improve the security level by using various additional randomization techniques. In this paper, we will show some concrete randomization techniques for quantum stream cipher. First we will sketch the framework of the deliberate signal randomization (DSR) that is realized by randomizing the signals deliberately with true-random numbers or with pseud-random numbers generated by a secret key. Secondly, we will consider about the deliberate error randomization (DER) by using concrete models. It will be shown that these randomization techniques enhance the security level of the quantum stream cipher.

A number of schemes that use quantum mechanics to preserve privacy are presented. In particular, anonymous broadcast channels, voting, and secure function evaluation are discussed. It is found that entangled quantum states can be useful in maintaining privacy.

We report on the implementation of a photon counting polarimeter based on a scheme known to be optimal for obtaining the polarization vector of ensembles of spin-1/2 quantum systems. We show how to use this polarimeter to estimate the complete polarization state for generic multi-photon states. State reconstruction using the polarimeter is illustrated by actual measurements on prepared ensembles of one- and two-photon systems. The rate at which the estimated polarization state converges to an asymptote state is also measured and presented.

Recently, quantum systems with D-dimensional bases states (D > 2) or quDits have attracted much attention in the context of fundamental tests of quantum theory and potential applications in quantum information. In this paper, we discuss several schemes for generating entangled states of two ququarts (four-dimensional quantum systems). The ququart in our scheme is based on frequency-nondegenerate biphoton states of spontaneous parametric down-conversion and we show how the entangled states between two ququarts can be generated with simple linear optical elements, such as an ordinary 50/50 beamsplitter, a polarizing beamsplitter, or a dichroic beamsplitter. We also show that our scheme is capable of generating postselection-free two-ququart entangled states efficiently.

Processing information quantum mechanically is known to enable new communication and computational scenarios that cannot be accessed with conventional information technology (IT). We present here a new approach to scalable quantum computing---a "qubus computer"---which realizes qubit measurement and quantum gates through interacting qubits with a quantum communication bus mode. The qubits could be "static" matter qubits or "flying" optical qubits, but the scheme we focus on here is particularly suited to matter qubits. Universal two-qubit quantum gates may be effected by schemes which involve measurement of the bus mode, or by schemes where the bus disentangles automatically and no measurement is needed. This approach enables a parity gate between qubits, mediated by a bus, enabling near-deterministic Bell state measurement and entangling gates. Our approach is therefore the basis for very efficient, scalable QIP, and provides a natural method for distributing such processing, combining it with quantum communication.

We report on two experiments implementing quantum communications primitives in linear optics systems: a secure Quantum Random Bit Generator (QRBG) and a multi-qubit gate based on Two-Photon Multiple-Qubit (TPMQ) quantum logic. In the first we use photons to generate random numbers and introduce and implement a physics-based estimation of the sequence randomness as opposed to the commonly used statistical tests. This scheme allows one to detect and neutralize attempts to eavesdrop or influence the random number sequence. We also demonstrate a C-SWAP gate that can be used to implement quantum signature and fingerprinting protocols. A source of momentum-entangled photons, remote state preparation, and a C-SWAP gate are the ingredients used for this proof-of-principle experiment. While this implementation cannot be used in field applications due to the limitations of TPMQ logic, it provides useful insights into this protocol.

Quantum information processes utilize the potential of quantum coherence to achieve improvements in communication and computation protocols. In order to develop appropriate technologies, it is therefore necessary to test the successful implementation of quantum coherent operations in experimental devices. In this presentation, it is shown how the quantum coherent performance of a device can be evaluated from complementary test measurements. Despite the limitation of test measurements to only two orthogonal basis sets of states, this method provides a surprisingly detailed and intuitively accessible picture of errors in quantum operations, making it possible to assess the quantum parallelism of non-classical operations in terms of the directly observable "classical" device properties.

Optical coherence theory for the complex envelopes of passband fields has been concerned, almost exclusively, with correlations that are all phase insensitive, despite decades of theoretical and experimental work on the generation and applications of light with phase-sensitive correlations. This paper begins the process of remedying that deficiency, by developing coherence theory for classical scalar fields with phase-sensitive fluctuations. In particular, the Wolf equations are extended to phase-sensitive fields, paraxial free-space propagation effects are analyzed, and a normal-mode decomposition for scalar fields of arbitrary coherence is established. The extension of this theory to the field-operator description that is needed to characterize non-classical light beams is briefly discussed.

Strong optical cross-phase modulation (XPM) for weak fields is tremendously important for optical quantum information (QI) processing and for all-optical switches in classical communication. A sufficiently large XPM would allow the design of deterministic controlled quantum gates for photonic qubits and thus enable universal optical quantum computation. Recently, several proposals have been brought forward to create large XPM using double electromagnetically induced transparency (DEIT) in which two weak signal light pulses travel at equally slow group velocity, but creating DEIT still poses an experimental challenge. We give a brief overview about DEIT and discuss its applications and limitations. A scheme that combines the best features of previous proposals and optimizes the large XPM parameter for DEIT schemes is outlined. Finally we devise a scheme to perform universal quantum information processing, which respects the bound on the achievable nonlinearity and addresses the requirement of quantum error correction.

Various protocols of quantum cryptography has been presented. Some of them can be used to transmit secret messages directly. In this letter we present a new kind of quantum cryptography protocol for direct transmission of classical and quantum messages based on Shamir's generic protocol on classical message encryption. Though Shamir's original idea has only realization of computational security and falls to middle-man attack, our protocol is theoretically secure based on properties of quantum entanglement and Boolean function, and can resist the middle-man attack.

We continue our previous program¹ where we introduced a set of quantum-based design rules directed at quantum engineers who design single-photon quantum communications and quantum imaging devices. Here, we report on experimental progress using SPAD (single photon avalanche diode) arrays of our design and fabricated in CMOS (complementary metal oxide semiconductor) technology. Emerging high-resolution imaging techniques based on SPAD arrays have proven useful in a variety of disciplines including bio-fluorescence microscopy and 3D vision systems. They have also been particularly successful for intra-chip optical communications implemented entirely in CMOS technology. More importantly for our purposes, a very low dark count allows SPADs to detect rare photon events with a high dynamic range and high signal-to-noise ratio. Our CMOS SPADs support multi-channel detection of photon arrivals with picosecond accuracy, several million times per second, due to a very short detection cycle. The tiny chip area means they are suitable for highly miniaturized quantum imaging devices and that is how we employ them in this paper. Our quantum path integral analysis of the Young-Afshar-Wheeler interferometer showed that Bohr's complementarity principle was not violated due the previously overlooked effect of photon bifurcation within the lens--a phenomenon consistent with our quantum design rules--which accounts for the loss of which-path information in the presence of interference. In this paper, we report on our progress toward the construction of quantitative design rules as well as some proposed tests for quantum imaging devices using entangled photon sources with our SPAD imager.

Quantum ghost imaging was explored by use of a chaotic laser light source, a photon effcient charged coupled device (CCD) camera, a stencil mask, a photon bucket detector, and computer processing. We investigated and successfully achieved quantum ghost imaging of the stencil letters ARL from macroscopic time integration scales of 1ms to 10ms. Importantly, quantum ghost images were obtained from photons which did not interact with the letter object. In addition to the timescale effect on ghost imaging we investigated the role of speckle spatial size in resolving images. Results are presented from our investigations of these components of the time and space correlations of photons emanating from chaotic laser light and ending in both the photon bucket detector arm and the CCD arm. Important applications for quantum imaging and quantum ghost imaging are also discussed.

Two-photon correlation phenomena of chaotic light, including the historical Hanbury Brown and Twiss effect, are essentially quantum effect of two-photon interference, instead of classical statistical correlation between intensity fluctuations. To support our view, we analyze a "ghost" imaging experiment with chaotic light for which the classical understanding does not give a satisfactory interpretation. We also provide a two-photon optical picture of ghost imaging with chaotic light in terms of two-photon phase-conjugate mirror which suggests lensless imaging applications for radiations for which no effective lens is available.

We discuss a model for time displaced entanglement, produced by taking one member of an entangled pair on a round trip at relativistic speeds, thus inducing a time-shift between the pair. We show that decoherence of the entangled pair is predicted. For non-maximal entanglement this then implies the ability to induce a non-unitary, non-linear quantum evolution. Although exhibiting unusual characteristics, we show that these evolutions cannot be dismissed on the basis of entropic or causal arguments.

As the quantum information field advances, the need for improved single-photon devices is becoming more critical. Quantum information systems are often limited by detector deadtime to count rates of a few MHz, at best. We present a multiplexed detection scheme that allows photon counting at higher rates than possible with single detectors. The system uses an array of detectors and an optical switch system to direct incoming photons to detectors known to be live. We model the system for realistic individual detector deadtimes and optical switching times. We show that such a system offers more promise than simply reducing the deadtime of an individual detector. We find that a system of N detectors with a given deadtime, can count photons at faster rates than a single detector with a deadtime reduced by 1/N, even if it were practical to make such a large improvement.

The bandwidth and the frequency correlations of quantum light can be considered as a resource for the implementation of new quantum information algorithms, and it should enable the applicability of quantum techniques not yet implemented. For that purpose, the control of the frequency correlations, and the bandwidth, of single and paired photons is an essential ingredient, since the optimum bandwidth, as well as the most appropriate type of frequency correlations for a specific quantum application, depend on the specific quantum information application under consideration. Here we elucidate and implement new strategies to tailor the frequency properties of quantum light. Such strategies, which are based on the use of non collinear spontaneous parametric down conversion (SPDC) configurations, include the generation of narrow and enhanced bandwidth quantum light, the control of the frequency correlations of paired photon, and the generation of heralded single photons with a high degree of purity from pairs of uncorrelated photons.

In this paper we will present some experimental researches about Quantum Communication performed in "Carlo Novero" Quantum Optics laboratory at INRiM (former IEN). After a general review of our studies, we will describe our recent researches on propagation of polarization entangled photons in optical fibres focused on the investigation of the effect of two-photon interference in the second-order Glauber's correlation function and on the characterization of this quantum channel as a Complete Positive (CP) map. We will then describe an innovative method, based on detectors operating in Geiger mode (on/off), for reconstructing the photon statistics of quantum optical states, presenting experimental data collected to test the extension of this method to multi-partite states.

We report an entanglement concentration scheme in which a maximally entangled Bell-state can be obtained from two pairs of partially (or non-maximally) entangled photons. Since this scheme is built only upon linear optics and does not require photon-number resolving detectors, it should be applicable in experimental implementations of various quantum information protocols which require maximally entangled Bell-states.

The working principle of quantum lithography has been demonstrated a few years ago: entangled N-photon systems can improve the spatial resolution of an imaging system by a factor of N, despite the classical Rayleigh limit. Recently, a number of experiments successfully simulated certain features of quantum imaging and two-photon interference-diffraction by using chaotic light in coincidence detection experiments. Can classical light simulate the effect of imaging type quantum lithography? This article attempts to provide an answer to this currently debated question.

We show the experimental observation of quantum states of light exhibiting nonclassical features obtained by single photon excitation of a thermal state. Such single-photon-added thermal states are the result of the single action of the creation operator on a mixed state that can be fully described classically. They show different degree of nonclassicality depending on the mean photon number of the original thermal state. The generated state is characterized by means of ultra-fast homodyne detection which allows us to reconstruct its density matrix and Wigner function by quantum tomography. We demonstrate the nonclassical behavior of single-photon added thermal states by an analysis of the negativity of the Wigner function.

A new configuration for optical coherence tomography is proposed that utilizes a phase-conjugate amplifier in conjunction with a Michelson interferometer to detect interference between two classical light fields with a non-zero phase-sensitive cross correlation. This imaging configuration - which we call phase-conjugate optical coherence tomography (PC-OCT) - shares the same factor-of-two axial resolution improvement and cancellation of even-order dispersion terms that are the key features of quantum optical coherence tomography, but without the necessity for non-classical signal and reference beams. Under appropriate conditions, PC-OCT can achieve a signal-to-noise ratio that is comparable to that of conventional optical coherence tomography.

We put forward a new scheme to tailor the frequency correlations of paired photons which allows their spectral properties to be tuned from correlation to anticorrelation, including uncorrelation. The method is based on the proper tailoring of the group velocities of all interacting waves through the use of beams with angular dispersion. The method can be implemented in materials and frequency bands where conventional solutions do not hold. This technique makes possible the generation of frequency correlated photons, heralded single photons with a high degree of purity from pairs of uncorrelated photons, and the suppression of distinguishing information contained in the frequency spectrum of polarization entangled photon pairs.

A Quantum Key Distribution (QKD) network can allow multi-user communication via secure key. Moreover, by actively switching communication nodes, one can achieve high key transmission rate for the selected nodes. However, the polarization properties of different fiber path are different and these properties also randomly drift over time. Therefore, polarization recovery after the switching and auto-compensation during key transmission are critical for the QKD network. In this work, we use programmable polarization controllers to implement polarization recovery and auto-compensation in the QKD network. We will also discuss its time limitation and future improvement.

We present an experimental study of decoherence of the ground energy levels of ⁸⁷Rb atoms in vapor cells. We measure the decoherence of the ground state using three different methods: measuring the decay constant of the storage of light in atomic vapor, the decay rates of transient coherence oscillations of the ground state, and the width of the electromagnetically induced transparency resonances. The measurements showed decoherence rates on the scale of 10⁴ s^-1.

Quantum communication in free space is the next challenge of telecommunications. Since we want to determine the outcome of quantum communication by means of single photons, we must understand how a single photon interacts with the atmosphere. In this brief article, some simulation results for realistic and generic atmospheric conditions are reported, a related experiment is considered and its results are described and discussed. Furthermore, the setup of a future experiment, currently under preparation, is described and analyzed.