This PDF file contains the front matter associated with SPIE Proceedings Volume 10409 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Photon-number-resolving measurements allow one to address quantum optics from the corpuscular angle of quantum physics. A number of classically non-intuitive features are typically expected from working in the photon-number Fock eigenbasis, such as nonpositive Wigner functions. In this talk, we report on the progress of two applications of photon-number-resolving measurements in quantum optics: quantum interferometry with photon-subtracted twin beams and quantum state tomography of Fock states created by heralded parametric downconversion.

We discuss the importance of permutation symmetry on coincidence rates at interferometer outputs, how permutations naturally lead partition the many-photon space, and how permutation symmetry reveals the connection between partial distinguishability, matrix immanants and the complexity of the coincidence landscape.

Multiphoton interference results in a wide variety of non-classical photon number statistics, including characteristic signatures of entanglement between two or more sets of optical modes. Here, we consider the photon number statistics observed after applying discrete Fourier transformations (DFTs) to bipartite entangled states generated using single photon sources and beam splitters. It is shown that the output photon number states of DFTs are eigenstates of a translational mode shifting operator in the input. The complex eigenvalues of the mode shift can be identified by a phase number K obtained from the output photon distribution. For each output distribution, the possible input states are limited to mode shift eigenstates with the same K-value. Using this mode shift rule, we can identify the quantum coherence between different photon number distributions in the input with experimentally observable K-values in the output of the DFT. In the case of multi-photon entanglement obtained by post-selection and beam splitting single photons, this coherence is non-local, resulting in correlated pairs of K-values that always sum up to zero. We can therefore observe both the correlations between the input photon number distributions and a complementary correlation between the output photon numbers of two DFTs to obtain a reliable characterization of the entanglement between the two multi-mode multi-photon systems. Importantly, the K-value allows a classification of large sets of possible photon number distributions, resulting in a significant simplification of the experimental evaluation of the multi-photon output statistics and opening up the road towards more efficient applications of non-classical multi-photon states

In this work, we show that measurement in quantum 2-design, such as mutually unbiased bases (MUBs) or symmetric, informationally complete states (SICs), improves the capability of detecting entangled states both theoretically and experimentally. On the theoretical side, we show that measurement in quantum 2-design can detect entangled states twice compared to entanglement witnesses. On the implementation side, we present the scheme of entanglement detection with two detectors only of a Hong-Ou-Mandel interferometer. The experimental scheme applies single-copy level measurement followed by post-processing of measurement outcomes, which is feasible with current technologies.

Youngs double slit experiment is one of the most celebrated achievements in quantum and classical optics; it provides experimental proof of the wave-particle duality of light. When the paths of the double slit are marked with orthogonal polarizations, the path information is revealed and no interference pattern is observed. However, the path information can be erased with a complimentary analysis of the polarization. Here we use hybrid entanglement between photons carrying orbital angular momentum and polarization to show that, just as in Young's experiment, the paths (OAM) marked with polarization do not lead to interference. However, when introducing the eraser (polarizer) which projects the polarization of one of the entangled photons onto a complementary polarization basis, the OAM (paths) are allowed to interfere, leading to the formation of azimuthal fringes whose frequency is proportional to the OAM content carried by the photon.

Quantum teleportation, a transfer protocol of quantum states, is the essence of many sophisticated quantum information protocols. There have been two complementary approaches to optical quantum teleportation: discrete variables (DVs) and continuous variables (CVs). However, both approaches have pros and cons. Here we take a “hybrid” approach to overcome the current limitations: CV quantum teleportation of DVs. This approach enabled the first realization of deterministic quantum teleportation of photonic qubits without post-selection. We also applied the hybrid scheme to several experiments, including entanglement swapping between DVs and CVs, conditional CV teleportation of single photons, and CV teleportation of qutrits. We are now aiming at universal, scalable, and fault-tolerant quantum computing based on these hybrid technologies.

We have developed a comprehensive simulator to study the polarization entangled quantum key distribution (QKD) system, which takes various imperfections into account. We assume that a type-II SPDC source using a PPLN-based nonlinear optical waveguide is used to generate entangled photon pairs and implements the BB84 protocol, using two mutually unbiased basis with two orthogonal polarizations in each basis. The entangled photon pairs are then simulated to be transmitted to both parties; Alice and Bob, through the optical channel, imperfect optical elements and onto the imperfect detector. It is assumed that Eve has no control over the detectors, and can only gain information from the public channel and the intercept resend attack. The secure key rate (SKR) is calculated using an upper bound and by using actual code rates of LDPC codes implementable in FPGA hardware. After the verification of the simulation results, such as the pair generation rate and the number of error due to multiple pairs, for the ideal scenario, available in the literature, we then introduce various imperfections. Then, the results are compared to previously reported experimental results where a BBO nonlinear crystal is used, and the improvements in SKRs are determined for when a PPLN-waveguide is used instead.

Single-photon sources are crucial components for the implementation of quantum communication protocols. However, photons emitted by some of the most popular types of realistic sources are spectrally broadband. Due to this drawback, the signal emitted from such sources is typically affected by the effect of temporal broadening during its propagation through telecommunication fibers which exhibit chromatic dispersion. Such problem can be observed e.g. when using sources based on the process of spontaneous parametric down-conversion (SPDC). In the case of long-distance quantum communication temporal broadening can significantly limit the efficiency of temporal filtering. It is a popular method, which is based on the reduction of the duration time of the detection window, used for decreasing the number of registered errors. In this work we analyzed the impact of the type of spectral correlation within a pair of photons produced by the SPDC source on the temporal width of those photons during their propagation in dispersive media. We found out that in some situations the width can be decreased by changing the typical negative spectral correlation into positive one or by reducing its strength. This idea can be used to increase the efficiency of the temporal filtering method. Therefore it can be applied in various implementations of quantum communication protocols. As an example of the application we subsequently analyzed the security of a quantum key distribution (QKD) scheme based on single photons. It was realized in the configuration with the source of photons located in the middle between the legitimate participants of a QKD protocol (called typically Alice and Bob). We demonstrated that when the information about the emission time of the photons produced by the SPDC source is not distributed to Alice and Bob, the maximal security distance can be considerably extended by using positively correlated photons, while in the opposite case strongly (no matter positively or negatively) correlated photons are optimal. We also found out that the results of our calculation may be very sensitive to the spectral widths of the photons produced by the SPDC source. In addition, we concluded that in realistic situation Alice and Bob would have to optimize their source over both the spectral widths of the generated photons and the type of spectral correlation in order to maximally extend the security distance. The results of our work are, in particular, important for the QKD systems utilizing commercial telecom fibers populated by strong classical signals. In those systems temporal filtering method can be used to reduce not only the dark counts registered by the detection system, but also the channel noise originating from the process of Raman scattering, which is the main factor limiting their performance.

Let us consider the experimental setup where SPDC source generates photon pairs which are subsequently coupled to single-mode fibers (SMFs). We assume that there are three parties involved: 1) Alice, possessing the photon pair source, detection system and the fiber connecting them, 2) Bob, who monitors the output of the second, long-distance fiber and 3) Eve, who can perform the most general collective attacks in order to acquire information which Alice and Bob wish to transfer. Typically, in fiber-based communication the chromatic dispersion is considered to be an obstacle, limiting the maximal distance at which information carrier can be securely transmitted. This phenomenon forces the trusted parties to define longer detection windows to avoid losing signal photons and increases the amount of detection noise that is being registered. We consider standard BB84 quantum key distribution protocol, based on the SPDC source located in between Alice and Bob. The parameters of standard realistic telecommunication fibers (SMF28e+) are take into account. The source emits photon which apart of being entangled in polarization degree of freedom are entangled in spectral domain. This is the key feature which allows one to reduce detection noise by manipulating the spectral correlation between the produced photons. In this way the maximal security distance can be increased by around 10%.

A power distribution of optical signals as the ciphertext of Y-00 quantum stream cipher transceivers should be uniform, even when the bit sequence of plaintext is a non-uniform sequence. For examining the uniformity, we experimentally measure powers of optical signals with 4096 intensity levels and calculate the ratio of signal numbers over and under the average power. The ratios for various kinds of bit sequences are 0.5 with measurement error of 1%, which is an evidence of the uniformity of the power distribution. In addition, the effectiveness of a randomization technique of overlapping selection keying for enhancing the security against the known plaintext attack is experimentally observed.

This paper is focused on the problem of Information Reconciliation (IR) for continuous variable Quantum Key Distribution (QKD). The main problem is quantization and assignment of labels to the samples of the Gaussian variables observed at Alice and Bob. Trouble is that most of the samples, assuming that the Gaussian variable is zero mean which is de-facto the case, tend to have small magnitudes and are easily disturbed by noise. Transmission over longer and longer distances increases the losses corresponding to a lower effective Signal to Noise Ratio (SNR) exasperating the problem. Here we propose to use Permutation Modulation (PM) as a means of quantization of Gaussian vectors at Alice and Bob over a d-dimensional space with d ≫ 1. The goal is to achieve the necessary coding efficiency to extend the achievable range of continuous variable QKD by quantizing over larger and larger dimensions. Fractional bit rate per sample is easily achieved using PM at very reasonable computational cost. Ordered statistics is used extensively throughout the development from generation of the seed vector in PM to analysis of error rates associated with the signs of the Gaussian samples at Alice and Bob as a function of the magnitude of the observed samples at Bob.

A unified treatment of typical optical modulation formats is given toward a future development of the quantum enigma cipher. In our analysis, the intensity shift keying (ISK), amplitude shift keying (ASK), phase shift keying (PSK), and quadrature amplitude modulation (QAM) coherent state signals are compared in terms of the masking number of potential signals through the numerical calculations of the square-root measurement. Through this comparison, we see that the use of ISK coherent state signal is the best choice for cryptographic purpose among these formats in achieving an appropriate balance between the number of signal photons and the number of quantum state signals.

Quantum technologies promise to overcome by far the limits of the classical schemes. However, the present challenge is to overpass the limits of proof of principle demonstrations to approach real applications. In this paper, we present an experiment which aims to bridge this gap in the field of quantum enhanced imaging. In particular, we realize a sub-shot noise wide field microscope based on spatially multi-mode non-classical photon number correlations in twin beams. The microscope produces real time images of 8000 pixels at full resolution, with noise reduced to the 80% of the shot noise level (for each pixel), hence able to image faint samples at low illumination level. The noise can be further reduced (less than 30% of the shot noise level) turning down the resolution. It demonstrates the best sensitivity per incident photon ever achieved in absorption microscopy.

The imaging through optical systems may have distortions called aberrations. It can be chromatic aberrations, an effect resulting from dispersion due to the impossibility to focus all colors to the same point or monochromatic aberrations, where the rays emerging from one object point will not all meet at a single image point. Thanks to an analogy between the quantum and classical intensity light correlations, the previous studies explored the ghost imaging under both viewpoint. Although, the first approach was the use of correlated-photon imaging for the cancellation of phase aberrations, some authors have suggested theoretical models for the cancellation of phase aberrations using classical light in the ghost-imaging scheme. However, a detailed experimental study of the cancelation of phase aberrations using classical light intensity correlation is still missing in the literature. In this work, we show that exploring correlations of fluctuations in speckle intensity it is possible to cancel out aberrations that may exist in the Fraunhofer plane of an optical system. The aberrations cancelation occurs independently of its shape and it does not need coordinate inversion. We use high-order intensity correlations to obtain high visibility. Therefore, we extended the quantum-classical analogy to the study of cancelation of phase aberrations showing an interesting and useful distinction from the quantum case. It is possible to embed images into speckle patterns, and to recover it though the spatial correlation function. Therefore, this effect can be useful in imaging through random media and microscopy, canceling inherent aberrations than can cause distortions in the image.

Ultra-precise measurements of various parameters such as the mass of nano-particles, magnetic fields or gravity can be attained by probing the phononic modes of a micro-mechanical oscillator with light. The sensitivity of such measurements is in part governed by the noise of the phononic mode as well as the noise of the probing light mode, so by decreasing the noise of the probe beam an enhanced sensitivity can be expected. We demonstrate this effect by using squeezed states of light where the quantum uncertainty of the relevant quadrature is reduced below the shot noise level. Using this squeezing-enhanced sensitivity effect, we demonstrate 1) improved feedback cooling of a phononic mode in a microtoroidal cavity and 2) improved sensing of a magnetic field using the coupling to a microtoroidal phononic mode via a magnetorestrictive material. We present our recent experimental results and discuss future directions.

In recent years there has been substantial progress in solid-state quantum information demonstrations based on diamond. This ranges from room-temperature multi-qubit quantum registers to spin-photon entanglement to loophole-free Bell tests. Most of these demonstrations are done with the nitrogen-vacancy (NV) center. Recently however the silicon-vacancy (SiV) center has shown a much better coupling to photons, owing to the suppression of spectral diffusion. Still more recently the germanium-vacancy (GeV) has shown much higher quantum efficiency the SiV. However there are tradeoffs. In this talk I will review the various options for quantum information devices in diamond. I will also discuss new options for the future, including designer color centers made possible by novel diamond growth techniques.

Two-mode squeezed light is an effective resource for quantum entanglement and shows a non-classical correlation between each optical mode. We are developing a two-mode squeezed light source to explore the possibility of quantum radar based on the quantum illumination theory. It is expected that the error probability for discrimination of target presence or absence is improved even in a lossy and noisy environment. We are also expecting to apply two-mode squeezed light source to quantum imaging. In this work we generated two-mode squeezed light and verify its quantum entanglement property towards quantum radar and imaging. Firstly we generated two independent single-mode squeezed light beams utilizing two sub-threshold optical parametric oscillators which include periodically-polled potassium titanyl phosphate crystals for the second order nonlinear interaction. Two single-mode squeezed light beams are combined using a half mirror with the relative optical phase of 90◦ between each optical field. Then entangled two-mode squeezed light beams can be generated. We observes correlation variances between quadrature phase amplitudes in entangled two-mode fields by balanced homodyne measurement. Finally we verified quantum entanglement property of two-mode squeezed light source based on Duan’s and Simon’s inseparability criterion.

We propose to use intensity correlation microscopy in combination with structured illumination to image quantum emitters that exhibit antibunching with a spatial resolution reaching far into the sub-classical regime. Combining intensity measurements and intensity auto correlations up to order m creates an effective PSF with FWHM shrunk by the factor p m. Structured Illumination microscopy on the other hand introduces a resolution improvement of factor 2 by use of the principle of moiré fringes. Here, we show that for linear low-intensity excitation and linear optical detection the simultaneous use of both techniques leads to an in theory unlimited resolution power with the improvement scaling favorably as m+sqrt m in dependence of the correlation order m. Hence, yielding this technique to be of the utmost interest for biological imaging. We present the underlying theory and simulations demonstrating the highly increased spatial superresolution, and point out requirements for an experimental implementation.

In the avid search for means to increase computational power in comparison to that which is currently available, quantum walks (QWs) have become a promising option with derived quantum algorithms providing an associated speed up compared to what is currently used for implementation in classical computers. It has additionally been shown that the physical implementation of QWs will provide a successful computational basis for a quantum computer. It follows that considerable drive for finding such means has been occurring over the 20+ years since its introduction with phenomena such as electrons and photons being employed. Principal problems encountered with such quantum systems involve the vulnerability to environmental influence as well as scalability of the systems. Here we outline how to perform the QW due to interference characteristics inherent in the phenomenon, to mitigate these challenges. We utilize the properties of vector beams to physically implement such a walk in orbital angular momentum space by manipulating polarization and exploiting the non-separability of such beams.

We formulate the sum uncertainty relations for compact classical Lie algebras of the type su (n), so (2n), so (2n + 1) and sp (n). Then we present resulting uncertainty relations explicitly for the su (2) ; su (3) and su (4) cases. We numerically verified our bound by choosing a large number of random vectors within an irrep of each of these algebras. We verify the bounds for several irreps. We discuss what type of states saturate su (n) bound for n = 2; 3 and 4, and compare these states with states that saturate more familiar products uncertainty relations.

Quantum systems have always been controlled in a manner to overpower the effects of the environment as they destroy the coherence and entanglement required for quantum protocols. We show here that by harnessing the quantum nature of the environment, it can be manipulated in a manner that it acts as a repository for storing and retreving measurement trajectories of the probing system. Such a macroscopic environment could also be used as a memory for quantum users to store information and securely share it.