This PDF file contains the front matter associated with SPIE Proceedings Volume 11507, including the Title Page, Copyright information, and Table of Contents.

Quantum illumination utilizing two-mode squeezed light beams is a promising target detection technology. It is expected that the error probability of target detection will be improved even under atmospheric disturbance such as loss and noise because of the non-classical property of the light source. One of the entangled beam, called the signal beam, is reflected by a target in atmospheric disturbance and then detected by homodyne measurement at the receiver. To improve detection efficiency, it is important to study optical wave propagation at various atmospheric conditions. We focused on fog as an example of atmospheric disturbance and investigated the effect of fog on near-infrared laser beam. We studied beam propagation using Mach-Zehnder interferometer including a chamber filled with a uniform fog. It was observed that the visibility of interference decreased with increasing fog density. Our experimental results and theoretical analysis indicate that the effect of uniform fog is mainly energy loss and does not change the spatial modes of light wave.

CPI is a novel imaging modality capable of addressing the intrinsic limitations of conventional plenoptic imaging - namely, the resolution loss and the sacrificed change of perspective, - while guaranteeing the typical advantages of plenotpic imaging: 3D imaging, refocusing of acquired pictures, in post-processing, and depth of field extension. In this work, we review a recently developed CPI scheme, named correlation plenoptic imaging between arbitrary planes and derive the algorithm for image refocusing.

It is now widely accepted that a global quantum internet will be developed in the future with many of the essential elements already demonstrated in the laboratory. We are at the stage where many of these elements are being integrated together, however the communities focus has been on small scale networks whose topology makes the of routing of quantum signals quite forward. Moving forward we need to explore how routing will work on this larger scale quantum networks especially when the networks entire topology is not known to all users of that network. We will discuss various routing options that are available and show these impose quite different constraints on the fundamental building blocks of the network. This will be illustrated with a number of examples including several with no classical analog.

A quantum internet holds promise for achieving communication tasks that seem to be intractable by the current internet. The required functionality of a physical layer for such a quantum internet is to distribute entanglement efficiently to clients over a quantum network. A fundamental building block to design such efficient distribution of entanglement is to bound capacities of such quantum internet protocols. In this talk, we present a set of efficient linear programs to bound quantum/private capacities of quantum internet protocols, as well as their analytic upper/lower bounds. Our linear program is applied to bipartite cases, multi-pair cases, and a multi-partite case, covering almost all known situations.

In the distribution of quantum states over a long distance, not only are quantum states corrupted by interactions with an environment but also a measurement setting should be re-aligned such that detection events can be ensured for the resulting states. We here present a method of protecting a measurement in quantum key distribution against the interactions quantum states experience during the transmission, without the verification of a channel. As a result, a receiver does not have to revise the measurement that has been prepared in a noiseless scenario since it would remain ever optimal. The measurement protection is achieved by applications of local unitary transformations before and after the transmission, that leads to a supermap transforming an arbitrary channel to a depolarization one. An experimental demonstration is presented with the polarization encoding on photonic qubits. It is shown that the security bounds for prepare-and-measure protocols can be improved.

Quantum secret sharing (QSS) is a cryptographic multiparty communication technique in which a secret is divided and shared among N parties and then securely reconstructed by (N-1) cooperating parties, making it perfect for storing and sharing highly sensitive data. Challenges in high dimensional state preparation, transformation and detection, the key steps of any QSS protocol, have so far hindered experimental realisation. Here, by taking advantage of the high-dimensional encoding space accessible by a photon's orbital angular momentum, we present a toolbox for realising practical high-dimensional single photon QSS schemes that are easily scalable in both dimension and number of participants. Our implementations realised a new record in both the number of participants (N=10) and the dimensionality (d=11), with the latter facilitating the transfer of 2.89 bits of information per photon. This work is an important step towards securely distributing information across a network of nodes.

The focus of this work is the evolution of a quantum network where each node is initialized with a unique data entry encoding classical information such as a serial number. Each node is then probabilistically swapped with one of its immediate neighbors in a specific, but undisclosed order, resulting in a redistribution of entries across the network. In foreseeable practice, Quantum systems prevent the reliable swap of quantum states due to decoherence and imperfect circuit fidelity, but in this network model we require that if each node is measured, all data entries remain unique. Originally motivated by the desire to develop a method which results in all entries only being able to move to their immediate neighbors, we present aspects of the underlying combinatorics, and solutions to either mitigate or potentially minimize the effects of larger travel distances.

Diamond optical emitters have been explored extensively in recent years for applications to quantum information and processing. Recently they have also been considered for quantum sensing, especially as nanocrystals in biological systems. However, for most of these applications, the performance of the diamond optical emitters is sensitive to the quality of the material. However, there are still important problems that still must be solved in order to realize the full potential of diamond. For example, existing growth and processing techniques (including ion implantation) are fundamentally probabilistic in nature and so cannot easily produce scalable quantum systems. To overcome these problems, we show a molecule-seeded growth technique that decouples the doping and growth processes. The result is near-deterministic creation of specific color centers in chemically pure, low-strain diamond.

Characterization of quantum states and detectors is a key task in rapidly emerging optical quantum science and technology. First, we introduce and experimentally demonstrate a noise-robust quantum state characterization protocol using photon-number-resolving (PNR) measurements. Unlike conventional continuous variable state tomography methods, our method utilizes computationally efficient semi-definite programming (SDP) and can be used to accurately reconstruct the state even after loss a known loss. The protocol is demonstrated for a weak coherent state as well as a single-photon Fock state. Next, we propose a method for characterizing a photodetector by directly reconstructing the Wigner functions of the detector’s Positive-Operator-Value-Measure (POVM) elements via weak-field homodyne technique. We also report our experimental progress on characterizing a superconducting transition-edge sensor for PNR measurements.

On-demand single photon sources are a crucial element of quantum key distribution and quantum computing. Such devices face major challenges in achieving high performance, which include realized necessary environmental isolation, preservation of coherence over time, and reliable triggering of multi-photon emission. Nanodiamonds exhibit uniquely good characteristics at room temperature, but emission rates are still limited. Here, we consider how nitrogen and Xenon color centers with electronic triggering can increase emission rates, by carefully modeling the interaction between rapid voltage pulses and the resulting photonic emission. If realized experimentally, this work will pave the way for short-wavelength infrared quantum communications.

This paper describes high-performance optical circuits operating on hyper-entangled quantum states. The concept is based on correlated photon pairs generated in such a way that their polarization, frequency or propagation paths are entangled and can be used to support quantum encryption protocols. Detection and processing of such signals requires hyper-spectral optical circuits capable of responding to non-classical features of light. Processing capabilities of these quantum circuits are straight-forward in theory, but physical implementation can still present a challenge for integrity of the quantum states. In this paper, we address experimental verification of entanglement characteristics at each step of building the quantum optical circuits and present a formal quantitative approach based on the quantum state tomography.

In this work we present a proof-of-concept of a photonic quantum random number generation scheme based on the entanglement between two internal degrees of freedom of single photons. The quantum correlations between momentum and polarization at the single photon level are verified by a Bell test in the CHSH form. The violation of this inequality not only ensures the entanglement, but also provides an estimation of the minimum entropy of the generated sequence. This allows to optimize the application of randomness extractor to obtain an unbiased sequence of random numbers.

Two photon absorption processes are known to be sensitive to energy-time entanglement between the absorbed photons. It should therefore be possible to characterize the energy-time entanglement by observing the two photon absorption cross-sections in a known medium controlled by external conditions. In this paper, we analyze the correspondence between two photon absorption and projective measurements of the two photon wavefunction. It is shown that the two photon absorption process is described by a projection onto an energy-time entangled two photon state, indicating that the absorption cross-section can provide direct evidence of entanglement between the absorbed photon pairs.

In this talk, our recent activities for nanofiber-integrated single light emitters for photonic quantum network. First, we report the coupling of various single light emitters, including diamond nanocrystals with color centers and hexagonal Boron Nitride flakes, to ultra-thin tapered optical fibers (nano optical fiber) with a diameter as small as 300 nm, keeping the transmittance of the fiber more than 90%. Second, for further improvement of the coupling efficiency, we introduce our development of nanofiber Bragg cavity (NFBC) with the mode volume of under 1 mu m^3 and repeatable tuning capability over more than 20 nm at visible wavelengths. The improvement of Q factor using He-ion beam milling will also be reported.

Developed and characterised for the first time is a quantum vector magnetometer using coherent population trapping (CPT) in ⁸⁷Rb vapour and external magnetic field compensation by 3-axis Helmholtz coils to stabilize the amplitude and spectral position of the CPT resonance excited on a magneto-sensitive (m_f ≠ 0) energy level transition. To measure the magnetic field components orthogonal to the magnetometer optical axis, resonance amplitude dependence upon the transverse field strength is used, which has a maximum at zero transverse field. The proposed approach allowed measurement of the external magnetic field vector with a sensitivity exceeding 500 pT/Hz^1/2.

Since the publication of the Quantum Amplitude Estimation (QAE) algorithm by Brassard et al., 2002, several variations have been proposed, such as Aaronson et al., 2019, Grinko et al., 2019, and Suzuki et al., 2020. The main difference between the original and the variants is the exclusion of Quantum Phase Estimation (QPE) by the latter. This difference is notable given that QPE is the key component of original QAE, but is composed of many operations considered expensive for the current NISQ era devices. We compare two recently proposed variants (Grinko et al., 2019 and Suzuki et al., 2020) by implementing them on the IBM Quantum device using Qiskit, an open source framework for quantum computing. We analyze and discuss advantages of each algorithm from the point of view of their implementation and performance on a quantum computer.