Quantum communications bring a paradigm change in internet security by using quantum resources to establish secure keys between parties. Present-day quantum communications networks are mainly point-to-point and use trusted nodes and key management systems to relay the keys. Future quantum networks, including the quantum internet, will have complex topologies in which groups of users are connected and communicate with each-other. Here we investigate several architectures for quantum communication networks. We show that photonic orbital angular momentum (OAM) can be used to route quantum information between different nodes. Starting from a simple, point-to-point network, we will gradually develop more complex architectures: point-to-multipoint, fully-connected and entanglement-distribution networks. As a particularly important result, we show that an $n$-node, fully-connected network can be constructed with a single OAM sorter and $n-1$ OAM values. Our results pave the way to construct complex quantum communication networks with minimal resources.

Reconstructing the state of a quantum system, called quantum state tomography, is an essential task in quantum technologies. Standard methods are inefficient, as they scale exponentially with the number of qubits. Better scaling meth-ods, such as SIC-POVM tomography, are not yet widespread due to their measurement complexity. As such, there is an urgent need to find efficient and compact devices for performing tomography of photonic states. Here we present simulations of photonic devices implementing SIC-POVM tomography in photonic chips. We obtain fidelities F = 0.8452 (for qubits) and F = 0.7609 (for qutrits). Our results show that we can design efficient and compact SIC-POVM tomography modules for integrated-photonics quantum chips. Since integrated photonics is a major quantum technology platform, we expect our results to be instrumental in the future development of compact, costeffective quantum devices. Applications include quantum communications, quantum computing, quantum sensing and quantum imaging.

Accelerator mass spectrometry (AMS) is a widely usedtechniquewith multiple applications, including geology, molecular biology,and archeology. In order to achieve a high dynamic range, AMS requirestandem accelerators and large magnets, which thus confines it to biglaboratories. Here we propose interferometric mass spectrometry (Interf-MS),a novel method of mass separation which uses quantum interference.Interf-MS employs the wave-like properties of the samples and as suchis complementary to AMS, in which samples are particle-like. Thiscomplementarity has two significant consequences: (i) in Interf-MSseparation is performed according to the absolute mass m, and not to the mass-to-charge ratio m/q, as in AMS; (ii) in Interf-MS the samples are in the low-velocityregime, in contrast to the high-velocity regime used in AMS. Potentialapplications of Interf-MS are compact devices for mobile applications,sensitive molecules that break at the acceleration stage and neutralsamples which are difficult to ionize.

Entangled photons are crucial for the development of quantum technologies and especially important in quantum communications. Hence it is paramount to have a reliable, high-fidelity source of entangled photons. Here we describe the construction and characterization of a polarization-entangled photon source. We generate maximally-entangled Bell states using a type-II PPKTP nonlinear crystal inside a Sagnac interferometer. We characterize the source in terms of brightness, visibility, Bell-CHSH test and we perform quantum-state tomography of the density matrix. Our source violates Bell-CHSH inequality $|S|\le 2$ by $n=22$ standard deviations. With visibilities up to ${\cal V}= 98.9\%$ and fidelity ${\cal F}= 97\%$, our source is highly competitive, with state-of-the-art performance. To the best of our knowledge, this is the first source of entangled-photons designed and build in Romania and as such, represents an important step for the development of quantum technologies in our country. We envisage that our results will stimulate the progress of quantum technologies in Romania and will educate the future generation of quantum engineers.

Quantum imaging, one of the pillars of quantum technologies, is well-suited to study sensitive samples which require low-light conditions, like biological tissues. In this context, interaction-free measurements (IFM) allow us infer the presence of an opaque object without the photon interacting with the sample. Current IFM schemes are designed for single-pixel objects, while real-life samples are structured, multi-pixel objects. Here we extend the IFM imaging schemes to multi-pixel, semi-transparent objects, by encoding the information about the pixels into an internal degree of freedom, namely orbital angular momentum (OAM). This allows us to image the pixels in parallel. Our solution exhibits a better theoretical efficiency than the single-pixel case. Our scheme can be extended to other degrees of freedom, like the photon radial quantum number, in order to image 1D and 2D objects.

Photons carrying orbital angular momentum (OAM) are excellent qudits and are widely used in several applications, such as long distance quantum communication, d-dimensional teleportation and high-resolution imaging and metrology. All these protocols rely on quantum tomography to characterise the OAM state, which currently requires complex measurements involving spatial light modulators and mode filters. To simplify the measurement and characterisation of OAM states, here we apply a recent tomography protocol [Asadian et al, Phys. Rev. A 94 010301 (2016)]. Our scheme for OAM tomography in d dimensions requires only a set of measurements on a path qubit, i.e., a two-dimensional system. This replaces the current complexity of OAM measurements by the ability to perform generalized Pauli operators Xd, Zd on OAM states. Our scheme can be adapted in principle to other degrees of freedom, thus opening the way for more complex qudit tomography.

Quantum technologies, such as quantum communication, quantum sensing, quantum imaging and quantum computation, need a platform which is flexible, miniaturisable and works at room temperature. Integrated photonics is a promising and fast-developing platform. This requires to develop the right tools to design and fabricate arbitrary photonic quantum devices. Here we present an algorithm which, starting from an-mode transformationU, designs a photonic device that approximatesU. Using this method we design integrated photonic devices which implement quantum gates with high fidelity. Apart from quantum computation, future applications include the design of photonic subroutines and embedded quantum devices. These custom-designed photonic chips will be able to implement a given algorithm as a single-block circuit and will be small, robust and fast compared to a fully-programmable processor.

Random numbers play a significant role in fields like scientific simulations and cryptography. Here we describe a physical random number generator based on the intrinsic randomness of quantum mechanics. We design a simple method to produce and analyze random sequences starting with a source of correlated/entangled photon pairs as an entropy source. We generate random bits from the coincidence rates of photon pairs in 3 steps: (i) generate correlated/entangled photon pairs, (ii) convert the coincidence rates from decimal to binary, (iii) apply a randomness extraction procedure (post-processing). In this approach, we have tested the influence of entanglement on the entire generation process. In order to obtain a good-quality random sequance, we have used and compared two different extractors in the post-processing step.

One of the main challenges in quantum technologies is the ability to control individual quantum systems. This task becomes increasingly difficult as the dimension of the system grows. Here we propose a general setup for cyclic permutations Xd in d dimensions, a major primitive for constructing arbitrary qudit gates. Using orbital angular momentum states as a qudit, the simplest implementation of the Xd gate in d dimensions requires a single quantum sorter Sd and two spiral phase plates. We then extend this construction to a generalised Xd(p) gate to perform a cyclic permutation of a set of d, equally spaced values {|[Formula: see text]〉, |[Formula: see text] + p〉, …, |[Formula: see text] + (d - 1)p〉} [Formula: see text] {|[Formula: see text] + p〉, |[Formula: see text] + 2p〉, …, |[Formula: see text]〉}. We find compact implementations for the generalised Xd(p) gate in both Michelson (one sorter Sd, two spiral phase plates) and Mach-Zehnder configurations (two sorters Sd, two spiral phase plates). Remarkably, the number of spiral phase plates is independent of the qudit dimension d. Our architecture for Xd and generalised Xd(p) gate will enable complex quantum algorithms for qudits, for example quantum protocols using photonic OAM states.

We propose an integrated polarization beam splitter based on a two-dimensional photonic crystal of polymer cylinders in air, with a TE bandgap centered on a wavelength of lambda = 780nm. The novel design is tailored for two photon absorption 3D laser direct writing in a polymeric photoresist. Using this fabrication method, we can obtain spatial features as small as 200 nm. Processing time can be as low as 15 minutes for a 300x300 mu m processed area and an overall cost of 15 eur. The device is comprised of a series of components such as linear waveguides, parabolic tapered waveguides and multimode interference couplers, in order to lower optical losses. In this paper, design steps and simulation results are presented and discussed. Experimental results supporting the chosen fabrication method and materials are presented and discussed in the form of a parametric study

Centre for Quantum Science and Technology, Macquarie Unive rsity, Sydney NSW 2109, Australia Institute for Quantum Computing, University of Waterloo, W aterloo, Ontario N2L 3G1, Canada Department of Applied Mathematics, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada Department of Physics & Astronomy, Macquarie University, S ydney NSW 2109, Australia Centre for Quantum Technologies, National University of Si ngapore, Singapore 117543

One of the main challenges in quantum technologies is the ability to control individual quantum systems. This task becomes increasingly difficult as the dimension of the system grows. Here we propose an efficient setup for cyclic permutations $X_d$ in $d$ dimensions, a major primitive for constructing arbitrary qudit gates. Using orbital angular momentum (OAM) states as a qudit, the simplest implementation of the $X_d$ gate in $d$ dimensions requires a single quantum sorter $S_d$ and two spiral phase plates (SPPs). We then extend this construction to a generalised $X_d(p)$ gate to perform a cyclic permutation of a set of $d$, equally spaced values {$l_0, l_0+p,..., l_0+(d-1)p$} $mapsto$ {$ l_0+p, l_0+2p,..., l_0 $}. We find efficient implementations for the generalised $X_d(p)$ gate in both Michelson (one sorter $S_d$, two SPPs) and Mach-Zehnder configurations (two sorters $S_d$, two SPPs). Remarkably, the number of spiral phase plates is independent of the qudit dimension $d$. Our architecture for $X_d$ and generalised $X_d(p)$ gate will enable complex quantum algorithms for qudits, for example quantum protocols using photonic OAM states.

This corrects the article DOI: 10.1038/ncomms5997.

Interferometry is a widely used technique for precision measurements in both classical and quantum contexts. One way to increase the precision of phase measurements, for example, in a Mach-Zehnder interferometer (MZI), is to use high-intensity lasers. In this paper we study the phase sensitivity of a MZI in two detection setups (difference intensity detection and single-mode intensity detection) and for three input scenarios (coherent, double coherent, and coherent plus squeezed vacuum). For the coherent and double coherent input, both detection setups can reach the quantum Cramer-Rao bound, although at different values of the optimal phase shift. The double coherent input scenario has the unique advantage of changing the optimal phase shift by varying the input power ratio.

Complex high-power laser installations which require multiple mirrors in high vacuum conditions also require multiple diagnostics points to ensure that the laser beam is properly aligned before starting experiments. In some cases, the diagnostics beam is acquired using transmitted (leaked) beam from the rear of the alignment mirrors, but the refractive index of these mirrors will induce a deviation of the diagnostics beam position, which itself varies according to the mirror relative inclination. Correcting this deviation needs to be implemented in automated and manual beam alignment systems, to avoid clipping and internal reflections which can be detrimental to laser system infrastructure. Fig. 1 – Conceptual representation of laser diagnostics beam displacement For implementing refraction correction into beam alignment algorithms, a optical study has been conducted at INFLPR CETAL petawatt laser system, taking several factors into consideration: laser parameters, beam transport system design, mirror constructive details and material parameters. Beam deviation tests have been conducted at various mirror inclinations and results were compared with calculated deviation interval. Fig. 2 – Calculated beam displacement variation function of mirror inclination (sapphire, 65 mm thickness, P-polarized laser beam)

Schrödinger's cat is one of the most striking paradoxes of quantum mechanics that reveals the counterintuitive aspects of the microscopic world. Here, I discuss the paradox in the framework of quantum information. Using a quantum networks formalism, I analyse the information flow between the atom and the cat. This reveals that the atom and the cat are connected only through a classical information channel: the detector clicks → the poison is released → the cat is killed. No amount of local operations and classical communication can entangle the atom and the cat, which are initially in a separable state. This casts a new light on the paradox. Quanta 2017; 6: 57–60.

Measuring the state of a quantum system is a fundamental process in quantum mechanics and plays an essential role in quantum information and quantum technologies. One method to measure a quantum observable is to sort the system in different spatial modes according to the measured value, followed by single-particle detectors on each mode. Examples of quantum sorters are polarizing beam-splitters (PBS) - which direct photons according to their polarization - and Stern-Gerlach devices. Here we propose a general scheme to sort a quantum system according to the value of any d-dimensional degree of freedom, such as spin, orbital angular momentum (OAM), wavelength etc. Our scheme is universal, works at the single-particle level and has a theoretical efficiency of 100%. As an application we design an efficient OAM sorter consisting of a single multi-path interferometer which is suitable for a photonic chip implementation.

Hidden-variable models aim to reproduce the results of quantum theory and to satisfy our classical intuition. Their refutation is usually based on deriving predictions that are different from those of quantum mechanics. Here instead we study the mutual compatibility of apparently reasonable classical assumptions. We analyze a version of the delayed-choice experiment which ostensibly combines determinism, independence of hidden variables on the conducted experiments, and wave-particle objectivity (the assertion that quantum systems are, at any moment, either particles or waves, but not both). These three ideas are incompatible with any theory, not only with quantum mechanics.

The Weltanschauung emerging from quantum theory clashes profoundly with our classical concepts. Quantum characteristics like superposition, entanglement, wave-particle duality, nonlocality, contextuality are difficult to reconcile with our everyday intuition. In this article I survey some aspects of quantum foundations and discuss intriguing connections with the foundations of mathematics.

Quantum information is a recently emerged, paradigm-changing field which lays the foundations for quantum technologies. In this article we overview this fascinating subject, introduce the main mathematical concepts (entanglement, teleportation etc) and discuss forthcoming technologies like quantum computation, quantum cryptography, quantum imaging and quantum metrology.