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We implement an all electrical controller for 3D feedback cooling of an optically levitated nanoparticle capable of reaching sub-Kelvin temperatures for the center of mass motion. The controller is based on an optimal policy where state estimation is made by delayed position measurements. The method offers a simplified path for pre-cooling and decoupling the transverse degrees of freedom of the nanoparticle. Numerical simulations show that in an improved setup with quantum limited detection, 3D ground state cooling and all electrical quantum control can be achieved.

We provide a tool for measuring the Stokes parameters and the degree of polarization of single photons by employing second order interference, namely the Hong-Ou-Mandel (HOM) interferometer. It is shown that the technique is able to distinguish a partially polarized photon where the polarization state is coupled to an internal degree of freedom, such as time of arrival, from partial polarization due to external entanglement with the environment. The method does not directly resort to any kind of polarization-selective components and therefore is not limited by the extinction ratio of polarizers. Moreover, the technique can be generalized to any two-level encoding of quantum information in single photons, such as time-bin or orbital angular momentum qubits.

Feedback control can be used to generate well-determined nonlinear effective potentials in an optical trap, a goal whose applications may range from non-equilibrium thermodynamics to the generation of non-Gaussian states of mechanical motion. Here, we investigate the action of an effective feedback-generated quartic potential on a levitated nanoparticle within the perturbation regime. The effects of feedback delay are discussed and predictions from the perturbation theory of a Brownian particle subjected to a quartic anharmonicity are experimentally verified.

In the "Beyond Moore's Law" era, with increasing edge intelligence, domain-specific computing embracing unconventional approaches will become increasingly prevalent. At the same time, adopting a variety of nanotechnologies will offer benefits in energy cost, computational speed, reduced footprint, cyber resilience, and processing power. The time is ripe for a roadmap for unconventional computing with nanotechnologies to guide future research, and this collection aims to fill that need. The authors provide a comprehensive roadmap for neuromorphic computing using electron spins, memristive devices, two-dimensional nanomaterials, nanomagnets, and various dynamical systems. They also address other paradigms such as Ising machines, Bayesian inference engines, probabilistic computing with p-bits, processing in memory, quantum memories and algorithms, computing with skyrmions and spin waves, and brain-inspired computing for incremental learning and problem-solving in severely resource-constrained environments. These approaches have advantages over traditional Boolean computing based on von Neumann architecture. As the computational requirements for artificial intelligence grow 50 times faster than Moore's Law for electronics, more unconventional approaches to computing and signal processing will appear on the horizon, and this roadmap will help identify future needs and challenges. In a very fertile field, experts in the field aim to present some of the dominant and most promising technologies for unconventional computing that will be around for some time to come. Within a holistic approach, the goal is to provide pathways for solidifying the field and guiding future impactful discoveries., Comment: 80 pages accepted in Nano Futures

Quantum Computing has been presenting major developments in the last few years, unveiling systems with a increasing number of qubits. However, unreliable quantum processes in universal quantum computers still represent one of the the greatest challenges to be overcome. Such obstacle has its source on noisy operations and interactions with the environment which introduce decoherence to a quantum system. In this article we verify whether a tool called Swap Test is able to identify decoherence. Our experimental results demonstrate that the Swap Test can be employed as an alternative to a full Quantum Process Tomography, with the advantage of not destroying the qubit under test, under certain circumstances, as long as some modifications are introduced.

Optical tweezers find applications in various fields, ranging from biology to physics. One of the fundamental steps necessary to perform quantitative measurements using trapped particles is the calibration of the tweezer's spring constant. This can be done through power spectral density analysis, from forward scattering detection of the particle's position. In this work, we propose and experimentally test simplifications to such measurement procedure, aimed at reducing post-processing of recorded data and dealing with acquisition devices that have frequency-dependent electronic noise. In the same line of simplifying the tweezer setup, we also present a knife-edge detection scheme that can substitute standard position-sensitive detectors.

Depolarization is one of the most important sources of error in a quantum communication link that can be introduced by the quantum channel. Even though standard quantum process tomography can, in theory, be applied to characterize this effect, in most real-world implementations depolarization cannot be distinguished from time-varying unitary transformations, especially when the time scales are much shorter than the detectors response time. In this paper, we introduce a method for distinguishing true depolarization from fast polarization rotations by employing Hong-Ou-Mandel interference. It is shown that the results are independent of the timing resolutions of the photodetectors., Comment: 9 pages, 4 figures

Traditional methods for measurement of Polarizing Beamsplitter (PBS) parameters, especially the extinction ratio, require highly polarized light sources, alignment procedures and/or experimental parameters that change over time, such as polarization rotations. In this work, a new method is presented, which employs unpolarized light and a Faraday Mirror. It is shown that precise extinction ratio and insertion loss values can be achieved in three single-sweep measurements, without any alignment requirements or time-varying signals of any kind., Comment: 6 pages, 3 figures

The complementarity relation between the visibility and the spectral distinguishability of frequency-displaced photonic wave-packets in a Hong-Ou-Mandel interferometer is studied. An experimental definition of $K$, the distinguishability parameter, is proposed and tested for the $K^2+\mathcal{V}^2\leq 1$ complementarity inequality when a consistent visibility parameter is defined. The results show that the spectral distinguishability is, indeed, complementary to the visibility and that the quantum aspect of the two-photon interference phenomenon can be examined by employing weak-coherent states., Comment: 8 pages, 6 figures, submitted to Optics Letters

We present a complete optoelectronic unit for polarization visualization, switching and control. The system is based on an FPGA unit and comprises: an acquisition unit containing an analog polarimeter and digital-to-analog converters, an FPGA capable of implementing an optimal algorithm for three-stage arbitrary polarization tracking, and an electronic driver with analog-to-digital converters capable of interfacing with Lithium-Niobate-based Polarization Controllers. The results, determined via simulation of real-parameter devices, show that fast polarization switch is achievable., Comment: 8 pages, 7 figures

We perform a high resolution Fourier Transform Spectroscopy of optical sources in the few-photon regime based on the phenomenon of two-photon interference in a beam splitter. From the heterodyne interferogram between test and reference sources it is possible to obtain the spectrum of the test source relative to that of the reference. The method proves to be a useful asset for spectral characterization of faint optical sources below the range covered by classical heterodyne beating techniques., Comment: 8 pages, 4 figures, to appear in Optics Letters, Vol. 41, No. 7 (2016)

We demonstrate a method for characterizing the coherence function of coherent states based on two-photon interference. Two states from frequency mismatched faint laser sources are fed into a Hong-Ou-Mandel interferometer and the interference pattern is fitted with the presented theoretical model for the quantum beat. The fitting parameters are compared to the classical optical beat when bright versions of the sources are used. The results show the equivalence between both techniques., Comment: 5 pages, 5 figures

We present a Heralded Photon Source based only on linear optics and weak coherent states. By time-tuning a Hong-Ou-Mandel interferometer fed with frequency-displaced coherent states, the output photons can be synchronously heralded following sub-Poisson statistics, which is indicated by the second-order correlation function ($g^2\left(0\right)=0.556$). The absence of phase-matching restrictions makes the source widely tunable, with 100-nm spectral tunability on the telecom bands. The technique presents yield comparable to state-of-the-art spontaneous parametric down-conversion-based sources, with high coherence and fiber-optic quantum communication compatibility., Comment: 9 pages, 7 figures

In this paper we analyze the impact of the spontaneous Raman scattered noise generated from multiple optical classical channels on a single quantum key distribution channel, all within the telecom C-band. We experimentally measure the noise generated from up to 14 continuous lasers with different wavelengths using the dense wavelength division multiplexing (DWDM) standard, in both propagation directions in respect to the QKD channel, over different standard SMF-28 fiber lengths. We then simulate the expected secure key generation rate for a decoy-states-based system as a function of distance under the presence of simultaneous telecom traffic with different modulation techniques, and show a severe penalty growing with the number of classical channels present. Our results show that, for in-band coexistence, the telecom channels should be distributed as close as possible from the quantum channel to avoid the Raman noise peaks. Operation far from the zero dispersion wavelength of the fiber is also beneficial as it greatly reduces the generation of four-wave mixing inside the quantum channel. Furthermore, narrow spectral filtering on the quantum channels is required due to the harsh limitations of performing QKD under real telecom environments, with the quantum and several classical channels coexisting in the same ITU-T C-band., Comment: Typos were fixed in a couple of equations which do not affect the results in the paper

We propose and experimentally demonstrate a scheme to render the detection apparatus of a Quantum Key Distribution system immune to the main classes of hacking attacks in which the eavesdropper explores the back-door opened by the single-photon detectors. The countermeasure is based on the creation of modes that are not deterministically accessible to the eavesdropper. We experimentally show that the use of beamsplitters and extra single-photon detectors at the receiver station passively creates randomized spatial modes that erase any knowledge the eavesdropper might have gained when using bright-light faked states. Additionally, we experimentally show a detector-scrambling approach where the random selection of the detector used for each measurement - equivalent to an active spatial mode randomization - hashes out the side-channel open by the detection efficiency mismatch-based attacks. The proposed combined countermeasure represents a practical and readily implementable solution against the main classes of quantum hacking attacks aimed on the single-photon detector so far, without intervening on the inner working of the devices., Comment: Accepted for publication in the Special Issue on Quantum Communication and Cryptography of the IEEE Journal of Selected Topics in Quantum Electronics (JSTQE)

According to quantum theory, two ensembles of quantum systems that are described by the same density operator are indistinguishable. For example, unpolarized light can be obtained either by an incoherent mixture of two orthogonal pure states or by tracing out a photon from a maximally polarization-entangled photon pair. In both cases, one is unable to guess with probability greater than 50% the outcome of any polarization measurement, but the reasons are conceptually different: whereas the first case is a matter of classical ignorance (the photons were prepared in a definite but unknown way), the second one is of the quantum ignorance kind - if one cannot access the other degrees of freedom of the quantum state, there is no information that could be used to predict a measurement result. We use these concepts to discuss the quantum-physical interpretation of partially polarized light from the point of view of quantum communication and whether a polarization scrambler can really depolarize light. A novel definition of the degree of polarization of a single photon which does not depend on classical ignorance is also provided., Comment: 6 pages, 2 figures

We report a procedure to detect mid-infrared single photons at 4.65 um via a two-stage scheme based on Sum Frequency Generation, using a Periodically Poled Lithium Niobate (PPLN) nonlinear crystal and a Silicon Avalanche Photodiode. An experimental investigation shows that, in addition to a high timing resolution, this technique yields a detection sensitivity of 1.24 pW with 63mW of net pump power., Comment: 9 pages, 2 figures, submitted to Optics Letters