Plenary

Quantum-Information Processing with Single-Photon Emitters

Semiconductor quantum dots have improved their optical performance dramatically in recent years, and today a clear pathway is laid out for constructing a deterministic and coherent photon-emitter interface by embedding quantum dots in photonic nanostructures [1]. Such an interface can be employed as an on-demand single-photon source for quantum-information applications, but more generally enables single-photon nonlinearities and deterministic quantum gates [2]. We will review the recent experimental progress on quantum dots coupled to nanophotonic waveguides and cavities enabling unique ways of engineering light-matter interaction. A single-photon coupling efficiency exceeding 98.4% is reported [3] and the indistinguishability of the emitted photons is extracted [4] and the fundamental limits exploited [5]. Furthermore, various out-coupling strategies for efficiently transferring single photons to an optical fiber are implemented [6]. The unique engineering potential of the nanophotonic waveguides is demonstrated by implementing a chiral quantum interface [7,8]. Finally, the experimental demonstration of a photonic switched controlled by a single spin coupled to a waveguide is discussed [9].

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Invited talk

Encoding a logical qubit in a trapped-ion oscillator

Quantum error correction requires storing qubits in systems with enlarged Hilbert spaces, on which measurements can be made without disturbing the stored information. While much focus is placed on implementing this in qubit arrays, such a space is also available in single harmonic oscillators. An oscillator codes which is optimal for many of the most prominent errors involves basis states which are a periodic array of squeezed states[1,2]. I will describe experiments in which we create, measure and manipulate logical information stored in such a code using the oscillatory motion of a single trapped atomic ion. These elements are implemented using a laser which couples the motion to an ancilliary pseudo-spin qubit stored in the internal electronic ion, which can be read out in a single shot with high fidelity. The sequence of spin-motion coupling and state readout allows us to measure modular variables of the position and momentum of the oscillator state, which is used for both the logical and stabilizer operator measurements of the code [3]. We demonstrate preparation of the cardinal states on the encoded Bloch sphere with a fidelity of 85%, and perform process tomography to characterize operations on the encoded qubit. The states and techniques developed in this work may be applied to sensing as well as for early stage studies of quantum error-correcting codes.

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Single photon Fock state ring down spectroscopy in bulk diamond

We present a new technique combining ultrafast Raman spectroscopy and time-correlated photon counting to measure the ring down time of a single phonon Fock state. We demonstrate our scheme with a bulk diamond sample, in which the optical phonon has a frequency close to 40 THz. Our technique and our setup are designed to be applicable to a wide range of material systems and can be used for phonon-sideband excitation in the presence of internal resonances or external cavities.

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Invited talk

Fundamental test of Quantum Mechanics in Space using Quantum Communications

Quantum states with a positive partial transpose are useful for metrology 

We show that multipartite quantum states that have a positive partial transpose with respect to all bipartitions of the particles can outperform separable states in linear interferometers. We introduce an iterative method to find such states. We present some examples for multipartite states and examine the scaling of the precision with the particle number. Some bipartite examples are also shown that possess an entanglement very robust to noise. We also discuss the relation of metrological usefulness to Bell inequality violation.

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Unconditional violation of shot-noise limit with two photon NOON states 

We demonstrate the first unconditional violation of the shot-noise limit in photonic NOON-state interferometry. Using ultrahigh-efficiency spontaneous downconversion photon-pair source and superconducting nanowire photon detectors we outperform ideal classical phase sensing measurement without employing postselection, or correction for loss and imperfections

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Invited talk 

Quantum control for quantum technologies: Tools, achievements, limitations

Quantum control is an important prerequisite for quantum devices. A major obstacle is the fact that a quantum system can never completely be isolated from its environment, and the interaction with the environment causes decoherence. Optimal control theory is a tool that can be used to identify control strategies in the presence of decoherence. I will show how to adapt optimal control theory to quantum information tasks for open quantum systems [1] and present examples for superconducting qubits [2,3]. The perspective on decoherence only as the adversary of quantum control is nevertheless too narrow. There exist a number of control tasks, such as cooling and measurement, that can only be achieved by an interplay of control and dissipation. I will show how to utilize optimal control theory to derive efficient cooling strategies when the timescales of coherent dynamics and dissipation are very different [1]. Our approach can be generalized to quantum reservoir engineering, opening up new avenues for control.

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Invited talk

Electro-optic temporal optical systems for spectral shaping of quantum light

Hallmarking quantum states: unified framework for coherence and correlations

Quantum coherence and correlations among subparties are often seen as separate properties of a quantum state. We propose a measure able to quantify both  the contribution derived by he tensor structure of the multipartite Hilbert space and the presence of coherence inside each of the subparties. Global coherence is responsible for the presence of distributed quantum correlations. Our framework also provides a simple physical interpretation of the  difference between total correlations and the sum of classical and quantum correlations obtained using relative-entropy--based quantifiers.

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Structure of the resource theory of quantum coherence

We provide a rigorous study of the recently established resource theory of coherence by finding minimal representation for incoherent  operations. This class is an important example for free operations within the coherence framework. We show that any incoherent operation on a single qubit can be implemented with at most 5 incoherent Kraus operators. The potential of these results is demonstrated by giving a full solution for the mixed-state conversion problem via incoherent operations on a single qubit. We also introduce Gibbs-preserving strictly incoherent operations and solve the mixed-state conversion problem for a single qubit for this class as well.

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Invited talk 

Dynamical and antidynamical Casimir effects via controlled artificial atoms

Circuit quantum electrodynamics (circuit QED) offers unprecedented possibilities to manipulate in situ the properties of mesoscopic systems composed of superconducting artificial atoms interacting with the electromagnetic field inside the waveguide resonator on a chip [1]. This solid-state architecture thus allows for the experimental study of some of the most fundamental physical processes, such as the lightmatter interaction at the level of a few photons. One example is the implementation of the dynamical, and antidynamical, Casimir effect and associated phenomena using actively controlled artificial atoms, which may serve both as the source and as the detector of modulation-induced radiation [2]. The broad term dynamical Casimir effect (DCE) refers to the generation from vacuum of excitations of some field (electromagnetic, in the majority of cases) due to time dependent boundary conditions, such as changes in the geometry or material properties of the system [3]. The antidynamical Casimir effect (ADCE) instead is a term coined to designate the coherent annihilation of excitations due to resonant external perturbation of system parameters, allowing for extraction of quantum work from nonvacuum states of some field [4]. Here we consider the dissipative singlequbit circuit QED architecture in which the atomic transition frequency undergoes a weak external time modulation. For sinusoidal modulation with linearly varying frequency we derive effective Hamiltonians that resemble the Landau-Zener problem of finite duration associated with a two or multilevel systems. The corresponding off-diagonal coupling coefficients originate either from the rotating or the counter-rotating terms in the Rabi Hamiltonian, depending on the values of the modulation frequency. We demonstrate that under this condition photon generation from vacuum via effective Landau-Zener transitions could be implemented with the current technology on the time scales of a few microseconds[4]. Unlike the harmonic dynamical Casimir effect implementations, our proposal does not require precise knowledge of the resonant modulation frequency to accomplish meaningful photon generation. Photon generation is not the only phenomenon induced by parametric modulations in circuit QED. Recently indeed it was shown that the counter-rotating terms can also be employed to annihilate excitations of the electromagnetic field from nonvacuum initial states (ADCE) [5]. We here demonstrate, analytically and numerically, that the ADCE rate can be increased of, at least, one order of magnitude, by replacing the qubit by an artificial three-level atom (qutrit) in a properly chosen configuration [6].

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Input-output for spin-photon coupling in Si double quantum dots

The interaction of qubits via microwave frequency photons enables long-distance qubit-qubit coupling and facilitates the realization of a large-scale quantum processor. However, qubits based on electron spins in semiconductor quantum dots have proven challenging to couple to microwave photons. In this work [1] we show that a sizable coupling for a single electron spin is possible via spin-charge hybridization using a magnetic field gradient in a silicon double quantum dot. Our predictions are in good agreement with recent measurements of strong spin-photon coupling rates of more than 10 MHz.

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Multidimensional entanglement and EPR-steering with atomic quantum memory 

We experimentally generate and characterize high-dimensional entangled and EPR-steerable state of single photon and collective atomic excitation stored in cold atoms quantum memory [1]. The high-dimensional character relies on a wide space of angular emission modes and the conjugate position-space of the atomic ensemble. The EPR-steering is demonstrated for the first time using single-photon resolving camera without accidentals subtraction, which offers parallel access to all coincidences. Furthermore, we show that entropic witness vastly outperforms the variance-based witness for the emerging domain of high-dimensional entanglement, even in the presence of highly undersampled data.

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A logarithmic optical detector for single photons and bright light

We demonstrate an optical pulse energy detector with a dynamic range of 140 dB. This allows us to perform direct detector calibration, and test the non-classicality of states of light with a single device. To do so, we exploit the saturation region of a time-multiplexed loop architecture coupled to a superconducting nanowire single photon detector (SNSPD). With this device, light levels from the noise floor of the SNSPD (far below the single photon level) to bright states (hundreds of nW) can be measured in a reliable way, while retaining high quantum efficiency in the low photon number regime. 

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Experimental demonstration of inhibited-coupling hollow-core fibers for shaping photon-pair time-frequency correlations 

We experimentally show how the multiband dispersion properties of gas-filled inhibited-coupling hollow-core fibers allow to control the spectral correlations of photon pairs generated through four-wave-mixing. The measured joint spectral intensity of two different fibers are shown to exhibit respectively a factorable state and a highly correlated state. Our simulations predict that very different Schmidt decomposition can be obtained in the same fiber by changing the gas pressure or pump properties. Such control can be useful to create a versatile photon pair source targeting both applications using factorable state (heralded single photon) and correlated states (spectral entanglement or QKD).

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8.9Gb/s real-time quantum random numbers with verified security

We present a 8.9Gbit/s real-time implementation of vacuum fluctuation based quantum random number generator. A fully quantum secure proof for the QRNG model is provided which takes into account finite-bandwidth effects of a realistic device. A metrological grade characterization of the detector completes the security verification. We implemented a Toeplitz extractor in a high performance FPGA chip. By using the parallel processing feature of the chip, we achieved high extraction efficiency.

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Organic molecules for integrated quantum photonics

Organic molecules of polyaromatic hydrocarbons were the first system in the solid state to show single photon emission. However they are still considered unconventional sources of non-classical light. I will try to show how they could effectively contribute to integrated quantum photonic platforms, discussing two of our recent experiments. In particular I will report on fluorescence coupling from a single molecule to a single-mode dielectric waveguide with 40% coupling efficiency. Next, I will present our recent results about the fabrication of single-molecule doped nancrystals, preserving the optical properties of the bulk system, i.e. negligible blinking and spectral diffusion.

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Towards hybrid systems of ultracold atoms and ions

Ultracold atoms and trapped ions have proven to be extremely valuable resources for getting insights on fundamental physical phenomena. Combining the strengths of the two can lead to advances in quantum simulations. We present an overview on ideas for experiments with Ba+ ions and Li atoms. We motivate the choice of mixture by presenting its advantages and the experimental challenges in reaching a long-lived coherent coupling among atoms and ions. We describe how ions can be used as impurities as well as localized probes in a Fermi gas, permitting novel studies of out-of equilibrium phenomena like the Anderson orthogonality catastrophe.

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High-density alkali vapours: a new platform for ensemble quantum optics 

In most atomic ensemble quantum optics, optical depth and coherence time cannot be simultaneously optimized; collisional decoherence imposes a trade-off between these two critical figures of merit. This trade-off can be avoided by working in the so-called SERF regime, at high densities low field strengths, with potentially large advantages for quantum technologies. We describe the first tests of quantum correlations, e.g., squeezing, in SERF systems, and experimental progress toward their use as quantum technology components.

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Plenary

Quantum optomechanics with levitated solids: sensing, simulation and the gravity-quantum interface

I will discuss our current experiments to achieve quantum optical control over motional states of levitated solids. This includes dielectric nanospheres coupled to Fabry-Perot cavities and nanophotonic structures, as well as micron-sized superconductors and magnets that will eventually be coupled to superconducting circuits. I will provide an overview of the status and challenges, and of the perspectives for such experiments for sensing, simulation and for novel tests of the interface between quantum physics and gravity.

Invited talk 

Seeing A Single Atom Where It Is Not

The precise determination of the position of sub-wavelength scale emitters and scatterers using far-field optical imaging techniques is of utmost importance for a wide range of applications in medicine, biology, astronomy, and physics. In this talk, I theoretically and experimentally show that, for a standard optical imaging system like an optical microscope, the image of an elliptically polarized point-like emitter does not coincide with the emitter's real position. Instead, even for perfect, aberration-free imaging with high numerical aperture, the image will in general be shifted. This can lead to a systematic error in the optical localization of emitters which exceeds the typical precision of super-localization microscopes by far. Moreover, for the case of small numerical aperture, the shift can in principle reach arbitrarily large values. Imaging a single trapped atom as well as a single gold nanoparticle, we experimentally demonstrate this effect and observe wavelength-scale shifts. Beyond its relevance for optical imaging, the demonstrated phenomenon may also occur for sources of other types of waves. Consequently, it can, e.g., impede the precision of the localization of remote objects with imaging radar and sonar as well as the future localization of stellar objects in gravitational wave astronomy.

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Thermometry of a single ion by high-resolution imaging

We measure the temperature of a single ion in a radio-frequency trap by imaging its resonance-fluorescence photons onto a camera. Our measurement scheme benefits from collecting the fluorescence photons with a deep parabolic mirror, resulting in a high imaging resolution as well as in a large signal-to-noise ratio. Our setup offers the potential to measure temperatures well below the Doppler limit. We furthermore determine the anomalous heating rate for our ion trap. Thanks to the properties of our measurement setup this can be done without heating the ion.

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Universal measurement-based quantum computation with opto-mechanical systems

We consider dissipative opto- and electro-mechanical quantum systems composed of a driven cavity mode interacting with a set of mechanical oscillators. We show that this set-up is rich enough to allow for universal continuous-variable measurement-based quantum computation. In particular, we propose a multi-tone driving scheme to generate mechanical cluster states. Then, we show that, measuring only the field leaking from the cavity, it is possible to realize the measurements needed to perform arbitrary Gaussian operations. Finally, we propose a scheme to prepare a mechanical cubic phase state, which is a resource enabling universal computation.

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Invited talk

Is coherence catalytic?

Quantum coherence, the ability to control the phases in superposition states is a resource, and it is of crucial importance, therefore, to understand how it is consumed in use. It has been suggested that catalytic coherence is possible, that is repeated use of the coherence without degradation or reduction in performance. The claim has particular relevance for quantum thermodynamics because, were it true, it would allow free energy that is locked in coherence to be extracted indenitely. We address this issue directly with a careful analysis of the proposal by Aberg [1]. We nd that coherence cannot be used catalytically, or even repeatedly without limit. Here we present an analysis of a proposal by Aberg that coherence is catalytic [1] or, perhaps more accurately, that it is a resource that can be used repeatedly without degradation in performance [2]. We ask, specically, whether the coherence in Aberg's proposal is indeed catalytic or repeatable and show that it is neither. Quantum coherence is a strictly nite resource. Repeated use inevitably degrades and ultimately consumes it. Once eliminated the residual coherence source performs no better than one prepared randomly. In the Aberg proposal this is reflected in the complete destruction of reservoir coherence following a single and ultimately inevitable error in the transfer of the phase reference to a qubit.

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Thermodynamic of trajectories for quantum harmonic networks

Describing the interaction of a quantum system with its environment is one of the key goals of modern quantum physics. Recently, a promising approach came to light, combining the quantum master equation formalism and large-deviation theory. It unravels the exchange statistic taking place between any quantum system governed by Lindblad master equation and its environment. I will present how joining this approach to quantum optics method the exchange statistics can be exactly retrieved in a context of quantum harmonic oscillator network governed by a quadratic master equation giving access. This allows determining anti-bunching features but also fluctuation theorems.

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Weak measurements and new perspectives in quantum measurement

Quantum measurements in weak coupling regime provide new interesting perspectives of significant applications both conceptual and practical. Here we present several experimental achievements exploiting weak values (ranging from sequential weak values to the connection of weak values and contextuality), and the first realization of protective measurements.  

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Invited talk 

Energetic and entropic footprints of quantum noise

Despite its strategic interest for the development of scalable computing architectures, the question of the energetic cost of quantum information processing is still in its infancy. Such energetic cost is expected to be tightly related to the amount of quantum noise that should be overcome to perform the computation. A proper framework to conduct these investigations is provided by stochastic thermodynamics, which has analyzed for years the fundamental relations existing between energy, information, and noise. It is a major challenge of quantum thermodynamics to extend these relations in the quantum realm where noise is of purely quantum origin, e.g. stems from quantum measurement and decoherence. In this talk we will present a new framework for quantum stochastic thermodynamics, that ultimately aims at answering these questions [1,2]. After recalling the general tools of stochastic thermodynamics, we will present the new concept of quantum heat. Quantum heat corresponds to the energetic fluctuations experienced by a quantum system, that are induced by measurement back-action and decoherence. We show that quantum heat provides the proper energy scale to estimate the cost of quantum control and feedback [2]. Finally, we evidence that quantum heat can become a resource in new kinds of genuinely quantum engines, extracting work from quantum measurement [1].

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Double poster exhibition with a complete food truck menu which includes:

- Salmorejo

-Burger with arugula and Mahonese cheese

- Gató with almonds or tiramisu foam

- Soft drink, water or beer

Bars of the building will serve coffee to the participants who show the CEWQO 2018 badge.

Click here to download the poster session 1 PDF

Click here to download the poster session 2 PDF

Invited talk

Theory on the optical spin-polarization loop of the nitrogen-vacancy center in diamond

Dopants in solids are promising candidates for implementations of quantum bits. In particular, the high-spin negatively charged nitrogen-vacancy defect (NV) in diamond has become a leading contender in solid-state quantum information processing1{4. The initialization and readout of the spin is based on the spin-selective decay of the photo-excited electron to the ground state which is mediated by spin-orbit coupling between excited states states and phonons, i.e. intersystem crossing (ISC). The operation of NV center is based on the ISC processes but still the microscopic theory on these processes has not been fully developed. Understanding the ISC processes might lead to an improved control of NV qubit. We will show that strong coupling between electrons and phonons in the excited state can naturally explain the observed multiple ISC rates in the excited state branch5. Furthermore, we will present a theory including electronphonon coupling and correlated electron states from ab initio calculations that accounts for the optical spinpolarization of NV center and also explains the absorption and photoluminescence spectra of the singlets. Our nding completes the theory of the loop of the optical spinpolarization cycle6.

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Cavity-Enhanced Nitrogen-Vacancy Magnetometry 

We report on the combination of a diamond sample and an optical cavity resonant with the pump field of nitrogen-vacancy centers. This combination allows us to reach an enhanced magnetic-field sensitivity and enables sensing by recording the remaining pump light level.

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Excitation transfer by quantum walks on disrupted carbon nanotube structures

We study an asymptotic transfer of an excitation on disrupted graphs representing carbon nanotubes. The dynamics of this open quantum system si described by a quantum walk with dynamical percolation. First, we present an analytical approach for determining transfer probabilites from one end of the tube to a sink placed on the other end. Then we combine it with a new approach for numerical simulations of quantum walks with percolation allowing for verification and further research on these large graphs.

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Superposition principle of spin-1/2 states in quantum suprematism representation

The superposition of two pure spin-1/2 states is expressed in terms of the probability addition rule for probabilities associated with the positions of three classical coins determining the density matrix of a pure qubit state. The combination of Triadas of Malevich's squares illustrating the nonlinear superposition rules for the classical coin probabilities in the quantum suprematism representation of the two-level atom states is discussed.

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Fock-State Superradiance in a Quantum Memory 

We present a theoretical and experimental study of superradiant effects on the extraction of information stored in a quantum memory. In our experiments, a quantum memory may contain one or two excitations of a collective atomic state, which are mapped into a Fock state of light with one or two photons during the reading process. We experimentally verified that, due to superradiance, the photon emission rate increases linearly with the number of atoms of the quantum memory, as theoretically predicted. We also verified that in the two excitations case both photons are emitted in the same spatiotemporal mode.

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A novel integrated platform for quantum storage of heralded single photons

Quantum memories, providing an interface between flying and stationary qubits, represent an essential ingredient towards the realization of a quantum internet. We propose a new platform for integrated optical memories. A laser-written waveguide is inscribed in a Pr3+-doped crystal. We demonstrate that the spectral properties of the ions are maintained and that the optical coherence remains in the same order of magnitude of the bulk crystal even after the fabrication procedure. We then perform storage of heralded single photons, thus proving that this platform works at the quantum regime and showing the longest single photon quantum storage in waveguide.

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Rambla, Major square, modernists buildings, patios, City Hall, Cathedral and Almudaina palace, Parc de la Mar, Born. The route will end nearby Sa Llotja, where you will find different options to have dinner.

Plenary

Quantum optics and information science in multi-dimensional photonics networks

Invited talk

Tailored Non-Gaussian Multimode States

In an all-optical setting, there are various approaches to quantum information protocols, often classified according to the way information is encoded and measured. The discrete variable approach relates to single photon or photon number resolving detectors, while the continuous variable approach (CV) implies homodyne detection to access the quadratures of the electromagnetic field. The major advantage of the latter is the deterministic generation of quantum resources, e.g., entanglement between up to millions of modes [1]. Such multimode entangled states, however, remain Gaussian, which implies that their CV properties can be simulated using classical computational resources. Hence, if a quantum information protocol is to manifest a quantum advantage, it requires non-Gaussian operations, which can be implemented incorporating single-photon addition or subtraction to a purely CV scheme [2]. In order to scale up CV quantum information approach it is then necessary to combine de-Gaussification techniques to multimode squeezing sources. Using a source of multimode squeezed states based on parametric down conversion of an optical frequency comb [3], we implement photon subtraction on a coherent superposition of time/frequency modes [4]. This is achieve using a sum-frequency process between the quantum source and an intenses gate beam. Full control on the gate beam time/frequency modes governs the modal decomposition of the process. Single photon detection on the sum-frequency beam heralds the generation of a multimode non-gaussian beam. State tomography is performed via homodyne detection, where the local oscillator beam is pulse shaped in order to achieve mode-dependent quadrature measurement. We demonstrate that negative Wigner function states can be obtained, choosing which of the eigenmodes of the quantum source is affected by the de-gaussification (see figure). Furthermore, in this multimode scenario, mode dependent detection allow for the generation of graph-states [5]. We study the effect of coherent photon subtraction onto these graph states [6, 7] and demonstrate experimentally that the non-gaussian characters induced by photon-subtraction spreads along the graph state, but that the spread is fundamentally limited by the nature of these states. This can serve as a guide to tailor large-scale non-Gaussian states for quantum information processing.

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Entangling macroscopic light states via delocalized single photon addition 

We present the experimental generation of entanglement between two distinct field modes by the delocalized addition of a single photon. We show that, in principle, one can preserve a high degree of entanglement even between macroscopically populated modes and illustrate this concept by adding a single photon to two modes containing coherent states of growing amplitude.

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Invited talk

Observation of the photon-blockade breakdown phase transition

Nonequilibrium phase transitions exist in damped-driven open quantum systems when the continuous tuning of an external parameter leads to a transition between two robust steady states. In second-order transitions this change is abrupt at a critical point, whereas in rst-order transitions the two phases can coexist in a critical hysteresis domain. We discuss the photon-blockade-breakdown phase transition in the case of the driven Jaynes-Cummings model for very strong coupling between the two-level atom (A) and the single radiation mode of a cavity (C). This phase transition has no continuous connection to the well-known semiclassical optical bistability. Nevertheless, the signature is a bistability of the strongly driven quantum system [1, 2], which appears as an unexpected peak in the transmission spectrum (see Fig. 1). Study on the time scales and the thermodynamic limit of this rst-order phase transition are presented. We report on the observation of an analogous rst-order dissipative quantum phase transition in a driven circuit quantum electrodynamics system. The two-level atom in the Jaynes{Cummings model is replaced by articial atoms, so-called transmon qubits. The observed experimental signature is the sharp peak in the transmission spectrum (Fig. 2), clearly indicating the breakdown of the photon blockade. Additionally, this experiment demonstrates the co-existence of the two stable attractors, both of which can be associated with a robust quasi-classical state. The measurement resolves nicely the continuously varying weights in the bimodal phase space distribution as the control parameter of the phase transition, i.e., the drive strength is scanned through the bistability range. We point out the signicance of the level structure of the articial atom that is coupled to the microwave stripline resonator mode.

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Invited talk

Non-Markovian Unruh effect

We study memory effects as information backflow for an accelerating two-level detector weakly interacting with a scalar field in the Minkowski vacuum. This is the framework of the well-known Unruh effect: the detector behaves as if it were in a thermal bath with a temperature proportional to its acceleration. Here we show that, if we release the usual assumption of an eternally uniformly accelerating system, and we instead consider the more realistic case in which a finite-size detector starts accelerating at a certain time, its dynamics may become non-Markovian. Our results are the first description of a relativistic quantum system in terms of information back-flow and non-Markovianity, and they show the existence of a direct link between the trajectory of the detector in Minkowski space and the presence or absence of memory effects.

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Precision measurements, entanglement and squeezing with continuous-variable systems

The Fisher information quantifies the sensitivity of a quantum state with respect to changes of a parameter and defines the precision limits for applications in interferometry and sensing. We present upper limits for the sensitivity of separable states which can be applied to continuous-variable systems, leading to an experimentally usable witness for mode entanglement. We further discuss approximate expressions in terms of the covariance matrix, which are especially convenient for Gaussian states. Finally, we discuss quantum enhancements in single-mode displacement sensors and present a phase-insensitive scheme for precision measurements of a displacement amplitude using Fock states.

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Continuous-Variable Sampling from Photon-Added or Photon-Subtracted Squeezed States

We introduce a new family of quantum circuits in continuous variables and we address the corresponding sampling problem. We show that, relying on the widely accepted conjecture that the polynomial hierarchy does not collapse, their output probability distribution cannot be efficiently simulated by a classical computer. These circuits are composed of input photon-subtracted (or photon-added) squeezed states, passive linear optics evolution, and eight-port homodyne detection. We obtain both a worst-case and an average-case hardness result. Hardness of Boson Sampling with eight-port homodyne detection is obtained as the zero squeezing limit of our model.

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Invited talk

Quantized Angular Momentum in Topological Optical Systems

Topological materials and topological effects have elicited significant interest, first in the condensed-matter community [1-3] and more recently in the photonics community [4-10]. The only systems with a robust topological protection in optics are those with a broken-time reversal symmetry [11, 12]. The topological phases of such systems are usually classified by a topological integer: the Chern number. In condensed-matter systems, the Chern number has a clear physical interpretation: it is the quantum of the Hall conductivity of a 2D electron gas [2, 13], and hence it determines the electronic transport for very low temperatures. In contrast, in optics the Chern index has not been linked to any physical entity, except that it is known that it gives the net number of gapless unidirectional edge states supported by an interface with a trivial material. In this talk, I will show that the photonic Chern number is the quantum of the light-angular momentum in a closed photonic insulator cavity. I prove that for a sufficiently large cavity, the spectral density of the quantum or thermal fluctuation-induced angular momentum is precisely quantized in the band-gaps of the bulk modes. The nontrivial value the light angular momentum expectation is due to a circulation of a heat current in closed orbits, which may occur even when a system is in a thermodynamic equilibrium with a large reservoir [14, 15, 16]. Interestingly, the proposed theory also applies to systems without a topological classification, and in such a context the angular momentum “quantum” is given by the net number of unidirectional edge modes supported by the cavity walls [17,18].

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Quantum spookiness at zero distance

Quantum contextuality generalizes Bell experiments to systems without space-like separation. I will present two experiments in which demonstrate such non-classical correlations in a local system - a single trapped-ion qutrit. In the first experiment, we study the limits of correlations available to quantum systems. We not only violate the most contextuality inequality with the largest quantum-classical gap (KCBS), but also reach the quantum bound of the available correlations. In the second experiment, we study the state-independent Yu-Oh inequality. We generate sustained non-classical correlations by performing over 50 million measurements in one sequence.

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High-fidelity entanglement between a trapped ion and a telecom photon via quantum frequency conversion 

We present a complete device that produces entangled states between an atomic Zeeman qubit in a single trapped 40Ca+ ion and the polarization state of a telecom photon with a Bell-state fidelity of 98.2+/-0.2%. We achieve this by combining a trapped-ion quantum node producing ion-photon entanglement with a fidelity of 98.3+/-0.3% and a polarization-preserving frequency converter connecting 854 nm to the telecom O-band. The converter, realized by difference-frequency generation in a PPLN waveguide embedded in single-crystal Mach-Zehnder-interferometer, combines 99.75+/-0.18% process fidelity for the polarization-state conversion, 26.5% external conversion efficiency and 11.4 photons/s conversion-induced unconditional background.

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Informational Approach To Multiparametrical Quantum Estimation: Superresolving Imaging With Higher-Order Correlations 

We present an informational approach to multi-parametrical quantum state/process estimation under the condition of parametrical locality, when the result of a particular measurement is dependent only on a particular limited subset of parameters. We show that “sliding window” approach can be developed both for linear and nonlinear estimation problems, and allows for drastic simplification of the parameter inference procedure. The complexity of the developed method can be linear on the total number of parameters. We apply the reconstruction procedure to a practically important example of quantum near-field imaging with high-order correlation functions and show resolution increase beyond empirical "Abbe limit".

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Golden Ratio entanglement in hexagonally poled nonlinear crystals 

We analyse the quantum state generated by parametric-down conversion in a hexagonally poled nonlinear photonic  crystal [1].  By proper angle tuning,  we realize a  peculiar 4-mode entanglement, where the overall process is equivalent to i) two independent parametric processes, with unbalanced gains gϕ and g/ϕ, where ϕ=(1+√5)/2 is the famous Golden Ratio, ii) followed by an unbalanced beam-splitter that mixes the two processes according to the Golden Ratio. We offer an interpetation of the occurrence of such particular number, based on the microscopic processes taking place.

[1] M. Levenius et al. Appl.Phys. Lett. 101, 121114 (2012)

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Benchmarking photon-subtracted graph states of multimode light

Remote shaping of photonic temporal modes via entanglement 

Photonic temporal modes are a versatile resource for deploying quantum communication protocols, as they provide an unbounded space for high-dimensional information encodings compatible with the optical fibre infrastructure and integrated waveguide devices. Here, we experimentally demonstrate a complete platform for controlled generation and manipulation of entanglement encoded in temporal modes of single photons. As a notable example, we use a tailored parametric downconversion source to generate temporal bell state. Then we remotely prepare the generated photons by projecting their entangled partner into a specific temporal mode.

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Quantum-limited spectrotemporal measurement through mode-selective sum-frequency generation 

The minimum measurable separation between two objects is usually limited by the point-spread function of the optical field on the imaging device. However, this limit can be overcome for incoherent mixtures of pulses through phase-sensitive mode measurements, which allow for the precise measurement of arbitrarily small separations. In this work, we extend these techniques to the time-frequency domain.  Our mode-selective spectrotemporal measurements are enabled by sum-frequency generation with shaped pulsed light in tailored nonlinear waveguides. We apply this technique to sub-bandwidth spectral shifts and sub-pulsewidth time shifts, demonstrating sensitivity in regimes where intensity measurements fail.

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Click Detection of Nonclassical Phase-Space Distributions 

We implement the direct sampling of negative phase-space functions of single-photon states via unbalanced homodyne measurement using click-counting detectors. The negativities significantly certify nonclassical light in the high-loss regime without relying on photon-number discrimination. We demonstrate the most significant certification of nonclassicality for two detection bins only. By contrast, the frequently applied Wigner function would fail to directly indicate such quantum characteristics. Therefore, we realize a robust and reliable approach for characterizing nonclassical light in phase space.

arXiv:1711.10962 [quant-ph].

PDF Abstract

Direct quantum process tomography via measuring sequential weak values of incompatible observables 

We present our experimental measurement of sequential weak values for incompatible observables. In particular, by making use of two-photon quantum interference for the measurement interaction, our experiment can be viewed as an unambiguous quantum implementation of sequential quantum weak value measurement. And also, we demonstrate that the sequential weak value measurement can be used to perform direct quantum process tomography of a qubit channel. 

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Quantum phase transitions and correlation detection in the measurement space

Discrete distributions in the 3-dim space of symmetric measurements are very useful for visualization and analysis of general properties of arbitrary states of macroscopic quantum systems. We show that the analytical properties of such distriutions can be used for characterization of long-range correlations and detection of quantum phase transitions. In the framework of our approach we analyze all-order phase transitions and percolations for some typical N-spin parameter-dependent Hamiltonians and provide a sensible critaria for determination of critical points and type of correlations in stable (under parameter change) regions.

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Pulsed continuous-variable optoelectromechanical transducer based on geometric phase effect 

We propose a transducer that uses a sequence of pulsed quantum nondemolition (QND) interactions of the two modes of radiation with a mechanical oscillator to effectively induce a QND coupling between the modes of radiation.  The properly engineered sequence of interactions drives the mechanical oscillator around a closed path in a phase space and thereby allows to trace the mechanical mode out of the interaction of the radiation modes, leaving the latter coupled.  Importantly, the coupling can be achieved regardless of the temperature of the noisy mechanical mode.  We prove the feasibility of transducer and robustness to imperfections.

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Accelerating optical quantum technologies with quantum-dot based devices

In this talk I will discuss how a single quantum dot can be positioned in an optical cavity in a fully controlled way so as to obtain single-photon sources with single-photon purity and indistinguishability over 99% and brightness exceeding by a factor 20 the one of currently used sources based on parametric down conversion, allowing for a considerable speed-up of quantum protocols, like boson sampling.

I will also present our progresses towards the development of deterministic two-photon gates with devices performing as nonlinear switches at the single-photon level, converting a coherent pulse into a highly non-classical light wave-packet.

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Multimode quantum memory in cold rubidium atoms 

Using a single-photon resolving camera, we demonstrate a wavevector multiplexed quantum memory based on a cold atomic ensemble. Observation of nonclassical correlations between Raman scattered photons is confirmed by an average value of the second-order correlation function 72 in 665 separated modes simultaneously.

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Implementation of Practical Unforgeable Quantum Money

Wiesner's quantum money consists in producing unforgeable quantum banknotes, thanks to the no-cloning theorem. Despite the protocol's central role in quantum cryptography, its implementation has remained elusive because of the lack of realistic protocols adapted to practical quantum storage devices and verification techniques. Here, we experimentally demonstrate a protocol that rigorously satisfies the security condition for unforgeability, using a practical system exploiting single-photon polarization encoding of highly attenuated coherent states of light for on-the-fly quantum money generation and readout. Our implementation is designed to be compatible with state-of-the-art quantum memories, which have been taken into account in the security analysis.

PDF Abstract

Plenary

Optomechanical Compass States

PDF Abstract

Photonic like Cooper Pairs

Photons are the elementary particles of light. Contrary to most particles, photons don't interact directly in vacuum. However, photon pairs may interact when propagating in a material. In this work, we demonstrate theoretically and experimentally that photon pairs may interact via virtual vibrations, meaning that the energy exchanged in the process doesn't correspond to a quantum of vibrational energy of the material. The same occurs for electrons in metals at low temperatures, giving rise to Cooper pairs. We have shown the analogue of this with light, an effective photon-photon interaction mediated by a virtual vibration, i.e, a photonic-like Cooper pair.

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On-Demand Microwave Generator of Shaped Single Photons 

We present results of a microwave single-photon generator where the emission rate of a superconducting qubit is controlled in real time and photon wavepackets are emitted with a shaped profile. Controlling the radiative properties of quantum emitters has important applications in quantum information science. The modulation is achieved by controlling the position of the nodes of vacuum modes in a semi-infinite transmission line shunted by a dc-SQUID. Besides its practical application as a photon source, this work enables fundamental studies of artificial atoms interacting with a quantum vacuum, leading to controlled dissipative processes and their effect in quantum information applications.

PDF Abstract

Invited talk

Quantum Microwave Photonics with Superconducting Circuits

Invited talk

Quantum optics in ordered atomic arrays

Dissipation is a pervasive problem in many areas of physics. In quantum optics, losses curb our ability to realize controlled and efficient interactions between photons and atoms, which are essential for many technologies ranging from quantum information processing to metrology. Spontaneous emission - in which photons are first absorbed by atoms and then rescattered into undesired channels - imposes a fundamental limit in the fidelities of many quantum applications, such as quantum memories and gates. Typically, it is assumed that this process occurs at a rate given by a single isolated atom. However, this assumption can be substantially violated: interference in photon emission and absorption generates correlations and entanglement among atoms, thus making dissipation a collective phenomenon. In this talk, I will discuss the physics of subradiance, a form of collective dissipation in which interference is destructive and atomic decay is inhibited. Exploiting subradiant states of ordered atomic arrays allows for the realization of a quantum memory with a photon retrieval fidelity that performs dramatically better with number of atoms than previously known bounds [1,2]. This single example illustrates how ordered arrays transcend the "standard model" of disordered atomic ensembles. Time permitting, I will also discuss collective effects in ordered chains of atoms with non-trivial hyperfine structure [3].

PDF Abstract

Multiple Transparency Windows and Fano interferences Induced by Dipole-Dipole Couplings 

We investigate the induced transparency phenomenon in an array of coupled two-level systems (TLS), where the dipole-dipole coupling plays the same role as the control field in electromagnetically induced transparency experiments with three-level atoms. We describe this physical phenomenon and investigate how it scales with the number of coupled TLS. In particular we have shown that the number of TLS determines the number of transparency windows allowed in this system. The ideas presented here are very general and can be implemented in different physical systems such as array of superconducting qubits, array of quantum dots, spin chains, optical lattices, etc.

PDF Abstract

Nonlinear photon-atom coupling in free space

Implementing nonlinear interactions between single photons and single atoms is at the forefront of optical physics. We adapt a super-resolution imaging technique, 4Pi microscopy, to efficiently couple light to a single atom [1]. We observe strong extinction of the incident field, and a modified photon statistics of the transmitted field -- indicating nonlinear interaction at the single-photon level. Our results pave the way to few-photon nonlinear optics with individual atoms in free space.

[1] Y.S. Chin, M. Steiner, C. Kurtsiefer, Nature Comm 8, 1200 (2017)

PDF Abstract

Incommensurate Crystals of Trapped Ions in Optical Cavities

We argue that a chain of trapped ion interacting with a cavity potential can simulate the commensurate-incommensurate transition predicted at two crystal interfaces. For this purpose we use a mean-field approach and analyse the phase diagram and the kinks' properties in presence of the long-range Coulomb interactions and of the external potential. We show that the commensurate-incommensurate transition can be observed in chains of dozen of ions in a harmonic trap at finite temperatures and identify the salient properties at the transition. Our study sets the stage for analysing quantum effects in the Frenkel-Kontorova model on an experimentally accessible setup.

PDF Abstract

Invited talk

Simulating quantum light propagation through atomic ensembles using matrix product states

Atomic ensembles constitute a powerful and versatile platform to realize a quantum interface between matter and light. Recently, a number of such interfaces have emerged, most prominently ensembles with atoms excited to high-lying Rydberg states, which enable strong nonlinear interactions between propagating photons. A largely open problem, which is difficult to treat both analytically and numerically, is whether these systems can produce exotic quantum many-body states of light, and the development of new techniques to solve for the out-of-equilibrium quantum dynamics as photons propagate and interact with atoms is highly desirable. Here, we describe a novel numerical approach, wherein a problem involving quasi one-dimensional light propagation is mapped to the dynamics of an open 1D interacting “spin” system describing the atomic internal degrees of freedom, where all photon correlations are obtained from those of the spins by a quantum input-output relation. The spin dynamics in turn are numerically solved using the powerful matrix product state ansatz, which avoids the exponentially large Hilbert space nominally associated with the spins. As two specific examples, we apply this formalism to investigate vacuum induced transparency, a phenomena where the different photon number components of a pulse propagate with number-dependent group velocity and separate at the output, and Rydberg EIT in the high-photon limit, where it becomes possible to generate pulse trains of single photons starting from continuous input fields.

PDF Abstract

During the social dinner there will take place a concert by Sergi Sellés Quartet, a jazz fusion band

Invited talk

New variance uncertainty relations for several observables

The uncertainty relations for an arbitrary set of N observables were derived for the first time by Robertson [1], and many their special cases and generalizations were given in the review [2]. Unfortunately, such inequalities are too complicated in the most general form, because they contain, in addition to N variances of the observables and N(N It is known that the minimal value of the product of variances of two non-commuting observables A and B is bigger than the standard Heisenberg-Robertson limit h[ ^ A; ^B ]ij2=4, if some constraints on the statistical properties of the quantum system are available, such as the correlation coefficient betwee two observables [3, 4, 5] or the degree of purity of the quantum system [2, 6]. Now I show how the uncertainty product is increased due to non-zero correlations between the selected two-variable system and some extra variables describing the ”external world”. They show that even if the mean value of the commutator between two operators is zero, nonetheless, the product of variances of the corresponding observables must be nonzero, if these observables are parts of some extended system. The proofs of the results can be found in Refs. [7, 8].

PDF Abstract

Direct observation of phase sensitive Hong-Ou-Mandel interference 

Quality of individual photons and their ability to interfere is traditionally tested by measuring the Hong-Ou-Mandel photon bunching effect. However, this measurement only tests the particle aspect of the quantum interference. We show that both the particle and the wave aspects of the quantum interference can be revelead in a single set of direct measurements. We experimentally test the newly proposed witness by applying it to a pair of independent single photons retrieved on demand. 

PDF Abstract

A Learning Scheme with Coherent State Amplification 

Quantum amplification is a crucial task in quantum information processing and its performance are fundamentally bounded by the laws of quantum mechanics. In this work, we propose a probabilistic linear amplifier for a known set of coherent states, that uses a feed-forward enacted learning strategy. The device’s figures of merit compare favourably with other systems. For example, the success probability-fidelity product is higher than the one for the unambiguous state discrimination. This simple system, is realized with classical resources, allowing for high repetition rates and it suitable for on-chip implementation.

PDF Abstract

Speeding-up the Decision Making of a Learning Agent Using an Ion Trap Quantum Processor

The decision-making process of a quantum learning agent within the projective simulation paradigm for machine learning is implemented with two atomic ions. This process is quadratically improved with respect to comparable classical learning agents.

PDF Abstract

Place: AULA MAGNA

• Natalia Bruno - Interfacing a single trapped atom to a pair of indistinguishable photons 

• Chiara Decaroli - A double junction segmented ion trap with integrated optical delivery 

• Kanupriya Sinha - Collective effects in Casimir-Polder forces 

Invited talk

Noisy quantum metrology

Identifying the properties a system with high precision is the primary goal of sensing: understanding what are the ultimate limits on what is achievable with given resources is the main issue in quantum metrology [1]. In this respect, phase estimation is by far the most investigated example, due to its relevance for practical applications. Choosing the optimal quantum state of the light probe can result in increased sensitivity with respect to what can be accomplished with purely classical means. However, the price to pay is a frailty of such resource states against any ungoverned and unwanted couplings with the external world. Quantum metrology in the presence of noise requires clever arrangements to restore possible advantages from the use of quantum resources. In any case, a thorough characterization of the actual noise is necessary to implement these strategies, and such characterisation might not be accessible as a precalibration, as, for instance, in time-varying cases. It is then important to design parameter estimation protocols that treat both unitary parameters, such as phases, and dissipative parameters, including loss or phase diffusion, with equal importance. Multiparameter estimation has been intensively studied over the last years, showing how a trade-off in the precision on individual parameters comes out in many practical instances. In addition, a multiparameter approach may also result in increased robustness in against small deviations of the designed probes from the optimal states [2]. In this talk we will discuss some recent results on quantum metrology in the presence of noise. We will start by discussing an experiment on the usefulness of ancilla-assisted protocols to mitigate the impact of some models of noise on phase estimation with qubits [3]. We will then proceed to present results on the application of entangling operations for the joint estimation of phase and phase diffusion with two qubits [4]. We will conclude with an estimation experiment in which the multiparameter approach is applited to obtaining a phase shift and, at the same time, a quantification of the quality of the probe that actually employed [5] (Fig. 1).

PDF Abstract

Novel light sources for correlated-imaging applications 

We present a scheme of computational imaging exploiting two novel kinds of correlated light: the spatio/spectral correlated twin beam states in the high-intensity domain and the super-thermal light obtained by performing the second-harmonics of a classical speckle field. In the first case, frequency correlations are made useful for ghost imaging by means of an imaging spectrometer that maps the different frequency in the spectrum into different spatial positions. The scheme is used for a proof-of-principle information protocol. In the second case, the super-thermal light divided at a beam splitter produces increased correlations and hence higher-visibility ghost images.

PDF Abstract

The usability of optical parametric amplification (OPA) of the light for astronomical imaging

The idea of a Quantum Telescope (QT) based on the Optical Parametric Amplification (OPA) of the light is aimed at bypassing diffraction limit for imaging of extended sources. We present a scheme of an OPA-based device and a semiclassical model of signal amplification by such device. We analyzed the efficiency of the OPA in increasing the angular resolution of imaging of extended targets and precise localization of a distant point source (astrometry).  According to our results, in both cases OPA is offering a gain in comparison to ``classical optics''.

PDF Abstract

Optical Bipartite Continuous Variable Entangled States carrying Orbital Angular Momentum 

We have generated a bipartite continuous variable entangled state travelling over helical modes. This is done by providing two polarisation entangled modes, produced by an OPO, with opposite amounts of orbital angular momentum. A liquid crystal-based optical device, the q-plate, couples polarisation and OAM degrees of freedoms. Due its Gaussian character, the entangled state is characterised by measuring its covariance matrix with a OAM-selective homodyne detection. The measured state fulfils different entanglement criteria. The entangled state is distinguishable by both d.o.f.: polarization and orbital angular momentum. The experimental method paves the way to the generation of multipartite continuous variable states.

PDF Abstract