National Institute of Optics

Ion acceleration driven by superintense laser pulses is attracting an impressive and steadily increasing effort. Motivations can be found in the applicative potential and in the perspective to investigate novel regimes as available laser intensities will be increasing. Experiments have demonstrated, over a wide range of laser and target parameters, the generation of multi-MeV proton and ion beams with unique properties such as ultrashort duration, high brilliance, and low emittance. An overview is given of the state of the art of ion acceleration by laser pulses as well as an outlook on its future development and perspectives. The main features observed in the experiments, the observed scaling with laser and plasma parameters, and the main models used both to interpret experimental data and to suggest new research directions are described.

We present the first results of the Fermilab National Accelerator Laboratory (FNAL) Muon g -2 Experiment for the positive muon magnetic anomaly a g -2=2. The anomaly is determined from the precision measurements of two angular frequencies. Intensity variation of high-energy positrons from muon decays directly encodes the difference frequency a between the spin-precession and cyclotron frequencies for polarized muons in a magnetic storage ring. The storage ring magnetic field is measured using nuclear magnetic resonance probes calibrated in terms of the equivalent proton spin precession frequency 0

We analyze in detail an algorithm for computing Liapunov exponents from an experimental time series. As an application, a hydrodynamic experiment is investigated.Received 24 April 1986DOI:https://doi.org/10.1103/PhysRevA.34.4971©1986 American Physical Society

In the 2015 review paper ‘Petawatt Class Lasers Worldwide’ a comprehensive overview of the current status of high-power facilities of ${>}200~\text{TW}$ was presented. This was largely based on facility specifications, with some description of their uses, for instance in fundamental ultra-high-intensity interactions, secondary source generation, and inertial confinement fusion (ICF). With the 2018 Nobel Prize in Physics being awarded to Professors Donna Strickland and Gerard Mourou for the development of the technique of chirped pulse amplification (CPA), which made these lasers possible, we celebrate by providing a comprehensive update of the current status of ultra-high-power lasers and demonstrate how the technology has developed. We are now in the era of multi-petawatt facilities coming online, with 100 PW lasers being proposed and even under construction. In addition to this there is a pull towards development of industrial and multi-disciplinary applications, which demands much higher repetition rates, delivering high-average powers with higher efficiencies and the use of alternative wavelengths: mid-IR facilities. So apart from a comprehensive update of the current global status, we want to look at what technologies are to be deployed to get to these new regimes, and some of the critical issues facing their development.

Single-photon-added coherent states are the result of the most elementary amplification process of classical light fields by a single quantum of excitation. Being intermediate between a single-photon Fock state (fully quantum-mechanical) and a coherent (classical) one, these states offer the opportunity to closely follow the smooth transition between the particle-like and the wavelike behavior of light. We report the experimental generation of single-photon-added coherent states and their complete characterization by quantum tomography. Besides visualizing the evolution of the quantum-to-classical transition, these states allow one to witness the gradual change from the spontaneous to the stimulated regimes of light emission.

Chiral edge states are a hallmark of quantum Hall physics. In electronic systems, they appear as a macroscopic consequence of the cyclotron orbits induced by a magnetic field, which are naturally truncated at the physical boundary of the sample. Here we report on the experimental realization of chiral edge states in a ribbon geometry with an ultracold gas of neutral fermions subjected to an artificial gauge field. By imaging individual sites along a synthetic dimension, encoded in the nuclear spin of the atoms, we detect the existence of the edge states and observe the edge-cyclotron orbits induced during quench dynamics. The realization of fermionic chiral edge states opens the door for edge state interferometry and the study of non-Abelian anyons in atomic systems.

Quantum walk represents one of the most promising resources for the simulation of physical quantum systems, and has also emerged as an alternative to the standard circuit model for quantum computing. Here we investigate how the particle statistics, either bosonic or fermionic, influences a two-particle discrete quantum walk. Such an experiment has been realized by exploiting polarization entanglement to simulate the bunching-antibunching feature of noninteracting bosons and fermions. To this scope a novel three-dimensional geometry for the waveguide circuit is introduced, which allows accurate polarization independent behavior, maintaining remarkable control on both phase and balancement.

Self-bound quantum droplets are a newly discovered phase in the context of ultracold atoms. In this Letter, we report their experimental realization following the original proposal by Petrov [Phys. Rev. Lett. 115, 155302 (2015)PRLTAO0031-900710.1103/PhysRevLett.115.155302], using an attractive bosonic mixture. In this system, spherical droplets form due to the balance of competing attractive and repulsive forces, provided by the mean-field energy close to the collapse threshold and the first-order correction due to quantum fluctuations. Thanks to an optical levitating potential with negligible residual confinement, we observe self-bound droplets in free space, and we characterize the conditions for their formation as well as their size and composition. This work sets the stage for future studies on quantum droplets, from the measurement of their peculiar excitation spectrum to the exploration of their superfluid nature.

Subharmonic bifurcations, generalized multistability, and chaotic behavior were found experimentally in a $Q$-switched C${\mathrm{O}}_{2}$ laser operating at 10.6 \ensuremath{\mu}m. Jumps between two strange attractors lead to a low-frequency ($\frac{1}{f}$ type) divergence in the power spectrum. This is the first experimental evidence of these phenomena in a quantum-optical molecular system. A theoretical model is also presented whose results are in good agreement with the experimental data.

Few years ago the possibility of coupling and inter-converting the spin and orbital angular momentum (SAM and OAM) of paraxial light beams in inhomogeneous anisotropic media was demonstrated. An important case is provided by wave-plates having a singular transverse pattern of the birefringent optical axis, with a topological singularity of charge q at the plate center, hence named "q-plates". The introduction of q-plates has given rise in a few years to a number of new results and to a significant progress in the field of orbital angular momentum of light. Particularly promising are the quantum photonic applications, because the polarization control of OAM allows the transfer of quantum information from the SAM qubit space to an OAM subspace of a photon and vice versa. In this paper, we review the development of the q-plate idea and some of the most significant results that have originated from it, and we will briefly touch on many other related findings concerning the interaction of the SAM and OAM of light.

We numerically study heat conduction in chains of nonlinear oscillators with time-reversible thermostats. A nontrivial temperature profile is found to set in, which obeys a simple scaling relation for increasing the number $N$ of particles. The thermal conductivity diverges approximately as ${N}^{1/2}$, indicating that chaotic behavior is not enough to ensure the Fourier law. Finally, we show that the microscopic dynamics ensures fulfillment of a macroscopic balance equation for the entropy production.

In this paper, a forensic tool able to discriminate between original and forged regions in an image captured by a digital camera is presented. We make the assumption that the image is acquired using a Color Filter Array, and that tampering removes the artifacts due to the demosaicking algorithm. The proposed method is based on a new feature measuring the presence of demosaicking artifacts at a local level, and on a new statistical model allowing to derive the tampering probability of each 2 × 2 image block without requiring to know a priori the position of the forged region. Experimental results on different cameras equipped with different demosaicking algorithms demonstrate both the validity of the theoretical model and the effectiveness of our scheme.

The propensity for synchronization of complex networks with directed and weighted links is considered. We show that a weighting procedure based upon the global structure of network pathways enhances complete synchronization of identical dynamical units in scale-free networks. Furthermore, we numerically show that very similar conditions hold also for phase synchronization of nonidentical chaotic oscillators.

The possibility of arbitrarily "adding" and "subtracting" single photons to and from a light field may give access to a complete engineering of quantum states and to fundamental quantum phenomena. We experimentally implemented simple alternated sequences of photon creation and annihilation on a thermal field and used quantum tomography to verify the peculiar character of the resulting light states. In particular, as the final states depend on the order in which the two actions are performed, we directly observed the noncommutativity of the creation and annihilation operators, one of the cardinal concepts of quantum mechanics, at the basis of the quantum behavior of light. These results represent a step toward the full quantum control of a field and may provide new resources for quantum information protocols.

The first microscopic artificial walker equipped with liquid-crystalline elastomer muscle is reported. The walker is fabricated by direct laser writing, is smaller than any known living terrestrial creatures, and is capable of several autonomous locomotions on different surfaces. Nature provides a valuable source of inspiration for many fields of research. From optical effects to engineering problems, organisms have often adapted elaborate solutions and strategies, being the result of many years of natural selection.1, 2 Within this context, there is a lot of interest in creating artificial structures (robots) that can walk,3 swim,4 and perform various tasks.5 Micrometer scale robots, in particular, have been proposed for applications, including drug delivery,6 biosensing,7 and microsurgery.8 Based on the development of artificial muscles, a variety of soft robots that mimic natural creatures have been designed.9-12 Such artificial creatures rely on power delivered from outside, while the entire mechanical actuation is due to the inner stress of the muscle. Differently from external field driven devices,6, 7 they approach naturally occurring living systems and are considered of broad scientific interest and technological value. Terrestrial soft robots have been realized with walking,9, 10 gripping,11 and camouflage12 functionalities, but still remain in the centimeter scale. Working in the microscopic scale, strong adhesion arising from the surface related forces (e.g., van der Waals and the capillary forces) is the limit of existing examples. Inspiringly, it has been found that in nature van der Waals and the capillary forces dominate the reversible adhesion between terrestrial living organisms and the surfaces in contact.13-16 Typical natural adhesion force per unit area has been experimentally measured to be 0.3–1.3 MPa17, 18 (on gecko foot spatula), while a typical muscle stress in nature is between 0.01 and 0.2 MPa (peak 0.8 MPa).19 While the entire creature size shrinks down to micrometers (muscle cross section getting close to body-environment contact area), the adhesion becomes comparable with the muscle force, thus posing a great difficulty for any movement. The balance between achievable muscle stress and adhesive forces thus sets the lower limit for the size of terrestrial free moving organisms, being of the order of 140 μm.20 Similarly, microscopic artificial creatures equipped with artificial muscles are struggling to overcome the same adhesion hurdle as their natural relatives. The different physics ruling on microscopic length scales (including domination of adhesion forces) is worth exploring in micrometer-sized robotic systems. In addition, the realization of large numbers of autonomous robots can allow the study of cooperative effects, and open up new strategies for self-assembly. Muscles (soft sections of the creatures) have always been well isolated from the environment, while hard crust and hairy-like surfaces often assist to reduce the surface contact zone and hence minimize the biological adhesion. Liquid crystalline elastomers (LCEs), that combine elastomeric properties with liquid crystalline orientational order, can reversibly deform in response to external stimuli with dramatic contraction (up to 400%) and with a stress compatible to natural muscles.21 Therefore, LCEs have been considered as artificial muscles with great potential for creating biomimetic soft robot.22 Recently developed technologies enable to control molecular order in the microscopic and macroscopic scale23, 24 (instinct related to actuation) and fabricate patterned actuator with micrometer-sized complex shapes.25 However, such soft materials with typical elastic moduli E in the MPa range,21 behave extremely sticky. All approaches to these artificial muscles in the microscopic scale have failed, as the natural adhesion poses a huge hurdle for robot's locomotion. To isolate the LCE muscle from the environment, and then solve the natural adhesion problem, we chose acrylic resin (IP-Dip) for fabrication of walker's limbs with a relative high E (≈4 GPa). The LCE composition shown in Figure 1a is chosen,25 and interestingly the light sensitive element (azo-dye) is designed to open a transparency window for 780 nm fs laser and two-photon absorption polymerization at 390 nm in the direct laser writing (DLW) system, and to be activated by green light. DLW is used to pattern the complex 3D hybrid robot structures. This technique allows to arbitrary design 3D robot structures with nanometer scale resolution by a well control of sample position and laser exposure condition. Moreover, it consents a single polymerization/crosslinking step for the LC monomer mixture. Light energy is used to power the walker, with no external force applied. Approaching biological systems, such obtained artificial creatures are capable of different autonomous movements influenced by the interaction with their environment—similar to existing walking creatures in nature subject to the same van der Waals forces. In the first stage, we prepared a LCE structure in order to characterize the optomechanical response of our materials, as reported in Figure 1. The mechanical response to light is fully reversible and consists in a contraction of around 20% along the director of the nematic liquid crystal elastomeric network (Figure 1b,c). For such specific elastomer it takes place via an intermediate heating step, which induces a phase transition (nematic–isotropic) in the material.23 Illuminating the overall environment with light from a modulated, 532 nm laser source, the LCE responds following the laser modulation frequency (Figure 1d). High response frequency, 1.8 kHz, as shown in Figure S1d, Supporting Information, can be obtained. Such response is the fastest in the reported LCE muscles. The modulated amplitude decreases with increasing frequency (Figure S1, Supporting Information). Maximum light induced stress is measured to be 260 ± 2 kPa, which should be compared to natural muscles (10–200 kPa, and up to 1 kHz).19, 26 These parameters perfectly fit the needing for the microscopic robot construction. In the fabrication process, an array of 3 μm high anchors were first fabricated on a polyimide (PI) coated, rubbed glass slide by DLW in IP-Dip (Figure 2a left). After removing the unsolidified resin (Figure 2a right), a 10 μm cell was formed by adding a PVA coated glass slide with parallel rubbing and infiltrated with the liquid-crystalline monomer mixture taking the nematic alignment along the rubbing.25 The LCE actuator, being the 60 × 30 × 10 μm3 walker's main body, was made by DLW, with one side gripped by the anchors (Figure 2b). Subsequently, the lower glass slide is removed, and the whole structures are developed in toluene to remove the unsolidified LC monomers. Anchors (nonswelled in toluene) ensure proper adhesion between LCE (swelled in toluene) and the upper glass slide during development. In the last step the legs were made with the same technique as the anchors. The leg has a round, 12 μm diameter bottom, 0.5 μm top, and is 10 μm high (Figure 2c). Conical shape is chosen to reduce the surface contact area, while 45° tilt creates the adhesion asymmetry necessary for walking (SEM image in Figure 2d,e). Once the walker has been detached from the substrate, it is capable to perform reversible deformation under a modulated laser beam (Figure 2f and Movie S1, Supporting Information). The artificial creature can automatically perform various locomotion highly depended on the interactions with the environment, which is demonstrated as follow. The walker was first set on a polyimide coated microscope glass slide surface. With the 532 nm laser beam chopped at 50 Hz, the body performs the contract-expand cycles following the laser modulation frequency. The light powered LCE actuator with 30 × 10 μm2 cross section generates up to 78 μN force and the estimated total contact area of the leg pair is ≈0.4 μm2 (inset in Figure 2e). Taking the maximum van der Waals attractive force27 FvdW = A/6πD03 per unit area (Hamaker constant A is typically between28 0.5 × 10−19 and 1 × 10−19 J) and assuming that the gap between the leg and the glass is D0 = 0.3 nm27 and the friction coefficient μ being in the range from 0.2 to 1.0,28 the maximum friction force F =μFvdW is between 7.9 and 78.5 μN. With the driving elastomeric force larger than the friction, the walker starts to slip. Local fluctuations of the adhesion on the nonuniform PI coating result in a random walking behavior—the instantaneous movement direction is determined by the friction differences from one leg to the other. An example of random walking trajectory is shown in Figure 3c (see Movie S2, Supporting Information). The friction strength on PI coating is relative high that the asymmetric tilted leg geometry does not provide any directional walking tendency. The walking velocity has an average value of some μms−1, while the maximum speed is observed to be around 22 μm s−1. In this mode, once a leg gets stuck to the surface, the walking terminates, and continuous rotation in one direction begins (Figure 3d–f, Movie S3, Supporting Information). The maximum rotation speed is measured in 1.5 rad s−1. In order to obtain directional walking on these length scales one has to overcome the local fluctuations in the adhesion forces. An example of this is reported in Figure 3g–i. The walker is positioned on a clean glass substrate and moves with a preferred direction defined by the leg tilt due to the shear off anisotropy, as previously demonstrated in biomimetic artificial adhesive structures.29 No leg sticking is present; instead the continuous actuation of the walker body results in the net walking movement in the leg tilt direction. Local adhesion fluctuation may also induce rotation of the body, followed however by automatic reorientation (Figure 3i, Movie S4, Supporting Information). The walking direction is very sensitive to the friction fluctuation due to the small mass and to the high adhesion strength. Therefore, a straight walking direction is difficult to maintain. The average walking velocity in this case is measured to be around 37 μm s−1. In the above case the walking direction is decided by the walker itself. It is also possible, on the other hand, to create an environment in which the direction is determined externally using patterned surfaces. On a blazed diffraction grating with 1300 lines per mm, the walker prefers to move in a certain direction, due to the interaction between different tilted nanoslope. A steady walking direction is maintained, independent from the leg tilt (Figure 3j–l)—the walker can actually rotate the full 360° while maintaining the net movement direction (Movie S5, Supporting Information). On such grating surface, a 10 times larger walking velocity (380 μm s−1) comparing to the previous cases has been observed. Yet another behavior—up to ≈1 cm (100 body lengths) long jumps—was obtained on a Teflon surface. Conical legs provide advantage for walking, however, they can easily get stuck into the micro-gully on the Teflon surface. For this reason, we have also designed the walker with a 100 × 50 × 10 μm3 body size and three sets of lamellar legs (see Figure S2, Supporting Information). By increasing the laser power, such robot approached to the maximum deformation and stored the light energy inside the LCE body as elastic potential. At some point the walker overcome the adhesion on one side, and the elastic body converted the elastic energy into kinetic energy. Thus, a sudden jump occurred as shown in Movie S6 in the Supporting Information. Inspired by fleas,30 jumping could be a more efficient way of transport for robots at the micrometer scale, where the influence of gravity and inertia are significantly reduced and the air drag dominates the kinetic energy loss (see the Experimental Section). In conclusion, we demonstrated the preparation of a microscopic walker entirely powered by light and based on LCE artificial muscles. The locomotion of such artificial creatures is influenced by their design and how they interact with the environment—random or directional walking, rotation or jumping is possible and demonstrated above. With an average muscle stress of 200 kPa, assuming 1 μm2 of the foot-surface contact area and taking adhesive forces per unit area of 1 MPa into account, a minimum muscle cross section of 5 μm2 is needed to overcome adhesion for every single step. This results in the lower estimate of moving terrestrial creatures of the order of tens of micrometers, already saturated by our walker. Note that, although a laser source was used for convenience in all the experiments, such laser is defocused and illuminates large areas and the LCE muscles can, in principle, use energy from any light source. Importantly, there is also no need to target specific individual parts of the robots with a light beam. The walkers simply absorb the light from their overall environment. Optimization of the body geometry, actuator shape, and action and the leg tilt angle may still result in improved locomotion performance. The method presented allows to create, in principle, large amounts of elastomer-based robots in a relatively easy manner and with a versatile approach. Variety of LCE-based autonomous robot can be expected, e.g., microswimmers, microjumpers, origami photonic devices, and so on. Materials: The monomer mixture contains 78 mol% of the LC monomer 1, 20 mol% of the LC crosslinker 2, 1 mol% of the azo dye 3, and 1 mol% of the photoinitiator (Irgacure 369) as reported by Zeng et al.25 Direct Laser Writing Fabrication: Fabrication of the walker was peformed in a Dip-in Laser Lithography system (Nanoscribe GmbH). Sample preparation and development followed the procedure presented in ref. 25. Characterization: For testing the LCE light response, a 60 × 30 × 10 μm3 LCE actuator was held on a glass tip. Two laser beams: 300 mW continuous 532 nm solid state laser and 2 mW He-Ne laser were focused on the center and edge of the structure, respectively, by a 20× (NA 0.4) objective. The 532 nm laser was chopped to induce the modulated deformation, while the transmitted He-Ne laser beam was detected by a photodiode as a signal related to the LCE deformation. Two glass tips (1 and 5 μm in diameter) were used to transfer the microwalker onto different substrates with the help of 3D manual translation stages. A chopped 532 nm laser was defocused by a 10× objective (NA = 0.2) to a spot of around 200 μm diameter and ≈10 W mm2 intensity for the walker actuation. As the walker body temperature exceeded 100 °C under laser illumination the environment could be assumed to be dry, and thus ignored capillary interactions. A scanning electron microscope (PHENOM-World) was used to observe the structures after sputter-coating them with a 10 nm gold layer. Light-Induced Stress Measurement: For measuring the light induced stress, a 5 × 2.5 × 0.05 mm3 LCE sample was hung vetically attached to a force sensor and a 532 nm laser illumination (5 W cm-2) was applied to induce contraction. A maximum stress of 260 ± 2 kPa was measured. Glass Treatment: The surface of the glass substrates was cleaned by rinsing in a 1:300 diluted HF water solution for 10 min. Jumping Dynamics Estimation: Assuming 50% of the elastic energy turned into kinetic energy the launch speed was ≈4.7 ms−1 (LCE Young's modulus E ≈ 1.3 MPa, density ρLCE = 1.16 g cm-3, and strain of 20%). The Reynolds number Re was calculated to be no more than 12 (Re = ρνD/μ, air density ρ = 1.2 kg m−3, velocity ν < 4.7 ms−1, approximate object diameter D = 40 μm, air dynamic viscosity μ = 18.6 μPa s) for the entire flight. In a low Reynolds number environment, the drag contributed to the majority of the energy lost. The research leading to these results has received funding from the IIT SEED project Microswim and the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC Grant Agreement No. 291349 on photonic microrobotics. H.Z. was supported by the ICTP Ph.D. Program. The authors gratefully thank the entire Optics of Complex Systems group at LENS for feedback and discussions. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Interferometers with atomic ensembles are an integral part of modern precision metrology. However, these interferometers are fundamentally restricted by the shot noise limit, which can only be overcome by creating quantum entanglement among the atoms. We used spin dynamics in Bose-Einstein condensates to create large ensembles of up to 10(4) pair-correlated atoms with an interferometric sensitivity -1.61(-1.1)(+0.98) decibels beyond the shot noise limit. Our proof-of-principle results point the way toward a new generation of atom interferometers.

This is a comprehensive discussion of complexity as it arises in physical, chemical, and biological systems, as well as in mathematical models of nature. Common features of these apparently unrelated fields are emphasised and incorporated into a uniform mathematical description, with the support of a large number of detailed examples and illustrations. The quantitative study of complexity is a rapidly developing subject with special impact in the fields of physics, mathematics, information science, and biology. Because of the variety of the approaches, no comprehensive discussion has previously been attempted. This book will be of interest to graduate students and researchers in physics (nonlinear dynamics, fluid dynamics, solid-state, cellular automata, stochastic processes, statistical mechanics and thermodynamics), mathematics (dynamical systems, ergodic and probability theory), information and computer science (coding, information theory and algorithmic complexity), electrical engineering and theoretical biology

The refractive indices of E7 liquid-crystal mixture were measured at six visible and two infrared (λ=1.55 and 10.6μm) wavelengths at different temperatures, using Abbe and wedged cell refractometer methods, respectively. The experimental data of the visible wavelengths fit the extended Cauchy equations well. Using the extended Cauchy equations, we can extrapolate the refractive indices of E7 to IR. The extrapolated results almost strike through the measured data. Thus, the extended Cauchy equations can be used to link the visible refractive indices to infrared, where the refractive index measurements are more difficult.

Quantum metrology bears a great promise in enhancing measurement precision, but is unlikely to become practical in the near future. Its concepts can nevertheless inspire classical or hybrid methods of immediate value. Here we demonstrate NOON-like photonic states of m quanta of angular momentum up to m=100, in a setup that acts as a ‘photonic gear’, converting, for each photon, a mechanical rotation of an angle θ into an amplified rotation of the optical polarization by mθ, corresponding to a ‘super-resolving’ Malus’ law. We show that this effect leads to single-photon angular measurements with the same precision of polarization-only quantum strategies with m photons, but robust to photon losses. Moreover, we combine the gear effect with the quantum enhancement due to entanglement, thus exploiting the advantages of both approaches. The high ‘gear ratio’ m boosts the current state of the art of optical non-contact angular measurements by almost two orders of magnitude. Beating the standard measurement limits is a goal of metrology, as it would allow for more precise estimation of physical quantities. Borrowing concepts from NOON-state quantum metrology, this work presents a single-photon scheme to measure rotation angles of light with super-resolution precision.

Electron-positron pair plasmas represent a unique state of matter, whereby there exists an intrinsic and complete symmetry between negatively charged (matter) and positively charged (antimatter) particles. These plasmas play a fundamental role in the dynamics of ultra-massive astrophysical objects and are believed to be associated with the emission of ultra-bright gamma-ray bursts. Despite extensive theoretical modelling, our knowledge of this state of matter is still speculative, owing to the extreme difficulty in recreating neutral matter-antimatter plasmas in the laboratory. Here we show that, by using a compact laser-driven setup, ion-free electron-positron plasmas with unique characteristics can be produced. Their charge neutrality (same amount of matter and antimatter), high-density and small divergence finally open up the possibility of studying electron-positron plasmas in controlled laboratory experiments.