## Bosonic Quantum Gases

Prof. Dr. Markus Oberthaler

Telefon: +49 6221 54 5170

markus.oberthaler@kip.uni-heidelberg.de

apl. Prof. Dr. Thomas Gasenzer

Telefon: +49 6221 54 5173

thomas.gasenzer@kip.uni-heidelberg.de

Dr. Martin Gärttner

Telefon: +49 6221 54 5185

martin.gaerttner@kip.uni-heidelberg.de

Maurus Hans

Telefon: +49 6221 54 5177

mhans@kip.uni-heidelberg.de

Dr. Philipp Hauke

Telefon: +49 6221 54 5185

philipp.hauke@kip.uni-heidelberg.de

Philipp Kunkel

Telefon: +49 6221 54 5178

philipp.kunkel@kip.uni-heidelberg.de

Stefan Lannig

Telefon: +49 6221 54 5178

stefan.lannig@kip.uni-heidelberg.de

Maximilian Prüfer

Telefon: +49 6221 54 5178

maximilian.pruefer@kip.uni-heidelberg.de

Christian-Marcel Schmied

Telefon: +49 6221 54 5186

christian-marcel.schmied@kip.uni-heidelberg.de

Helmut Strobel

Telefon: +49 6221 54 5177

helmut.strobel@kip.uni-heidelberg.de

Celia Viermann

Telefon: +49 6221 54 5177

celia.viermann@kip.uni-heidelberg.de

**You can find the lastest results of the project in:**

### Observation of universal dynamics in a spinor Bose gas far from equilibrium

Paper, HD-KIP 18-143, 2018 PDF-Datei### Spatially distributed multipartite entanglement enables EPR steering of atomic clouds

Paper, HD-KIP 18-84, 2018, Science (360) 413--416 PDF-DateiA key resource for distributed quantum-enhanced protocols is entanglement between spatially separated modes. However, the robust generation and detection of entanglement between spatially separated regions of an ultracold atomic system remain a challenge. We used spin mixing in a tightly confined Bose-Einstein condensate to generate an entangled state of indistinguishable particles in a single spatial mode. We show experimentally that this entanglement can be spatially distributed by self-similar expansion of the atomic cloud. We used spatially resolved spin read-out to reveal a particularly strong form of quantum correlations known as Einstein-Podolsky-Rosen (EPR) steering between distinct parts of the expanded cloud. Based on the strength of EPR steering, we constructed a witness, which confirmed genuine 5-partite entanglement.

### Low-energy effective theory of non-thermal fixed points in a multicomponent Bose gas

Paper, HD-KIP 18-80, 2018, arXiv:1807.10228 [cond-mat.quant-gas] PDF-DateiNon-thermal fixed points in the evolution of a quantum many-body system quenched far out of equilibrium manifest themselves in a scaling evolution of correlations in space and time. We develop a low-energy effective theory of non-thermal fixed points in a bosonic quantum many-body system by integrating out long-wave-length density fluctuations. The system consists of N distinguishable spatially uniform Bose gases with O(N)×U(1)-symmetric interactions. The effective theory describes interacting Goldstone modes of the total and relative-phase excitations. It is similar in character to the non-linear Luttinger-liquid description of low-energy phonons in a single dilute Bose gas, with the markable difference of a universal non-local coupling function depending, in the large-N limit, only on momentum, single-particle mass, and density of the gas. Our theory provides a perturbative description of the non-thermal fixed point, technically easy to apply to experimentally relevant cases with a small number of fields N. Numerical results for N=3 allow us to characterize the analytical form of the scaling function and confirm the analytically predicted scaling exponents. The fixed point which is dominated by the relative phases is found to be Gaussian, while a non-Gaussian fixed point is anticipated to require scaling evolution with a distinctly lower power of time.

### Prescaling in a far-from-equilibrium Bose gas

Paper, HD-KIP 18-81, 2018, arXiv:1807.07514 [cond-mat.quant-gas] PDF-DateiNon-equilibrium conditions give rise to a class of universally evolving low-energy configurations of fluctuating dilute Bose gases at a non-thermal fixed point. While the fixed point and thus full scaling in space and time is generically only reached at very long evolution times, we here propose that systems can show prescaling much earlier, on experimentally accessible time scales. During the prescaling evolution, some well-measurable short-distance properties of the spatial correlations already scale with the universal exponents of the fixed point while others still show scaling violations. Prescaling is characterized by the evolution obeying already, to a good approximation, the conservation laws which are associated with the asymptotically reached non-thermal fixed point, defining its belonging to a specific universality class. In our simulations, we consider N=3 spatially uniform three-dimensional Bose gases of particles labeled, e.g., by different hyperfine magnetic quantum numbers, with identical inter- and intra-species interactions. In this system, the approach of a non-thermal fixed point is marked by low-energy phase excitations self-similarly redistributing towards smaller wave numbers. During prescaling, the full U(N) symmetry of the model is broken while the conserved transport, reflecting the remaining U(1) symmetries, leads to the buildup of a rescaling quasicondensate distribution.

### Quenches near criticality of the quantum Ising chain-power and limitations of the discrete truncated Wigner approximation

Paper, HD-KIP 18-76, 2018, arXiv:1805.11881 [cond-mat.quant-gas] PDF-DateiThe semi-classical discrete truncated Wigner approximation (dTWA) has recently been proposed as a simulation method for spin-1/2 systems. While it appears to provide a powerful approach which shows promising results in higher dimensions and for systems with long-range interactions, its performance is still not well understood in general. Here we perform a systematic benchmark on the one-dimensional transverse-field Ising model and point to limitations of the approximation arising after sudden quenches into the quantum critical regime. Our procedure allows to identify the limitations of the semi-classical simulations and with that to determine the regimes and questions where quantum simulators can provide information which is inaccessible to semi-classics.

### Active SU(1,1) atom interferometry

Paper, HD-KIP 17-74, 2017, Quantum Sci. Technol. (2) 044009 PDF-DateiActive interferometers use amplifying elements for beam splitting and recombination. We experimentally implement such a device by using spin exchange in a Bose–Einstein condensate. The two interferometry modes are initially empty spin states that get spontaneously populated in the process of parametric amplification. This nonlinear mechanism scatters atoms into both modes in a pairwise fashion and generates a non-classical state. Finally, a matched second period of spin exchange is performed that nonlinearly amplifies the output signal and maps the phase onto readily detectable first moments. Depending on the accumulated phase this nonlinear readout can reverse the initial dynamics and deamplify the entangled state back to empty spin states. This sequence is described in the framework of SU(1,1) mode transformations and compared to the SU(2) angular momentum description of passive interferometers.

### Universal equilibrium scaling functions at short times after a quench

Paper, HD-KIP 17-30, 2017, Phys. Rev. E (96) 022110 PDF-DateiBy analyzing spin-spin correlation functions at relatively short distances, we show that equilibrium near-critical properties can be extracted at short times after quenches into the vicinity of a quantum critical point. The time scales after which equilibrium properties can be extracted are sufficiently short so that the proposed scheme should be viable for quantum simulators of spin models based on ultracold atoms or trapped ions. Our results, analytic as well as numeric, are for one-dimensional spin models, either integrable or nonintegrable, but we expect our conclusions to be valid in higher dimensions as well.

### Experimental observation of the Poincaré-Birkhoff scenario in a driven many-body quantum system

Paper, HD-KIP 17-12, 2017, Phys. Rev. A (95) 011602 PDF-DateiAccessing the connection between classical chaos and quantum many-body systems has been a long-standing experimental challenge. Here, we investigate the onset of chaos in periodically driven two-component Bose-Einstein condensates, whose small quantum uncertainties allow for exploring the phase space with high resolution. By analyzing the uncertainties of time-evolved many-body states, we find signatures of elliptic and hyperbolic periodic orbits generated according to the Poincaré-Birkhoff theorem, and the formation of a chaotic region at increasing driving strengths. The employed fluctuation analysis allows for probing the phase-space structure by use of only short-time quantum dynamics.

### Quantum-Enhanced Sensing Based on Time Reversal of Nonlinear Dynamics

Paper, HD-KIP 16-50, 2016, Physical Review Letters (117) 013001 PDF-DateiWe experimentally demonstrate a nonlinear detection scheme exploiting time-reversal dynamics that disentangles continuous variable entangled states for feasible readout. Spin-exchange dynamics of Bose-Einstein condensates is used as the nonlinear mechanism which not only generates entangled states but can also be time reversed by controlled phase imprinting. For demonstration of a quantum-enhanced measurement we construct an active atom SU(1,1) interferometer, where entangled state preparation and nonlinear readout both consist of parametric amplification. This scheme is capable of exhausting the quantum resource by detecting solely mean atom numbers. Controlled nonlinear transformations widen the spectrum of useful entangled states for applied quantum technologies.

### Nonlinear dressed states at the miscibility-immiscibility threshold

Paper, HD-KIP 15-101, 2015, Physical Review A (92) 056314 PDF-DateiThe dynamical evolution of spatial patterns in a complex system can reveal the underlying structure and stability of stationary states. As a model system we employ a two-component Bose-Einstein condensate at the transition from miscible to immiscible with the additional control of linear interconversion. Excellent agreement is found between the detailed experimental time evolution and the corresponding numerical mean-field computations. Analyzing the dynamics of the system, we find clear indications of stationary states that we term nonlinear dressed states. A steady-state bifurcation analysis reveals a smooth connection of these states with dark-bright soliton solutions of the integrable two-component Manakov model.

### Observation of Scaling in the Dynamics of a Strongly Quenched Quantum Gas

Paper, HD-KIP 15-92, 2015, PHYSICAL REVIEW LETTERS (115) 245301 PDF-DateiWe report on the experimental observation of scaling in the time evolution following a sudden quench into the vicinity of a quantum critical point. The experimental system, a two-component Bose gas with coherent exchange between the constituents, allows for the necessary high level of control of parameters as well as the access to time-resolved spatial correlation functions. The theoretical analysis reveals that when quenching the system close to the critical point, the energy introduced by the quench leads to a short-time evolution exhibiting crossover reminiscent of the finite-temperature critical properties in the system’s universality class. Observing the time evolution after a quench represents a paradigm shift in accessing and probing experimentally universal properties close to a quantum critical point and allows in a new way benchmarking of quantum many-body theory with experiments.

### Twist-and-turn spin squeezing in Bose-Einstein condensates

Paper, HD-KIP 15-48, 2015, PHYSICAL REVIEW A (92) 023603 PDF-DateiWe demonstrate experimentally an alternative method for the dynamic generation of atomic spin squeezing, building on the interplay between linear coupling and nonlinear phase evolution. Since the resulting quantum dynamics can be seen as rotation and shear on the generalized Bloch sphere, we call this scheme twist-and-turn. This is closely connected to an underlying instability in the classical limit of this system. The short-time evolution of the quantum state is governed by a fast initial spreading of the quantum uncertainty in one direction, accompanied by squeezing in the orthogonal axis. We find an optimal value of ξ

^{2}_{S}=−7.1(3) dB in a single Bose-Einstein condensate and scalability of the squeezing to more than 10^{4}particles with ξ^{2}_{S}=−2.8(4) dB.### Double-well atom trap for fluorescence detection at the Heisenberg limit

Paper, HD-KIP 15-07, 2015, PHYSICAL REVIEW A (91) 013412 PDF-DateiWe experimentally demonstrate an atom number detector capable of simultaneous detection of two mesoscopic ensembles with single-atom resolution. Such a sensitivity is a prerequisite for quantum metrology at a precision approaching the Heisenberg limit. Our system is based on fluorescence detection of atoms in a hybrid trap in which a dipole barrier divides a magneto-optical trap into two separated wells. We introduce a noise model describing the various sources contributing to the measurement error and report a limit of up to 500 atoms for single-atom resolution in the atom number difference.

### Scalable Spin Squeezing for Quantum-Enhanced Magnetometry with Bose-Einstein Condensates

Paper, HD-KIP 14-66, 2014, Physical Review Letters (113) 5 PDF-DateiA major challenge in quantum metrology is the generation of entangled states with a macroscopic atom number. Here, we demonstrate experimentally that atomic squeezing generated via nonlinear dynamics in Bose-Einstein condensates, combined with suitable trap geometries, allows scaling to large ensemble sizes. We achieve a suppression of fluctuations by 5.3(5) dB for 12 300 particles, from which we infer that similar squeezing can be obtained for more than 10

^{7}atom. With this resource, we demonstrate quantum-enhanced magnetometry by swapping the squeezed state to magnetically sensitive hyperfine levels that have negligible nonlinearity. We find a quantum-enhanced single-shot sensitivity of 310(47) pT for static magnetic fields in a probe volume as small as 90 μm^{3}.### Fisher information and entanglement of non-Gaussian spin states

Paper, HD-KIP 14-57, 2014, SCIENCE (345) 424-427 PDF-DateiEntanglement is the key quantum resource for improving measurement sensitivity beyond classical limits. However, the production of entanglement in mesoscopic atomic systems has been limited to squeezed states, described by Gaussian statistics. Here, we report on the creation and characterization of non-Gaussian many-body entangled states. We develop a general method to extract the Fisher information, which reveals that the quantum dynamics of a classically unstable system creates quantum states that are not spin squeezed but nevertheless entangled. The extracted Fisher information quantifies metrologically useful entanglement, which we confirm by Bayesian phase estimation with sub–shot-noise sensitivity. These methods are scalable to large particle numbers and applicable directly to other quantum systems.

### Accurate Atom Counting in Mesoscopic Ensembles

Paper, HD-KIP 13-97, 2013, PHYSICAL REVIEW LETTERS (111) 253001 PDF-DateiMany cold atom experiments rely on precise atom number detection, especially in the context of quantum-enhanced metrology where effects at the single particle level are important. Here, we investigate the limits of atom number counting via resonant fluorescence detection for mesoscopic samples of trapped atoms. We characterize the precision of these fluorescence measurements beginning from the single-atom level up to more than one thousand. By investigating the primary noise sources, we obtain single-atom resolution for atom numbers as high as 1200. This capability is an essential prerequisite for future experiments with highly entangled states of mesoscopic atomic ensembles.

### Optimized absorption imaging of mesoscopic atomic clouds

Paper, HD-KIP 13-37, 2013, Applied Physics B-Laseres and Optics (online)We report on the optimization of high-intensity absorption imaging for small Bose–Einstein condensates. The imaging calibration exploits the linear scaling of the quantum projection noise with the mean number of atoms for a coherent spin state. After optimization for atomic clouds containing up to 300 atoms, we find an atom number resolution of Δ det =3.7 Δ det = 3.7 atoms, mainly limited by photon shot noise and radiation pressure.

### Measuring and engineering entropy and spin squeezing in weakly linked Bose-Einstein condensates

Paper, HD-KIP 13-25, 2013, NEW JOURNAL OF PHYSICS (15) 8 PDF-DateiWe propose a method to infer the single-particle entropy of bosonic atoms in an optical lattice and to study the local evolution of entropy, spin squeezing, and entropic inequalities for entangle- ment detection in such systems. This method is based on experimentally feasible measurements of non-nearest-neighbour coherences. We study a specific example of dynamically controlling atom tunneling between selected sites and show that this could potentially also improve the metrologically relevant spin squeezing.

### Dynamic generation of spin-squeezed states in bosonic Josephson junctions

Paper, HD-KIP 12-56, 2012, Phys. Rev. Lett. A (86) 023615 PDF-DateiWe analyze the formation of squeezed states in a condensate of ultracold bosonic atoms confined by a double-well potential. The emphasis is set on the dynamical formation of such states from initially coherent many-body quantum states. Two cases are described: the squeezing formation in the evolution of the system around the stable point, and in the short-time evolution in the vicinity of an unstable point. The latter is shown to produce highly squeezed states on very short times. On the basis of a semiclassical approximation to the Bose-Hubbard Hamiltonian, we are able to predict the amount of squeezing, its scaling with N, and the speed of coherent spin formation with simple analytical formulas which successfully describe the numerical Bose-Hubbard results. This method of producing highly squeezed spin states in systems of ultracold atoms is compared to other standard methods in the literature.

### Atomic homodyne detection of continuous variable entangled twin-atom states

Paper, HD-KIP 11-85, 2011, NATURE (480) 219-223 PDF-DateiHistorically, the completeness of quantum theory has been questioned using the concept of bipartite continuous-variable entanglement. The non-classical correlations (entanglement) between the two subsystems imply that the observables of one subsystem are determined by the measurement choice on the other, regardless of the distance between the subsystems. Nowadays, continuous-variable entanglement is regarded as an essential resource, allowing for quantum enhanced measurement resolution, the realization of quantum teleportation and quantum memories or the demonstration of the Einstein–Podolsky–Rosen paradox. These applications rely on techniques to manipulate and detect coherences of quantum fields, the quadratures. Whereas in optics coherent homodyne detection of quadratures is a standard technique, for massive particles a corresponding method was missing. Here we report the realization of an atomic analogue to homodyne detection for the measurement of matter-wave quadratures. The application of this technique to a quantum state produced by spin-changing collisions in a Bose–Einstein condensate reveals continuous-variable entanglement, as well as the twin-atom character of the state. Our results provide a rare example of continuous-variable entanglement of massive particles. The direct detection of atomic quadratures has applications not only in experimental quantum atom optics, but also for the measurement of fields in many-body systems of massive particles.

### Rabi Flopping Induces Spatial Demixing Dynamics

Paper, HD-KIP 11-76, 2011, PHYSICAL REVIEW LETTERS (107) 193001 PDF-DateiWe experimentally investigate the mixing and demixing dynamics of Bose-Einstein condensates in the presence of a linear coupling between two internal states. The observed amplitude reduction of the Rabi oscillations can be understood as a result of demixing dynamics of dressed states as experimentally confirmed by reconstructing the spatial profile of dressed state amplitudes. The observations are in quantitative agreement with numerical integration of coupled Gross-Pitaevskii equations without free parameters, which also reveals the criticality of the dynamics on the symmetry of the system. Our observations demonstrate new possibilities for changing effective atomic interactions and studying critical phenomena.

### Local and spatially extended sub-Poisson atom-number fluctuations in optical lattices

Paper, HD-KIP 11-67, 2011, PHYSICAL REVIEW A (84) 011609 PDF-DateiWe demonstrate that ultracold interacting bosonic atoms in an optical lattice with large on-site population show sub-Poisson on-site and intersite atom-number fluctuations. The experimental observations agree with numerical predictions of the truncated Wigner approximation. The correlations persist in the presence of multimode atom dynamics and even over large spatially extended samples involving several sites.

### Two-Well Bose-Einstein Condensates and Strategies for Einstein-Podolsky-Rosen Entanglement

Paper, HD-KIP 11-26, 2011, Phys. Rev. Lett. (106) 4Criteria suitable for measuring entanglement between two different potential wells in a Bose-Einstein condensation are evaluated. We show how to generate the required entanglement, utilizing either an adiabatic two-mode or a dynamic four-mode interaction strategy, with techniques that take advantage of s-wave scattering interactions to provide the nonlinear coupling. The dynamic entanglement method results in an entanglement signature with spatially separated detectors, as in the Einstein-Podolsky-Rosen paradox.

### Einstein-Podolsky-Rosen Correlations of Ultracold Atomic Gases

Paper, HD-KIP 11-22, 2011, Phys. Rev. Lett. (106) 4 PDF-DateiWe demonstrate that collective continuous variables of two species of trapped ultracold bosonic gases can be Einstein-Podolsky-Rosen-correlated (entangled) via inherent interactions between the species. We propose two different schemes for creating these correlations—a dynamical scheme and a static scheme analogous to two-mode squeezing in quantum optics. We quantify the correlations by using known measures of entanglement and study the effect of finite temperature on these quantum correlations.

### Classical Bifurcation at the Transition from Rabi to Josephson Dynamics

Paper, HD-KIP 10-62, 2010, Phys. Rev. Lett. (105, 204101) 4 PDF-DateiWe report on the experimental demonstration of the internal bosonic Josephson effect in a rubidium spinor Bose-Einstein condensate. The measurement of the full time dynamics in phase space allows the characterization of the theoretically predicted π-phase modes and quantitatively confirms analytical predictions, revealing a classical bifurcation. Our results suggest that this system is a model system which can be tuned from classical to the quantum regime and thus is an important step towards the experimental investigation of entanglement generation close to critical points.

### Multiple atomic dark solitons in cigar-shaped Bose-Einstein condensates

Paper, HD-KIP 10-16, 2010, Phys. Rev. A (81, 063604) 15 PDF-DateiWe consider the stability and dynamics of multiple dark solitons in cigar-shaped Bose-Einstein condensates. Our study is motivated by the fact that multiple matter-wave dark solitons may naturally form in such settings as per our recent work [Phys. Rev. Lett. 101, 130401 (2008)]. First, we study the dark soliton interactions and show that the dynamics of well-separated solitons (i.e., ones that undergo a collision with relatively low velocities) can be analyzed by means of particle-like equations of motion. The latter take into regard the repulsion between solitons (via an effective repulsive potential) and the confinement and dimensionality of the system (via an effective parabolic trap for each soliton). Next, based on the fact that stationary, well-separated dark multisoliton states emerge as a nonlinear continuation of the appropriate excited eigenstates of the quantum harmonic oscillator, we use a Bogoliubov-de Gennes analysis to systematically study the stability of such structures. We find that for a sufficiently large number of atoms, multiple soliton states are dynamically stable, while for a small number of atoms, we predict a dynamical instability emerging from resonance effects between the eigenfrequencies of the soliton modes and the intrinsic excitation frequencies of the condensate. Finally, we present experimental realizations of multisoliton states including a three-soliton state consisting of two solitons oscillating around a stationary one and compare the relevant results to the predictions of the theoretical mean-field model.

### Two-mode Bose gas: Beyond classical squeezing

Paper, HD-KIP 10-15, 2010, Phys. Rev. A (81, 063605) 14 PDF-DateiThe dynamical evolution of squeezing correlations in an ultracold Bose-Einstein condensate distributed across two modes is investigated theoretically in the framework of the Bose-Hubbard model. It is shown that the eigenstates of the Hamiltonian do not exploit the full region allowed by Heisenberg’s uncertainty relation for number and phase fluctuations. The development of nonclassical correlations and relative number squeezing is studied in the transition from the Josephson to the Fock regime. Comparing the full quantum evolution with classical statistical simulations allows us to identify quantum aspects of the squeezing formation. In the quantum regime, the measurement of squeezing allows us to distinguish even and odd total particle numbers.

### Nonlinear atom interferometer surpasses classical precision limit

Paper, HD-KIP 10-10, 2010 PDF-DateiInterference is fundamental to wave dynamics and quantum mechanics. The quantum wave properties of particles are exploited in metrology using atom interferometers, allowing for high-precision inertia measurements

^{1,2}. Furthermore, the state-of-the-art time standard is based on an interferometric technique known as Ramsey spectroscopy. However, the precision of an interferometer is limited by classical statistics owing to the finite number of atoms used to deduce the quantity of interest^{3}. Here we show experimentally that the classical precision limit can be surpassed using nonlinear atom interferometry with a Bose–Einstein condensate. Controlled interactions between the atoms lead to non-classical entangled states within the interferometer; this represents an alternative approach to the use of non-classical input states^{4, 5, 6, 7, 8}. Extending quantum interferometry^{9}to the regime of large atom number, we find that phase sensitivity is enhanced by 15 per cent relative to that in an ideal classical measurement. Our nonlinear atomic beam splitter follows the ‘one-axis-twisting’ scheme^{10}and implements interaction control using a narrow Feshbach resonance. We perform noise tomography of the quantum state within the interferometer and detect coherent spin squeezing with a squeezing factor of -8.2 dB (refs 11–15). The results provide information on the many-particle quantum state, and imply the entanglement of 170 atoms^{16}.### Ultrakalte Quantenpendel

Paper, HD-KIP 10-06, 2010 PDF-DateiDas Phänomen des Tunnelns – das Durchdringen von Barrieren – ist ein Paradebeispiel dafür, dass sich Teilchen in der Quantenwelt anders verhalten als in der klassischen. Kürzlich gelang es erstmals, das Tunneln makroskopischer Atomgase im Experiment direkt zu beobachten. Unter gewissen Näherungen lässt sich das komplizierte Vielteilchensystem mit einem einzigen Freiheitsgrad beschreiben, dessen Bewegungsgleichung derjenigen des Fadenpendels entspricht.

### Dynamic control and probing of many-body decoherence in double-well Bose-Einstein condensates

Paper, HD-KIP 09-97, 2009, Phys. Rev. A (80) 053613### Squeezing and entanglement in a Bose-Einstein condensate

Paper, HD-KIP 08-28, 2008 PDF-DateiEntanglement, a key feature of quantum mechanics, is a resource that allows the improvement of precision measurements beyond the conventional bound attainable by classical means. This results in the standard quantum limit, which is reached in today"s best available sensors of various quantities such as time and position. Many of these sensors are interferometers in which the standard quantum limit can be overcome by using quantum-entangled states (in particular spin squeezed states) at the two input ports. Bose–Einstein condensates of ultracold atoms are considered good candidates to provide such states involving a large number of particles. Here we demonstrate spin squeezed states suitable for atomic interferometry by splitting a condensate into a few parts using a lattice potential. Site-resolved detection of the atoms allows the measurement of the atom number difference and relative phase, which are conjugate variables. The observed fluctuations imply entanglement between the particles, a resource that would allow a precision gain of 3.8 dB over the standard quantum limit for interferometric measurements.