The most general quantum object that can be shared between two distant parties is a bipartite channel, as it is the basic element to construct all quantum circuits. In general, bipartite channels can produce entangled states, and can be used to simulate quantum operations that are not local. While much effort over the last two decades has been devoted to the study of entanglement of bipartite states, very little is known about the entanglement of bipartite channels. In this work, we rigorously study the entanglement of bipartite channels as a resource theory of quantum processes. We present an infinite and complete family of measures of dynamical entanglement, which gives necessary and sufficient conditions for convertibility under local operations and classical communication. Then we focus on the dynamical resource theory where free operations are positive partial transpose (PPT) superchannels, but we do not assume that they are realized by PPT pre- and postprocessing. This leads to a greater mathematical simplicity that allows us to express all resource protocols and the relevant resource measures in terms of semidefinite programs. Along the way, we generalize the negativity from states to channels, and introduce the max-logarithmic negativity, which has an operational interpretation as the exact asymptotic entanglement cost of a bipartite channel. Finally, we use the non-positive partial transpose (NPT) resource theory to derive a no-go result: it is impossible to distill entanglement out of bipartite PPT channels under any sets of free superchannels that can be used in entanglement theory. This allows us to generalize one of the long-standing open problems in quantum information—the NPT bound entanglement problem—from bipartite states to bipartite channels. It further leads us to the discovery of bound entangled positive operator valued measures.

Beyond future applications, quantum networks open interesting fundamental perspectives, notably novel forms of quantum correlations. In this Letter we discuss quantum correlations in networks from the perspective of the underlying quantum states and their entanglement. We address the questions of which states can be prepared in the so-called triangle network, consisting of three nodes connected pairwise by three sources. We derive necessary criteria for a state to be preparable in such a network, considering both cases where the sources are statistically independent and classically correlated. This shows that the network structure imposes strong and nontrivial constraints on the set of preparable states, fundamentally different from the standard characterization of multipartite quantum entanglement.

We study the trapping of a ground-state cesium atom in a small region around the midpoint between two coupled identical parallel optical nanofibers. We suggest using a blue-detuned guided light field in the odd ${\mathcal{E}}_z$-sine array mode to produce an optical potential with a local minimum of exactly zero at the midpoint between the nanofibers. We find that the effects of the van der Waals potential on the total trapping potential around the minimum point are not significant when the fiber separation distance and the power of the guided light field are large. For appropriate realistic parameters, a net trapping potential with a significant depth of about 1 mK, a large scattering-limited coherence time of several seconds, and a large recoil-heating-limited trap lifetime of several hours can be obtained. We investigate the dependencies of the trapping potential on the power of the guided light field, the fiber radius, the wavelength of light, and the fiber separation distance.

Alkali-metal atomic magnetometers suffer from heading errors in geomagnetic fields as the measured magnetic field depends on the orientation of the sensor with respect to the field. In addition to the nonlinear Zeeman splitting, the difference between Zeeman resonances in the two hyperfine ground states can also generate heading errors depending on initial spin polarization. We examine heading errors in an all-optical scalar magnetometer that uses free precession of polarized ${}^{87}\mathrm{Rb}$ atoms by varying the direction and magnitude of the magnetic field at different spin-polarization regimes. In the high-polarization limit where the lower hyperfine ground state $F=1$ is almost depopulated, we show that heading errors can be corrected with an analytical expression, reducing the errors by up to two orders of magnitude in earth's field. We also verify the linearity of the measured Zeeman precession frequency with the magnetic field. With lower spin polarization, we find that the splitting of the Zeeman resonances for the two hyperfine states causes beating in the precession signals and nonlinearity of the measured precession frequency with the magnetic field. We correct for the frequency shifts by using the unique probe geometry where two orthogonal probe beams measure opposite relative phases between the two hyperfine states during the spin precession.

A nontraditional quantum heat engine based on electromagnetically induced transparency has recently been suggested and experimentally demonstrated in ultracold atoms. In more practical setups with warm atoms, thermal atomic motions might hamper this heat engine mechanism. We here show that a Doppler-broadened atomic sample can still behave like an engine. However, only photons emitted in the direction of the coupling laser have the same brightness as for the Doppler-free engine, while the larger the angular deviation of emission, the lower the brightness. Our results suggest that the lower brightness can be seen as an act of Doppler broadening as if the thermal occupation number of the reservoir serving as entropy sink is increased, and the quantum heat engines may be feasible in warm atomic interfaces.

Quantum memory and entanglement are both the essential resources for long-distance quantum communication. Among the schemes reported to date, time-bin encoding is widely applied due to its advantage of robustness against decoherence in optical fibers. Despite enormous progress, at present the time-bin entanglement with cold-atom-ensemble-based quantum memory has to be realized in a well-isolated environment and at ultralow temperature, and the challenges for its generation in a room-temperature regime remain due to the strong noise and decoherence induced by the atomic motion and collision. Here, we report a time-bin entanglement built in room-temperature quantum memory. We propose and experimentally demonstrate a dual-addressing scheme to eliminate the strong quantum correlation between two time bins for generation of the entangled state. By using two all-fiber integrated Mach-Zehnder interferometers, we analyze the Bell inequality on the phase and obtain a visibility enabling a violation of the Clauser-Horne-Shimony-Holt Bell inequality.

We present a composably secure protocol allowing $n$ parties to test an entanglement generation resource controlled by a possibly dishonest party. The test consists only in local quantum operations and authenticated classical communication once a state is shared among them and provides composable security, namely, it can be used as a secure subroutine by $n$ honest parties within larger communication protocols to test if a source is sharing quantum states that are at least $\epsilon$-close to the Greenberger-Horne-Zeilinger (GHZ) state. This claim follows previous results on multipartite entanglement verification where the security was studied in the usual game-based model. Here we improve the protocol to make it more suitable for practical use in a quantum network and we study its security in the abstract cryptography framework to highlight composability issues and avoid hidden assumptions. This framework is a top-to-bottom theory that makes explicit any piece of information that each component (party or resource) gets at every time step of the protocol. Moreover, any security proof, which amounts to showing indistinguishability between an ideal resource having the desired security properties (up to local simulation) and the concrete resource representing the protocol, is composable for free in this setting. This allows us to readily compose our basic protocol in order to create a composably secure multiround protocol enabling honest parties to obtain a state close to a GHZ state or an abort signal, even in the presence of a noisy or malicious source. Our protocol can typically be used as a subroutine in a quantum Internet protocol, to securely share a GHZ state within the network before performing a communication or computation protocol.

Controlling light-matter-based quantum systems in the strong coupling regime allows for exploring quantum simulation of many-body physics in current architectures. For instance, the atom-field interaction in a cavity QED network provides control and scalability for quantum information processing. Here, we propose the control of single- and two-body Jaynes-Cummings systems in a nonequilibrium scenario, which allows us to establish conditions for the coherent and incoherent interchange of polariton branches. Our findings provide a systematic approach to manipulate the interchange of polaritons, which we apply to reveal insights into the transition between Mott-insulator- and superfluid-like states. Furthermore, we study the asymmetry in the absorption spectrum by triggering the cavity and atomic losses as a function of the atom-cavity detuning and the photon's hopping.

We investigate the properties of a strongly interacting imbalanced mixture of bosonic ${}^{41}\mathrm{K}$ impurities immersed in a Fermi sea of ultracold ${}^6\mathrm{Li}$ atoms. This enables us to explore the Fermi polaron scenario for large impurity concentrations including the case where they form a Bose-Einstein condensate. The system is characterized by means of radio-frequency injection spectroscopy and interspecies interactions are widely tunable by means of a well-characterized Feshbach resonance. We find that the energy of the Fermi polarons formed in the thermal fraction of the impurity cloud remains rather insensitive to the impurity concentration, even as we approach equal densities for both species. The apparent insensitivity to high concentration is consistent with a theoretical prediction, based on Landau's quasiparticle theory, of weak effective interaction between the polarons. The condensed fraction of the bosonic ${}^{41}\mathrm{K}$ gas is much denser than its thermal component, which leads to a break-down of the Fermi polaron description. Instead, we observe a new branch in the radio-frequency spectrum with a small energy shift, which is consistent with the presence of Bose polarons formed by ${}^6\mathrm{Li}$ fermions inside the ${}^{41}\mathrm{K}$ condensate. A closer investigation of the behavior of the condensate by means of Rabi oscillation measurements supports this observation, indicating that we have realized Fermi and Bose polarons, two fundamentally different quasiparticles, in one cloud.

We study experimentally accessible lower bounds on entanglement measures based on entropic uncertainty relations. Experimentally quantifying entanglement is highly desired for applications of quantum simulation experiments to fundamental questions, e.g., in quantum statistical mechanics and condensed-matter physics. At the same time it poses a significant challenge because the evaluation of entanglement measures typically requires the full reconstruction of the quantum state, which is extremely costly in terms of measurement statistics. We derive an improved entanglement bound for bipartite systems, which requires measuring joint probability distributions in only two different measurement settings per subsystem, and demonstrate its power by applying it to currently operational experimental setups for quantum simulation with cold atoms. Examining the tightness of the derived entanglement bound, we find that the set of pure states for which our relation is tight is strongly restricted. We show that, for measurements in mutually unbiased bases, the only pure states that saturate the bound are maximally entangled states on a subspace of the bipartite Hilbert space (this includes product states). We further show that our relation can also be employed for entanglement detection using generalized measurements, i.e., when not all measurement outcomes can be resolved individually by the detector. In addition, the impact of local conserved quantities on the detectable entanglement is discussed.

In the past decade or so, there has been a growing number of papers postulating a correspondence relating quantum cellular automata (QCAs) to quantum field theories (QFTs). If valid, this would relate two important but seemingly unconnected physical systems and offer the potential for a deeper understanding of both. For example, it could lead to algorithms for simulating QFTs on a quantum computer. Moreover, it is possible that nature actually obeys a QCA model and it is the corresponding QFT that is a mere approximation. However, verifying the QCA/QFT correspondence beyond the single-particle case has proven challenging, especially in more than one spatial dimension. Here we construct quantum cellular automata that, in the long-wavelength limit, yield a multiparticle theory for free fermions and bosons in three spatial dimensions: the Dirac and Maxwell field theories, i.e., free QED.

We present a quantum-state reconstruction procedure to retrieve a rotational wave packet created via nonadiabatic excitation by ultrashort pulses with controlled polarization. Here a unidirectionally rotating ensemble of CO molecules, adiabatically cooled to be populated only in $J$ = 0 as an initial state, has been prepared by skewed-polarization pulse-pair excitation. The two-dimensional sections of the molecular-axis probability distribution have been captured by time-delayed Coulomb explosion with the space-slice ion-imaging setup, recently developed in the present research group [K. Mizuse, R. Fujimoto, and Y. Ohshima, Rev. Sci. Instrum. 90, 103107 (2019)]. The time- and angle-dependent distribution has been subjected to the least-squares regression to provide a unique set of the amplitudes and phases for the eigenstates that constitute the wave packet. The reconstruction allows us to make a movie of the three-dimensional probability distribution and evaluate any physical properties, e.g., the magnitude of rotational angular momentum and its degree of orientation. The determined phases also serve as a sensitive probe for the excitation and the quantum interference to control the directionality of the rotational wave packet.

A nonreciprocal phonon laser in a spinning microwave magnomechanical system is proposed. The system consists of a spinning microwave resonator coupled with an yttrium iron garnet sphere. The Fizeau light-dragging effect caused by the spinning of the resonator leads to a significant difference in the mechanical gain and the threshold power for driving the resonator from the opposite directions, which results in a nonreciprocal phonon laser. The nonreciprocal phonon laser is highly tunable by the spinning speed and direction of the resonator. These results provide an experimentally feasible approach for exploring various nonreciprocal effects in cavity magnomechanical systems, and may find applications in photon, magnon, and phonon manipulations in many-body coupled systems.

With the emerging possibility to obtain emissions of triples of Greenberger-Horne-Zeilinger–entangled photons via direct parametric generation, we study here bright emissions of this kind which involve higher-order emissions of two triples, three triples, etc. Such states would constitute a natural generalization of the four-mode (two beams plus polarization) squeezed vacuum. We show how to avoid technical difficulties related to straight-ahead generalization of the usual approximate description of parametric down-conversion which uses a classical pump field. Using Padé approximation, we turn the first terms of the nonconverging perturbation expansion into elements of a converging series. This allows us to study nonclassicality of the new bright generalized Greenberger-Horne-Zeilinger states. We present both violations of local realism by such a radiation and simple entanglement indicators tailored for this case.

Quantum computers are capable of efficiently contracting unitary tensor networks, a task that is likely to remain difficult for classical computers. For instance, networks based on matrix product states or the multiscale entanglement renormalization ansatz can be contracted on a small quantum computer to aid the simulation of a large quantum system. However, without the ability to selectively reset qubits, the associated spatial cost can be exorbitant. In this paper, we propose a protocol that can unitarily reset qubits when the circuit has a common convolutional form, thus dramatically reducing the spatial cost for implementing the contraction algorithm on general near-term quantum computers. This protocol generates fresh qubits from used ones by partially applying the time-reversed quantum circuit over qubits that are no longer in use. In the absence of noise, we prove that the state of a subset of these qubits becomes $|0\cdots 0\rangle$, up to an error exponentially small in the number of gates applied. We also provide numerical evidence that the protocol works in the presence of noise and formulate a condition under which the noise-resilience follows rigorously.

We demonstrate experimentally a method of varying the degree of directionality in laser-induced molecular rotation. To control the ratio between the number of clockwise and counterclockwise rotating molecules (with respect to a fixed laboratory axis), we change the polarization ellipticity of the laser field of an optical centrifuge. The experimental data, supported by numerical simulations, show that the degree of rotational directionality can be varied in a continuous fashion between unidirectional and bidirectional rotation. The control can be executed with no significant loss in the total number of rotating molecules. The technique could be used for studying the effects of orientation of the molecular angular momentum on molecular collisions and chemical reactions. It could also be utilized for controlling magnetic and optical properties of gases, as well as for the enantioselective detection of chiral molecules.

General relativity predicts the existence of closed timelike curves (CTCs), along which an object could travel to its own past. A consequence of CTCs is the failure of determinism, even for classical systems: one initial condition can result in multiple evolutions. Here we introduce a quantum formulation of a classic example, where a billiard ball can travel along two possible trajectories: one unperturbed and one, along a CTC, where it collides with its past self. Our model includes a vacuum state, allowing the ball to be present or absent on each trajectory, and a clock, which provides an operational way to distinguish the trajectories. We apply the two foremost quantum theories of CTCs to our model: Deutsch's model (D-CTCs) and postselected teleportation (P-CTCs). We find that D-CTCs reproduce the classical solution multiplicity in the form of a mixed state, while P-CTCs predict an equal superposition of the two trajectories, supporting a conjecture by Friedman et al. [Phys. Rev. D 42, 1915 (1990)].

Quantum coherence is one of the key features that fuels applications for which quantum mechanics exceeds the power of classical physics. This explains the considerable efforts that were undertaken to quantify coherence via quantum resource theories. An application of the resulting framework to concrete technological tasks is however largely missing. Here, we address this problem and connect the ability of an operation to detect or create coherence to the performance of interferometric experiments.

Polyatomic molecules in strong laser fields can undergo substantial nuclear motion within tens of femtoseconds. Ion imaging methods based on dissociation or Coulomb explosion therefore have difficulty faithfully recording the geometry dependence of the field ionization that initiates the dissociation process. Here we compare the strong-field double ionization and subsequent dissociation of water (both ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$) in 10-fs and 40-fs 800-nm laser pulses. We find that 10-fs pulses turn off before substantial internuclear motion occurs, whereas rapid internuclear motion can take place during the double ionization process for 40-fs pulses. The short-pulse measurements are consistent with a simple tunnel ionization picture whose predictions help interpret the motion observed in the long-pulse measurements.

The solving of linear systems provides a rich area to investigate the use of nearer-term, noisy, intermediate-scale quantum computers. In this work, we discuss hybrid quantum-classical algorithms for heavily skewed linear systems for overdetermined and underdetermined cases. Our input model is such that the columns or rows of the matrix defining the linear system are given via quantum circuits of polylogarithmic depth and the number of circuits is much smaller than their Hilbert space dimension. Our algorithms have polylogarithmic dependence on the dimension and polynomial dependence in other natural quantities. In addition, we present an algorithm for the special case of a factorized linear system with run time polylogarithmic in the respective dimensions. At the core of these algorithms is the Hadamard test and in the second part of this paper, we consider the optimization of the circuit depth of this test. Given an $n$-qubit and $d$-depth quantum circuit $\mathcal{C}$, we can approximate $\langle 0|\mathcal{C}|0\rangle$ using $(n+s)$ qubits and $O\left({log}_{2}s+d{log}_2(n/s)+d\right)$-depth quantum circuits, where $s\le n$. In comparison, the standard implementation requires $n+1$ qubits and $O\left(dn\right)$ depth. Lattice geometries underlie recent quantum supremacy experiments with superconducting devices. We also optimize the Hadamard test for an $(l_1\times l_2)$ lattice with $l_1\times l_2=n$ and can approximate $\langle 0|\mathcal{C}|0\rangle$ with $(n+1)$ qubits and $O\left(d\left(l_1+l_2\right)\right)$-depth circuits. In comparison, the standard depth is $O\left(d{n}^2\right)$ in this setting. Both of our optimization methods are asymptotically tight in the case of one-depth quantum circuits $\mathcal{C}$.