Author(s): Rachel Berkowitz

Experiments reveal new details of the process by which contaminants in the ocean could reach the atmosphere through the bursting of bubbles in foam.

[Physics 16, 17] Published Fri Feb 03, 2023

Categories: Physics

Author(s): Katherine Wright

Ice nucleation in freezing drops can suddenly increase the drops’ velocity via a rocket-like mechanism.

[Physics 16, s18] Published Fri Feb 03, 2023

Categories: Physics

Author(s): Diego Tancara, Hossein T. Dinani, Ariel Norambuena, Felipe F. Fanchini, and Raúl Coto

Quantum machine learning is a growing research field that aims to perform machine learning tasks assisted by a quantum computer. Kernel-based quantum machine learning models are paradigmatic examples where the kernel involves quantum states, and the Gram matrix is calculated from the overlap between…

[Phys. Rev. A 107, 022402] Published Fri Feb 03, 2023

Disorder-free localization (DFL) is an ergodicity breaking mechanism that has been shown to occur in lattice gauge theories in the quench dynamics of initial states spanning an extensive number of gauge superselection sectors. Whether DFL is intrinsically a quantum interference effect or can arise classically has hitherto remained an open question whose resolution is pertinent to further understanding the far-from-equilibrium dynamics of gauge theories. In this work, we utilize cellular automaton circuits to model the quench dynamics of large-scale quantum link model (QLM) formulations of $(1+1)$D quantum electrodynamics, showing excellent agreement with the exact quantum case for small system sizes. Our results demonstrate that DFL persists in the thermodynamic limit as a purely classical effect arising from the finite-size regularization of the gauge-field operator in the QLM formulation, and that quantum interference, though not a necessary condition, may be employed to enhance DFL.

Quantum entanglement of weak interaction gauge bosons produced at colliders can be explored by computing the corresponding polarization density matrix. To this end, we consider the Higgs boson decays $H\to W W^*$ and $H\to Z Z^*$, in which $W^*$ and $Z^*$ are off-shell states, and the $WW$, $WZ$ and $ZZ$ di-boson production in proton collisions. The polarization density matrix of the di-boson state is determined by the amplitude of the production process and can be experimentally reconstructed from the angular distribution of the momenta of the charged leptons into which the gauge boson decays. We show that a suitable instance of the Bell inequality is violated in the Higgs boson decays to a degree that can be tested with high accuracy at the LHC already with present data. The same Bell inequality is violated in the production of $WW$ and $ZZ$ boson pairs for invariant masses above 900 GeV and scattering angles close to $\pi/2$ in the center of mass frame. In this case high luminosity LHC data are needed to detect violations of the Bell inequality with sufficient accuracy. We also analyze the prospects for detecting Bell inequality violations in di-boson final states at future $e^+e^-$ and muon colliders. A further observable that provides a lower bound on the amount of polarization entanglement in the di-boson system is computed for each of the examined processes. The analytic expressions for the polarization density matrices are presented in full in an Appendix. We also provide the unitary matrices required in the optimization procedure necessary in testing the Bell inequalities.

Entanglement entropies of two-dimensional gapped ground states are expected to satisfy an area law, with a constant correction term known as the topological entanglement entropy (TEE). In many models, the TEE takes a universal value that characterizes the underlying topological phase. However, the TEE is not truly universal: it can differ even for two states related by constant-depth circuits, which are necessarily in the same phase. The difference between the TEE and the value predicted by the anyon theory is often called the spurious topological entanglement entropy. We show that this spurious contribution is always nonnegative, thus the value predicted by the anyon theory provides a universal lower bound. This observation also leads to a definition of TEE which is invariant under constant-depth quantum circuits.

Multi-photon entanglement plays a central role in optical quantum technologies. One way to entangle two photons is to prepare them in orthogonal internal states, for example, in two polarisations, and then send them through a balanced beam splitter. Post-selecting on the cases where there is one photon in each output port results in a maximally entangled state. This idea can be extended to schemes for the post-selected generation of larger entangled states. Typically, switching between different types of entangled states require different arrangements of beam splitters and so a new experimental setup. Here, we demonstrate a simple and versatile scheme to generate different types of genuine tripartite entangled states with only one experimental setup. We send three photons through a three-port splitter and vary their internal states before post-selecting on certain output distributions. This results in the generation of tripartite W, G and GHZ states. We obtain fidelities of up to $(87.3 \pm 1.1)\%$ with regard to the respective ideal states, confirming a successful generation of genuine tripartite entanglement.

Symmetric multiport splitters are versatile tools in optical quantum information processing. They can be used for studying multiparticle scattering, studying distinguishability and mixedness, and also for the generation of multipartite entangled quantum states. Here, we show that N-photon N-mode Greenberger-Horne-Zeilinger (GHZ) states can be generated using symmetric multiport beam splitters. Varying the input states' internal degrees of freedom and post-selecting onto certain photon-number distributions allows the probabilistic generation of GHZ states with arbitrary photon numbers. We present two novel schemes, one for odd and one for even numbers of photons, to generate GHZ states using symmetric multiport splitters and compare them to a strategy utilizing a 2N-port network as well as the standard post-selection method.

We study the dynamics of a pair of optomechanical systems interacting dissipatively with a wave guide in a unidirectional way. We investigate the behaviour of both classical and quantum correlations established between the two mechanical modes both in the transient and in the stationary regime. We find that a constant amount of steady correlations can exists at long times. We furthermore analyze the power spectrum of the output guide field and we show how from such spectrum it is possible to reconstruct the spectra of each single mirror. Finally we show that that, thanks to the unidirectional coupling, a temperature gradient between the mirrors depending on the frequencies detuning is established .

Weak values and Kirkwood--Dirac (KD) quasiprobability distributions have been independently associated with both foundational issues in quantum theory and advantages in quantum metrology. We propose simple quantum circuits to measure weak values, KD distributions, and density matrix spectra without the need for post-selection. This is achieved by measuring unitary-invariant, relational properties of quantum states, as functions of Bargmann invariants. Our circuits also enable direct experimental implementation of various applications of KD distributions, such as out-of-time-ordered correlators (OTOCs) and the quantum Fisher information in post-selected parameter estimation, among others. This results in a unified view of nonclassicality in all those tasks. In particular, we discuss how negativity and imaginarity of Bargmann invariants relate to set coherence.

Based on a microscopic model, we use a functional integral approach to evaluate the quantum interaction energy between two neutral atoms. Each atom is coupled to the electromagnetic (EM) field via a dipole term, generated by an electron bound to the nucleus via a harmonic potential. We show that the resulting expression for the energy becomes the Van der Waals interaction energy at the first non-trivial order in an expansion in powers of the fine structure constant, encompassing both the long and short distance behaviours. We also explore the opposite, strong-coupling limit, which yields a result for the interaction energy as well as a threshold for the existence of a vacuum decay probability, manifested here as an imaginary part for the effective action.

In the weak-coupling limit, we also study the effect of using a general central potential for the internal structure of the atoms.

Multi-qubit parity measurements are at the core of many quantum error correction schemes. Extracting multi-qubit parity information typically involves using a sequence of multiple two-qubit gates. In this paper, we propose a superconducting circuit device with native support for multi-qubit parity-controlled gates (PCG). These are gates that perform rotations on a parity ancilla based on the multi-qubit parity operator of adjacent qubits, and can be directly used to perform multi-qubit parity measurements. The circuit consists of a set of concatenated Josephson ring modulators and effectively realizes a set of transmon-like qubits with strong longitudinal nearest-neighbor couplings. PCGs are implemented by applying microwave drives to the parity ancilla at specific frequencies. We investigate the scheme's performance with numerical simulation using realistic parameter choices and decoherence rates, and find that the device can perform four-qubit PCGs in 30 ns with process fidelity surpassing 99%. Furthermore, we study the effects of parameter disorder and spurious coupling between next-nearest neighboring qubits. Our results indicate that this approach to realizing PCGs constitute an interesting candidate for near-term quantum error correction experiments.

Engines are systems and devices that convert one form of energy into another, typically into a more useful form that can perform work. In the classical setup, physical, chemical, and biological engines largely involve the conversion of heat into work. This energy conversion is at the core of thermodynamic laws and principles and is codified in textbook material. In the quantum regime, however, the principles of energy conversion become ambiguous, since quantum phenomena come into play. As with classical thermodynamics, fundamental principles can be explored through engines and refrigerators, but, in the quantum case, these devices are miniaturized and their operations involve uniquely quantum effects. Our work provides a broad overview of this active field of quantum engines and refrigerators, reviewing the latest theoretical proposals and experimental realizations. We cover myriad aspects of these devices, starting with the basic concepts of quantum analogs to the classical thermodynamic cycle and continuing with different quantum features of energy conversion that span many branches of quantum mechanics. These features include quantum fluctuations that become dominant in the microscale, non-thermal resources that fuel the engines, and the possibility of scaling up the working medium's size, to account for collective phenomena in many-body heat engines. Furthermore, we review studies of quantum engines operating in the strong system-bath coupling regime and those that include non-Markovian phenomena. Recent advances in thermoelectric devices and quantum information perspectives, including quantum measurement and feedback in quantum engines, are also presented.

In ``Distributed quantum sensing with mode-entangled spin-squeezed atomic states" Nature (2022), Malia et. al. claim to improve the precision of a network of clocks by using entanglement. In particular, by entangling a clock network with up to four nodes, a precision 11.6\,dB better than the quantum projection noise limit (i.e. precision without any entanglement) is reported. These claims are incorrect, Malia et. al. do not achieve an improved precision with entanglement. Here we show their demonstration is more than two orders of magnitude worse than the quantum projection noise limit.

Optical solitons are known to be classically stable objects which are robust to perturbations. In this work, we show that due to quantum mechanical effects, an optical soliton that is initially in a classical soliton coherent state will shed photons into the continuum and hence decay. The standard formulation of the quantized soliton uses the linearized version of the quantum nonlinear Schrodinger equation in the background of the classical soliton, and the quantized soliton remains stable in this approximation. We show that if higher-order interaction terms are taken into account, the soliton is no longer stable, and its photon number decreases quadratically as a function of the number of soliton cycles. We compute the power spectrum for the continuum radiation and find a narrow band that is localized about the initial soliton momentum with a cut-off that is inversely proportional to the initial soliton width.

The recently proposed Quantum Neuron Born Machine (QNBM) has demonstrated quality initial performance as the first quantum generative machine learning (ML) model proposed with non-linear activations. However, previous investigations have been limited in scope with regards to the model's learnability and simulatability. In this work, we make a considerable leap forward by providing an extensive deep dive into the QNBM's potential as a generative model. We first demonstrate that the QNBM's network representation makes it non-trivial to be classically efficiently simulated. Following this result, we showcase the model's ability to learn (express and train on) a wider set of probability distributions, and benchmark the performance against a classical Restricted Boltzmann Machine (RBM). The QNBM is able to outperform this classical model on all distributions, even for the most optimally trained RBM among our simulations. Specifically, the QNBM outperforms the RBM with an improvement factor of 75.3x, 6.4x, and 3.5x for the discrete Gaussian, cardinality-constrained, and Bars and Stripes distributions respectively. Lastly, we conduct an initial investigation into the model's generalization capabilities and use a KL test to show that the model is able to approximate the ground truth probability distribution more closely than the training distribution when given access to a limited amount of data. Overall, we put forth a stronger case in support of using the QNBM for larger-scale generative tasks.

We discuss the procedure for obtaining measurement-based implementations of quantum algorithms given by quantum circuit diagrams and how to reduce the required resources needed for a given measurement-based computation. This forms the foundation for quantum computing on photonic systems in the near term. To demonstrate that these ideas are well grounded we present three different problems which are solved by employing a measurement-based implementation of the variational quantum eigensolver algorithm (MBVQE). We show that by utilising native measurement-based gates rather than standard gates, such as the standard CNOT, MBQCs may be obtained that are both shallow and have simple connectivity while simultaneously exhibiting a large expressibility. We conclude that MBVQE has promising prospects for resource states that are not far from what is already available today.

One of the most exciting quantum emulation [1] breakthroughs was the first analog signal-based emulation of a universal quantum computer [2]. This yielded a very interesting paper, but no practical use - even for theorists. The reason for this was that a signal duration of the approximate age of the universe (13.77 billion years) could accommodate only about 95 qubits. We propose a new scheme with the following properties: 1) a pair of oscillators or sinusioidal wave sources must be sufficient to emulate n superimposed states with the ability to be identifiably mixed or entangled, 2) the time required to perform a measurement of a state must not scale poorly with the complexity of the state, 3) a fixed set of hardware components must be sufficient to emulate a system of a significant number of qubits, and 4) at least as much must be knowable about an emulated quantum state as is expected to be measurable in a theoretical quantum computing system. We achieve a design whose time complexity scales favourably based on a new method of encoding quantum information into classical signals, but only anticipate the feasibility of encodings of up to 20 qubits with modern electrical hardware.

We calculate repulsive Casimir forces between metallic and magnetic plates and quantitatively probe the magnetic plate's properties as tuning knobs for the repulsion. Namely, the plate's thickness and its permittivity and permeability at vanishing frequency. We also explore the effect of temperature on the repulsion and transition distance between attractive and repulsive interactions. We show how the parameters can be tuned to allow repulsion at sub-micron separation regimes, making it potentially accessible to known high-resolution measurement techniques using magnetic van der Waals materials.

The non-Hermitian models, which are symmetric under parity (P) and time-reversal (T) operators, are the cornerstone for the fabrication of new ultra-sensitive optoelectronic devices. However, providing the gain in such systems usually demands precise contorol of nonlinear processes, limiting their application. In this paper, to bypass this obstacle, we introduce a class of time-dependent non-Hermitian Hamiltonians (not necessarily Floquet) that can describe a two-level system with temporally modulated on-site potential and couplings. We show that implementing an appropriate non-Unitary gauge transformation converts the original system to an effective one with a balanced gain and loss. This will allow us to derive the evolution of states analytically. Our proposed class of Hamiltonians can be employed in different platforms such as electronic circuits, acoustics, and photonics to design structures with hidden PT-symmetry potentially without imaginary onsite amplification and absorption mechanism to obtain an exceptional point.