Author(s): Brian B. Zhou, Paul C. Jerger, V. O. Shkolnikov, F. Joseph Heremans, Guido Burkard, and David D. Awschalom

Although geometric phases in quantum evolution are historically overlooked, their active control now stimulates strategies for constructing robust quantum technologies. Here, we demonstrate arbitrary single-qubit holonomic gates from a single cycle of nonadiabatic evolution, eliminating the need to ...

[Phys. Rev. Lett. 119, 140503] Published Mon Oct 02, 2017

Author(s): Lincoln D. Carr and Simon L. Cornish

The creation of ultracold molecules that have both an electric and a magnetic dipole moment offers new ways to explore physics in the many-body quantum regime.

[Physics 10, 107] Published Mon Oct 02, 2017

Categories: Physics

Author(s): N. S. Maslova, P. I. Arseyev, and V. N. Mantsevich

The time evolution of an initially prepared entangled state in the system of coupled quantum dots has been analyzed by means of two different theoretical approaches: equations of motion for all orders localized electron correlation functions, considering interference effects, and kinetic equations f...

[Phys. Rev. A 96, 042301] Published Mon Oct 02, 2017

Fractons are emergent particles which are immobile in isolation, but which can move together in dipolar pairs or other small clusters. These exotic excitations naturally occur in certain quantum phases of matter described by tensor gauge theories. Previous research has focused on the properties of small numbers of fractons and their interactions, effectively mapping out the "Standard Model" of fractons. In the present work, however, we consider systems with a finite density of either fractons or their dipolar bound states, with a focus on the $U(1)$ fracton models. We study some of the phases in which emergent fractonic matter can exist, thereby initiating the study of the "condensed matter" of fractons. We begin by considering a system with a finite density of fractons, which we show can exhibit microemulsion physics, in which fractons form small-scale clusters emulsed in a phase dominated by long-range repulsion. We then move on to study systems with a finite density of mobile dipoles, which have phases analogous to many conventional condensed matter phases. We focus on two major examples: Fermi liquids and quantum Hall phases. A finite density of fermionic dipoles will form a Fermi surface and enter a Fermi liquid phase. Interestingly, this dipolar Fermi liquid exhibits a finite-temperature phase transition, corresponding to an unbinding transition of fractons. Finally, we study chiral two-dimensional phases corresponding to dipoles in "quantum Hall" states of their emergent magnetic field. We study numerous aspects of these generalized quantum Hall systems, such as their edge theories and ground state degeneracies.

The work performed by a classical electromagnetic field on a quantum dipole is well known in quantum optics. The absorbed power linearly depends on the time derivative of the average dipole moment, in that case. The following problem, however, still lacks an answer: can the most elementary electromagnetic pulse, consisting of a single-photon state, perform work on a quantum dipole? As a matter of fact, the average quantum dipole moment exactly vanishes in such a scenario. In this paper, we present a method that positively answers to this question, by combining techniques from the fields of quantum machines and open quantum systems. Quantum work here is defined as the unitary contribution to the energy variation of the quantum dipole. We show that this quantum work corresponds to the energy spent by the photon pulse to dynamically Stark shift the dipole. The non-unitary contribution to the dipole energy is defined here as a generalized quantum heat. We show that this generalized quantum heat is the energy corresponding to out-of-equilibrium photon absorption and emission. Finally, we reveal connexions between the quantum work and the generalized quantum heat transferred by a single photon and those by a low-intensity coherent field.

We investigate the validity of two common assumptions in the modelling of superconducting circuits: first, that the superconducting qubits are pointlike, and second, that the UV behaviour of the transmission line is not relevant to the qubit dynamics. We show that in the experimentally accessible ultra-strong coupling regime and for short (but attainable) times, the use of an inaccurate cutoff model (such as sharp, or none at all) could introduce very significant inaccuracies in the model's predictions.

An effective toy model for an ideal one-dimensional nonstationary cavity is taken to be the starting point to derive a fitting markovian master equation for the corresponding leaky cavity. In the regime where the generation of photons via the dynamical Casimir effect is bounded, the master equation thus constructed allows us to investigate the effects of decoherence on the average number of Casimir photons and their quantum fluctuations through the second-order correlation function.

The theory of the asymptotic manipulation of pure bipartite quantum systems can be considered completely understood: The rates at which bipartite entangled states can be asymptotically transformed into each other are fully determined by a single number each, the respective entanglement entropy. In the multi-partite setting, similar questions of the optimally achievable rates of transforming one pure state into another are notoriously open. This seems particularly unfortunate in the light of the revived interest in such questions due to the perspective of experimentally realizing multi-partite quantum networks. In this work, we report substantial progress by deriving surprisingly simple upper and lower bounds on the rates that can be achieved in asymptotic multi-partite entanglement transformations. These bounds are based on and develop ideas of entanglement combing, state merging, and assisted entanglement distillation. We identify cases where the bounds coincide and hence provide the exact rates. As an example, we bound rates at which resource states for the cryptographic scheme of quantum secret sharing can be distilled from arbitrary pure tri-partite quantum states, providing further scope for quantum internet applications beyond point-to-point.

One of the interesting topics in quantum contextuality is the construction for various non-contextual inequalities. By introducing a new structure called hyper-graph, we present a general method, which seems to be analytic and extensible, to derive the non-contextual inequalities for the qutrit systems. Based on this, several typical families of non-contextual inequalities are discussed. And our approach may also help us to simplify some state-independent proofs for quantum contextuality in one of our recent works.

We report development and microwave characterization of rf SQUID (Superconducting QUantum Interference Device) qubits, consisting of an aluminium-based Josephson junction embedded in a superconducting loop patterned from a thin film of TiN with high kinetic inductance. Here we demonstrate that the systems can offer small physical size, high anharmonicity, and small scatter of device parameters. The hybrid devices can be utilized as tools to shed further light onto the origin of film dissipation and decoherence in phase-slip nanowire qubits, patterned entirely from disordered superconducting films.

One-time programs, computer programs which self-destruct after being run only once, are a powerful building block in cryptography and would allow for new forms of secure software distribution. However, ideal one-time programs have been proved to be unachievable using either classical or quantum resources. Here we relax the definition of one-time programs to allow some probability of error in the output and show that quantum mechanics offers security advantages over purely classical resources. We introduce a scheme for encoding probabilistic one-time programs as quantum states with prescribed measurement settings, explore their security, and experimentally demonstrate various one-time programs using measurements on single-photon states. These include classical logic gates, computing the parity of a hidden set of bits, and a program to solve Yao's millionaires problem. By combining quantum and classical technology, we demonstrate the quantum techniques to enhance computing capabilities even before full-scale quantum computers are available.

One intriguing issue in the nucleon spin decomposition problem is the existence of two types of decompositions, which are representably characterized by two different orbital angular momenta (OAMs) of quarks. The one is the manifestly gauge-invariant mechanical OAM, while the other is the so-called gauge-invariant canonical (g.i.c.) OAM, the concept of which was introduced by Chen et al. To get a deep insight into the difference of these two decompositions, it is therefore vitally important to understand the the physical meanings of the above two OAMs correctly. Also to be clarified is the implication of the gauge symmetry that is immanent in the concept of g.i.c. OAM. We find that the famous Landau problem provides us with an ideal tool to answer these questions owing to its analytically solvable nature. After deriving a complete relation between the standard eigen-functions of the Landau Hamiltonian in the Landau gauge and in the symmetric gauge, we try to unravel the physics of the the canonical OAM and the mechanical OAM, by paying special attention to their gauge-dependence. We also argue that, different from the mechanical OAM of the electron, the canonical OAM or its gauge-invariant version would not correspond to any direct observables at least in the Landau problem. Also briefly discussed is the uniqueness or non-uniqueness problem of the nucleon spin decomposition, which arises from the arbitrariness in the definition of the so-called physical component of the gauge field.

We study a one-dimensional photonic resonator lattice with Kerr nonlinearity under the dynamic modulation. With an appropriate choice of the modulation frequency and phase, we find that this system can be used to create anyons from photons. By coupling the resonators with external waveguides, the anyon characteristics can be explored by measuring the transport property of the photons in the external waveguides.

Numerous molecular systems, including solutions, proteins, and composite materials, can be modeled using mixed-resolution representations, of which the quantum mechanics/molecular mechanics (QM/MM) approach has become the most widely used. However, the QM/MM approach often faces a number of challenges, including the slow sampling of the large configuration space for the MM part, the high cost of repetitive QM computations for changing coordinates of atoms in the MM surroundings, and a difficulty in providing a simple, qualitative interpretation of numerical results in terms of the influence of the molecular environment upon the active QM region. In this paper, we address these issues by combining QM/MM modeling with the methodology of "bottom-up" coarse-graining (CG) to provide the theoretical basis for a systematic quantum-mechanical/coarse-grained molecular mechanics (QM/CG-MM) mixed resolution approach. A derivation of the method is presented based on a combination of statistical mechanics and quantum mechanics, leading to an equation for the effective Hamiltonian of the QM part, a central concept in the QM/CG-MM theory. A detailed analysis of different contributions to the effective Hamiltonian from electrostatic, induction, dispersion and exchange interactions between the QM part and the surroundings is provided, serving as a foundation for a potential hierarchy of QM/CG-MM methods varying in their accuracy and computational cost. A relationship of the QM/CG-MM methodology to other mixed resolution approaches is also discussed.

We use strong complementarity to introduce dynamics and symmetries within the framework of CQM, which we also extend to infinite-dimensional separable Hilbert spaces: these were long-missing features, which open the way to a wealth of new applications. The coherent treatment presented in this work also provides a variety of novel insights into the dynamics and symmetries of quantum systems: examples include the extremely simple characterisation of symmetry-observable duality, the connection of strong complementarity with the Weyl Canonical Commutation Relations, the generalisations of Feynman's clock construction, the existence of time observables and the emergence of quantum clocks.

Furthermore, we show that strong complementarity is a key resource for quantum algorithms and protocols. We provide the first fully diagrammatic, theory-independent proof of correctness for the quantum algorithm solving the Hidden Subgroup Problem, and show that strong complementarity is the feature providing the quantum advantage. In quantum foundations, we use strong complementarity to derive the exact conditions relating non-locality to the structure of phase groups, within the context of Mermin-type non-locality arguments. Our non-locality results find further application to quantum cryptography, where we use them to define a quantum-classical secret sharing scheme with provable device-independent security guarantees.

All in all, we argue that strong complementarity is a truly powerful and versatile building block for quantum theory and its applications, and one that should draw a lot more attention in the future.

In this work, we apply the Cole's non-standard form of the FDTD to solve the time dependent Schr\"odinger equation. We deduce the equations for the non-standard FDTD considering an electronic wave function in the presence of potentials which can be higher or lower in comparison with the energy of the electron. The non-standard term is found to be almost the same, except for a sine functions which is transformed to a hyperbolic sine function,as the argument is imaginary when the potential has higher energy than the electron. Perfectly Matched Layers using this methodology are also presented.

We present the design of a passive, on-chip microwave circulator based on a ring of superconducting tunnel junctions. We investigate two distinct physical realisations, based on either Josephson junctions (JJ) or quantum phase slip elements (QPS), with microwave ports coupled either capacitively (JJ) or inductively (QPS) to the ring structure. A constant bias applied to the center of the ring provides the symmetry breaking (effective) magnetic field, and no microwave or rf bias is required. We find that this design offers high isolation even when taking into account fabrication imperfections and environmentally induced bias perturbations and find a bandwidth in excess of 500 MHz for realistic device parameters.

Fidelity mechanics is formalized as a framework to investigate quantum critical phenomena in quantum many-body systems. This is achieved by introducing fidelity temperature to properly quantify quantum fluctuations, which, together with fidelity entropy and fidelity internal energy, constitute three basic state functions in fidelity mechanics, thus enabling us to formulate analogues of the four thermodynamic laws and Landauer's principle at zero temperature. Fidelity flows are defined and may be interpreted as an alternative form of renormalization group flows. Thus, both stable and unstable fixed points are characterized in terms of fidelity temperature and fidelity entropy: divergent fidelity temperature for unstable fixed points and zero fidelity temperature and (locally) maximal fidelity entropy for stable fixed points. In addition, an inherently fundamental role of duality is clarified, resulting in a canonical form of the Hamiltonian in fidelity mechanics. Dualities, together with symmetry groups and factorizing fields, impose the constraints on a fidelity mechanical system, thus shaping fidelity flows from an unstable fixed point to a stable fixed point.

A detailed analysis of fidelity mechanical state functions is presented for the quantum XY model, the transverse field quantum Ising chain in a longitudinal field, the spin-$1/2$ XYZ model and the XXZ model in a magnetic field.

We show that under the weak measurement scheme, the double-slit experiment can produce an interference pattern even when one of the slits is completely blocked. The initial and final states are corpuscular, whilst the intermediate states are wave-like, in that it exhibits an interference pattern. Remarkably, the interference pattern is measured to be vertically polarised, whilst simultaneously the individual photons are measured to be horizontally polarised. We call this the \textit{phantom slit} effect. The phantom slit is the dual of the quantum Cheshire cat.

Technological development in the implementation of optical resonators strongly coupled to two-level systems has made it increasingly easy to access non-perturbative regimes known as ultra or deep strong coupling, where the coupling rate is of the order of the frequency of the bare modes and counter rotating terms in the Hamiltonian play a relevant role. In this work, we address the adequacy of the single-mode Rabi model to describe non- perturbative light-matter coupling. In the case of resonators with harmonic spectra, we discuss how the single- mode approximation yields unphysical results leading to superluminal signalling. We show that the multi-mode description of the field, necessary to account for light propagation at finite speed, yields physical observables, such as transition energies or population dynamics, that differ radically from their single-mode counterpart already in the ultrastrong coupling regime. Our analysis of the multi-mode Rabi model also reveals phenomena of fundamental interest on the dynamics of the electric field inside the cavity, where a free photonic wavefront and a bound state of virtual photons are shown to coexist.