## Physics

### The quantum theory of time, the block universe and human experience. (arXiv:1804.05177v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Advances in our understanding of the physical universe have impacted dramatically on how we view ourselves. Right at the core of all modern thinking about the universe is the assumption that dynamics is an elemental feature that exists without question. However, ongoing research into the quantum nature of time is challenging this view: my recently-introduced quantum theory of time suggests that dynamics may be a phenomenological consequence of a fundamental violation of time reversal symmetry. I show here that there is consistency between the new theory and the block universe view. I also discuss the new theory in relation to the human experience of existing in the present moment, able to reflect on the past and contemplate a future that is yet to happen.

Categories: Journals, Physics

### Adaptive quantum tomography of high-dimensional bipartite systems. (arXiv:1804.05226v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Adaptive measurements have recently been shown to significantly improve the performance of quantum state and process tomography. However, the existing methods either cannot be straightforwardly applied to high-dimensional systems or are prohibitively computationally expensive. Here we propose and experimentally implement a novel tomographic protocol specially designed for the reconstruction of high-dimensional quantum states. The protocol shows qualitative improvement in infidelity scaling with the number of measurements and is fast enough to allow for complete state tomography of states with dimensionality up to 36.

Categories: Journals, Physics

### Optimizing a Polynomial Function on a Quantum Simulator. (arXiv:1804.05231v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Gradient descent method, as one of the major methods in numerical optimization, is the key ingredient in many machine learning algorithms. As one of the most fundamental way to solve the optimization problems, it promises the function value to move along the direction of steepest descent. For the vast resource consumption when dealing with high-dimensional problems, a quantum version of this iterative optimization algorithm has been proposed recently[arXiv:1612.01789]. Here, we develop this protocol and implement it on a quantum simulator with limited resource. Moreover, a prototypical experiment was shown with a 4-qubit Nuclear Magnetic Resonance quantum processor, demonstrating a optimization process of polynomial function iteratively. In each iteration, we achieved an average fidelity of 94\% compared with theoretical calculation via full-state tomography. In particular, the iterative point gradually converged to the local minimum. We apply our method to multidimensional scaling problem, further showing the potentially capability to yields an exponentially improvement compared with classical counterparts. With the onrushing tendency of quantum information, our work could provide a subroutine for the application of future practical quantum computers.

Categories: Journals, Physics

### Summoning, No-Signaling and Relativistic Bit Commitments. (arXiv:1804.05246v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Summoning is a task between two parties, Alice and Bob, with distributed networks of agents in space-time. Bob gives Alice a random quantum state, known to him but not her, at some point. She is required to return the state at some later point, belonging to a subset defined by communications received from Bob at other points. Many results about summoning, including the impossibility of unrestricted summoning tasks and the necessary conditions for specific types of summoning tasks to be possible, follow directly from the quantum no-cloning theorem and the relativistic no-superluminal-signalling principle. The impossibility of cloning devices can be derived from the impossibility of superluminal signalling and the projection postulate, together with assumptions about the devices' location-independent functioning. In this qualified sense, known summoning results follow from the causal structure of space-time and the properties of quantum measurements. Bounds on the fidelity of approximate cloning can be similarly derived. Bit commitment protocols and other cryptographic protocols based on the no-summoning theorem can thus be proven secure against some classes of post-quantum but non-signalling adversaries.

Categories: Journals, Physics

### Kicked Hall Systems: Quantum-Dynamical and Spectral Manifestations of Generic Superweak Chaos. (arXiv:1804.05280v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Classical "kicked Hall systems" (KHSs), i.e., periodically kicked charges in the presence of uniform magnetic and electric fields that are perpendicular to each other and to the kicking direction, have been introduced and studied recently. It was shown that KHSs exhibit, under generic conditions, the phenomenon of "superweak chaos" (SWC), i.e., for small kick strength $\kappa$ a KHS behaves as if this strength were effectively $\kappa^2$ rather than $\kappa$. Here we investigate quantum-dynamical and spectral manifestations of this generic SWC. We first derive general expressions for quantum effective Hamiltonians for the KHSs. We then show that the phenomenon of quantum antiresonance (QAR), i.e., "frozen" quantum dynamics with flat quasienergy (QE) bands, takes place for integer values of a scaled Planck constant $\hbar_{\rm s}$ and under the same generic conditions for SWC. This appears to be the most generic occurrence of QAR in quantum systems. The vicinity of QAR is shown to correspond semiclassically to SWC. A global spectral manifestation of SWC is the fact that a scaled QE spectrum as function of $\hbar_{\rm s}$, at fixed small value of $\kappa /\hbar_{\rm s}$, features an approximately "doubled" structure. In the case of standard (cosine) potentials, this structure is that of a universal (parameters-independent) double Hofstadter butterfly. Also, for standard potentials and for small $\hbar_{\rm s}$ (semiclassical regime), the evolution of the kinetic-energy expectation value exhibits a relatively slow quantum-diffusive behavior having universal features. These approximate spectral and quantum-dynamical universalities agree with predictions from the effective Hamiltonian.

Categories: Journals, Physics

### Modified Unruh effect from Generalized Uncertainty Principle. (arXiv:1804.05282v1 [hep-th])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

We consider a generalized uncertainty principle (GUP) corresponding to a deformation of the fundamental commutator obtained by adding a term quadratic in the momentum. From this GUP, we compute corrections to the Unruh effect and related Unruh temperature, by first following a heuristic derivation, and then a more standard field theoretic calculation. In the limit of small deformations, we recover the thermal character of the Unruh radiation. Corrections to the temperature at first order in the deforming parameter are compared for the two approaches, and found to be in agreement as for the dependence on the cubic power of the acceleration of the reference frame. The dependence of the shifted temperature on the frequency is also pointed out and discussed.

Categories: Journals, Physics

### Topological valleytronics: Brought to light. (arXiv:1804.05323v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

The topological valley Hall effect was predicted as a consequence of the bulk topology of electronic systems. Now it has been observed in photonic crystals, showing that both topology and valley are innate to classical as well as quantum systems.

The discovery of counterflow in opposite valleys not only demonstrates that the valley can be a novel carrier of information and energy --- particularly valuable for systems without charge and spin --- but also exemplifies that symmetry-protected band topology can be universal to both quantum and classical systems.

Categories: Journals, Physics

### ZX-Rules for 2-qubit Clifford+T Quantum Circuits. (arXiv:1804.05356v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

ZX-calculus is a high-level graphical formalism for qubit computation. In this paper we give the ZX-rules that enable one to derive all equations between 2-qubit Clifford+T quantum circuits. Our rule set is only a small extension of the rules of stabilizer ZX-calculus, and substantially less than those needed for the recently achieved universal completeness. One of our rules is new, and we expect it to also have other utilities.

These ZX-rules are much simpler than the complete of set Clifford+T circuit equations due to Selinger and Bian, which indicates that ZX-calculus provides a more convenient arena for quantum circuit rewriting than restricting oneself to circuit equations. The reason for this is that ZX-calculus is not constrained by a fixed unitary gate set for performing intermediate computations.

Categories: Journals, Physics

### The Geometric Speed Limit of Accessible Quantum State Preparation. (arXiv:1804.05399v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

We analyze state preparation within a restricted space of local control parameters between adiabatically connected states of control Hamiltonians. We formulate a conjecture that the time integral of energy fluctuations over the protocol duration is bounded from below by the geodesic length set by the quantum geometric tensor. The conjecture implies a geometric lower bound for the quantum speed limit (QSL). We prove the conjecture for arbitrary sufficiently slow protocols using adiabatic perturbation theory, and show that the bound is saturated by geodesic protocols, which keep the energy variance constant along the trajectory. Our conjecture implies that any optimal unit-fidelity protocol, even those which drive the system far from equilibrium, are fundamentally constrained by the quantum geometry of adiabatic evolution. When the control space includes all possible couplings, spanning the full Hilbert space, we recover the well-known Mandelstam-Tamm bound. However, using only accessible local controls to anneal in complex models such as glasses, or target individual excited states in quantum chaotic systems, the geometric bound for the QSL can be exponentially large in the system size due to a diverging geodesic length. We validate our conjecture both analytically by constructing counter-diabatic and fast-forward protocols for a three-level system, and numerically in non-integrable spin chains using optimal control.

Categories: Journals, Physics

### Universality property of local weak interactions and its application for interferometric alignment. (arXiv:1804.05400v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

The modification of the effect of interactions of a particle as a function of its pre- and postselected states is analyzed theoretically and experimentally. The universality property of this modification in the case of local interactions of a spatially pre- and postselected particle has been found. It allowed to define an operational approach for characterization of the presence of a quantum particle in a particular place: the way it modifies the effect of local interactions. The experiment demonstrating this universality property provides an efficient interferometric alignment method, in which the beam on a single detector throughout one phase scan yields all misalignment parameters.

Categories: Journals, Physics

### Simulating Quantum Circuits by Shuffling Paulis. (arXiv:1804.05404v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Verification of NISQ era quantum devices demands fast classical simulation of large noisy quantum circuits. We present an algorithm based on the stabilizer formalism that can efficiently simulate noisy stabilizer circuits. Additionally, the protocol can efficiently simulate a large set of multi-qubit mixed states that are not mixtures of stabilizer states. The existence of these 'bound states' was previously only known for odd-dimensional systems like qutrits. The algorithm also has the favorable property that circuits with depolarizing noise are simulated much faster than unitary circuits. This work builds upon a similar algorithm by Bennink et al. (Phys. Rev. A 95, 062337) and utilizes a framework by Pashayan et al. (Phys. Rev. Lett. 115, 070501).

Categories: Journals, Physics

### Simple 2.5 GHz time-bin quantum key distribution. (arXiv:1804.05426v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

We present a 2.5 GHz quantum key distribution setup with the emphasis on a simple experimental realization. It features a three-state time-bin protocol based on a pulsed diode laser and a single intensity modulator. Implementing an efficient one-decoy scheme and finite-key analysis, we achieve record breaking secret key rates of 1.5 kbps over 200 km of standard optical fiber.

Categories: Journals, Physics

### Comparison study of quantum computing methods for simulating the Hamiltonian of the water molecule. (arXiv:1804.05453v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

We compare recently proposed different quantum simulation methods to simulate the ground state energy of the Hamiltonian for the water molecule on a quantum computer. The methods include the phase estimation algorithm based on Trotter decomposition, phase estimation algorithm based on the direct implementation of Hamiltonian, direct measurement based on the implementation of the Hamiltonian and the variational quantum eigensolver. After deriving the Hamiltonian, we first explain how each method works and then compare the simulation results in terms of the accuracy and the gate complexity for the ground state of the molecule with different O-H bond lengths. Moreover, we present the analytical analyses of the error and the gate-complexity for each method. While the required number of qubits for each method is almost the same, the number of gates and the error depend on the coefficients of the local Hamiltonians forming the Hamiltonian of the system and the desired accuracy. In conclusion, while the standard Trotter decomposition yields more accurate ground state energies for different bond lengths, the direct methods provide more efficient circuit implementations in terms of the gate complexity.

Categories: Journals, Physics

### Locality of Edge States and Entanglement Spectrum from Strong Subadditivity. (arXiv:1804.05457v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

We consider two-dimensional states of matter satisfying an uniform area law for entanglement. We show that the topological entanglement entropy is equal to the minimum relative entropy distance of the edge state of the system to the set of thermal states of local models. The argument is based on strong subadditivity of quantum entropy. For states with zero topological entanglement entropy, in particular, the formula gives locality of the edge states as thermal states of local Hamiltonians. It also implies that the entanglement spectrum of a region is equal to the spectrum of a one-dimensional local thermal state on the boundary of the region. Our result gives a precise information-theoretic interpretation for topological entanglement entropy as the number of bits of information needed to describe the non-local degrees of freedom of edge states.

Categories: Journals, Physics

### Spin manipulation and spin dephasing in quantum dot integrated with a general slanting magnetic field. (arXiv:1804.05476v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

A slanting magnetic field is usually used to realize a slight hybridization between the spin and the orbital degrees of freedom in a semiconductor quantum dot, such that the spin is manipulable by an external oscillating electric field. Here we show that, the longitudinal slanting field mediates a longitudinal driving term in the electric-dipole spin resonance, such that the spin population inversion exhibits a modulated Rabi oscillation. Fortunately, we can minimize this modulation by increasing the statical magnetic field. The longitudinal slanting field also mediates an interaction between the spin and the $1/f$ charge noise, which causes the spin pure dephasing. Choosing proper spectrum function strength, the spin dephasing time is about $T^{*}_{2}=17$ $\mu$s and the spin echo time is about $T^{\rm echo}_{2}=85$ $\mu$s in a Si quantum dot, in good agreement with experimental observations. We also propose several strategies to alleviate the spin dephasing, such as lowering the experimental temperature, reducing the quantum dot size, engineering the slanting field, and using the dynamical decoupling scheme.

Categories: Journals, Physics

### Violation of a causal inequality in a spacetime with definite causal order. (arXiv:1804.05498v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Processes with an indefinite causal structure may violate a causal inequality, which quantifies quantum correlations that arise from a lack of causal order. In this paper, we show that when the inequalities are analysed with a Gaussian-localised field theoretic definition of particles and labs, the causal indeterminacy of the fields themselves allows a causal inequality to be violated within the causal structure of Minkowski spacetime. We quantify the violation of the inequality and determine the optimal ordering of observers.

Categories: Journals, Physics

### On Entropy for general quantum systems. (arXiv:1804.05579v1 [math-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

In these notes we will give an overview and road map for a definition and characterization of (relative) entropy for both classical and quantum systems. In other words, we will provide a consistent treatment of entropy which can be applied within the recently developed Orlicz space based approach to large systems. This means that the proposed approach successfully provides a refined framework for the treatment of entropy in each of classical statistical physics, Dirac's formalism of Quantum Mechanics, large systems of quantum statistical physics, and finally also for Quantum Field Theory.

Categories: Journals, Physics

### Diamond nano-pyramids with narrow linewidth SiV centers for quantum technologies. (arXiv:1804.05583v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Color centers in diamond are versatile solid state atomic-like systems suitable for quantum technological applications. In particular, the negatively charged silicon vacancy center (SiV) can exhibit a narrow photoluminescence (PL) line and lifetime-limited linewidth in bulk diamonds at cryogenic temperature. We present a low-temperature study of chemical vapour deposition (CVD)-grown diamond nano-pyramids containing SiV centers. The PL spectra feature a bulk-like zero-phonon line with ensembles of SiV centers, with a linewidth below 10 GHz which demonstrates very low crystal strain for such a nano-object.

Categories: Journals, Physics

### Purity Temperature Dependent for Coupled Harmonic Oscillators. (arXiv:1804.05595v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

We consider the thermal aspect of a system composed of two coupled harmonic oscillators and study the corresponding purity. We initially consider a situation where the system is brought to a canonical thermal equilibrium with a heat-bath at temperature $T$. We adopt the path integral approach and introduce the evolution operator to calculate the density matrix and subsequently the reduced matrix density. It is used to explicitly determine the purity in terms of different physical quantities and therefore study some limiting cases related to temperature as well as other parameters. Different numerical results are reported and discussed in terms of the involved parameters of our system.

Categories: Journals, Physics

### Are quantum thermodynamic machines better than classical?. (arXiv:1804.05659v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-04-17 05:33

Interesting effects arise in cyclic machines where both heat and ergotropy transfer take place between the energising bath and the system (the working fluid). Such effects correspond to unconventional decompositions of energy exchange between the bath and the system into heat and work, respectively, resulting in efficiency bounds that may surpass the Carnot efficiency. However, these effects are not directly linked with quantumness, but rather with heat and ergotropy, the likes of which can be realised without resorting to quantum mechanics.

Categories: Journals, Physics