## Journals

### Optomechanical transistor with mechanical gain. (arXiv:1801.05666v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We study an optomechanical transistor, where an input field can be transferred and amplified unidirectionally in a cyclic three-mode optomechanical system. In this system, the mechanical resonator is coupled simultaneously to two cavity modes. We show that it only requires a finite mechanical gain to achieve the nonreciprocal amplification. Here the nonreciprocity is caused by the phase difference between the linearized optomechanical couplings that breaks the time-reversal symmetry of this system. The amplification arises from the mechanical gain, which provides an effective phonon bath that pumps the mechanical mode coherently. This effect is analogous to the stimulated emission of atoms, where the probe field can be amplified when its frequency is in resonance with that of the anti-Stokes transition. We show that by choosing optimal parameters, this optomechanical transistor can reach perfect unidirectionality accompanied with strong amplification. In addition, the presence of the mechanical gain can result in ultra-long delay in the phase of the probe field, which provides an alternative to controlling light transport in optomechanical systems.

Categories: Journals, Physics

### Quantum gravity: a geometrical perspective. (arXiv:1801.05689v1 [gr-qc])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We present a theory of quantum gravity that combines a geometrical formulation of quantum field theory in space-time with classical Einstein's general relativity. This approach is based on the geometrization of quantum mechanics proposed in refs.[1,2] and combines quantum and gravitational effects into a global curvature of the Finsler's space associated to the 4N-dimensional configuration space of a N-particle system. In order to make this theory compatible with general relativity, the quantum effects are described in the framework of quantum field theory, where a covariant definition of 'simultaneity' for many-body systems is introduced through the definition of a suited foliation of space-time. As for Einstein's classical gravitation theory, the particles dynamics is finally described by means of a geodesic equation in a curved space-time manifold.

Categories: Journals, Physics

### Quantum Guiding Equation Can Determine Classical Behavior of Earth for Large Quantum Numbers. (arXiv:1801.05691v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

For quantum systems, we expect to see classical behavior at the limit of large quantum numbers. Hence, we apply Bohmian approach to describe the Earth evolution around the Sun. We obtain possible trajectories of the Earth system with different initial conditions which converge to a definite stable orbit after a given time, known as the Kepler orbit. The trajectories have resulted from the guiding $p=\nabla S$ equation in Bohmian mechanics which relates the momentum of the system to the phase part of the wave function. Except for some special situations, Bohmian trajectories are not Newtonian in character. We show that the classical behavior of the Earth can be described as the consequence of the guiding equation at the limit of large quantum numbers.

Categories: Journals, Physics

### Engineering Quantum Spin Liquids and Many-Body Majorana States with a Driven Superconducting Box Circuit. (arXiv:1801.05698v1 [cond-mat.str-el])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We design a driven superconducting box with four spins-1/2 (qubits) such that coupled devices can give insight on the occurrence of quantum spin liquids and many-body Majorana states. Within one box or island, we introduce a generalized nuclear magnetic resonance protocol and study numerically the dynamics in time, as well as dissipation effects on spins, to probe Majorana braiding and to detect the gauge fields. Coupling boxes allow to realize quantum spin liquid phases of Kitaev Z2 spin models in various geometries with applications in the toric code. We further present an implementation of the Sachdev-Ye-Kitaev model in coupled ladder systems.

Categories: Journals, Physics

### Number state filtered coherent state. (arXiv:1801.05704v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Number state filtering in coherent states leads to sub-Poissonian photon statistics. These states are more suitable for phase estimation when compared with the coherent states. Nonclassicality of these states is quantified in terms of the negativity of the Wigner function and the entanglement potential. Filtering of the vacuum from a coherent state is almost like the photon-addition. However, filtering makes the state more resilient against dissipation than photon-addition. Vacuum state filtered coherent states perform better than the photon-added coherent states for a two-way quantum key distribution protocol. A scheme to generate these states in multi-photon atom-field interaction is presented.

Categories: Journals, Physics

### L-state solutions of a new four-parameter 1/r^2 singular radial non-conventional potential via asymptotic iteration method. (arXiv:1801.05713v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

In the present work, we give a numerical solution of the radial Schr\"odinger equation for new four-parameter radial non-conventional potential, which was introduced by Alhaidari. In our calculations, we applied the asymptotic iteration method (AIM) to calculate the eigenvalues of the potential for arbitrary parameters and any l state. It is found that this method gives highly accurate results that compares favorably with other. Moreover, some new results were presented in this paper.

Categories: Journals, Physics

### Exact quantum query complexity of weight decision problems. (arXiv:1801.05717v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

The weight decision problem, which requires to determine the Hamming weight of a given binary string, is a natural and important problem, with applications in cryptanalysis, coding theory, fault-tolerant circuit design and so on. In particular, both Deutsch-Jozsa problem and Grover search problem can be interpreted as special cases of weight decision problems. In this work, we investigate the exact quantum query complexity of weight decision problems, where the quantum algorithm must always output the correct answer. More specifically we consider a partial Boolean function which distinguishes whether the Hamming weight of the length-$n$ input is $k$ or it is $l$. Our contribution includes both upper bounds and lower bounds for the precise number of queries. Furthermore, for most choices of $(\frac{k}{n},\frac{l}{n})$ and sufficiently large $n$, the gap between our upper and lower bounds is no more than one. To get the results, we first build the connection between Chebyshev polynomials and our problem, then determine all the boundary cases of $(\frac{k}{n},\frac{l}{n})$ with matching upper and lower bounds, and finally we generalize to other cases via a new \emph{quantum padding} technique. This quantum padding technique can be of independent interest in designing other quantum algorithms.

Categories: Journals, Physics

### Linear confinement of a scalar particle in a G\"odel-type spacetime. (arXiv:1801.05718v1 [gr-qc])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Based on the studies of confinement of quarks, we introduce a linear scalar potential into the relativistic quantum dynamics of a scalar particle. Then, we analyse the linear confinement of a relativistic scalar particle in a G\"odel-type spacetime in the presence of a topological defect. We consider a G\"odel-type spacetime associated with null curvature, i.e., the Som-Raychaudhuri spacetime, which is characterized by the presence of vorticity in the spacetime. Then, we search for analytical solutions to the Klein-Gordon equation and analyse the influence of the topology of the cosmic string and the vorticity on the relativistic energy levels.

Categories: Journals, Physics

### A compressed classical description of quantum states. (arXiv:1801.05721v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We show how to approximately represent a quantum state using the square root of the usual amount of classical memory. The classical representation of an n-qubit state $\psi$ consists of its inner products with $O(\sqrt{2^n})$ stabilizer states. A quantum state initially specified by its $2^n$ entries in the computational basis can be compressed to this form in time $O(2^n \mathrm{poly}(n))$, and, subsequently, the compressed description can be used to additively approximate the expectation value of an arbitrary observable. Our compression scheme directly gives a new protocol for the vector in subspace problem with randomized one-way communication complexity that matches (up to polylogarithmic factors) the best known upper bound, due to Raz. We obtain an exponential improvement over Raz's protocol in terms of computational efficiency.

Categories: Journals, Physics

### Entanglement between a photonic time-bin qubit and a collective atomic spin excitation. (arXiv:1801.05723v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Entanglement between light and matter combines the advantage of long distance transmission of photonic qubits with the storage and processing capabilities of atomic qubits. To distribute photonic states efficiently over long distances several schemes to encode qubits have been investigated -- time-bin encoding being particularly promising due to its robustness against decoherence in optical fibers. Here, we demonstrate the generation of entanglement between a photonic time-bin qubit and a single collective atomic spin excitation (spin-wave) in a cold atomic ensemble, followed by the mapping of the atomic qubit onto another photonic qubit. A magnetic field that induces a periodic dephasing and rephasing of the atomic excitation ensures the temporal distinguishability of the two time-bins and plays a central role in the entanglement generation. To analyse the generated quantum state, we use largely imbalanced Mach-Zehnder interferometers to perform projective measurements in different qubit bases and verify the entanglement by violating a CHSH Bell inequality.

Categories: Journals, Physics

### Avoiding apparent signaling in Bell tests for quantitative applications. (arXiv:1801.05739v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Bell tests have become a powerful tool for quantifying security, randomness, entanglement, and many other properties, as well as for investigating fundamental physical limits. In all these cases, the specific experimental value of the Bell parameter is important as it leads to a quantitative conclusion. However, most experimental implementations aiming for high values of the Bell parameter suffer from the defect of showing signaling. This signaling can be attributed to systematic errors occurring due to weaknesses in the experimental designs. Here we point out the importance, for quantitative applications, to identify and address this problem. We present a set of experiments with polarization-entangled photons in which we point out common sources of systematic errors and demonstrate approaches to avoid them. This allows us to establish a reliable estimate for the Bell parameter.

Categories: Journals, Physics

### Reconstruction of quantum theory from universal filters. (arXiv:1801.05798v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

This paper reconstructs quantum theory from operational principles starting from something we dub an effect theory, a generalisation of operational probabilistic theories that does not have any monoidal structure and does not a priori refer to the real numbers, allowing a study of more exotic theories. Our first postulates require the existence of initial filters and final compressions for effects, a variation of the ideal compression axiom of Chiribella et al. (2011). Restricting to the standard operational setting (real probabilities, finite-dimensional) we show that this leads to the existence of spectral decompositions of effects in terms of sharp effects. In the presence of two additional postulates that are relatively standard (composition of pure maps being pure again, and the existence of a transitive symmetry group) we show that the systems must be Euclidean Jordan algebras. Finally we add an "observability of energy" condition (Barnum et al. 2014) to complete the reconstruction of quantum theory.

Categories: Journals, Physics

### Measurement uncertainty relations: characterising optimal error bounds for qubits. (arXiv:1512.00104v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

In standard formulations of the uncertainty principle, two fundamental features are typically cast as impossibility statements: two noncommuting observables cannot in general both be sharply defined (for the same state), nor can they be measured jointly. The pioneers of quantum mechanics were acutely aware and puzzled by this fact, and it motivated Heisenberg to seek a mitigation, which he formulated in his seminal paper of 1927. He provided intuitive arguments to show that the values of, say, the position and momentum of a particle can at least be unsharply defined, and they can be measured together provided some approximation errors are allowed. Only now, nine decades later, a working theory of approximate joint measurements is taking shape, leading to rigorous and experimentally testable formulations of associated error tradeoff relations. Here we briefly review this new development, explaining the concepts and steps taken in the construction of optimal joint approximations of pairs of incompatible observables. As a case study, we deduce measurement uncertainty relations for qubit observables using two distinct error measures. We provide an operational interpretation of the error bounds and discuss some of the first experimental tests of such relations.

Categories: Journals, Physics

### Double-slit experiment in momentum space. (arXiv:1606.04732v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Young's classic double-slit experiment demonstrates the reality of interference when waves and particles travel simultaneously along two different spatial paths. Here, we propose a double-slit experiment in momentum space, realized in the free-space elastic scattering of vortex electrons. We show that this process proceeds along two paths in momentum space, which are well localized and well separated from each other. For such vortex beams, the (plane-wave) amplitudes along the two paths acquire adjustable phase shifts and produce interference fringes in the final angular distribution. We argue that this experiment can be realized with the present day technology. We show that it gives experimental access to the Coulomb phase, a quantity which plays an important role in all charged particle scattering but which usual scattering experiments are insensitive to.

Categories: Journals, Physics

### Lorentz quantum mechanics. (arXiv:1701.00057v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We present a theoretical framework called Lorentz quantum mechanics, where the dynamics of a system is a complex Lorentz transformation in complex Minkowski space. In contrast, in usual quantum mechanics, the dynamics is the unitary transformation in Hilbert space. In our Lorentz quantum mechanics, there exist three types of states, space-like, light-like, and time-like. Fundamental aspects are explored in parallel to the usual quantum mechanics, such as matrix form of a Lorentz transformation, construction of Pauli-like matrices for spinors. We also investigate the adiabatic evolution in this mechanics, as well as the associated Berry curvature and Chern number. Three typical physical systems, where this Lorentz quantum dynamics can arise, are presented. They are one dimensional fermion gas, Bose-Einstein condensate (or superfluid), and one dimensional antiferromagnet.

Categories: Journals, Physics

### Proton tunneling in hydrogen bonds and its implications in an induced-fit model of enzyme catalysis. (arXiv:1703.00789v2 [physics.chem-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

The role of proton tunneling in biological catalysis is investigated here within the frameworks of quantum information theory and thermodynamics. We consider the quantum correlations generated through two hydrogen bonds between a substrate and a prototypical enzyme that first catalyzes the tautomerization of the substrate to move on to a subsequent catalysis, and discuss how the enzyme can derive its catalytic potency from these correlations. In particular, we show that classical changes induced in the binding site of the enzyme spreads the quantum correlations among all of the four hydrogen-bonded atoms thanks to the directionality of hydrogen bonds. If the enzyme rapidly returns to its initial state after the binding stage, the substrate ends in a new transition state corresponding to a quantum superposition. Open quantum system dynamics can then naturally drive the reaction in the forward direction from the major tautomeric form to the minor tautomeric form without needing any additional catalytic activity. We find that in this scenario the enzyme lowers the activation energy so much that there is no energy barrier left in the tautomerization, even if the quantum correlations quickly decay.

Categories: Journals, Physics

### Conversion of Thermal Equilibrium States into Superpositions of Macroscopically Distinct States. (arXiv:1703.05034v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

A simple procedure for obtaining superpositions of macroscopically distinct states is proposed and analyzed. We find that a thermal equilibrium state can be converted into such a state when a single global measurement of a macroscopic observable, such as the total magnetization, is made. This method is valid for systems with macroscopic degrees of freedom and finite (including zero) temperature. The superposition state is obtained with a high (low) probability when the measurement is made with a high (low) resolution. We find that this method is feasible in an experiment.

Categories: Journals, Physics

### One-sided Measurement-Device-Independent Quantum Key Distribution. (arXiv:1704.04371v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Measurement-device-independent quantum key distribution (MDI-QKD) protocol was proposed to remove all the detector side channel attacks, while its security relies on the trusted encoding systems. Here we propose a one-sided MDI-QKD (1SMDI-QKD) protocol, which enjoys detection loophole-free advantage, and at the same time weakens the state preparation assumption in MDI-QKD. The 1SMDI-QKD can be regarded as a modified MDI-QKD, in which Bob's encoding system is trusted, while Alice's is uncharacterized. For the practical implementation, we also provide a scheme by utilizing coherent light source with an analytical two decoy state estimation method. Simulation with realistic experimental parameters shows that the protocol has a promising performance, and thus can be applied to practical QKD applications.

Categories: Journals, Physics

### Machine-learning-assisted correction of correlated qubit errors in a topological code. (arXiv:1705.07855v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

A fault-tolerant quantum computation requires an efficient means to detect and correct errors that accumulate in encoded quantum information. In the context of machine learning, neural networks are a promising new approach to quantum error correction. Here we show that a recurrent neural network can be trained, using only experimentally accessible data, to detect errors in a widely used topological code, the surface code, with a performance above that of the established minimum-weight perfect matching (or blossom) decoder. The performance gain is achieved because the neural network decoder can detect correlations between bit-flip (X) and phase-flip (Z) errors. The machine learning algorithm adapts to the physical system, hence no noise model is needed. The long short-term memory layers of the recurrent neural network maintain their performance over a large number of quantum error correction cycles, making it a practical decoder for forthcoming experimental realizations of the surface code.

Categories: Journals, Physics

### Implementing Parrondo's paradox with two coin quantum walks. (arXiv:1702.05927v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Parrondo's paradox is ubiquitous in games, ratchets and random walks.The apparent paradox, devised by J.~M.~R.~Parrondo, that two losing games $A$ and $B$ can produce an winning outcome has been adapted in many physical and biological systems to explain their working. However, proposals on demonstrating Parrondo's paradox using quantum walks failed {for large number of steps}. In this work, we show that instead of a single coin if we consider a two coin initial state which may or may not be entangled, we can observe a genuine Parrondo's paradox with quantum walks. Further we focus on reasons for this and pin down the asymmetry in initial two-coin state or asymmetry in shift operator, either of which are necessary for observing a genuine Parrondo's paradox. We extend our work to a 3-coin initial state too with similar results. The implications of our work for observing quantum ratchet like behavior using quantum walks is also discussed.

Categories: Journals, Physics