## Physics

### Expanding possibilities of quantum state engineering and amplifying optical Schrodinger kitten state. (arXiv:1905.07334v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We demonstrate an optical method to engineer optical Schrodinger cat states (SCSs) of large amplitude in the range from 2 to 3 with high fidelity close to 0.99. The approach uses the {\alpha}-representation of the SCSs in infinite Hilbert with base displaced number states characterized by the displacement amplitude {\alpha}. An arbitrary {\alpha}-representation of SCSs enables to manipulate the amplitudes in wider range of parameters that which greatly expands the possibilities for generation of the states. We consider a general optical scheme for implementation of the conditioned states close to SCSs with linear optics methods and detectors projecting unitarily transformed initial state onto target. Different input states (number and coherent states, Schr\"odinger kitten states) are selected as input.

Categories: Journals, Physics

### Eternal life of entropy in non-Hermitian quantum systems. (arXiv:1905.07348v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We find a new effect for the behaviour of Von Neumann entropy. For this we derive the framework for describing Von Neumann entropy in non-Hermitian quantum systems and then apply it to a simple interacting PT symmetric bosonic system. We show that our model is well defined even in the PT broken regime with the introduction of a time-dependent metric and that it displays three distinct behaviours relating to the PT symmetry of the original time-independent Hamiltonian. When the symmetry is unbroken, the entropy undergoes rapid decay to zero (so-called "sudden death") with a subsequent revival. At the exceptional point it decays asymptotically to zero and when the symmetry is spontaneously broken it decays asymptotically to a finite constant value ("eternal life").

Categories: Journals, Physics

### Observation of Multimode Strong Coupling of Cold Atoms to a 30-m Long Optical Resonator. (arXiv:1905.07353v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We report on the observation of multimode strong coupling of a small ensemble of atoms interacting with the field of a 30-m long fiber resonator containing a nanofiber section. The collective light--matter coupling strength exceeds the free spectral range and the atoms couple to consecutive longitudinal resonator modes. The measured transmission spectra of the coupled atom-resonator system provide evidence of this regime, realized with a few hundred atoms with an intrinsic single-atom cooperativity of 0.26. These results are the starting point for studies in a new setting of light-matter interaction, with strong quantum non-linearities and a new type of dynamics.

Categories: Journals, Physics

### On the Philosophical Foundations of Physics: An Experimentalist's Perspective. (arXiv:1905.07359v1 [physics.hist-ph])

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

Since I first became enthralled with physics as a teenager, I've been intrigued by the philosophical aspects of the discipline. As I approached the end of my career as an experimental physicist and observational astronomer (I'm now retired), I decided to return to these philosophical matters, some of which were still perturbing me, to see if I could finally make enough sense of them to quiet my discomfort. I've more or less succeeded in this quest in large part, I believe, because of my experimentalist background and a concomitant proclivity for pragmatic explanation. My purpose in this essay is to sketch a pragmatic worldview with which one might be able to approach fundamental philosophical and interpretational problems. You might ask how I can possibly expect to say anything meaningful in the less than 50 pages of the present essay? On the other hand, it might be an advantage to avoid the depth and precision that would limit flexibility in dealing with the philosophical conundrums I seek to resolve. In any case, I here offer my thoughts on the philosophical foundations of physics.

Categories: Journals, Physics

### Maximally correlated matter from many-body quantum coherence. (arXiv:1905.07361v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We construct quantum coherence resource theories in symmetrized Fock space (QCRTF) that unify previous frameworks for the analysis of coherence in discrete-variable (DV) and continuous variable (CV) quantum systems. Unlike traditional finite dimensional or CV quantum coherence resource theories, QCRTF can be made independent of the single-particle basis and allows to quantify coherence within and between particle number sectors. We provide physical justification for the definition of the set of free quantum channels in QCRTF by showing that: 1. in a basis-independent QCRTF, the free channels are associated with Stinespring isometries that preserve the set of free states, and 2. an energy density constraint can be imposed on the free channels that still allows for a wide range of protocols to be implemented within the resource theory. The QCRTF framework is utilized to calculate the optimal asymptotic distillation rate of maximally correlated states both for particle number conserving resource states and resource states of indefinite particle number. In particular, we show that energy density preserving manipulations of bosonic insulating states allow the extraction of a uniform superposition of maximally correlated states from a state of maximal bosonic coherence with asymptotically unit efficiency.

Categories: Journals, Physics

### A Case of Quantum Scattering. (arXiv:1905.07369v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We analyze the results of an experimental setup that consist of two statistically independent laser beams that cross, interfere and end at detectors. At the beam intersection we place a thin wire at the center of a dark interference fringe and analyze the complementarity inequality. We find that the complementarity inequality is fulfilled provided we include a scattering interaction. We find that this interaction is implicit in the formalism of quantum theory; however, this interaction is active only when the conservation laws are satisfied.

Categories: Journals, Physics

### Efficient color code decoders in $d\geq 2$ dimensions from toric code decoders. (arXiv:1905.07393v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We introduce an efficient decoder of the color code in $d\geq 2$ dimensions, the Restriction Decoder, which uses any $d$-dimensional toric code decoder combined with a local lifting procedure to find a recovery operation. We prove that the Restriction Decoder successfully corrects errors in the color code if and only if the corresponding toric code decoding succeeds. We also numerically estimate the Restriction Decoder threshold for the color code in two and three dimensions against the bit-filp and phase-flip noise with perfect syndrome extraction. We report that the 2D color code threshold $p_{\textrm{2D}} \approx 10.2\%$ on the square-octagon lattice is on a par with the toric code threshold on the square lattice.

Categories: Journals, Physics

### Compensation of polarization dependent loss using noiseless amplification and attenuation. (arXiv:1905.07398v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

Polarization dependent loss (PDL) is a serious problem that hinders the transfer of polarization qubits through quantum networks. Recently it has been shown that the detrimental effects of PDL on qubit fidelity can be compensated for with the introduction of an additional passive PDL element that rebalances the polarization modes of the transmitted qubit. This procedure works extremely well when the output of the system is postselected on photon detection. However, in cases where the qubit might be needed for further analysis this procedure introduces unwanted vacuum terms into the state. Here we present procedures for the compensation of the effects of PDL using noiseless amplification and attenuation. Each of these techniques introduces a heralding signal into the correction procedure that significantly reduces the vacuum terms in the final state. When detector inefficiency and dark counts are included in the analysis noiseless amplification remains superior, in terms of the fidelity of the final state, to both noiseless attenuation and passive PDL compensation for detector efficiencies greater than 40%.

Categories: Journals, Physics

### Study on a Spinorial Representation of Linear Canonical Transformations. (arXiv:1711.04975v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

This work is a continuation of our previous works concerning linear canonical transformations and phase space representation of quantum theory. It is mainly focused on the description of an approach which allows to establish spinorial representation of linear canonical transformations. This description is started with the presentation of a suitable parameterization of linear canonical transformations which permits to represent them with special pseudo-orthogonal transformations in an operator space. Then the establishment of the spinorial representation is deduced using the well-known relation existing between special pseudo-orthogonal and spin groups. The cases of one dimension and general multidimensional theory are both studied.

Categories: Journals, Physics

### Pairwise Concurrence in Cyclically Symmetric Quantum States. (arXiv:1802.06877v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We provide an initial characterization of pairwise concurrence in quantum states which are invariant under cyclic permutations of party labeling. We prove that maximal entanglement can be entirely described by adjacent pairs, then give explicit descriptions of those states in specific subsets of 4 and 5 qubit states - X states. We also construct a monogamy bound on shared concurrences in the same subsets in 4 and 5 qubits, finding that above non-maximal entanglement thresholds, no other entanglements are possible.

Categories: Journals, Physics

### Experimental demonstration of Pauli-frame randomization on a superconducting qubit. (arXiv:1803.01818v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

The realization of quantum computing's promise despite noisy imperfect qubits relies, at its core, on the ability to scale cheaply through error correction and fault-tolerance. While fault-tolerance requires relatively mild assumptions about the nature of the errors, the overhead associated with coherent and non-Markovian errors can be orders of magnitude larger than the overhead associated with purely stochastic Markovian errors. One proposal, known as Pauli frame randomization, addresses this challenge by randomizing the circuits so that the errors are rendered incoherent, while the computation remains unaffected. Similarly, randomization can suppress couplings to slow degrees of freedom associated with non-Markovian evolution. Here we demonstrate the implementation of circuit randomization in a superconducting circuit system, exploiting a flexible programming and control infrastructure to achieve this with low effort. We use high-accuracy gate-set tomography to demonstrate that without randomization the natural errors experienced by our experiment have coherent character, and that with randomization these errors are rendered incoherent. We also demonstrate that randomization suppresses signatures of non-Markovianity evolution to statistically insignificant levels. This demonstrates how noise models can be shaped into more benign forms for improved performance.

Categories: Journals, Physics

### Open quantum systems with local and collective incoherent processes: Efficient numerical simulation using permutational invariance. (arXiv:1805.05129v5 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

The permutational invariance of identical two-level systems allows for an exponential reduction in the computational resources required to study the Lindblad dynamics of coupled spin-boson ensembles evolving under the effect of both local and collective noise. Here we take advantage of this speedup to study several important physical phenomena in the presence of local incoherent processes, in which each degree of freedom couples to its own reservoir. Assessing the robustness of collective effects against local dissipation is paramount to predict their presence in different physical implementations. We have developed an open-source library in Python, the Permutational-Invariant Quantum Solver (PIQS), which we use to study a variety of phenomena in driven-dissipative open quantum systems. We consider both local and collective incoherent processes in the weak, strong, and ultrastrong-coupling regimes. Using PIQS, we reproduced a series of known physical results concerning collective quantum effects and extended their study to the local driven-dissipative scenario. Our work addresses the robustness of various collective phenomena, e.g., spin squeezing, superradiance, quantum phase transitions, against local dissipation processes.

Categories: Journals, Physics

### Bayesian Deep Learning on a Quantum Computer. (arXiv:1806.11463v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

Bayesian methods in machine learning, such as Gaussian processes, have great advantages com-pared to other techniques. In particular, they provide estimates of the uncertainty associated with a prediction. Extending the Bayesian approach to deep architectures has remained a major challenge. Recent results connected deep feedforward neural networks with Gaussian processes, allowing training without backpropagation. This connection enables us to leverage a quantum algorithm designed for Gaussian processes and develop a new algorithm for Bayesian deep learning on quantum computers. The properties of the kernel matrix in the Gaussian process ensure the efficient execution of the core component of the protocol, quantum matrix inversion, providing an at least polynomial speedup over classical algorithms. Furthermore, we demonstrate the execution of the algorithm on contemporary quantum computers and analyze its robustness with respect to realistic noise models.

Categories: Journals, Physics

### Non-Markovianity through quantum coherence in an all-optical setup. (arXiv:1807.05378v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We propose an all-optical experiment to quantify non-Markovianity in an open quantum system through quantum coherence of a single quantum bit. We use an amplitude damping channel implemented by an optical setup with an intense laser beam simulating a single-photon polarization. The optimization over initial states required to quantify non-Markovianity is analytically evaluated. The experimental results are in a very good agreement with the theoretical predictions.

Categories: Journals, Physics

### Combined tensor network/cluster expansion method using logic gates: Illustrated for (bi-)excitons by a single layer MoS$_2$ model system. (arXiv:1807.09036v2 [cond-mat.mes-hall] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

Carriers such as electrons and holes inside the Brillouin zone of complex semiconducting materials can form bound states (excitons, biexcitons etc.). For obtaining the corresponding eigenstates (e.g. through Wannier or Bethe Salpeter equation) and dynamics (e.g. cluster expansion) the number of involved electrons and holes as well as the accuracy is limited by the appearing high dimensional tensors (i.e. wavefunctions or correlations). These tensors can be efficiently represented and manipulated via tensor network methods. We show how tensor networks formulated via classic logic gates can be used to treat electron-hole complexes inside the Brillouin zone. The method is illustrated for the exciton and biexciton states of a single layer transition metal dichalcogenide MoS$_2$ like model system.

Categories: Journals, Physics

### Dynamical Ginzburg criterion for the quantum-classical crossover of the Kibble-Zurek mechanism. (arXiv:1807.10611v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We introduce a simple criterion for lattice models to predict quantitatively the crossover between the classical and the quantum scaling of the Kibble-Zurek mechanism, as the one observed in a quantum $\phi^4$-model on a 1D lattice [Phys. Rev. Lett. 116, 225701 (2016)]. We corroborate that the crossover is a general feature of critical models on a lattice, by testing our paradigm on the quantum Ising model in transverse field for arbitrary spin-$s$ ($s \geq 1/2$) in one spatial dimension. By means of tensor network methods, we fully characterize the equilibrium properties of this model, and locate the quantum critical regions via our dynamical Ginzburg criterion. We numerically simulate the Kibble-Zurek quench dynamics and show the validity of our picture, also according to finite-time scaling analysis.

Categories: Journals, Physics

### Observation of Multiphoton Frequency Conversion in Superconducting Circuits. (arXiv:1808.01303v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

Multiphoton up/down conversion in a transmon circuit, driven by a pair of microwaves tuned near and far off the qubit resonance, has been observed. The experimental realization of these high order non-linear processes is accomplished in the three-photon regime, when the transmon is coupled to weak bichromatic microwave fields with the same Rabi frequencies. A many-mode Floquet formalism, with longitudinal coupling, is used to simulate the quantum interferences in the absorption spectrum that manifest the multiphoton pumping processes in the transmon qubit. An intuitive graph theoretic approach is used to introduce effective Hamiltonians that elucidate main features of the Floquet results. The analytical solutions also illustrate how controllability is achievable for desired single- or multiphoton pumping processes in a wide frequency range.

Categories: Journals, Physics

### Photonic systems with two-dimensional landscapes of complex refractive index via time-dependent supersymmetry. (arXiv:1808.08225v2 [math-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We present a framework for the construction of solvable models of optical settings with genuinely two-dimensional landscapes of refractive index. The solutions of the associated non-separable Maxwell equations in paraxial approximation are found with the use of the time-dependent supersymmetry. We discuss peculiar theoretical aspects of the construction. Sufficient conditions for existence of localized states are discussed. Localized solutions vanishing for large $|\vec{x}|$, that we call light dots, as well as the guided modes that vanish exponentially outside the wave guides are constructed. We consider different definitions of the parity operator and analyze general properties of the $\mathcal{PT}$-symmetric systems, e.g. presence of localized states or existence of symmetry operators. Despite the models with parity-time symmetry are of the main concern, the proposed framework can serve for construction of non-$\mathcal{PT}$-symmetric systems as well. We explicitly illustrate the general results on a number of physically interesting examples, e.g. wave guides with periodic fluctuation of refractive index or with a localized defect, curved wave guides, two coupled wave guides or a uniform refractive index system with a localized defect.

Categories: Journals, Physics

### A cavity-QED simulator of slow and fast scrambling. (arXiv:1810.00866v2 [cond-mat.quant-gas] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

We study information scrambling, as diagnosed by the out-of-time order correlations (OTOCs), in a system of large spins collectively interacting via spatially inhomogeneous and incommensurate exchange couplings. The model is realisable in a cavity QED system in the dispersive regime. Fast scrambling, signalled by an exponential growth of the OTOCs, is observed when the couplings do not factorise into the product of a pair of local interaction terms, and at the same time the state of the spins points initially coplanar to the equator of the Bloch sphere. When one of these conditions is not realised, OTOCs grow algebraically with an exponent sensitive to the orientation of the spins in the initial state. The impact of initial conditions on the scrambling dynamics is attributed to the presence of a global conserved quantity, which critically slows down the evolution for initial states close to the poles of the Bloch sphere.

Categories: Journals, Physics

### Universal spectra of random Lindblad operators. (arXiv:1811.12282v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2019-05-20 23:24

To understand typical dynamics of an open quantum system in continuous time, we introduce an ensemble of random Lindblad operators, which generate Markovian completely positive evolution in the space of density matrices. Spectral properties of these operators, including the shape of the spectrum in the complex plane, are evaluated by using methods of free probabilities and explained with non-Hermitian random matrix models. We also demonstrate universality of the spectral features. The notion of ensemble of random generators of Markovian qauntum evolution constitutes a step towards categorization of dissipative quantum chaos.

Categories: Journals, Physics