# Quantum Physics

## New submissions

[ total of 65 entries: 1-65 ]
[ showing up to 2000 entries per page: fewer | more ]

### New submissions for Fri, 23 Aug 19

[1]
Title: Sorting topological stabilizer models in three dimensions
Comments: 14+29 pages, 7+22 figures, 3+34 tables
Subjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el)

The S-matrix invariant is known to be complete for translation invariant topological stabilizer models in two spatial dimensions, as such models are phase equivalent to some number of copies of toric code. In three dimensions, much less is understood about translation invariant topological stabilizer models due to the existence of fracton topological order. Here we introduce bulk commutation quantities inspired by the 2D S-matrix invariant that can be employed to coarsely sort 3D topological stabilizer models into qualitatively distinct types of phases: topological quantum field theories, foliated or fractal type-I models with rigid string operators, or type-II models with no string operators.

[2]
Title: Automated quantum programming via reinforcement learning for combinatorial optimization
Subjects: Quantum Physics (quant-ph); Machine Learning (cs.LG)

We develop a general method for incentive-based programming of hybrid quantum-classical computing systems using reinforcement learning, and apply this to solve combinatorial optimization problems on both simulated and real gate-based quantum computers. Relative to a set of randomly generated problem instances, agents trained through reinforcement learning techniques are capable of producing short quantum programs which generate high quality solutions on both types of quantum resources. We observe generalization to problems outside of the training set, as well as generalization from the simulated quantum resource to the physical quantum resource.

[3]
Title: Many-body systems as resources for universal fault tolerant quantum computation
Subjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el)

A universal quantum computer will inevitably rely on error correction to be fault tolerant. Most error correcting schemes are based on stabilizer circuits which fail to provide universal quantum computation. An extra quantum resource, in the form of magic states, is needed in conjunction with stabilizer circuits to perform universal quantum computation. However, creating, distilling, and preserving high quality magic states are not easy. Here, we show that quantum many-body systems are promising candidates to mine high quality magic states by considering transverse field anisotropic XY spin chains. In particular, we provide an analytic formula for the magic content of the qubits in the symmetry broken ground state of the XY spin chain, and show that there are two distinct scaling behaviors for magic near criticality. Moreover, we find an exact point in the phase diagram of the XY model at which every qubit of the system are pure H-states. This point represents a factorizable broken-symmetry ground state of the model. This is an excellent demonstration that many-body systems, even in the absence of ground state entanglement, are resourceful for fault tolerant universal quantum computation.

[4]
Title: Random Quantum Batteries
Comments: 7 pages double column + 14 supplementary material single column
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Statistical Mechanics (cond-mat.stat-mech); Mathematical Physics (math-ph)

Quantum nano-devices are fundamental systems in quantum thermodynamics that have been the subject of profound interest in recent years. Among these, quantum batteries play a very important role. In this paper we lay down a theory of random quantum batteries and provide a systematic way of computing the average work and work fluctuations in such devices by investigating their typical behavior. We show that the performance of random quantum batteries exhibits typicality and depends only on the spectral properties of the time evolving operator, the initial state and the measuring Hamiltonian. At given revival times a random quantum battery features a quantum advantage over classical random batteries. Our method is particularly apt to be used both for exactly solvable models like the Jaynes-Cummings model or in perturbation theory, e.g., systems subject to harmonic perturbations. We also study the setting of quantum adiabatic random batteries.

[5]
Title: Measurement reduction in variational quantum algorithms
Subjects: Quantum Physics (quant-ph)

Variational quantum algorithms are promising applications of noisy intermediate-scale quantum (NISQ) computers. These algorithms consist of a number of separate prepare-and-measure experiments that estimate terms in the Hamiltonian. The number of separate measurements required can become overwhelmingly large for problems at the scale of NISQ hardware that may soon be available. We approach this problem from the perspective of contextuality, and use unitary partitioning (developed independently by Izmaylov \emph{et al.}) to define VQE procedures in which additional unitary operations are appended to the ansatz preparation to reduce the number of measurements. This approach may be scaled to use all coherent resources available after ansatz preparation. We also study the use of asymmetric qubitization to implement the additional coherent operations with lower circuit depth. We investigate this technique for lattice Hamiltonians, random Pauli Hamiltonians, and electronic structure Hamiltonians. We find a constant factor speedup for lattice and random Pauli Hamiltonians. For electronic structure Hamiltonians, we prove that the linear term reduction with respect to the number of orbitals, which has been previously observed in numerical studies, is always achievable. We show that unitary partitioning applied to the plane-wave dual basis representation of fermionic Hamiltonians offers only a constant factor reduction in the number of terms. Finally, we show that noncontextual Hamiltonians are equivalent to commuting Hamiltonians by giving a reduction via unitary partitioning.

[6]
Title: The Classical Complexity of Gaussian Boson Sampling
Subjects: Quantum Physics (quant-ph)

We introduce an exact classical algorithm for simulating Gaussian Boson Sampling (GBS). The complexity of the algorithm is exponential in the number of photons detected, which is itself a random variable. For a fixed number of modes, the complexity is in fact equivalent to that of calculating output probabilities, up to constant prefactors. The simulation algorithm can be extended to other models such as GBS with threshold detectors, GBS with displacements, and sampling linear combinations of Gaussian states. In the specific case of encoding non-negative matrices into a GBS device, our method leads to an approximate sampling algorithm with polynomial runtime. We implement the algorithm, making the code publicly available as part of Xanadu's The Walrus library, and benchmark its performance on GBS with random Haar interferometers and with encoded Erd\H{o}s-Renyi graphs.

[7]
Title: Closing gaps of a quantum advantage with short-time Hamiltonian dynamics
Subjects: Quantum Physics (quant-ph)

Demonstrating a quantum computational speedup is a crucial milestone for near-term quantum technology. Recently, quantum simulation architectures have been proposed that have the potential to show such a quantum advantage, based on commonly made assumptions. The key challenge in the theoretical analysis of this scheme - as of other comparable schemes such as boson sampling - is to lessen the assumptions and close the theoretical loopholes, replacing them by rigorous arguments. In this work, we prove two open conjectures for these architectures for Hamiltonian quantum simulators: anticoncentration of the generated probability distributions and average-case hardness of exactly evaluating those probabilities. The latter is proven building upon recently developed techniques for random circuit sampling. For the former, we develop new techniques that exploit the insight that approximate 2-designs for the unitary group admit anticoncentration. We prove that the 2D translation-invariant, constant depth architectures of quantum simulation form approximate 2-designs in a specific sense, thus obtaining a significantly stronger result. Our work provides the strongest evidence to date that Hamiltonian quantum simulation architectures are classically intractable.

[8]
Title: Experimental investigation of Markovian and non-Markovian channel addition
Subjects: Quantum Physics (quant-ph)

The study of memory effects in quantum channels helps in developing characterization methods for open quantum systems and strategies for quantum error correction. Two main sets of channels exist, corresponding to system dynamics with no memory (Markovian) and with memory (non-Markovian). Interestingly, these sets have a non-convex geometry, allowing one to form a channel with memory from the addition of memoryless channels and vice-versa. Here, we experimentally investigate this non-convexity in a photonic setup by subjecting a single qubit to a convex combination of Markovian and non-Markovian channels. We use both divisibility and distinguishability as criteria for the classification of memory effects, with associated measures. Our results highlight some practical considerations that may need to be taken into account when using memory criteria to study system dynamics given by the addition of Markovian and non-Markovian channels in experiments.

[9]
Title: Quantum Neimark-Sacker bifurcation
Subjects: Quantum Physics (quant-ph); Chaotic Dynamics (nlin.CD)

Recently, it has been demonstrated that asymptotic states of open quantum system can undergo qualitative changes resembling pitchfork, saddle-node, and period doubling classical bifurcations. Here, making use of the periodically modulated open quantum dimer model, we report and investigate a quantum Neimark-Sacker bifurcation. Its classical counterpart is the birth of a torus (an invariant curve in the Poincar\'{e} section) due to instability of a limit cycle (fixed point of the Poincar\'{e} map). The quantum system exhibits a transition from unimodal to bagel shaped stroboscopic distributions, as for Husimi representation, as for observables. The spectral properties of Floquet map experience changes reminiscent of the classical case, a pair of complex conjugated eigenvalues approaching a unit circle. Quantum Monte-Carlo wave function unraveling of the Lindblad master equation yields dynamics of single trajectories on "quantum torus" and allows for quantifying it by rotation number. The bifurcation is sensitive to the number of quantum particles that can also be regarded as a control parameter.

[10]
Title: Flexible S-money token schemes
Subjects: Quantum Physics (quant-ph); Cryptography and Security (cs.CR)

S-money [Proc. R. Soc. A 475, 20190170 (2019)] schemes define virtual tokens designed for networks with relativistic or other trusted signalling constraints. The tokens allow near-instant verification and guarantee unforgeability without requiring quantum state storage. We present refined two stage S-money schemes. The first stage, which may involve quantum information exchange, generates private user token data. In the second stage, which need only involve classical communications, users determine the valid presentation point, without revealing it to the issuer. This refinement allows the user to determine the presentation point anywhere in the causal past of all valid presentation points. It also allows flexible transfer of tokens among users without compromising user privacy.

[11]
Title: Weak Gravitational Field Effects on Optical Bell Tests
Subjects: Quantum Physics (quant-ph)

The technological refinement of experimental techniques has recently allowed the generation of two-photon polarization entangled states at low Earth orbit, which have been subsequently applied to quantum communications. This achievement paves the way to study the interplay between General Relativity and Quantum Mechanics in new setups. Here, we study the generation of two-photon entangled states via large scale Franson and Hugged interferometric arrays in the presence of a weak gravitational field. We show that for certain configurations of the arrays, an entangled state emerges as a consequence of the gravitational time delay. We also show that the aforementioned arrays generate entanglement and violate the Clauser-Horne-Shymony-Holt inequality under suitable conditions even in the presence of frequency dispersion.

[12]
Title: Dynamical behaviour of coupled atom-cavity systems in the single excitation limit
Subjects: Quantum Physics (quant-ph); Optics (physics.optics)

We investigate the time evolution of the photon-detection probability at various output ports of an all-fiber coupled cavity-quantum-electrodynamics (cavity-QED) system. The setup consists of two atoms trapped separately in the field of two nanofiber cavities that are connected by a standard optical fiber. We find that the normal-mode picture captures well the main features of the dynamics. However, a more accurate description based on the diagonalization of a non-Hermitian Hamiltonian reveals the origin of small yet significant features in the spontaneous emission spectra.

[13]
Title: Electron-positron pair production in frequency modulated laser fields
Subjects: Quantum Physics (quant-ph); High Energy Physics - Phenomenology (hep-ph)

The momentum spectrum and the number density of created electron-positron pairs in a frequency modulated laser field are investigated using quantum kinetic equation. It is found that the momentum spectrum presents obvious interference pattern. This is an imprint of the frequency modulated field on the momentum spectrum, because the momentum peaks correspond to the pair production process by absorbing different frequency component photons. Moreover, the interference effect can also be understood qualitatively by analyzing turning point structures. The study of the pair number density shows that the number density is very sensitive to modulation parameters and can be enhanced by over two orders of magnitude for certain modulation parameters, which may provide a new way to increase the number of created electron-positron pairs in future experiments.

[14]
Title: Genuine Measure of Multipartite Entanglement and its Monogamy Relation
Authors: Yu Guo, Lin Zhang
Subjects: Quantum Physics (quant-ph)

To quantify entanglement, many entanglement measures have been proposed so far. However, much less is known in the multipartite case, and even the existing multipartite measures are still studied by virtue of the postulates of bipartite case. Namely, there is no genuine multipartite measure of entanglement indeed by now. We establish here a strict frame for multipartite entanglement measure: apart from the axioms of bipartite measure, a genuine multipartite measure should additionally satisfy the \textit{unification condition} and the \textit{hierarchy condition}. We then come up with a monogamy formula under the genuine multipartite entanglement measure. Our approach is a great improvement and complementary to the entanglement measures in literatures up to now. Consequently, we propose multipartite entanglement measures which are extensions of entanglement of formation, concurrence, tangle and negativity, respectively. We show that these extended measures are monogamous as multipartite measures. Especially, as a by-product, a long standing conjecture is confirmed---the entanglement of formation is shown to be additive. Alternative candidates of multipartite entanglement measures in terms of fidelity and its variants are also explored. We show that these fidelity-induced entanglement measures are monogamous as bipartite measures and can be extended as genuine tripartite entanglement monotones.

[15]
Title: Optimal Control for the Quantum Simulation of Nuclear Dynamics
Subjects: Quantum Physics (quant-ph); Nuclear Theory (nucl-th)

We propose a method for enacting the unitary time propagation of two interacting neutrons at leading order of chiral effective field theory by efficiently encoding the nuclear dynamics into a single multi-level quantum device. The emulated output of the quantum simulation shows that, by applying a single gate that draws on the underlying characteristics of the device, it is possible to observe multiple cycles of the nucleons' dynamics before the onset of decoherence. Owing to the signal's longevity, we can then extract spectroscopic properties of the simulated nuclear system. This allows us to validate the encoding of the nuclear Hamiltonian and the robustness of the simulation in the presence of quantum-hardware noise by comparing the extracted spectroscopic information to exact calculations. This work paves the way for transformative calculations of dynamical properties of nuclei on near-term quantum devices.

[16]
Title: Apoptosis of moving, non-orthogonal basis functions in many-particle quantum dynamics
Subjects: Quantum Physics (quant-ph)

Due to the exponential increase of the numerical effort with the number of degrees of freedom, moving basis functions have a long history in quantum dynamics. In addition, spawning of new basis functions is routinely applied. Here we advocate the opposite process: the programmed removal of motional freedom of selected basis functions. This is a necessity for converged numerical results wrt the size of a non-orthogonal basis, because generically two or more states approach each other too closely early on, rendering unstable the matrix inversion, required to make the equations of motion explicit. An application to the sub-Ohmic spin-boson model demonstrates the power of the proposed methodology.

[17]
Title: Quantum key distribution with correlated sources
Subjects: Quantum Physics (quant-ph)

Implementation security is a critical problem in quantum key distribution (QKD). With the advent of measurement-device-independent QKD, all security loopholes of the measurement unit have been closed. Securing the source, however, remains an elusive issue. Despite the tremendous progress made by developing security proofs that accommodate most typical source imperfections, such proofs usually disregard the effect of pulse correlations. That is, they disregard the fact that the state of an emitted signal can depend on the signals selected previously. Here, we close this gap by introducing a simple yet general method to prove the security of QKD with arbitrary pulse correlations. Our method is compatible with those security proofs that accommodate all the other source imperfections, thus paving the way towards achieving implementation security in QKD with arbitrary flawed devices. Moreover, we introduce a new security proof, which we call the reference technique, that provides high performance in the presence of source imperfections.

[18]
Title: Pretty simple bounds on quantum state discrimination
Authors: Ashley Montanaro
Subjects: Quantum Physics (quant-ph)

We show that the quantum measurement known as the pretty good measurement can be used to identify an unknown quantum state picked from any set of $n$ mixed states that have pairwise fidelities upper-bounded by a constant below 1, given $O(\log n)$ copies of the unknown state, with high success probability in the worst case. If the unknown state is promised to be pure, there is an explicit measurement strategy which solves this worst-case quantum state discrimination problem with $\widetilde{O}(\|G\|)$ copies, where $G$ is the Gram matrix of the states.

[19]
Title: Nonreciprocal transition between two nondegenerate energy levels
Comments: 6 pages + Supplemental Materials (6 pages), 5 figures
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Optics (physics.optics)

Stimulated emission and absorption are two fundamental processes of light-matter interaction, and the coefficients of the two processes should be equal in general. However, we will describe a generic method to realize significant difference between the stimulated emission and absorption coefficients of two nondegenerate energy levels, which we refer to as nonreciprocal transition. As a simple implementation, a cyclic three-level atom system, comprising two nondegenerate energy levels and one auxiliary energy level, is employed to show nonreciprocal transition via a combination of synthetic magnetism and reservoir engineering. Moreover, a single-photon nonreciprocal transporter is proposed using two one dimensional semi-infinite coupled-resonator waveguides connected by an atom with nonreciprocal transition effect. Our work opens up a route to design atom-mediated nonreciprocal devices in a wide range of physical systems.

[20]
Title: Variational spin-squeezing algorithms on programmable quantum sensors
Subjects: Quantum Physics (quant-ph); Atomic Physics (physics.atom-ph)

Arrays of atoms trapped in optical tweezers combine features of programmable analog quantum simulators with atomic quantum sensors. Here we propose variational quantum algorithms, tailored for tweezer arrays as programmable quantum sensors, capable of generating entangled states on-demand for precision metrology. The scheme is designed to generate metrological enhancement by optimizing it in a feedback loop on the quantum device itself, thus preparing the best entangled states given the available quantum resources. We apply our ideas to generate spin-squeezed states on Sr atom tweezer arrays, where finite-range interactions are generated through Rydberg dressing. The complexity of experimental variational optimization of our quantum circuits is expected to scale favorably with system size. We numerically show our approach to be robust to noise, and surpassing known protocols.

[21]
Title: Signatures of Many-Particle Interference
Comments: Tutorial / Pedagogical review; 67 pages, 11 figures
Subjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)

This Tutorial will introduce the mathematical framework for describing systems of identical particles, and explain the notion of indistinguishability. We will then focus our attention on dynamical systems of free particles and formally introduce the concept of many-particle interference. Its impact on many-particle transition probabilities is computationally challenging to evaluate, and it becomes rapidly intractable for systems with large numbers of identical particles. Hence, this Tutorial will build up towards alternative, more efficient methods for observing signatures of many-particle interference. A first type of signatures relies on the detection of a highly sensitive -but also highly fragile-processes of total destructive interference that occurs in interferometers with a high degree of symmetry. A second class of signatures is based on the statistical features that arise when we study the typical behaviour of correlations between a small number of the interferometer's output ports. We will ultimately show how these statistical signatures of many-particle interference lead us to a statistical version of the Hong-Ou-Mandel effect.

[22]
Title: Supervised learning with a quantum classifier using a multi-level system
Subjects: Quantum Physics (quant-ph)

We propose a quantum classifier, which can classify data under the supervised learning scheme using a quantum feature space. The input feature vectors are encoded in a single qu$N$it (a $N$ level quantum system), as opposed to more commonly used entangled multi-qubit systems. For training we use the much used quantum variational algorithm -- a hybrid quantum-classical algorithm -- in which the forward part of the computation is performed on a quantum hardware whereas the feedback part is carried out on a classical computer. We introduce "single shot training" in our scheme, with all input samples belonging to the same class being used to train the classifier simultaneously. This significantly speeds up the training procedure and provides an advantage over classical machine learning classifiers. We demonstrate successful classification of popular benchmark datasets with our quantum classifier and compare its performance with respect to some classical machine learning classifiers. We also show that the number of training parameters in our classifier is significantly less than the classical classifiers.

[23]
Title: Exact annihilation energy and proper decay time solution of a para-positronium system
Subjects: Quantum Physics (quant-ph); High Energy Physics - Theory (hep-th); Atomic Physics (physics.atom-ph)

Para positronium composed by an electron-antielectron pair is an unstable system decaying into two high energetic gamma photons via self annihilation process, due to the conservation of the charge conjugation parity in electromagnetically interacting systems. Therefore, the spectrum covering all fundamental properties of the para-positronium system includes an imaginary part corresponding to the proper decay time besides the real parts corresponding to the total annihilation energy and binding energy, simultaneously. The para-positronium can be regarded as relativistic two body system in which there exist a Coulomb interaction force between the oppositely charged particles. Because of the annihilation condition, ($l=0$), and total spin of the system, ($S=0$), the problem is solved in 1+1 dimensional spacetime background by using fully covariant relativistic two body equation, without any approximation. Adopting the obtained spectra to an electron-antielectron pair we find total annihilation energy, binding energy and proper decay time of the para-positronium system. Since the obtained spectra shows the fascinating properties of the system, our findings can shed light to medical monitoring processes, positron annihilation spectroscopy in any system and gamma-ray laser studies.

[24]
Title: Two-photon blockade and photon-induced tunneling generated by squeezing
Comments: 20 pages, 12 figures, 3 tables
Subjects: Quantum Physics (quant-ph)

Inspired by the recent experiment of Hamsen \emph{et al.} [Phys. Rev. Lett. 118, 133604 (2017)], which demonstrated two-photon blockade in a driven nonlinear system (composed of a harmonic cavity with a driven atom), we show that two-photon blockade and other nonstandard types of photon-blockade and photon-induced tunneling can be generated in a driven harmonic cavity without an atom or any other kind of nonlinearity, but instead coupled to a nonlinear (i.e., squeezed) reservoir. We also simulate these single- and two-photon effects with squeezed coherent states and displaced squeezed thermal states.

[25]
Title: Improving the dynamics of quantum sensors with reinforcement learning
Subjects: Quantum Physics (quant-ph); Machine Learning (cs.LG)

Recently proposed quantum-chaotic sensors achieve quantum enhancements in measurement precision by applying nonlinear control pulses to the dynamics of the quantum sensor while using classical initial states that are easy to prepare. Here, we use the cross entropy method of reinforcement learning to optimize the strength and position of control pulses. Compared to the quantum-chaotic sensors in the presence of superradiant damping, we find that decoherence can be fought even better and measurement precision can be enhanced further by optimizing the control. In some examples, we find enhancements in sensitivity by more than an order of magnitude. By visualizing the evolution of the quantum state, the mechanism exploited by the reinforcement learning method is identified as a kind of spin-squeezing strategy that is adapted to the superradiant damping.

### Cross-lists for Fri, 23 Aug 19

[26]  arXiv:1908.08040 (cross-list from math.OC) [pdf, other]
Title: Quantum Algorithms for Portfolio Optimization
Subjects: Optimization and Control (math.OC); Portfolio Management (q-fin.PM); Quantum Physics (quant-ph)

We develop the first quantum algorithm for the constrained portfolio optimization problem. The algorithm has running time $\widetilde{O} \left( n\sqrt{r} \frac{\zeta \kappa}{\delta^2} \log \left(1/\epsilon\right) \right)$, where $r$ is the number of positivity and budget constraints, $n$ is the number of assets in the portfolio, $\epsilon$ the desired precision, and $\delta, \kappa, \zeta$ are problem-dependent parameters related to the well-conditioning of the intermediate solutions. If only a moderately accurate solution is required, our quantum algorithm can achieve a polynomial speedup over the best classical algorithms with complexity $\widetilde{O} \left( \sqrt{r}n^\omega\log(1/\epsilon) \right)$, where $\omega$ is the matrix multiplication exponent that has a theoretical value of around $2.373$, but is closer to $3$ in practice. We also provide some experiments to bound the problem-dependent factors arising in the running time of the quantum algorithm, and these experiments suggest that for most instances the quantum algorithm can potentially achieve an $O(n)$ speedup over its classical counterpart.

[27]  arXiv:1908.08051 (cross-list from cond-mat.stat-mech) [pdf, other]
Title: Measurement-induced criticality in random quantum circuits
Subjects: Statistical Mechanics (cond-mat.stat-mech); Disordered Systems and Neural Networks (cond-mat.dis-nn); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)

We investigate the critical behavior of the entanglement transition induced by projective measurements in (Haar) random unitary quantum circuits. Using a replica approach, we map the calculation of the entanglement entropies in such circuits onto a two-dimensional statistical mechanics model. In this language, the area- to volume-law entanglement transition can be interpreted as an ordering transition in the statistical mechanics model. We derive the general scaling properties of the entanglement entropies and mutual information near the transition using conformal invariance. We analyze in detail the limit of infinite on-site Hilbert space dimension in which the statistical mechanics model maps onto percolation. In particular, we compute the exact value of the universal coefficient of the logarithm of subsystem size in the $n$th R\'enyi entropies for $n \geq 1$ in this limit using relatively recent results for conformal field theory describing the critical theory of 2D percolation, and we discuss how to access the generic transition at finite on-site Hilbert space dimension from this limit, which is in a universality class different from 2D percolation. We also comment on the relation to the entanglement transition in Random Tensor Networks, studied previously in Ref. 1.

[28]  arXiv:1908.08062 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: Half-integer quantized response in strongly driven quantum systems
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

A spin strongly driven by two incommensurate tones can pump energy from one drive to the other at a quantized average rate, in close analogy with the quantum Hall effect. The quantized pumping is a pre-thermal effect with a lifetime that diverges as the drive frequencies approach zero. We study the transition between the pumping and non-pumping pre-thermal states. The transition is sharp at zero frequency and is characterized by a Dirac point in the instantaneous band structure parametrized by the drive phases. We show that the pumping rate is half-integer quantized at the transition and present universal Kibble-Zurek scaling functions for energy transfer processes in the low frequency regime. Our results identify qubit experiments to measure the universal linear and non-linear response of a Dirac point.

[29]  arXiv:1908.08100 (cross-list from cond-mat.quant-gas) [pdf, ps, other]
Title: Normal modes for N identical particles: A study of the evolution of collective behavior from few-body to many-body
Authors: D.K. Watson
Comments: 24 pages, 7 figures, 2 appendices
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

Normal mode dynamics are ubiquitous underlying the motions of diverse systems from rotating stars to crystal structures. These behaviors are composed of simple collective motions of particles which move with the same frequency and phase, thus encapsulating many-body effects into simple dynamic motions. In regimes such as the unitary regime for ultracold Fermi gases, a single collective mode can dominate, leading to simple behavior as seen in superfluidity. I investigate the evolution of collective motion as a function of N for five types of normal modes obtained from an L=0 group theoretic solution of a general Hamiltonian for confined, identical particles. I show using simple analytic forms that the collective behavior of few-body systems, with the well known motions of molecular equivalents such as ammonia and methane, evolves smoothly to the collective motions expected for large N ensembles. The transition occurs at quite low values of N. I study a Hamiltonian known to support collective behavior, the Hamiltonian for Fermi gases in the unitary regime. I analyze the evolution of both frequencies and the coefficients that mix the radial and angular coordinates which both depend on interparticle interactions. This analysis reveals two phenomena that could contribute to the viability of collective behavior. First the mixing coefficients go to zero or unity, i.e. no mixing, as N becomes large resulting in solutions that do not depend on the details of the interparticle potential as expected for this unitary regime, and that manifest the symmetry of an underlying approximate Hamiltonian. Second, the five normal mode frequencies which are all close for low values of N, separate as N increases, creating large gaps that can, in principle, offer stability to collective behavior if mechanisms to prevent the transfer of energy to other modes exist (such as low temperature) or can be constructed.

[30]  arXiv:1908.08172 (cross-list from cond-mat.quant-gas) [pdf, other]
Title: Exact results for the boundary energy of one-dimensional bosons
Comments: five pages and one figure
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Mathematical Physics (math-ph); Quantum Physics (quant-ph)

We study bosons in a one-dimensional hard wall box potential. In the case of contact interaction, the system is exactly solvable by Bethe ansatz, as first shown by Gaudin in 1971. Although contained in the exact solution, the boundary energy for this problem is only approximately calculated by Gaudin at the leading order at weak repulsion. Here we derive an exact integral equation that enables one to calculate the boundary energy in the thermodynamic limit at an arbitrary interaction. We then solve such equation and find the asymptotic results for the boundary energy at weak and strong interaction. The analytical results obtained from Bethe ansatz are in agreement with the ones found by other complementary methods, including quantum Monte Carlo simulations. We study the universality of the boundary energy in the regime of small gas parameter by making a comparison with the exact solution for the hard rod gas.

[31]  arXiv:1908.08249 (cross-list from cond-mat.mes-hall) [pdf]
Title: Electric and Magnetic Field Nano-Sensing Using a New, Atomic-like Qubit in a Carbon Nanotube
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

Quantum sensing techniques have been successful in pushing the sensitivity limits in numerous fields, and hold great promise for scanning probes that study nano-scale devices and novel materials. However, forming a nano-scale qubit that is simple and robust enough to be placed on a scanning tip, and sensitive enough to detect various physical observables, is still a great challenge. Here we demonstrate a conceptually new qubit implementation in a carbon nanotube that achieves these requirements. In contrast to the prevailing semiconducting qubits that use electronic states in double quantum dots, our qubit utilizes the natural electronic wavefunctions in a single quantum dot. Using an ultraclean nanotube we construct a qubit from two wavefunctions with significantly different magnetic moments and spatial charge distributions, making it sensitive to both magnetic and electric fields. We use an array of gates to directly image these wavefunctions and demonstrate their localized moments. Owing to their different spatial structure, these wavefunctions also show radically different transport properties, giving us a simple transport-based qubit readout mechanism. Due to its narrow coherence-limited transition, the qubit demonstrates significantly better electric field detection sensitivity than a single electron transistor. Moreover, with the same qubit we demonstrate simultaneous probing of magnetic fields with DC sensitivity comparable to that of NV centers. Our technique has minimal requirements for device complexity, which can be implemented using a number of straightforward fabrication methods. These features make this atomic-like qubit a powerful new tool that enables a variety of new nanoscale imaging experiments.

[32]  arXiv:1908.08306 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: Conditional Teleportation of Quantum-Dot Spin States
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

Among the different experimental platforms for quantum information processing, individual electron spins in semiconductor quantum dots stand out for their long coherence times and potential for scalable fabrication. The past years have witnessed substantial progress in the capabilities of spin qubits. However, coupling between distant electron spins, which is required for quantum error correction, presents a challenge, and this goal remains the focus of intense research. Quantum teleportation is a canonical method to transmit qubit states, but it has not previously been implemented in quantum-dot spin qubits. Here, we present a conditional quantum teleportation protocol for electron spin qubits in semiconductor quantum dots. We demonstrate this method, which relies on a recently-developed technique to distribute entangled states of spin qubits, through conditional teleportation of spin eigenstates, entanglement swapping, and gate teleportation. This method is a promising addition to the quantum-dot spin-qubit toolbox, and it will alleviate many of the challenges associated with long-distance coupling between spins and open the door to scalable spin-based quantum information processing.

[33]  arXiv:1908.08367 (cross-list from cond-mat.quant-gas) [pdf, other]
Title: Nonzero temperature dynamics of a repulsive two-component Fermi gas
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We study spin-dipole oscillations of a binary fermionic mixture at nonzero temperatures. We apply the atomic-orbital method combined with the Monte Carlo technique based sampling to probe finite temperatures. Our results agree quantitatively with recent experiment, G. Valtolina et al., Nat. Phys. 13, 704 (2017), showing the appearance of the ferromagnetic phase at stronger repulsion between components when the temperature is increased.

[34]  arXiv:1908.08373 (cross-list from physics.bio-ph) [pdf, other]
Title: Structure and efficiency in bacterial photosynthetic light-harvesting
Subjects: Biological Physics (physics.bio-ph); Quantum Physics (quant-ph)

Photosynthetic organisms use networks of chromophores to absorb sunlight and deliver the energy to reaction centres, where charge separation triggers a cascade of chemical steps to store the energy. We present a detailed model of the light-harvesting complexes in purple bacteria, including explicit interaction with sunlight; energy loss through radiative and non-radiative processes; and dephasing and thermalizing effects of coupling to a vibrational bath. An important feature of the model is that we capture the effect of slow vibrational modes by introducing time-dependent disorder. Our model describes the experimentally observed high efficiency of light harvesting, despite the absence of long-range quantum coherence. The one-exciton part of the quantum state fluctuates due to slow vibrational changes, but remains highly mixed at all times. This lack of long-range coherence suggests a relatively minor role for structure in determining the efficiency of bacterial light harvesting. To investigate this we built hypothetical models with randomly arranged chromophores, but still observed high efficiency when typical nearest-neighbour distances are comparable with those found in nature. This helps to explain the efficiency of energy transport in organisms whose chromophore networks differ widely in structure, while also suggesting new design criteria for efficient artificial light-harvesting devices.

[35]  arXiv:1908.08443 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: All-microwave holonomic control of an electron-nuclear two-qubit register in diamond
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

We present a theoretical scheme that allows to perform a universal set of holonomic gates on a two qubit register, formed by a $^{13}$C nuclear spin coupled to the electron spin of a nitrogen-vacancy center in diamond. Strong hyperfine interaction between the electron spin and the spins of the first three shells of $^{13}$C atoms allows to operate the state of the register on the submicrosecond timescale using microwave pulses only. We describe the system and the operating regime analytically and numerically, as well as simulate the initialization protocols.

[36]  arXiv:1908.08496 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: Cavity electromechanics with parametric mechanical driving
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con); Quantum Physics (quant-ph)

Microwave optomechanical circuits have been demonstrated in the past years to be extremely powerfool tools for both, exploring fundamental physics of macroscopic mechanical oscillators as well as being promising candidates for novel on-chip quantum limited microwave devices. In most experiments so far, the mechanical oscillator is either used as a passive device element and its displacement is detected using the superconducting cavity or manipulated by intracavity fields. Here, we explore the possibility to directly and parametrically manipulate the mechanical nanobeam resonator of a cavity electromechanical system, which provides additional functionality to the toolbox of microwave optomechanical devices. In addition to using the cavity as an interferometer to detect parametrically modulated mechanical displacement and squeezed thermomechanical motion, we demonstrate that parametric modulation of the nanobeam resonance frequency can realize a phase-sensitive parametric amplifier for intracavity microwave photons. In contrast to many other microwave amplification schemes using electromechanical circuits, the presented technique allows for simultaneous cooling of the mechanical element, which potentially enables this type of optomechanical microwave amplifier to be quantum-limited.

[37]  arXiv:1908.08514 (cross-list from hep-th) [pdf, ps, other]
Title: Reflections on Virasoro circuit complexity and Berry phase
Authors: Ibrahim Akal
Subjects: High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)

Recently, the notion of circuit complexity defined in symmetry group manifolds has been related to geometric actions which generally arise in the coadjoint orbit method in representation theory and play an important role in geometric quantization. On the other hand, it is known that there exists a precise relation between geometric actions and Berry phases defined in group representations. Motivated by these connections, we elaborate on a relation between circuit complexity and the group theoretic Berry phase. As the simplest setup relevant for holography, we discuss the case of two dimensional conformal field theories. In the large central charge limit, we identify the computational cost function with the Berry connection in the unitary representation of the Virasoro group. We then use the latter identification to express the Berry phase in terms of the Virasoro circuit complexity. The former can be seen as the holonomy of the Berry connection along the path in the group manifold which defines the protocol. In addition, we derive a proportionality relation between Virasoro circuit complexity and the logarithm of the inner product between a particularly chosen reference state and the prepared target state. In this sense, the logarithmic formula turns out to be approximating the complexity up to some additive constant if the building blocks of the circuit are taken to be the underlying symmetry gates. Predictions based on this formula have recently been shown to coincide with the holographic complexity proposals and the path integral optimization procedure. The found connections may therefore help to better understand such coincidences. We also discuss that our findings, put together with earlier observations, may suggest a connection between the Virasoro Berry phase and the complexity measure in the path integral optimization proposal.

[38]  arXiv:1908.08519 (cross-list from physics.data-an) [pdf, other]
Title: Unfolding as Quantum Annealing
Comments: 8 pages, 5 figures, the source code is available at this https URL
Subjects: Data Analysis, Statistics and Probability (physics.data-an); High Energy Physics - Experiment (hep-ex); Quantum Physics (quant-ph)

High-energy physics is replete with hard computational problems and it is one of the areas where quantum computing could be used to speed up calculations. We present an implementation of likelihood-based regularized unfolding on a quantum computer. The inverse problem is recast in terms of quadratic unconstrained binary optimization (QUBO), which has the same form of the Ising hamiltonian and hence it is solvable on a programmable quantum annealer. We tested the method using a model that captures the essence of the problem, and compared the results with a baseline method commonly used in precision measurements at the Large Hadron Collider (LHC) at CERN. The unfolded distribution is in very good agreement with the original one. We also show how the method can be extended to include the effect of nuisance parameters representing sources of systematic uncertainties affecting the measurement.

### Replacements for Fri, 23 Aug 19

[39]  arXiv:1612.01269 (replaced) [pdf, ps, other]
Title: Time Dependent Rindler Hamiltonian Eigen States in Momentum Space
Comments: nine pages REVTEX file with six .eps figures (included) new conclusion is added
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Astrophysical Phenomena (astro-ph.HE); Quantum Physics (quant-ph)
[40]  arXiv:1708.07832 (replaced) [pdf, other]
Title: Cold hybrid ion-atom systems
Comments: Review article. 63 pages, 49 figures, over 400 references
Journal-ref: Rev. Mod. Phys. 91, 035001 (2019)
Subjects: Atomic Physics (physics.atom-ph); Quantum Gases (cond-mat.quant-gas); Chemical Physics (physics.chem-ph); Quantum Physics (quant-ph)
[41]  arXiv:1711.05159 (replaced) [pdf, ps, other]
Title: Classical Control, Quantum Circuits and Linear Logic in Enriched Category Theory
Comments: 28 pages, 3 figures; Mathematical Foundations of Program Semantics XXXIII
Subjects: Logic in Computer Science (cs.LO); Programming Languages (cs.PL); Category Theory (math.CT); Operator Algebras (math.OA); Quantum Physics (quant-ph)
[42]  arXiv:1802.09712 (replaced) [pdf, ps, other]
Title: Mapping a quantum walk by tuning the coupling coefficient
Comments: 4 pages, 4 figures. Substantial revision
Subjects: Quantum Physics (quant-ph); Optics (physics.optics)
[43]  arXiv:1805.01251 (replaced) [pdf, other]
Title: Hyperfine level structure in nitrogen-vacancy centers near the ground-state level anticrossing
Comments: Significant changes from v2: 1) Reinserted treatment of high-density (200ppm) sample; 2) updated paper to version accepted for publication
Journal-ref: Phys. Rev. B 100, 075204 (2019)
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[44]  arXiv:1809.08535 (replaced) [pdf, other]
Title: Optimal quantum refrigeration in strained graphene
Comments: 8 pages, 7 figures, accepted for publication in Journal of Physical Chemistry C
Journal-ref: Journal of Physical Chemistry C (2019)
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Chemical Physics (physics.chem-ph); Quantum Physics (quant-ph)
[45]  arXiv:1810.01444 (replaced) [pdf, other]
Title: Regenesis and quantum traversable wormholes
Authors: Ping Gao, Hong Liu
Subjects: High Energy Physics - Theory (hep-th); Statistical Mechanics (cond-mat.stat-mech); General Relativity and Quantum Cosmology (gr-qc); Chaotic Dynamics (nlin.CD); Quantum Physics (quant-ph)
[46]  arXiv:1810.03150 (replaced) [pdf, other]
Title: Fluctuation Theorems for a Quantum Channel
Journal-ref: Phys. Rev. X 9, 031029 (2019)
Subjects: Quantum Physics (quant-ph)
[47]  arXiv:1810.05627 (replaced) [pdf, other]
Title: Fundamental limits on key rates in device-independent quantum key distribution
Subjects: Quantum Physics (quant-ph)
[48]  arXiv:1810.07144 (replaced) [pdf, other]
Title: Scalable Emulation of Stoquastic Hamiltonians with Room Temperature p-bits
Subjects: Quantum Physics (quant-ph)
[49]  arXiv:1811.09053 (replaced) [pdf, other]
Title: Quantifying the nonclassicality of pure dephasing
Journal-ref: Nature Communications 10, 3794 (2019)
Subjects: Quantum Physics (quant-ph)
[50]  arXiv:1812.04374 (replaced) [pdf, other]
Title: Quantum correlations for anonymous metrology
Comments: 10 pages, 2 figures; version to be published in Quantum
Subjects: Quantum Physics (quant-ph)
[51]  arXiv:1902.00129 (replaced) [pdf, ps, other]
Title: A time-reversible quantum causal model
Authors: Jacques Pienaar
Comments: 9 pages. Minor improvements to previous draft
Subjects: Quantum Physics (quant-ph)
[52]  arXiv:1903.00969 (replaced) [pdf, other]
Title: Microwave-based Arbitrary CPHASE Gates for Transmon Qubits
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[53]  arXiv:1904.00470 (replaced) [pdf, ps, other]
Title: Generation of NOON states in waveguide arrays
Subjects: Quantum Physics (quant-ph)
[54]  arXiv:1904.04681 (replaced) [pdf, other]
Title: Benchmarking maximum-likelihood state estimation with an entangled two-cavity state
Journal-ref: Phys. Rev. Lett. 123, 060404 (2019)
Subjects: Quantum Physics (quant-ph)
[55]  arXiv:1904.05205 (replaced) [pdf, other]
Title: Long-distance entanglement in Motzkin and Fredkin spin chains
Authors: Luca Dell'Anna
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
[56]  arXiv:1905.06728 (replaced) [pdf, ps, other]
Title: Implementing perceptron models with qubits
Subjects: Quantum Physics (quant-ph)
[57]  arXiv:1906.05808 (replaced) [pdf, other]
Title: Transmission spectra of an ultrastrongly coupled qubit-dissipative resonator system
Subjects: Quantum Physics (quant-ph)
[58]  arXiv:1907.06953 (replaced) [pdf, other]
Title: Gravitationally induced entanglement between two quantum walkers
Comments: 7 pages, 9 figures, updated version with effect of noise and supplementary information
Subjects: Quantum Physics (quant-ph); High Energy Physics - Theory (hep-th)
[59]  arXiv:1908.01864 (replaced) [pdf, ps, other]
Title: Entanglement distillation of boundary states of large N SU(N)1, Chern-Simons theory and Riemann surfaces
Comments: v2. Typo in the title corrected
Subjects: High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
[60]  arXiv:1908.02754 (replaced) [pdf, other]
Title: Quantum Overlapping Tomography
Comments: 7 pages, 1 figure; v2: New appendix added, new measurement estimates
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
[61]  arXiv:1908.03828 (replaced) [pdf, ps, other]
Title: Pauli Matrices: A Triple of Accardi Complementary Observables
Comments: 6 pages, clearer exposition, new abstract, one new reference
Subjects: Mathematical Physics (math-ph); Quantum Physics (quant-ph)
[62]  arXiv:1908.04231 (replaced) [pdf, other]
Title: Analyzing the structure of basic quantum knowledge for instruction
Comments: 22 pages, 9 figures, 1 table
Subjects: Physics Education (physics.ed-ph); Quantum Physics (quant-ph)
[63]  arXiv:1908.04779 (replaced) [pdf]
Title: Improved circuits for a biologically-inspired random pulse computer
Subjects: Emerging Technologies (cs.ET); Quantum Physics (quant-ph)
[64]  arXiv:1908.06423 (replaced) [pdf, other]
Title: Quantum Mechanics of Particle on a torus knot: Curvature and Torsion Effects