## Physics

### How accurate is density functional theory at predicting dipole moments? An assessment using a new database of 200 benchmark values. (arXiv:1709.05075v1 [physics.chem-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

Dipole moments are perhaps the simplest global measure of the accuracy of the electron density of a polar molecule. To directly assess the accuracy of modern density functionals for calculating dipole moments, we have developed a database of 200 benchmark dipole moments, using coupled cluster theory through triple excitations, extrapolated to the complete basis set limit. This new database is used to assess the performance of 76 popular density functionals. The results suggest that the best functionals are typically hybrids and they yield dipole moments within about 6% regularized RMS errors versus the reference values, which is quite competitive with the 4% regularized RMS error produced by coupled cluster singles and doubles. Some functionals however exhibit large outliers and local functionals in general perform less well than hybrids.

Categories: Journals, Physics

### Long-range maximal entanglement in Majorana edge modes of a Kitaev tube. (arXiv:1709.05086v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We study the edge modes of a finite-size Kitaev model on the square lattice with periodic boundary conditions in one direction and open boundary conditions in the other. Based on the fact that the Majorana representation of Kitaev model is equivalent to a brick wall model, the model in a finite-length cylindrical geometry is shown to support perfect Majorana bound states which is in strong localization limit, characterized by a edge-mode fermionic operator. In the framework of edge-mode pseudospin analysis, we find that the edge modes exhibit long-range maximal entanglement.

Categories: Journals, Physics

### Oscillation collapse in coupled quantum van der Pol oscillators. (arXiv:1709.05093v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

The classical self-oscillations can collapse merely due to mutual couplings. We investigate this oscillation collapse in quantum van der Pol oscillators. For a pair of quantum oscillators, the steady-state mean phonon number is shown to be lower than in the corresponding classical model with a Gaussian white noise that mimics quantum noise. We further show within the mean-field theory that a number of globally coupled oscillators undergo a transition from the synchronized periodic motion to the collective oscillation collapse. A quantum many-body simulation suggests that the increase in the number of oscillators leads to a lower steady-state mean phonon number, bounded below by the mean-field result.

Categories: Journals, Physics

### Semiclassical approach to the work distribution. (arXiv:1709.05115v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

Work in closed quantum systems is usually defined by a two-point measurement. This definition of work is compatible with quantum fluctuation theorems but it fundamentally differs from its classical counterpart. In this paper, we study the correspondence principle in quantum chaotic systems. We derive a semiclassical expression of the work distribution for chaotic systems undergoing a general, finite time, process. This semiclassical distribution converges to the classical distribution in the usual classical limit. We show numerically that, for a particle inside a chaotic cavity, the semiclassical distribution provides a good approximation to quantum distribution.

Categories: Journals, Physics

### Nonequilibrium Work Relation from Schroedinger's Unrecognized Probability Theory. (arXiv:1709.05169v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

Jarzynski's nonequilibrium work relation can be understood as the realization of the (hidden) time-generator reciprocal symmetry satisfied for the conditional probability function. To show this, we introduce wave functions to express a classical probability theory called reciprocal process and derive a mathematical relation using the symmetry. We further discuss that the descriptions by the usual Markov process from an initial equilibrium state are indistinguishable from those by the reciprocal process. Then the Jarzynski relation is obtained from the mathematical relation for the Markov processes described by the Fokker-Planck, Kramers and relativistic Kramers equations.

Categories: Journals, Physics

### Quantum Analysis and Thermodynamic Operator Relations in Stochastic Energetics. (arXiv:1709.05170v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We introduce a model of the quantum Brownian motion coupled to a classical neat bath by using the operator differential proposed in the quantum analysis. We then define the heat operator by adapting the idea of the stochastic energetics. The introduced operator satisfies the relations which are analogous to the first and second laws of thermodynamics.

Categories: Journals, Physics

### Quantum coherent transport in a three-arm beam splitter and a Braess paradox. (arXiv:1709.05171v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

The Braess paradox encountered in classical networks is a counterintuitive phenomenon when the flow in a road network can be impeded by adding a new road or, more generally, the overall net performance can degrade after addition of an extra available choice. In this work, we discuss the possibility of a similar effect in a phase-coherent quantum transport and demonstrate it by example of a simple Y-shaped metallic fork. To reveal the Braess-like partial suppression of the charge flow in such device, it is proposed to transfer two outgoing arms into a superconducting state. We show that the differential conductance-vs-voltage spectrum of the hybrid fork structure varies considerably when the extra link between the two superconducting leads is added and it can serve as an indicator of quantum correlations which manifest themselves in the quantum Braess paradox.

Categories: Journals, Physics

### Demonstration of two-atom entanglement with ultrafast optical pulses. (arXiv:1709.05179v1 [physics.atom-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We demonstrate quantum entanglement of two trapped atomic ion qubits using a sequence of ultrafast laser pulses. Unlike previous demonstrations of entanglement mediated by the Coulomb interaction, this scheme does not require confinement to the Lamb-Dicke regime and can be less sensitive to ambient noise due to its speed. To elucidate the physics of an ultrafast phase gate, we generate a high entanglement rate using just 10 pulses, each of $\sim20$ ps duration, and demonstrate an entangled Bell-state with $(76\pm1)$% fidelity. These results pave the way for entanglement operations within a large collection of qubits by exciting only local modes of motion.

Categories: Journals, Physics

### Security of subcarrier wave quantum key distribution against the collective beam-splitting attack. (arXiv:1709.05186v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We consider a subcarrier wave quantum key distribution (QKD) system, where the quantum en- coding is carried by weak sidebands generated to a coherent optical beam by means of an electrooptic phase modulation. We study the security of two protocols, B92 and BB84, against one of the most powerful attacks on the systems of this class: the collective beam splitting attack. We show that a subcarrier wave QKD system with realistic parameters is capable of distributing a cryptographic key over large distances. We show also that a modification of the BB84 protocol, with discrimina- tion of only one state in each basis, performs not worse than the original BB84 protocol for this class of QKD systems, which brings a significant simplification to the development of cryprographic networks on the basis of considered technique.

Categories: Journals, Physics

### Observing pure effects of counter-rotating terms without ultrastrong coupling: A single photon can simultaneously excite two qubits. (arXiv:1709.05199v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

The coherent process that a single photon simultaneously excites two qubits has recently been theoretically predicted by [https://link.aps.org/doi/10.1103/PhysRevLett.117.043601 {Phys. Rev. Lett. 117, 043601 (2016)}]. We propose a different approach to observe a similar dynamical process based on a superconducting quantum circuit, where two coupled flux qubits longitudinally interact with the same resonator. We show that this simultaneous excitation of two qubits (assuming that the sum of their transition frequencies is close to the cavity frequency) is related to the counter-rotating terms in the dipole-dipole coupling between two qubits, and the standard rotating-wave approximation is not valid here. By numerically simulating the adiabatic Landau-Zener transition and Rabi-oscillation effects, we clearly verify that the energy of a single photon can excite two qubits via higher-order transitions induced by the longitudinal couplings and the counter-rotating terms. Compared with previous studies, the coherent dynamics in our system only involves one intermediate state and, thus, exhibits a much faster rate. We also find transition paths which can interfere. Finally, by discussing how to control the two longitudinal-coupling strengths, we find a method to observe both constructive and destructive interference phenomena in our system.

Categories: Journals, Physics

### Noise spectra in balanced optical detectors based on transimpedance amplifiers. (arXiv:1709.05213v1 [physics.ins-det])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We present a thorough theoretical analysis and experimental study of the shot and electronic noise spectra of a balanced optical detector based on an operational amplifier (OA) connected in a transimpedance scheme. We identify and quantify the primary parameters responsible for the limitations of the circuit, in particular the bandwidth and shot-to-electronic noise clearance. We find that the shot noise spectrum can be made consistent with the second order Butterworth filter, while the electronic noise grows linearly with the second power of the frequency. Good agreement between the theory and experiment is observed, however the capacitances of the operational amplifier input and the photodiodes appear significantly higher than those specified in manufacturers' datasheets. This observation is confirmed by independent tests.

Categories: Journals, Physics

### Measuring the single-photon temporal-spectral wave function. (arXiv:1709.05248v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

Temporal-spectral modes of light provide a fundamental window into the nature of atomic and molecular systems and offer robust means for information encoding. Methods to precisely characterize the temporal-spectral state of light at the single-photon level thus play a central role in understanding quantum emitters and are a key requirement for quantum technologies that harness single-photon states. Here we demonstrate an optical reference-free method, which melds techniques from ultrafast metrology and single-photon spectral detection, to characterize the temporal-spectral state of single photons. This provides a robust, wavelength-tunable approach for rapid characterization of pulsed single-photon states that underpins emerging optical quantum technologies based upon the temporal-spectral mode structure of quantum light.

Categories: Journals, Physics

### Explicit Inverse Confluent Vandermonde Matrices with Applications to Exponential Quantum Operators. (arXiv:1709.05257v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

The Cayley-Hamilton problem of expressing functions of matrices in terms of only their eigenvalues is well-known to simplify to finding the inverse of the confluent Vandermonde matrix. Here, we give a highly compact formula for the inverse of any matrix, and apply it to the confluent Vandermonde matrix, achieving in a single equation what has only been achieved by long iterative algorithms until now. As a prime application, we use this result to get a simple formula for explicit exponential operators in terms of only their eigenvalues, with an emphasis on application to finite discrete quantum systems with time dependence. This powerful result permits explicit solutions to all Schr\"odinger and von Neumann equations for time-commuting Hamiltonians, and explicit solutions to any degree of approximation in the non-time-commuting case. The same methods can be extended to general finite discrete open systems to get explicit quantum operations for time evolution using effective joint systems, and the exact solution of all finite discrete Baker-Campbell-Hausdorff formulas.

Categories: Journals, Physics

### On converse bounds for classical communication over quantum channels. (arXiv:1709.05258v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We explore several new converse bounds for classical communication over quantum channels in both the one-shot and asymptotic regime. First, we show that the Matthews-Wehner meta-converse bound for entanglement assisted classical communication can be achieved by activated, no-signalling assisted codes, suitably generalizing a result for classical channels. Second, we derive a new efficiently computable meta-converse on the amount of information unassisted codes can transmit over a single use of a quantum channel. As an applications, we provide a finite resource analysis of classical communication over quantum erasure channels, including the second-order and moderate deviation asymptotics. Third, we explore the asymptotic analogue of our new meta-converse, the $\Upsilon$-information of the channel. We show that its regularization is an upper bound on the capacity that is generally tighter than the entanglement-assisted capacity and other known efficiently computable strong converse bounds. For covariant channels we show that the $\Upsilon$-information is a strong converse bound.

Categories: Journals, Physics

### Coherence and non-classicality of quantum Markov processes. (arXiv:1709.05267v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

Although quantum coherence is a basic trait of quantum mechanics, the presence of coherences in the quantum description of a certain phenomenon does not rule out the possibility to give an alternative description of the same phenomenon in purely classical terms. Here, we give definite criteria to determine when and to what extent quantum coherence is equivalent to non-classicality. We prove that a Markovian multi-time statistics obtained from repeated measurements of a non-degenerate observable cannot be traced back to a classical statistics if and only if the dynamics is able to generate coherences and to subsequently turn them into populations. Furthermore, we show with simple examples that such connection between quantum coherence and non-classicality is generally absent if the statistics is non-Markovian.

Categories: Journals, Physics

### High-fidelity quantum state preparation using neighboring optimal control. (arXiv:1709.05279v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We present an approach to single-shot high-fidelity preparation of an $n$-qubit state based on neighboring optimal control theory. This represents a new application of the neighboring optimal control formalism which was originally developed to produce single-shot high-fidelity quantum gates. To illustrate the approach, and to provide a proof-of-principle, we use it to prepare the two qubit Bell state $|\beta_{01}\rangle = (1/\sqrt{2})\left[\, |01\rangle + |10\rangle\,\right]$ with an error probability $\epsilon\sim 10^{-6}$ ($10^{-5}$) for ideal (non-ideal) control. Using standard methods in the literature, these high-fidelity Bell states can be leveraged to fault-tolerantly prepare the logical state $|\overline{\beta}_{01}\rangle$.

Categories: Journals, Physics

### Hardware-Efficient Bosonic Quantum Error-Correcting Codes Based on Symmetry Operators. (arXiv:1709.05302v1 [quant-ph])

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We establish a symmetry-operator framework for designing quantum error correcting~(QEC) codes based on fundamental properties of the underlying system dynamics. Based on this framework, we propose three hardware-efficient bosonic QEC codes that are suitable for $\chi^{(2)}$-interaction based quantum computation: the $\chi^{(2)}$ parity-check code, the $\chi^{(2)}$ embedded error-correcting code, and the $\chi^{(2)}$ binomial code, all of which detect photon-loss or photon-gain errors by means of photon-number parity measurements and then correct them via $\chi^{(2)}$ Hamiltonian evolutions and linear-optics transformations. Our symmetry-operator framework provides a systematic procedure for finding QEC codes that are not stabilizer codes. The $\chi^{(2)}$ binomial code is of special interest because, with $m\le N$ identified from channel monitoring, it can correct $m$-photon loss errors, $m$-photon gain errors, and $(m-1)$th-order dephasing errors using logical qudits that are encoded in $O(N)$ photons. In comparison, other bosonic QEC codes require $O(N^2)$ photons to correct the same degree of bosonic errors. Such improved photon-efficiency underscores the additional error-correction power that can be provided by channel monitoring. We develop quantum Hamming bounds for photon-loss errors in the code subspaces associated with the $\chi^{(2)}$ parity-check code and the $\chi^{(2)}$ embedded error-correcting code, and we prove that these codes saturate their respective bounds. Our $\chi^{(2)}$ QEC codes exhibit hardware efficiency in that they address the principal error mechanisms and exploit the available physical interactions of the underlying hardware, thus reducing the physical resources required for implementing their encoding, decoding, and error-correction operations, and their universal encoded-basis gate sets.

Categories: Journals, Physics

### Bipartite qutrit local realist inequalities and the robustness of their quantum mechanical violation. (arXiv:1605.08913v6 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

Distinct from the type of local realist inequality (known as the Collins-Gisin-Linden-Massar-Popescu or CGLMP inequality) usually used for bipartite qutrit systems, we formulate a new set of local realist inequalities for bipartite qutrits by generalizing Wigner's argument that was originally formulated for the bipartite qubit singlet state. This treatment assumes existence of the overall joint probability distributions in the underlying stochastic hidden variable space for the measurement outcomes pertaining to the relevant trichotomic observables, satisfying the locality condition and yielding the measurable marginal probabilities. Such generalized Wigner inequalities (GWI) do not reduce to Bell-CHSH type inequalities by clubbing any two outcomes, and are violated by quantum mechanics (QM) for both the bipartite qutrit isotropic and singlet states using trichotomic observables defined by six-port beam splitter as well as by the spin-$1$ component observables. The efficacy of GWI is then probed in these cases by comparing the QM violation of GWI with that obtained for the CGLMP inequality. This comparison is done by incorporating white noise in the singlet and isotropic qutrit states. It is found that for the six-port beam splitter observables, QM violation of GWI is more robust than that of the CGLMP inequality for singlet qutrit states, while for isotropic qutrit states, QM violation of the CGLMP inequality is more robust. On the other hand, for the spin-$1$ component observables, QM violation of GWI is more robust for both the type of states considered.

Categories: Journals, Physics

### Light-induced fractional quantum Hall phases in graphene. (arXiv:1612.08748v2 [cond-mat.str-el] UPDATED)

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

We show how to realize two-component fractional quantum Hall phases in monolayer graphene by optically driving the system. A laser is tuned into resonance between two Landau levels, giving rise to an effective tunneling between these two synthetic layers. Remarkably, because of this coupling, the interlayer interaction at non-zero relative angular momentum can become dominant, resembling a hollow-core pseudo-potential. In the weak tunneling regime, this interaction favors the formation of singlet states, as we explicitly show by numerical diagonalization, at fillings $\nu=1/2$ and $\nu=2/3$. We discuss possible candidate phases, including the Haldane-Rezayi phase, the interlayer Pfaffian phase, and a Fibonacci phase. This demonstrates that our method may pave the way towards the realization of non-Abelian phases, as well as the control of topological phase transitions, in graphene quantum Hall systems using optical fields and integrated photonic structures.

Categories: Journals, Physics

### Nonlinear interactions and non-classical light. (arXiv:1701.01403v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Mon, 2017-09-18 19:37

Non-classical concerns light whose properties cannot be explained by classical electrodynamics and which requires invoking quantum principles to be understood. Its existence is a direct consequence of field quantization; its study is a source of our understanding of many quantum phenomena. Non-classical light also has properties that may be of technological significance. We start this chapter by discussing the definition of non-classical light and basic examples. Then some of the most prominent applications of non-classical light are reviewed. After that, as the principal part of our discourse, we review the most common sources of non-classical light. We will find them surprisingly diverse, including physical systems of various sizes and complexity, ranging from single atoms to optical crystals and to semiconductor lasers. Putting all these dissimilar optical devices in the common perspective we attempt to establish a trend in the field and to foresee the new cross-disciplinary approaches and techniques of generating non-classical light.

Categories: Journals, Physics