## Physics

### Coherence makes quantum systems magical. (arXiv:1801.04807v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

Two primary facets of quantum technological advancement that hold great promise are quantum communication and quantum computation. For quantum communication, the canonical resource is entanglement. For quantum gate implementation, the resource is 'magic' in an auxiliary system. It has already been shown that quantum coherence is the fundamental resource for the creation of entanglement. We argue on the similar spirit that quantum coherence is the fundamental resource when it comes to the creation of magic. This unifies the two strands of modern development in quantum technology under the common underpinning of existence of quantum superposition, quantified by the coherence in quantum theory. We also attempt to obtain magic monotones inspired from coherence monotones and vice versa. We further study the interplay between quantum coherence and magic in a qutrit system and that between quantum entanglement and magic in a qutrit-qubit setting.

Categories: Journals, Physics

### Coupling Two Spin Qubits with a High-Impedance Resonator. (arXiv:1801.04858v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

Fast, high-fidelity single and two-qubit gates are essential to building a viable quantum information processor, but achieving both in the same system has proved challenging for spin qubits. We propose and analyze an approach to perform a long-distance two-qubit controlled phase (CPHASE) gate between two singlet-triplet qubits using an electromagnetic resonator to mediate their interaction. The qubits couple longitudinally to the resonator, and by driving the qubits near the resonator's frequency they can be made to acquire a state-dependent geometric phase that leads to a CPHASE gate independent of the initial state of the resonator. Using high impedance resonators enables gate times of order 10 ns while maintaining long coherence times. Simulations show average gate fidelities of over 96% using currently achievable experimental parameters and over 99% using state-of-the-art resonator technology. After optimizing the gate fidelity in terms of parameters tuneable in-situ, we find it takes a simple power-law form in terms of the resonator's impedance and quality and the qubits' noise bath.

Categories: Journals, Physics

### Mutual information of optical communication in phase conjugating Gaussian channels. (arXiv:1801.03811v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

In all practical communication channels, the code word consist of Gaussian states and the measurement strategy is often a Gaussian detector such as homodyning or heterodyning. In this paper we investigate the communication performance using a phase conjugated alphabet and joint Gaussian detection in an phase-insensitive amplifying channel. We find that a communication scheme consisting of a phase conjugating alphabet of coherent states and a joint detection strategy, significantly outperforms a standard coherent state strategy based in individual detection. Moreover, we show that the performance can be further enhanced by using entanglement and that the performance is completely independent on the gain of the phase-insensitively amplifying channel.

Categories: Journals, Physics

### Real-Time Kalman Filter: Cooling of an Optically Levitated Nanoparticle. (arXiv:1712.07921v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

We demonstrate that a Kalman filter applied to estimate the position of an optically levitated nanoparticle, and operated in real-time within a Field Programmable Gate Array (FPGA), is sufficient to perform closed-loop parametric feedback cooling of the centre of mass motion to sub-Kelvin temperatures. The translational centre of mass motion along the optical axis of the trapped nanoparticle has been cooled by three orders of magnitude, from a temperature of 300K to a temperature of 162 +/- 15mK.

Categories: Journals, Physics

### Collision models in quantum optics. (arXiv:1712.04994v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

Quantum collision models (CMs) provide advantageous case studies for investigating major issues in open quantum systems theory, and especially quantum non-Markovianity. After reviewing their general definition and distinctive features, we illustrate the emergence of a CM in a familiar quantum optics scenario. This task is carried out by highlighting the close connection between the well-known input-output formalism and CMs. Within this quantum optics framework, usual assumptions in the CMs' literature - such as considering a bath of non-interacting yet initially correlated ancillas - have a clear physical origin.

Categories: Journals, Physics

### Current fluctuations in general open quantum systems and crossover to the isolated thermodynamic limit. (arXiv:1712.01068v2 [cond-mat.stat-mech] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

We consider the open quantum set-up of a system connected to two baths with slightly different temperatures and chemical potentials. First, we obtain the open-system current fluctuation-dissipation relations (OCFDR) for this set-up. In doing so, we also obtain a general expression for the non-equilibrium steady state density matrix in the linear response regime. The only assumptions are time reversal and time translation symmetries of the set-up and open-system thermalization. The Onsager relation for thermoelectric transport coefficients is recovered if the system Hamiltonian does not have long range terms. Then, we argue and demonstrate that, the OCFDR and the standard dc Green-Kubo formula which involve the isolated system in the thermodynamic limit have the following relation. They are two different limits of the integrated total current autocorrelation of the open-system in equilibrium. This quantity, which is related to the equilibrium current noise in the open system, can show a crossover between the open-system behavior and the isolated thermodynamic limit behavior as a function of system-size and observation time, even with strong system-bath coupling. This new kind of out-of-equilibrium crossover provides a way to quantify the reaching of isolated thermodynamic limit in real set-ups via current fluctuations.

Categories: Journals, Physics

### Robust Macroscopic Quantum Measurements in the presence of limited control and knowledge. (arXiv:1711.01105v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

Quantum measurements have intrinsic properties which seem incompatible with our everyday-life macroscopic measurements. Macroscopic Quantum Measurement (MQM) is a concept that aims at bridging the gap between well understood microscopic quantum measurements and macroscopic classical measurements. In this paper, we focus on the task of the polarization direction estimation of a system of $N$ spins $1/2$ particles and investigate the model some of us proposed in Barnea et al., 2017. This model is based on a von Neumann pointer measurement, where each spin component of the system is coupled to one of the three spatial components direction of a pointer. It shows traits of a classical measurement for an intermediate coupling strength. We investigate relaxations of the assumptions on the initial knowledge about the state and on the control over the MQM. We show that the model is robust with regard to these relaxations. It performs well for thermal states and a lack of knowledge about the size of the system. Furthermore, a lack of control on the MQM can be compensated by repeated "ultra-weak" measurements.

Categories: Journals, Physics

### Vacuum source-field correlations and advanced waves in quantum optics. (arXiv:1710.06207v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

The solution to the wave equation as a Cauchy problem with prescribed fields at an initial time t=0 is purely retarded. Similarly, in the quantum theory of radiation the specification of Heisenberg picture photon annihilation and creation operators at time t>0 in terms of operators at t=0 automatically yields purely retarded source-fields. However, we show that two-time quantum correlations between the retarded source-fields of a stationary dipole and the quantum vacuum-field possess advanced wave-like contributions. Despite their advanced nature, these correlations are perfectly consistent with Einstein causality. It is shown that while they do not significantly contribute to photo-detection amplitudes in the vacuum state, they do effect the statistics of measurements involving the radiative force experienced by a point charge in the field of the dipole. Specifically, the dispersion in the charge's momentum is found to increase with time. This entails the possibility of obtaining direct experimental evidence for the existence of advanced waves in physical reality, and provides yet another signature of the quantum nature of the vacuum.

Categories: Journals, Physics

### On Landauer's principle and bound for infinite systems. (arXiv:1710.00910v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

Landauer's principle provides a link between Shannon's information entropy and Clausius' thermodynamical entropy. We set up here a basic formula for the incremental free energy of a quantum channel, possibly relative to infinite systems, naturally arising by an Operator Algebraic point of view. By the Tomita-Takesaki modular theory, we can indeed describe a canonical evolution associated with a quantum channel state transfer. Such evolution is implemented both by a modular Hamiltonian and a physical Hamiltonian, the latter being determined by its functoriality properties. This allows us to make an intrinsic analysis, extending our QFT index formula, but without any a priori given dynamics; the associated incremental free energy is related to the logarithm of the Jones index and is thus quantised. This leads to a general lower bound for the incremental free energy of an irreversible quantum channel which is half of the Landauer bound, and to further bounds corresponding to the discrete series of the Jones index. In the finite dimensional context, or in the case of DHR charges in QFT, where the dimension is a positive integer, our lower bound agrees with Landauer's bound.

Categories: Journals, Physics

### Precise measurement of coupling strength and high temperature quantum effect in a nonlinearly coupled qubit-oscillator system. (arXiv:1708.01737v4 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

We study the coherence dynamics of a qubit coupled to a harmonic oscillator with both linear and quadratic interactions. As long as the linear coupling strength is much smaller than the oscillator frequency, the long time behavior of the coherence is dominated by the quadratic coupling strength $g_2$. The coherence decays and revives at a period $2\pi/g_2$, with the width of coherence peak decreases as the temperature increases, hence providing a way to measure $g_2$ precisely without cooling. Unlike the case of linear coupling, here the coherence dynamics never reduces to the classical limit in which the oscillator is classical. Finally, the validity of linear coupling approximation is discussed and the coherence under Hahn-echo is evaluated

Categories: Journals, Physics

### Experimental High-Dimensional Einstein-Podolsky-Rosen Steering

Author(s): Qiang Zeng, Bo Wang, Pengyun Li, and Xiangdong Zhang

Steering nonlocality is the fundamental property of quantum mechanics, which has been widely demonstrated in some systems with qubits. Recently, theoretical works have shown that the high-dimensional (HD) steering effect exhibits novel and important features, such as noise suppression, which appear ...

[Phys. Rev. Lett. 120, 030401] Published Tue Jan 16, 2018

Categories: Journals, Physics

### Effect of Cherenkov radiation on localized-state interaction

Author(s): A. G. Vladimirov, S. V. Gurevich, and M. Tlidi

We study theoretically the interaction of temporal localized states in all fiber cavities and microresonator-based optical frequency comb generators. We show that Cherenkov radiation emitted in the presence of third-order dispersion breaks the symmetry of the localized structures interaction and gre...

[Phys. Rev. A 97, 013816] Published Tue Jan 16, 2018

Categories: Journals, Physics

### Analytical modeling of soliton interactions in a nonlocal nonlinear medium analogous to gravitational force

Author(s): Shihao Zeng, Manna Chen, Ting Zhang, Wei Hu, Qi Guo, and Daquan Lu

We illuminate an analytical model of soliton interactions in lead glass by analogizing to a gravitational force system. The orbits of spiraling solitons under a long-range interaction are given explicitly and demonstrated to follow Newton's second law of motion and the Binet equation by numerical si...

[Phys. Rev. A 97, 013817] Published Tue Jan 16, 2018

Categories: Journals, Physics

### Quarter-flux Hofstadter lattice in a qubit-compatible microwave cavity array

Author(s): Clai Owens, Aman LaChapelle, Brendan Saxberg, Brandon M. Anderson, Ruichao Ma, Jonathan Simon, and David I. Schuster

Topological and strongly correlated materials are exciting frontiers in condensed-matter physics, married prominently in studies of the fractional quantum Hall effect [H. L. Stormer et al., Rev. Mod. Phys. 71, S298 (1999)], There is an active effort to develop synthetic materials where the microsco...

[Phys. Rev. A 97, 013818] Published Tue Jan 16, 2018

Categories: Journals, Physics

### Magnetic dipole-dipole interaction induced by the electromagnetic field

Author(s): Jiaxuan Wang, Hui Dong, and Sheng-Wen Li

We give a derivation for the indirect interaction between two magnetic dipoles induced by the quantized electromagnetic field. It turns out that the interaction between permanent dipoles directly returns to the classical form; the interaction between transition dipoles does not directly return to th...

[Phys. Rev. A 97, 013819] Published Tue Jan 16, 2018

Categories: Journals, Physics

### Fast macroscopic-superposition-state generation by coherent driving

Author(s): Emi Yukawa, G. J. Milburn, and Kae Nemoto

We propose a scheme to generate macroscopic superposition states (MSSs) in spin ensembles, where a coherent driving field is applied to accelerate the generation of macroscopic superposition states. The numerical calculation demonstrates that this approach allows us to generate a superposition of tw...

[Phys. Rev. A 97, 013820] Published Tue Jan 16, 2018

Categories: Journals, Physics

### Enhancement of the quadrupole interaction of an atom with the guided light of an ultrathin optical fiber

Author(s): Fam Le Kien, Tridib Ray, Thomas Nieddu, Thomas Busch, and Síle Nic Chormaic

We investigate the electric quadrupole interaction of an alkali-metal atom with guided light in the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber. We calculate the quadrupole Rabi frequency, the quadrupole oscillator strength, and their enhancement factors. In the examp...

[Phys. Rev. A 97, 013821] Published Tue Jan 16, 2018

Categories: Journals, Physics

### Optical screwdriving induced by the quantum spin Hall effect of surface plasmons near an interface between strongly chiral material and air

Author(s): Qiang Zhang, Junqing Li, Xingguang Liu, Demissie J. Gelmecha, and Weigang Zhang

We reveal a behavior of screwdriving of a surface plasmonic field formed near a surface of strongly chiral material, where lateral force and torque can be provided by the quantum spin Hall effect of light simultaneously. We study the role of chirality strength of the material in optical manipulating...

[Phys. Rev. A 97, 013822] Published Tue Jan 16, 2018

Categories: Journals, Physics

### Viewpoint: Spin Gyroscope is Ready to Look for New Physics

APS Physics - Tue, 2018-01-16 11:00

Author(s): Derek F. Jackson Kimball

An enhanced version of a magnetometer based on atomic spins could be used to search for theoretically predicted exotic fields with ultrahigh sensitivity.

[Physics 11, 5] Published Tue Jan 16, 2018

Categories: Physics

### Quantum-key-distribution protocol with pseudorandom bases

PRA: Quantum information - Tue, 2018-01-16 11:00

Author(s): A. S. Trushechkin, P. A. Tregubov, E. O. Kiktenko, Y. V. Kurochkin, and A. K. Fedorov

Quantum key distribution (QKD) offers a way for establishing information-theoretical secure communications. An important part of QKD technology is a high-quality random number generator for the quantum-state preparation and for post-processing procedures. In this work, we consider a class of prepare...

[Phys. Rev. A 97, 012311] Published Tue Jan 16, 2018

Categories: Journals, Physics