## Physics

### Preservation of quantum coherence under Lorentz boost for narrow uncertainty wave packets. (arXiv:1705.09265v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We consider the effect of relativistic boosts on single particle Gaussian wave packets. The coherence of the wave function as measured by the boosted observer is studied as a function of the momentum and the boost parameter. Using various formulations of coherence it is shown that in general the coherence decays with the increase of the momentum of the state, as well as the boost applied to it. Employing a basis-independent formulation, we show however, that coherence may be preserved even for large boosts applied on narrow uncertainty wave packets. Our result is exemplified quantitatively for practically realizable neutron wave functions.

Categories: Journals, Physics

### Flux-dependent localisation in a disordered flat-band lattice. (arXiv:1705.09268v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

Physical systems with high ground state degeneracy, such as electrons in large magnetic fields [1, 2] and geometrically frustrated spins [3], provide a rich playground for exploring emergent many-body phenomena. Quantum simulations with cold atoms offer new prospects for exploring complex phases arising from frustration and interactions [4-7] through the direct engineering of these ingredients in a well-controlled environment [8, 9]. Advances in band structure engineering, through the use of sophisticated lattice potentials made from interfering lasers, have allowed for explorations of kagome [10] and Lieb [11] lattice structures that support high-degeneracy excited energy bands. The use of internal states as synthetic dimensions [12] offers even greater flexibility in creating nontrivial band structures [13-17]. Here, using synthetic lattices based on laser-coupled atomic momentum states, we perform the first exploration of high-degeneracy ground bands in a cold atom system. By combining nearest- and next-nearest-neighbour tunnellings, we form an effective zigzag lattice that naturally supports kinetic frustration and nearly-flat quartic energy bands. In a quartic band structure with time-reversal symmetry (TRS) broken by a synthetic magnetic flux, the quench dynamics of our atoms reveal hallmark signatures of spin-momentum locking. Under preserved TRS, we demonstrate the extreme sensitivity of a nearly-flat ground band to added disorder. Furthermore, we show that the ground state localisation properties are strongly modified by an added flux, relating to a flux-dependent mobility edge due to multi-range tunnelling. Our work constitutes the first quantum simulation study on the interplay of topology and disorder [18], and opens the door to studying emergent phenomena driven by frustrated kinetics and atomic interactions

Categories: Journals, Physics

### Non-Equilibrium Steady State generated by a moving defect: the supersonic threshold. (arXiv:1705.09270v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We consider the dynamics of a system of free fermions on a 1D lattice in the presence of a defect moving at constant velocity. The defect has the form of a localized time-dependent variation of the chemical potential and induces at long times a non-equilibrium steady state (NESS), which spreads around the defect. We present a general formulation which allows recasting the time-dependent protocol in a scattering problem on a static potential. We obtain a complete characterization of the NESS. In particular, we show a strong dependence on the defect velocity and the existence of a sharp threshold when such velocity exceeds the speed of sound. Beyond this value, the NESS is not produced and remarkably the defect travels without significantly perturbing the system. We present an exact solution for a $\delta-$like defect traveling with an arbitrary velocity and we develop a semiclassical approximation which provides accurate results for \cmmnt{arbitrary} smooth defects.

Categories: Journals, Physics

### Hartree Corrections in a Mean-field Limit for Fermions with Coulomb Interaction. (arXiv:1609.04754v2 [math-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We consider the many-body dynamics of fermions with Coulomb interaction in a mean-field scaling limit where the kinetic and potential energy are of the same order for large particle numbers. In the considered limit the spatial variation of the mean-field is small. We prove two results about this scaling limit. First, due to the small variation, i.e., small forces, we show that the many-body dynamics can be approximated by the free dynamics with an appropriate phase factor with the conjectured optimal error term. Second, we show that the Hartree dynamics gives a better approximation with a smaller error term. In this sense, assuming that the error term in the first result is optimal, we derive the Hartree equations from the many-body dynamics with Coulomb interaction in a mean-field scaling limit.

Categories: Journals, Physics

### Structure of irreducibly covariant quantum channels for finite groups. (arXiv:1610.05657v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We obtain an explicit characterization of linear maps, in particular, quantum channels, which are covariant with respect to an irreducible representation ($U$) of a finite group ($G$), whenever $U \otimes U^c$ is simply reducible (with $U^c$ being the contragradient representation). Using the theory of group representations, we obtain the spectral decomposition of any such linear map. The eigenvalues and orthogonal projections arising in this decomposition are expressed entirely in terms of representation characteristics of the group $G$. This in turn yields necessary and sufficient conditions on the eigenvalues of any such linear map for it to be a quantum channel. We also obtain a wide class of quantum channels which are irreducibly covariant by construction. For two-dimensional irrreducible representations of the symmetric group $S(3)$, and the quaternion group $Q$, we also characterize quantum channels which are both irreducibly covariant and entanglement breaking.

Categories: Journals, Physics

### Nonadiabatic holonomic quantum computation with dressed-state qubits. (arXiv:1612.03575v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

Implementing holonomic quantum computation is a challenging task as it requires complicated interaction among multilevel systems. Here we propose to implement nonadiabatic holonomic quantum computation based on dressed-state qubits in circuit QED. An arbitrary holonomic single-qubit gate can be conveniently achieved using external microwave fields and tuning their amplitudes and phases. Meanwhile, nontrivial two-qubit gates can be implemented in a coupled-cavities scenario assisted by a grounding SQUID with tunable interaction, where the tuning is achieved by modulating the ac flux threaded through the SQUID. In addition, our proposal is directly scalable, up to a two-dimensional lattice configuration. In the present scheme, the dressed states involve only the lowest two levels of each transmon qubit and the effective interactions exploited are all of resonant nature. Therefore, we release the main difficulties for physical implementation of holonomic quantum computation on superconducting circuits.

Categories: Journals, Physics

### Incoherent-mediator for quantum state transfer in the ultrastrong coupling regime. (arXiv:1612.06806v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We study quantum state transfer between two qubits coupled to a common quantum bus that is constituted by an ultrastrong coupled light-matter system. By tuning both qubit frequencies on resonance with a forbidden transition in the mediating system, we demonstrate a high-fidelity swap operation even though the quantum bus is thermally populated. This proposal may have applications on hot quantum information processing within the context of ultrastrong coupling regime of light-matter interaction.

Categories: Journals, Physics

### Moderate Deviation Analysis for Classical-Quantum Channels and Quantum Hypothesis Testing. (arXiv:1701.03195v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

In this work, we study the tradeoffs between the error probabilities of classical-quantum channels and the blocklength $n$ when the transmission rates approach the channel capacity at a rate slower than $1/\sqrt{n}$, a research topic known as moderate deviation analysis. We show that the optimal error probability vanishes under this rate convergence. Our main technical contributions are a tight quantum sphere-packing bound, obtained via Chaganty and Sethuraman's concentration inequality in strong large deviation theory, and asymptotic expansions of error-exponent functions. Moderate deviation analysis for quantum hypothesis testing is also established. The converse directly follows from our channel coding result, while the achievability relies on a martingale inequality.

Categories: Journals, Physics

### Genesis of Karl Popper's EPR-Like Experiment and its Resonance amongst the Physics Community in the 1980s. (arXiv:1701.09178v2 [physics.hist-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

I present the reconstruction of the involvement of Karl Popper in the community of physicists concerned with foundations of quantum mechanics, in the 1980s. At that time Popper gave active contribution to the research in physics, of which the most significant is a new version of the EPR thought experiment, alleged to test different interpretations of quantum mechanics. The genesis of such an experiment is reconstructed in detail, and an unpublished letter by Popper is reproduced in the present paper to show that he formulated his thought experiment already two years before its first publication in 1982. The debate stimulated by the proposed experiment as well as Popper's role in the physics community throughout 1980s is here analysed in detail by means of personal correspondence and publications.

Categories: Journals, Physics

### Aspects of Symmetry, Topology and Anomalies in Quantum Matter. (arXiv:1602.05569v1 [cond-mat.str-el] CROSS LISTED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

In this thesis, we explore the aspects of symmetry, topology and anomalies in quantum matter with entanglement from both condensed matter and high energy theory viewpoints. The focus of our research is on the gapped many-body quantum systems including symmetry-protected topological states and topologically ordered states. Chapter 1. Introduction. Chapter 2. Geometric phase, wavefunction overlap, spacetime path integral and topological invariants. Chapter 3. Aspects of Symmetry. Chapter 4. Aspects of Topology. Chapter 5. Aspects of Anomalies. Chapter 6. Quantum Statistics and Spacetime Surgery. Chapter 7. Conclusion: Finale and A New View of Emergence-Reductionism. (Thesis supervisor: Prof. Xiao-Gang Wen)

Categories: Journals, Physics

### Quantum thermodynamic cycle with quantum phase transition. (arXiv:1705.08625v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

With the Lipkin-Meshkov-Glick (LMG) model as an illustration, we construct a thermodynamic cycle composed of two isothermal processes and two isomagnetic field processes and study the thermodynamic performance of this cycle accompanied by the quantum phase transition (QPT). We find that for a finite particle system working below the critical temperature, the efficiency of the cycle is capable of approaching the Carnot limit when the external magnetic field \lambda_{1} corresponding to one of the isomagnetic processes reaches the crosspoint of the ground states' energy level, which can become critical point of the QPT in large N limit. Our analysis proves that the system's energy level crossings at low temperature limits can lead to significant efficiency improvement of the quantum heat engine. In the case of the thermodynamics limit, analytical partition function is obtained to study the efficiency of the cycle at high and low temperature limits. At low temperatures, when the magnetic fields of the isothermal processes are located on both sides of the critical point of the QPT, the cycle obtains the maximum efficiency and the Carnot efficiency can be achieved. This observation demonstrate that the QPT of the LMG model below critical temperature is beneficial to the thermodynamic cycle's operation.

Categories: Journals, Physics

### Entanglement spectrum of the degenerative ground state of Heisenberg ladders in a time-dependent magnetic field. (arXiv:1705.08675v2 [cond-mat.str-el] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We investigate of the relationship between the entanglement and subsystem Hamiltonians in the perturbative regime of strong coupling between subsystems. One of the two conditions that guarantees the proportionality between these Hamiltonians obtained by using the nondegenerate perturbation theory within the first order is that the unperturbed ground state has a trivial entanglement Hamiltonian. Furthermore, we study the entanglement Hamiltonian of the Heisenberg ladders in a time-dependent magnetic field using the degenerate perturbation theory, where couplings between legs are considered as a perturbation. In this case, when the ground state is two-fold degenerate, and the entanglement Hamiltonian is proportional to the Hamiltonian of a chain within first-order perturbation theory, even then also the unperturbed ground state has a nontrivial entanglement spectrum.

Categories: Journals, Physics

### Quantum versus classical simultaneity in communication complexity. (arXiv:1705.07211v2 [cs.CC] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We present a bipartite partial function, whose communication complexity is $O((\log n)^2)$ in the model of quantum simultaneous message passing and $\tilde\Omega(\sqrt n)$ in the model of randomised simultaneous message passing.

In fact, our function has a poly-logarithmic protocol even in the (restricted) model of quantum simultaneous message passing without shared randomness, thus witnessing the possibility of qualitative advantage of this model over randomised simultaneous message passing with shared randomness. This can be interpreted as the strongest known $-$ as of today $-$ example of "super-classical" capabilities of the weakest studied model of quantum communication.

Categories: Journals, Physics

### Quantum Emitters in Two-dimensional Structured Reservoirs in the Non-Perturbative Regime. (arXiv:1705.06673v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We show that the coupling of quantum emitters to a two-dimensional reservoir with a simple band structure gives rise to exotic quantum dynamics with no analogue in other scenarios and which can not be captured by standard perturbative treatments. In particular, for a single quantum emitter with its transition frequency in the middle of the band we predict an exponential relaxation at a rate different from that predicted by the Fermi's Golden rule, followed by overdamped oscillations and slow relaxation decay dynamics. This is accompanied by directional emission into the reservoir. This directionality leads to a modification of the emission rate for few emitters and even perfect subradiance, i.e., suppression of spontaneous emission, for four quantum emitters.

Categories: Journals, Physics

### Markovian and Non-Markovian Dynamics of Quantum Emitters coupled to Two-dimensional Structured Reservoirs. (arXiv:1705.06677v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

The interaction of quantum emitters with structured baths modifies both their individual and collective dynamics. In Gonz\'alez-Tudela \emph{et al} we show how exotic quantum dynamics emerge when QEs are spectrally tuned around the middle of the band of a two-dimensional structured reservoir, where we predict the failure of perturbative treatments, anisotropic interactions and novel super and subradiant behaviour. In this work, we provide further analysis of that situation, together with a complete analysis for the quantum emitter dynamics in spectral regions different from the center of the band.

Categories: Journals, Physics

### Non-Markovianity and coherence of a moving qubit inside a leaky cavity. (arXiv:1705.00887v4 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

Non-Markovian features of a system evolution, stemming from memory effects, may be utilized to transfer, storage, and revive basic quantum properties of the system states. It is well known that an atom qubit undergoes non-Markovian dynamics in high quality cavities. We here consider the qubit-cavity interaction in the case when the qubit is in motion inside a leaky cavity. We show that, owing to the inhibition of the decay rate, the coherence of the traveling qubit remains closer to its initial value as time goes by compared to that of a qubit at rest. We also demonstrate that quantum coherence is preserved more efficiently for larger qubit velocities. This is true independently of the evolution being Markovian or non-Markovian, albeit the latter condition is more effective at a given value of velocity. We however find that the degree of non-Markovianity is eventually weakened as the qubit velocity increases, despite a better coherence maintenance.

Categories: Journals, Physics

### Entanglement concentration with different measurement in a 3-mode optomechanical system. (arXiv:1703.05629v5 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

In this work, we perform a series of phonon counting measurement with different methods in a 3-mode optomechanical system, and we compare the difference of the entanglement after measurement. In this article we focus on the two cases: imperfect measurement and on-off measurement. We find that whatever measurement you take, the entanglement will increase. The size of entanglement enhancement is the largest in perfect measurement, second in the imperfect measurement, and it is not obvious in the on-off measurement. We are sure that the more precise measurement information, the larger entanglement concentration.

Categories: Journals, Physics

### Synchronization of an optomechanical system to an external drive. (arXiv:1703.04344v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

Optomechanical systems driven by an effective blue detuned laser can exhibit self-sustained oscillations of the mechanical oscillator. These self-oscillations are a prerequisite for the observation of synchronization. Here, we study the synchronization of the mechanical oscillations to an external reference drive. We study two cases of reference drives: (1) An additional laser applied to the optical cavity; (2) A mechanical drive applied directly to the mechanical oscillator. Starting from a master equation description, we derive a microscopic Adler equation for both cases, valid in the classical regime in which the quantum shot noise of the mechanical self-oscillator does not play a role. Furthermore, we numerically show that, in both cases, synchronization arises also in the quantum regime. The optomechanical system is therefore a good candidate for the study of quantum synchronization.

Categories: Journals, Physics

### Enhanced Quantization: The Right way to Quantize Everything. (arXiv:1702.04713v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

Canonical quantization relies on Cartesian, canonical, phase-space coordinates to promote to Hermitian operators, which also become the principal ingredients in the quantum Hamiltonian. While generally appropriate, this procedure can also fail, e.g., for covariant, quartic, scalar fields in five-and-more spacetime dimensions (and possibly four spacetime dimensions as well), which become trivial; such failures are normally blamed on the `problem' rather than on the 'quantization procedure'. In Enhanced Quantization the association of $c$-numbers to $q$-numbers is chosen very differently such that: (i) there is no need to seek classical, Cartesian, phase-space coordinates; (ii) every classical, contact transformation is applicable and no change of the quantum operators arises; (iii) a new understanding of the importance of 'Cartesian coordinates' is established; and (iv) although discussed elsewhere in detail, the procedures of enhanced quantization offer fully acceptable solutions yielding non-trivial results for quartic scalar fields in four-and-more spacetime dimensions. In early sections, this paper offers a wide-audience approach to the basic principles of Enhanced Quantization using simple examples; later, several significant examples are cited for a deeper understanding. An historical note concludes the paper.

Categories: Journals, Physics

### Coherence number as a discrete quantum resource. (arXiv:1702.03219v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2017-05-26 10:45

We introduce a new discrete coherence monotone named the \emph{coherence number}, which is a generalization of the coherence rank to mixed states. After defining the coherence number in a similar manner to the Schmidt number in entanglement theory, we present a necessary and sufficient condition of the coherence number for a coherent state to be converted to an entangled state of nonzero $k$-concurrence (a member of the generalized concurrence family with $2\le k \le d$). It also turns out that the coherence number is a useful measure to understand the process of Grover search algorithm. We showed that the coherence number is a simple and clear measure for the success probability of the process. Then we analyze the depletion pattern of $C_c^{(N)}$, the last member of the generalized coherence concurrence family. It is an optimal coherence resource for the Grover algorithm in the sense that it is completely consumed to finish the searching task.

Categories: Journals, Physics