Author(s): Karol Bartkiewicz, Grzegorz Chimczak, and Karel Lemr

We describe a direct method for experimental determination of the negativity of an arbitrary two-qubit state with 11 measurements performed on multiple copies of the two-qubit system. Our method is based on the experimentally accessible sequences of singlet projections performed on up to four qubit …

[Phys. Rev. A 95, 022331] Published Wed Feb 22, 2017

Author(s): Xue-Ke Song, Fu-Guo Deng, Lucas Lamata, and J. G. Muga

A nonrelativistic system such as an ultracold trapped ion may perform a quantum simulation of a Dirac equation dynamics under specific conditions. The resulting Hamiltonian and dynamics are highly controllable, but the coupling between momentum and internal levels poses some difficulties to manipula…

[Phys. Rev. A 95, 022332] Published Wed Feb 22, 2017

We propose a general protocol for low-control refrigeration and thermometry of thermal qubits, which can be implemented using electronic spins in diamond. The refrigeration is implemented by a probe, consisting of a network of spins with two-body XXZ interactions. The protocol involves two operations: (i) free evolution of the probe; and (ii) a swap gate between one spin in the probe and the thermal qubit we wish to cool. We show that if the initial state of the probe falls within a suitable range, then the cooling protocol will always succeed, with an efficiency that depends on the rate of spin dephasing and the swap gate fidelity. Furthermore, measuring the probe after it has cooled many qubits provides an estimate of their temperature. We provide a specific example where the probe is a Heisenberg spin chain, and suggest a physical implementation using electronic spins in diamond. Here the probe is constituted of nitrogen vacancy (NV) centers, while the thermal qubits are dark spins. By using a novel pulse sequence, a chain of NV centers can be made to evolve according to a Heisenberg Hamiltonian. This proposal allows for a range of applications, such as NV-based nuclear magnetic resonance of photosensitive molecules kept in a dark spot on a sample, and it opens up new possibilities for the study of quantum thermodynamics, environment-assisted sensing, and many-body physics.

Essential to the description of a quantum system are its local degrees of freedom, which enable the interpretation of subsystems and dynamics in the Hilbert space. While a choice of local tensor factorization of the Hilbert space is often implicit in the writing of a Hamiltonian or Lagrangian, the identification of local tensor factors is not intrinsic to the Hilbert space itself. Instead, the only basis-invariant data of a Hamiltonian is its spectrum, which does not manifestly determine the local structure. This ambiguity is highlighted by the existence of dualities, in which the same energy spectrum may describe two systems with very different local degrees of freedom. We argue that in fact, the energy spectrum alone almost always encodes a unique description of local degrees of freedom when such a description exists, allowing one to explicitly identify local subsystems and how they interact. In special cases, multiple dual local descriptions can be extracted from a given spectrum, but generically the local description is unique.

Quantum integrable systems, such as the interacting Bose gas in one dimension and the XXZ quantum spin chain, have an extensive number of local conserved quantities that endow them with exotic thermalization and transport properties. We review recently introduced hydrodynamic approaches for such integrable systems in detail and extend them to finite times and arbitrary initial conditions. We then discuss how such methods can be applied to describe non-equilibrium steady states involving ballistic heat and spin currents. In particular, we show that the spin Drude weight in the XXZ chain, previously accessible only by heuristic Bethe ansatz techniques, may be evaluated from hydrodynamics in very good agreement with density-matrix renormalization group calculations. This agreement is a strong check on the equivalence between the generalized hydrodynamics resulting from the infinite set of conservation laws in this model on the one hand, and the Bethe-Boltzmann equation in terms of the pseudo-momentum distribution on the other.

Spin qubits hosted in silicon (Si) quantum dots (QD) are attractive due to their exceptionally long coherence times and compatibility with the silicon transistor platform. To achieve electrical control of spins for qubit scalability, recent experiments have utilized gradient magnetic fields from integrated micro-magnets to produce an extrinsic coupling between spin and charge, thereby electrically driving electron spin resonance (ESR). However, spins in silicon QDs experience a complex interplay between spin, charge, and valley degrees of freedom, influenced by the atomic scale details of the confining interface. Here, we report experimental observation of a valley dependent anisotropic spin splitting in a Si QD with an integrated micro-magnet and an external magnetic field. We show by atomistic calculations that the spin-orbit interaction (SOI), which is often ignored in bulk silicon, plays a major role in the measured anisotropy. Moreover, inhomogeneities such as interface steps strongly affect the spin splittings and their valley dependence. This atomic-scale understanding of the intrinsic and extrinsic factors controlling the valley dependent spin properties is a key requirement for successful manipulation of quantum information in Si QDs.

We investigate the interplay between skew information of the subsystem and local quantum uncertainty of the corresponding bipartite system. Skew information can be converted to local quantum uncertainty by quantum operations whose Kraus operators commute with the observable, and the local quantum uncertainty created in the process is bounded above by the skew information. The skew information can also be extracted from local quantum uncertainty by quantum steering, and local quantum uncertainty is the upper bound of the skew information in the process.

This paper considers the problem of implementing a previously proposed distributed direct coupling quantum observer for a closed linear quantum system. By modifying the form of the previously proposed observer, the paper proposes a possible experimental implementation of the observer plant system using a non-degenerate parametric amplifier and a chain of optical cavities which are coupled together via optical interconnections. It is shown that the distributed observer converges to a consensus in a time averaged sense in which an output of each element of the observer estimates the specified output of the quantum plant.

The effects of different quantum feedback types on the estimation precision of the detection efficiency are studied. It is found that the precision can be more effective enhanced by a certain feedback type through comparing these feedbacks and the precision has a positive relation with detection efficiency for the optimal feedback when the system reach the state of dynamic balance. In addition, the bigger the proportion of is the higher the precision is and we will not obtain any information about the parameter to be estimated if is chosen as initial state for the feedback type {\lambda}{\sigma}_z.

The linear coupling of a rotating heat bath to a quantum field is studied in the framework of the Markovian master equation for the field's non-unitary time evolution. The bath's rotation induces population inversion for the field's low-energy modes. For bosons, this leads to superradiance, an irreversible process in which some of the bath's kinetic energy is extracted by spontaneous and stimulated emission. We find the energy and entropy balance for such systems and apply our results to the theory of black hole radiation. We also comment on how this relates to classical self-oscillations such as the hydrodynamic Kelvin-Helmholtz instability.

One of the peculiar features in quantum mechanics is that a superposition of macroscopically distinct states can exits. In optical system, this is highlighted by a superposition of coherent states (SCS), i.e. a superposition of classical states. Recently this highly nontrivial quantum state and its variant have been demonstrated experimentally. Here we demonstrate the superposition of coherent states in quantum measurement which is also a key concept in quantum mechanics. More precisely, we propose and implement a projection measurement onto the arbitrary superposition of the SCS bases in optical system. The measurement operators are reconstructed experimentally by a novel quantum detector tomography protocol. Our device is realized by combining the displacement operation and photon counting, well established technologies, and thus has implications in various optical quantum information processing applications.

With progress in quantum technology more sophisticated quantum annealing devices are becoming available. While they offer new possibilities for solving optimization problems, their true potential is still an open question. As the optimal design of adiabatic algorithms plays an important role in their assessment, we illustrate the aspects and challenges to consider when implementing optimization problems on quantum annealing hardware based on the example of the traveling salesman problem (TSP). We demonstrate that tunneling between local minima can be exponentially suppressed if the quantum dynamics are not carefully tailored to the problem. Furthermore we show that inequality constraints, in particular, present a major hurdle for the implementation on analog quantum annealers. We finally argue that programmable digital quantum annealers can overcome many of these obstacles and can - once large enough quantum computers exist - provide an interesting route to using quantum annealing on a large class of problems.

The most general structure (in matrix form) of a single-qubit gate is presented. Subsequently, used that to obtain a set of conditions for testing (a) whether a given 2-qubit gate is genuinely a 2-qubit gate, i.e., not decomposable into two single qubit gates and (b) whether a given single qubit gate is self-inverse? Relevance of the results reported here is discussed in the context of optimization of reversible and quantum circuits, especially for the optimization of quantum cost. A systematic procedure is developed for the identification of the non-decomposable 2-qubit gates. Such a non-decomposable 2-qubit gate along with all possible single qubit gates form a universal quantum gate library. Further, some possible applications of the present work are also discussed.

The aim of the current work is the research of the influence of the tilted magnetic field direction on excited states of a two-dimensional (2D) hydrogen atom. It was discovered that the quantum chaos appears with an increasing angle $\alpha$ between the magnetic field direction and the normal to the atomic plane. It is characterized by the repulsion of levels leading to the destruction of the shell structure and by changing the spectrum statistical properties. The evolution of the spatial distribution of the square of the absolute value of the wave function at an increasing angle $\alpha$ was identified. The differences of obtained dependencies of energies for various excited states on the tilt angle $\alpha$ at a wide range of the magnetic field strength were described.

Understanding how quantum resources can be quantified and distributed over many parties has profound applications in quantum communication. As one of the most intriguing features of quantum mechanics, Einstein-Podolsky-Rosen (EPR) steering is a useful resource for secure quantum networks. By reconstructing the covariance matrix of a continuous variable four-mode square Gaussian cluster state subject to asymmetric loss, we quantify the amount of bipartite steering with a variable number of modes per party, and verify recently introduced monogamy relations for Gaussian steerability, which establish quantitative constraints on the security of information shared among different parties. We observe a very rich structure for the steering distribution, and demonstrate one-way EPR steering of the cluster state under Gaussian measurements, as well as one-to-multi-mode steering. Our experiment paves the way for exploiting EPR steering in Gaussian cluster states as a valuable resource for multiparty quantum information tasks.

Wave-particle duality is a typical example of Bohr's principle of complementarity that plays a significant role in quantum mechanics. Previous studies used visibility to quantify wave property and used path information to quantify particle property. However, coherence is the core and basis of the interference phenomena of wave. If we use it to characterize wave property, which will be useful to strengthen the understanding of wave-particle duality. A recent theoretical work [Phys. Rev. Lett. 116, 160406 (2016)] found two relations between wave property quantified by coherence in different measure and particle property. Here, we demonstrated the wave-particle duality based on two coherence measures quantitatively for the first time. The path information can be obtained by the discrimination of detector states encoded in polarization of photons corresponding each path and mutual information between detector states and the outcome of the measurement performed on them. We obtain wave property quantified by coherence in l1 measure and relative entropy measure using tomography of photon state that encoded in paths.

We propose a reformulation of quantum field theory (QFT) as a Lorentz invariant statistical field theory. This rewriting embeds a collapse model within an interacting QFT and thus provides a possible solution to the measurement problem. Additionally, it relaxes structural constraints on standard QFTs and hence might open the way to future mathematically rigorous constructions. Finally, because it shows that collapse models can be hidden within QFTs, this article calls for a reconsideration of the dynamical program, as a possible underpinning rather than as a modification of quantum theory.

Quantum discord refers to an important aspect of quantum correlations for bipartite quantum systems. In our earlier works we have shown that corresponding to every graph (combinatorial) there are quantum states whose properties are reflected in the structure of the corresponding graph. Here, we attempt to develop a graph theoretic study of quantum discord that corresponds to a necessary and sufficient condition of zero quantum discord states which says that the blocks of density matrix corresponding to a zero quantum discord state are normal and commute with each other. These blocks have a one to one correspondence with some specific subgraphs of the graph which represents the quantum state. We obtain a number of graph theoretic properties representing normality and commutativity of a set of matrices which are indeed arising from the given graph. Utilizing these properties we define graph theoretic measures for normality and commutativity that results a formulation of graph theoretic quantum discord. We identify classes of quantum states with zero discord using the said formulation.

We analyze the stimulated (emission/absorption) interaction of an electron quantum wavepacket with coherent radiation, using perturbation theory and numerical solution of Schrodinger equation. The analysis applies to a wide class of free electron radiative interaction schemes, and exemplified for Smith-Purcell radiation. Though QED theory and experiments indicate that spontaneous emission of radiation by a free electron is independent of its dimensions, we show that wavepacket dimensions do affect the stimulated radiative interaction in a certain range. We identify a critical electron drift length away from the electron source-$z_G$, which depends on the electron energy and radiation wavelength $\lambda$ only, and defines two different operating ranges: When the drift length is $L_D>z_G$, wavepacket phase and dimension dependent acceleration/deceleration is fundamentally impossible because of the wavepacket spread. For $L_D<z_G$ such acceleration is possible. There we bridge the transition to classical "point particle" linear acceleration, corresponding to the case of a wavepacket spatial distribution of standard deviation short relative to $\lambda$. An independent quantum effect is the significance of the quantum recoil in stimulated radiative interaction. When the recoil is significant the wavepacket analysis emulates the results of FEL theory in the quantum regime and of quantum momentum recoil sidebands in PINEM. We use the platform for discussing the fundamental physics question of measurability of the quantum wavepacket size.

We find that the solution of the polar angular differential equation can be written as the universal associated Legendre polynomials. Its generating function is applied to obtain an analytical result for a class of interesting integrals involving complicated argument. The present method can in principle be generalizable to the integrals involving other special functions. As an illustration we also study a typical Bessel integral with a complicated argument.