Author(s): Iagoba Apellaniz, Matthias Kleinmann, Otfried Gühne, and Géza Tóth

Given a quantum state, how can one efficiently determine its fitness to metrological applications? Here it is shown how to accomplish this task by estimating the quantum Fisher information with very few measurements.

[Phys. Rev. A 95, 032330] Published Tue Mar 28, 2017

Author(s): Xiao-Long Hu, Yi-Heng Zhou, Zong-Wen Yu, and Xiang-Bin Wang

Most of existing protocols for the decoy-state measurement-device-independent quantum key distribution use vacuum sources or extremely weak sources, but the laboratory cannot give real vacuum sources or precise extremely weak sources. We present a four-intensity protocol without using any of them. O…

[Phys. Rev. A 95, 032331] Published Tue Mar 28, 2017

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A significant hurdle towards realization of practical and scalable quantum computing is to protect the quantum states from inherent noises during the computation. In physical implementation of quantum circuits, a long-distance interaction between two qubits is undesirable since, it can be interpreted as a noise. Therefore, multiple quantum technologies and quantum error correcting codes strongly require the interacting qubits to be arranged in a nearest neighbor (NN) fashion. The current literature on converting a given quantum circuit to an NN-arranged one mainly considered chained qubit topologies or Linear Nearest Neighbor (LNN) topology. However, practical quantum circuit realizations, such as Nuclear Magnetic Resonance (NMR), may not have an LNN topology. To address this gap, we consider an arbitrary qubit topology. We present an Integer Linear Programming (ILP) formulation for achieving minimal logical depth while guaranteeing the nearest neighbor arrangement between the interacting qubits. We substantiate our claim with studies on diverse network topologies and prominent quantum circuit benchmarks.

We derive the Hilbert space formalism of quantum mechanics from epistemic principles. A key assumption is that a physical theory that relies on entities or distinctions that are unknowable in principle gives rise to wrong predictions. An epistemic formalism is developed, where concepts like individual and collective knowledge are used, and knowledge may be actual or potential. The physical state $S$ corresponds to the collective potential knowledge. The state $S$ is a subset of a state space $\mathcal{S}=\{Z\}$, such that $S$ always contains several elements $Z$, which correspond to unattainable states of complete potential knowledge of the world. The evolution of $S$ cannot be determined in terms of the individual evolution of the elements $Z$, unlike the evolution of an ensemble in classical phase space. The evolution of $S$ is described in terms of sequential time $n\in \mathbf{\mathbb{N}}$, which is updated according to $n\rightarrow n+1$ each time potential knowledge changes. In certain experimental contexts $C$, there is initial knowledge at time $n$ that a given series of properties $P,P',\ldots$ will be observed within a given time frame, meaning that a series of values $p,p',\ldots$ of these properties will become known. At time $n$, it is just known that these values belong to predefined, finite sets $\{p\},\{p'\},\ldots$. In such a context $C$, it is possible to define a complex Hilbert space $\mathcal{H}_{C}$ on top of $\mathcal{S}$, in which the elements are contextual state vectors $\bar{S}_{C}$. Born's rule to calculate the probabilities to find the values $p,p',\ldots$ is derived as the only generally applicable such rule. Also, we can associate a self-adjoint operator $\bar{P}$ with eigenvalues $\{p\}$ to each property $P$ observed within $C$. These operators obey $[\bar{P},\bar{P}']=0$ if and only if the precise values of $P$ and $P'$ are simultaneoulsy knowable.

Angle-resolved (AR) RABBIT measurements offer a high information content measurement scheme, due to the presence of multiple, interfering, ionization channels combined with a phase-sensitive observable in the form of angle and time-resolved photoelectron interferograms. In order to explore the characteristics and potentials of AR-RABBIT, a perturbative 2-photon model is developed; based on this model, example AR-RABBIT results are computed for model and real systems, for a range of RABBIT schemes. These results indicate some of the phenomena to be expected in AR-RABBIT measurements, and suggest various applications of the technique in photoionization metrology.

This study analyzed the scar-like localization in the time-average of a timeevolving wavepacket on the desymmetrized stadium billiard. When a wavepacket is launched along the orbits, it emerges on classical unstable periodic orbits as a scar in the stationary states. This localization along the periodic orbit is clarified through the semiclassical approximation. It essentially originates from the same mechanism of a scar in stationary states: the piling up of the contribution from the classical actions of multiply repeated passes on a primitive periodic orbit. To create this enhancement, several states are required in the energy range, which is determined by the initial wavepacket.

We construct a linear system non-local game which can be played perfectly using a limit of finite-dimensional quantum strategies, but which cannot be played perfectly on any finite-dimensional Hilbert space, or even with any tensor-product strategy. In particular, this shows that the set of (tensor-product) quantum correlations is not closed. The constructed non-local game provides another counterexample to the "middle" Tsirelson problem, with a shorter proof than our previous paper (though at the loss of the universal embedding theorem). We also show that it is undecidable to determine if a linear system game can be played perfectly with a limit of finite-dimensional quantum strategies.

An optically levitated nonspherical nanoparticle can exhibit both librational and translational vibrations due to orientational and translational confinements of the optical tweezer, respectively. Usually, the frequency of its librational mode in a linearly-polarized optical tweezer is much larger than the frequency of its translational mode. Because of the frequency mismatch, the intrinsic coupling between librational and translational modes is very weak in vacuum. Here we propose a scheme to couple its librational and center-of-mass modes with an optical cavity mode. By adiabatically eliminating the cavity mode, the beam splitter Hamiltonian between librational and center-of-mass modes can be realized. We find that high-fidelity quantum state transfer between the librational and translational modes can be achieved with practical parameters. Our work may find applications in sympathetic cooling of multiple modes and quantum information processing.

Within the recent reformulation of quantum mechanics where a potential function is not required, we show how to reconstruct the potential so that a correspondence with the standard formulation could be established. However, severe restriction is placed by the correspondence on the kinematics of such problems.

We propose two deterministic secure quantum communication (DSQC) protocols employing three-qubit GHZ-like states and five-qubit Brown states as quantum channels for secure transmission of information in units of two bits and three bits using multipartite teleportation schemes developed here. In these schemes, the sender's capability in selecting quantum channels and the measuring bases leads to improved qubit efficiency of the protocols.

The uncertainty relation is a fundamental limit in quantum mechanics and is of great importance to quantum information processing as it relates to quantum precision measurement. Due to interactions with the surrounding environment, a quantum system will unavoidably suffer from decoherence. Here, we investigate the dynamic behaviors of the entropic uncertainty relation of an atom-cavity interacting system under a bosonic reservoir during the crossover between Markovian and non-Markovian regimes. Specifically, we explore the dynamic behavior of the entropic uncertainty relation for a pair of incompatible observables under the reservoir-induced atomic decay effect both with and without quantum memory. We find that the uncertainty dramatically depends on both the atom-cavity and the cavity-reservoir interactions, as well as the correlation time, $\tau$, of the structured reservoir. Furthermore, we verify that the uncertainty is anti-correlated with the purity of the state of the observed qubit-system. We also propose a remarkably simple and efficient way to reduce the uncertainty by utilizing quantum weak measurement reversal. Therefore our work offers a new insight into the uncertainty dynamics for multi-component measurements within an open system, and is thus important for quantum precision measurements.

We establish the equivalence between the loss of coherence due to mixing in a quantum system and the loss of information after performing a projective measurement. Subsequently, it is demonstrated that the quantum discord, a measure of correlation for the bipartite system $\rho_{Alice\leftarrow Bob}$, is identical to the minimum difference (over all projectors {|i><i|}) between local coherence (LQICC monotone) on Bob side and coherence of the reduced density matrix $\rho^B$.