We propose a scheme of fast three-qubit Toffoli quantum gate for ultracold neutral-atom qubits. The scheme is based on the Stark-tuned three-body F\"{o}rster resonances, which we have observed in our recent experiment [D.B.Tretyakov et al., Phys.Rev.Lett. 119, 173402 (2017)]. The three-body resonance corresponds to a transition when the three interacting atoms change their states simultaneously, and it occurs at a different dc electric field with respect to the two-body F\"{o}rster resonance. A combined effect of three-body and two-body F\"{o}rster interactions in external electric and magnetic fields near the three-body resonance results in complex coherent behavior of the populations and phases of collective states of a three-atom system. We have found that it is possible to obtain experimental conditions suitable to implement three-qubit Toffoli gate with 96.8\% fidelity and less than 3~$\mu$s duration.

We derive an effective field theory for general chaotic two-dimensional conformal field theories with a large central charge. The theory is a specific and calculable instance of a more general framework recently proposed in [1]. We discuss the gauge symmetries of the model and how they relate to the Lyapunov behaviour of certain correlators. We calculate the out-of-time-ordered correlators diagnosing quantum chaos, as well as certain more fine-grained higher-point generalizations, using our Lorentzian effective field theory. We comment on potential future applications of the effective theory to real-time thermal physics and conformal field theory.

Hyperentangled Bell-state analysis (HBSA) is critical for high-capacity quantum communication. Here we design two effective schemes for error-heralded deterministic generation and self-assisted complete analysis of hyperentangled Bell states for two-photon systems in both the polarization and spatial-mode degrees of freedom, assisted by single-sided quantum-dot-cavity systems. We construct an error-heralded block with a singly charged quantum dot inside a single-sided optical microcavity, two circular polarization beam splitters, one half-wave plate, and one single-photon detector, in which the errors due to imperfect interactions between photons and quantum dot systems can be heralded. With this error-heralded block, the fidelity of our two schemes for hyperentangled Bell-state generation and complete HBSA can reach unit one. What interesting is that by using the measured spatial-mode state to assist the analysis of the polarization state, our complete HBSA scheme works in a self-assisted way, which greatly simplifies the analysis process and largely relaxes the requirements on nonlinearities. These advantages make our schemes much easier to implement experimentally, and have more practical applications in long-distance high-capacity quantum communication.

Producing and certifying entanglement between distant qubits is a highly desirable skill for quantum information technologies. Here we propose a new strategy to monitor and characterize entanglement genesis in a half parity measurement setup, that relies on the continuous readout of an energetic observable which is the half-parity observable itself. Based on a quantum-trajectory approach, we theoretically analyze the statistics of energetic fluctuations for a pair of continuously monitored qubits. We quantitatively relate these energetic fluctuations to the rate of entanglement produced between the qubits, and build an energetic-based estimator to assess the presence of entanglement in the circuit. Remarkably, this estimator is valid at the single-trajectory level. Our work paves the road towards a fundamental understanding of the stochastic energetic processes associated with entanglement genesis, and opens new perspectives for witnessing non-local quantum correlations thanks to thermodynamic quantities.

Compiling quantum algorithms for near-term quantum computers (accounting for connectivity and native gate alphabets) is a major challenge that has received significant attention both by industry and academia. Avoiding the exponential overhead of classical simulation of quantum dynamics will allow compilation of larger algorithms, and a strategy for this is to evaluate an algorithm's cost on a quantum computer. To this end, we propose quantum-assisted quantum compiling (QAQC). In QAQC, we use the overlap between a target unitary $U$ and a trainable unitary $V$ as the cost function to be evaluated on the quantum computer. More precisely, to ensure that QAQC scales well with problem size, our cost function involves not only the global overlap ${\rm Tr} (V^\dagger U)$ but also the local overlaps with respect to individual qubits. We introduce novel short-depth quantum circuits to quantify the terms in our cost function, and we present both gradient-free and gradient-based approaches to minimizing this function. As a demonstration of QAQC, we compile various one-qubit gates on IBM's and Rigetti's quantum computers into their respective native gate alphabets. Future applications of QAQC include algorithm depth compression, black-box compiling, noise mitigation, and benchmarking.

We demonstrate dynamical control of the superradiant transition of cavity-BEC system via periodic driving of the pump laser. We show that the dominant density wave order of the superradiant state can be suppressed, and that the subdominant competing order of Bose-Einstein condensation emerges in the steady state. Furthermore, we show that additional, non-equilibrium density wave orders, which do not exist in equilibrium, can be stabilized dynamically. Finally, for strong driving, chaotic dynamics emerges.

Author(s): Gianfranco Bertone and Dan Hooper

The standard model of modern cosmology is unthinkable without dark matter, although direct detections are still missing. A broad perspective of how dark matter was postulated and became accepted is presented, from prehistory, over observations of galaxy clusters, galaxy rotation curves, the search for baryonic dark matter, possible alternative explanations via modified gravity, up to the hunt for dark matter particles. The interplay is described between observational discoveries and theoretical arguments which led finally to the adoption of this paradigm.

[Rev. Mod. Phys. 90, 045002] Published Mon Oct 15, 2018

Author(s): Hoi-Kwan Lau, Alexander Eisfeld, and Jan-Michael Rost

We theoretically study the radiation-induced interaction between the mechanical motion of an oscillating mirror and a remotely trapped atomic cloud. When illuminated by continuous-wave radiation, the mirror motion will induce red and blue sideband radiation, which respectively increases and reduces ...

[Phys. Rev. A 98, 043827] Published Mon Oct 15, 2018

Author(s): Ze-an Peng, Guo-qing Yang, Qing-lin Wu, and Gao-xiang Li

Frequency-resolved correlation (FRC) is investigated in fluorescent emission radiated from a Λ-type atomic system by employing weak coupling regime between quantum emitter and cavities, in which each cavity substituting a Lorentzian filter is assumed to pass through only one fluorescent photon at a ...

[Phys. Rev. A 98, 043828] Published Mon Oct 15, 2018

Author(s): Jinze Wu, Jinhong Liu, Yanyan He, Yueying Zhang, Junxiang Zhang, and Shiyao Zhu

We investigate the manipulation of the quantum interference (QI) in an electromagnetically induced transparency (EIT) system via phase fluctuations and their correlation of interacting fields. We show that the field fluctuation correlation and atomic dephasing rate have a similar effect to atomic co...

[Phys. Rev. A 98, 043829] Published Mon Oct 15, 2018

Author(s): Yaniv Eliezer, Alon Bahabad, and Boris A. Malomed

We introduce the one-dimensional PT-symmetric Schrödinger equation, with complex potentials in the form of the canonical superoscillatory and suboscillatory functions known in quantum mechanics and optics. While the suboscillatorylike potential always generates an entirely real eigenvalue spectrum, ...

[Phys. Rev. A 98, 043830] Published Mon Oct 15, 2018

Author(s): Juuso Manninen, Muhammad Asjad, Risto Ojajärvi, Petri Kuusela, and Francesco Massel

We propose here a scheme based on the measurement of quadrature phase coherence that is aimed at testing the Clauser-Horne-Shimony-Holt Bell inequality in an optomechanical setting. Our setup is constituted by two optical cavities dispersively coupled to a common mechanical resonator. We show that i...

[Phys. Rev. A 98, 043831] Published Mon Oct 15, 2018

Author(s): Shulin Wang, Bing Wang, and Peixiang Lu

We investigate the PT-symmetric Talbot effect in a temporal mesh lattice constructed by two coupled fiber loops, in which the PT symmetry is introduced through temporally controlling the gain and loss of the loops. The Talbot self-imaging exists only if the period of input pulse train is chosen as t...

[Phys. Rev. A 98, 043832] Published Mon Oct 15, 2018

Author(s): A. M. Zheltikov

The nonlinear Schrödinger equation (NSE) provides a powerful tool for the analysis of ultrafast nonlinear-optical dynamics, including a vast class of optical solitons. Here, we show, however, that the photon-number integral of the NSE differs from the physical number of photons, conserved by more ge...

[Phys. Rev. A 98, 043833] Published Mon Oct 15, 2018

Researchers observe new features in the band structure of multilayer graphene that point to enhanced electron interactions.

[Physics] Published Mon Oct 15, 2018

Categories: Physics

Author(s): Ryo Namiki, Akira Kitagawa, and Takuya Hirano

We have developed a method to calculate a secret key rate of a continuous-variable quantum-key-distribution scheme using four coherent states and postselection for a general model of Gaussian attacks. We assume that the transmission line and detection process are described by a pair of Gaussian chan...

[Phys. Rev. A 98, 042319] Published Mon Oct 15, 2018

Author(s): Shang Yu, Chang-Jiang Huang, Jian-Shun Tang, Zhih-Ahn Jia, Yi-Tao Wang, Zhi-Jin Ke, Wei Liu, Xiao Liu, Zong-Quan Zhou, Ze-Di Cheng, Jin-Shi Xu, Yu-Chun Wu, Yuan-Yuan Zhao, Guo-Yong Xiang, Chuan-Feng Li, Guang-Can Guo, Gael Sentís, and Ramon Muñoz-Tapia

Detecting a change point is a crucial task in statistics that has been recently extended to the quantum realm. A source state generator that emits a series of single photons in a default state suffers an alteration at some point and starts to emit photons in a mutated state. The problem consists in ...

[Phys. Rev. A 98, 040301(R)] Published Mon Oct 15, 2018

Author(s): Hongyi Zhou, Pei Zeng, Mohsen Razavi, and Xiongfeng Ma

Continuous-variable quantum cryptographic systems, including random number generation and key distribution, are often based on coherent detection. The essence of the security analysis lies in the randomness quantification. Previous analyses employ a semiquantum picture, where the strong local oscill...

[Phys. Rev. A 98, 042321] Published Mon Oct 15, 2018

Author(s): Mario Berta, Fernando G. S. L. Brandão, Christian Majenz, and Mark M. Wilde

We define the deconstruction cost of a tripartite quantum state on systems ABE as the minimum rate of noise needed to apply to the AE systems, such that there is negligible disturbance to the marginal state on the BE systems, while the system A of the resulting state is locally recoverable from the ...

[Phys. Rev. A 98, 042320] Published Mon Oct 15, 2018

Author(s): Shashank Gupta, Shounak Datta, and A. S. Majumdar

A tripartite quantum network is said to be bilocal if two independent sources produce a pair of bipartite entangled states. Quantum nonbilocal correlation emerges when the central party which possesses two particles from two different sources performs Bell-state measurement on them and nonlocality i...

[Phys. Rev. A 98, 042322] Published Mon Oct 15, 2018