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1. Resonant catalysis of thermally activated chemical reactions with vibrational polaritons

   https://doi.org/10.1038/s41467-019-12636-1  

   Campos-Gonzalez-Angulo, Jorge A. ; Ribeiro, Raphael F. ; Yuen-Zhou, Joel ( October 2019 , Nature Communications)

   Interaction between light and matter results in new quantum states whose energetics can modify chemical kinetics. In the regime of ensemble vibrational strong coupling (VSC), a macroscopic number$$N$$$N$of molecular transitions couple to each resonant cavity mode, yielding two hybrid light–matter (polariton) modes and a reservoir of$$N-1$$$N-1$dark states whose chemical dynamics are essentially those of the bare molecules. This fact is seemingly in opposition to the recently reported modification of thermally activated ground electronic state reactions under VSC. Here we provide a VSC Marcus–Levich–Jortner electron transfer model that potentially addresses this paradox: although entropy favors the transit through dark-state channels, the chemical kinetics can be dictated by a few polaritonic channels with smaller activation energies. The effects of catalytic VSC are maximal at light–matter resonance, in agreement with experimental observations.
2. Quantum state control on the chemical reactivity of a transition metal vanadium cation in carbon dioxide activation

   https://doi.org/10.1039/C9CP00575G  

   Chang, Yih Chung ; Xu, Yuntao ; Ng, Cheuk-Yiu ( March 2019 , Physical Chemistry Chemical Physics)

   By combining a newly developed two-color laser pulsed field ionization-photoion (PFI-PI) source and a double-quadrupole–double-octopole (DQDO) mass spectrometer, we investigated the integral cross sections ( σ s) of the vanadium cation (V + ) toward the activation of CO 2 in the center-of-mass kinetic energy ( E cm ) range from 0.1 to 10.0 eV. Here, V + was prepared in single spin–orbit levels of its lowest electronic states, a 5 D J ( J = 0–4), a 5 F J ( J = 1–5), and a 3 F J ( J = 2–4), with well-defined kinetic energies. For both product channels VO + + CO and VCO + + O identified, V + (a 3 F 2,3 ) is found to be greatly more reactive than V + (a 5 D 0,2 ) and V + (a 5 F 1,2 ), suggesting that the V + + CO 2 reaction system mainly proceeds via a “weak quintet-to-triplet spin-crossing” mechanism favoring the conservation of total electron spins. In addition, no J -state dependence was observed. The distinctive structures of the quantum electronic state selected integral cross sections observed as a function of E cm and the electronic state of themore »V + ion indicate that the difference in the chemical reactivity of the title reaction originated from the quantum-state instead of energy effects. Furthermore, this work suggests that the selection of the quantum electronic states a 3 F J ( J = 2–4) of the transition metal V + ion can greatly enhance the efficiency of CO 2 activation.« less
3. Quantum codes from neural networks

   https://doi.org/10.1088/1367-2630/ab6cdd  

   Bausch, Johannes ; Leditzky, Felix ( February 2020 , New Journal of Physics)

   We examine the usefulness of applying neural networks as a variational state ansatz for many-body quantum systems in the context of quantum information-processing tasks. In the neural network state ansatz, the complex amplitude function of a quantum state is computed by a neural network. The resulting multipartite entanglement structure captured by this ansatz has proven rich enough to describe the ground states and unitary dynamics of various physical systems of interest. In the present paper, we initiate the study of neural network states in quantum information-processing tasks. We demonstrate that neural network states are capable of efficiently representing quantum codes for quantum information transmission and quantum error correction, supplying further evidence for the usefulness of neural network states to describe multipartite entanglement. In particular, we show the following main results: (a) neural network states yield quantum codes with a high coherent information for two important quantum channels, the generalized amplitude damping channel and the dephrasure channel. These codes outperform all other known codes for these channels, and cannot be found using a direct parametrization of the quantum state. (b) For the depolarizing channel, the neural network state ansatz reliably finds the best known codes given by repetition codes. (c)more »Neural network states can be used to represent absolutely maximally entangled states, a special type of quantum error-correcting codes. In all three cases, the neural network state ansatz provides an efficient and versatile means as a variational parametrization of these highly entangled states.

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4. Quantifying the magic of quantum channels

   https://doi.org/10.1088/1367-2630/ab451d  

   Wang, Xin ; Wilde, Mark M. ; Su, Yuan ( October 2019 , New Journal of Physics)

   To achieve universal quantum computation via general fault-tolerant schemes, stabilizer operations must be supplemented with other non-stabilizer quantum resources. Motivated by this necessity, we develop a resource theory for magic quantum channels to characterize and quantify the quantum ‘magic’ or non-stabilizerness of noisy quantum circuits. For qudit quantum computing with odd dimensiond, it is known that quantum states with non-negative Wigner function can be efficiently simulated classically. First, inspired by this observation, we introduce a resource theory based on completely positive-Wigner-preserving quantum operations as free operations, and we show that they can be efficiently simulated via a classical algorithm. Second, we introduce two efficiently computable magic measures for quantum channels, called the mana and thauma of a quantum channel. As applications, we show that these measures not only provide fundamental limits on the distillable magic of quantum channels, but they also lead to lower bounds for the task of synthesizing non-Clifford gates. Third, we propose a classical algorithm for simulating noisy quantum circuits, whose sample complexity can be quantified by the mana of a quantum channel. We further show that this algorithm can outperform another approach for simulating noisy quantum circuits, based on channel robustness. Finally, we explore themore »threshold of non-stabilizerness for basic quantum circuits under depolarizing noise.

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5. Melting of charge order in the low-temperature state of an electronic ferroelectric-like system

   https://doi.org/10.1038/s41535-020-0217-5  

   Hassan, Nora M. ; Thirunavukkuarasu, Komalavalli ; Lu, Zhengguang ; Smirnov, Dmitry ; Zhilyaeva, Elena I. ; Torunova, Svetlana ; Lyubovskaya, Rimma N. ; Drichko, Natalia ( March 2020 , npj Quantum Materials)

   Strong electronic interactions can drive a system into a state with a symmetry breaking. Lattice frustration or competing interactions tend to prevent symmetry breaking, leading to quantum disordered phases. In spin systems frustration can produce a spin liquid state. Frustration of a charge degree of freedom also can result in various exotic states, however, experimental data on these effects is scarce. In this work we demonstrate how in a Mott insulator on a weakly anisotropic triangular lattice a charge ordered state melts on cooling down to low temperatures. Raman scattering spectroscopy finds that$$\kappa$$$\kappa$-(BEDT-TTF)$${}_{2}$$${}_2$Hg(SCN)$${}_{2}$$${}_2$Cl enters an insulating “dipole solid” state at$$T=30\,{\mathrm{K}}$$$T=30K$, but below$$T=15\,{\mathrm{K}}$$$T=15K$the order melts, while preserving the insulating energy gap. Based on these observations, we suggest a phase diagram relevant to other quantum paraelectric materials.