## Epitaxial III-V Quantum Dots for Quantum Optical Information S&T: Pros and Cons

These and other proofs of concept make of III-V QDs amenable for highly integrated quantum optical information technologies, yet, to bring expectations to reality, rather important technical and fundamental problems must be solved first. In this talk, I will introduce some of the current challenges in the field and describe how our activities at the MBE: Quantum Nanostructures Group (IMM-CSIC) try to address them [9-10].

References:

[1] P. Michler et al, «A Quantum Dot Single-Photon Turnstile Device», Science, 290 2282 (2000)

[2] C. Santori et al, «Indistinguishable photons from a single-photon device», Nature 419, 594 (2002)

[3] M. Birowosuto et al, «Fast Purcell-Enhanced Single Photon Source in 1,550-mm Telecom Band from a Resonant Quantum Dot-Cavity Coupling». Scientific Reports 2, 321 (2012)

[4] J. Kim et al «Two-Photon Interference from a Bright Single-Photon Source at Telecom Wavelengths». Optica 3, 577 (2016)

[5] Z. Yuan et al, «Electrically Driven Single-Photon Source», Science 295, 102, (2002)

[6] C. L. Salter et al, «An entangled-light-emitting diode», Nature 465, 594, (2010)

[7] E. D. Kim et al., «Fast Spin Rotations by Optically Controlled Geometric Phases in a Charge-Tunable InAs Quantum Dot», Phys. Rev. Lett. 104, 167401 (2010)

[8] W. Liu et al., «In situ tunable g factor for a single electron confined inside an InAs quantum dot», Phys. Rev. B 84 121304 (2011)

[9] J. Herranz et al «Role of re-growth interface preparation process for spectral linewidth reduction of single InAs site-controlled quantum dots». Nanotechnology 26 195301 (2015)

[10] J. M. Llorens et al., «Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology», Applied Physics Letters 107, 183101 (2015)

## Edge states at phase boundaries and their stability

I will show that the appearance of edge states is related with the type of boundary conditions describing the effective model and the difficulties that might arise when one considers different boundary conditions. For doing that I will consider two different situations, scalar theories and fermionic theories. Surprisingly, in the latter case, there is a threshold size for the sample below which the edge states disappear.

## Modifications of molecular structure and reactions under strong light-matter coupling

While most models of strong coupling are based on two-level systems, this is far from a realistic description for molecules with many nuclear (rovibrational) degrees of freedom. The influence of strong coupling on these internal degrees of freedom has only come into focus recently.

Pioneering experiments have shown modifications of material properties and chemical reaction rates under strong coupling, which cannot be explained by simple two-level models. In order to address this mismatch, we developed a first-principles model combining the tools of cavity QED with well-known molecular models in order to fully take into account electronic, nuclear and photonic degrees of freedom.

I will first discuss the applicability of the Born-Oppenheimer approximation, which is challenged by the introduction of the new intermediate timescale of energy exchange between the molecule(s) and the field. We then show how photochemical reactions such as photoisomerization can be almost completely suppressed under strong coupling. Surprisingly, this suppression works more efficiently when many molecules are coupled to a single light mode due to a “collective protection” effect within the delocalized polaritonic state.

## El computador cuántico

## Dissipative long-range entanglement generation between electronic spins

## Symmetry-Protected Heat Transport in Quantum Hall Physics

of practical and fundamental importance. Here, we will discuss stationary

properties of a two-dimensional boson topological insulator coupled to two

thermal baths in the quantum open-system formalism [1]. Novel phenomena

appear like chiral edge heat currents that are the out-of-equilibrium

counterparts of the zero-temperature edge currents. A new set of discrete

symmetries protect these topological heat currents, differing from the

zero-temperature limit, and with a purely dissipative origin. Remarkably,

one of these currents flows opposite to the decreasing external temperature

gradient. As the starting point, we will review some basics about quantum

Hall physics and consider the case of a single external reservoir showing

prominent results like thermal erasure effects and topological thermal

currents. Finally we will comment about the possibility to experimentally

observe these new phenomenology with platforms like photonics chips and

optical lattices.

[1] A. Rivas and M. A. Martin-Delgado, Topological Heat Transport and

Symmetry-Protected Boson Currents, arXiv:1606.07651

## Zero-Mode Rotating Surface States in 3D Dirac and Weyl Semimetals under Radiation

Reference:

J. González, R.A. Molina, PRL 116, 156803 (2016).

## Polaritons: classical and/or quantum aspects

(1) Microcavities, Kavokin et al., Oxford University Press 2011

(2) Entangling a polariton with one photon: effect of interactions at the single-particle level, Cuevas et al., arXiv:1609.01244

(3) Exciting Polaritons with Quantum Light, López Carreño et al., Phys. Rev. Lett. 115:196402 2015

## Simulating spin-boson models with trapped ions

Trapped atomic ions provide a clean and highly controllable system where dynamical quantities are directly accessible. In this talk I will present a method to simulate the dynamics of spin-boson models with macroscopic and non-Markovian environments with trapped ions.

## Machine Learning: Between the hype and the new possibilities

## Quantum simulation with a boson sampling circuit

We have studied a system that consists of 2M matter qubits that interact through a boson sampling circuit, i.e., an M-port interferometer, embedded in two di erent architectures. We have proven that, under the conditions required to derive a master equation, the qubits evolve according to e ective bipartite XY spin Hamiltonians, with or without local and collective dissipation terms. This opens the door to the simulation of any bipartite spin or hard-core boson models and exploring dissipative phase transitions as the competition between coherent and incoherent exchange of excitations. We have also shown that, in the purely dissipative regime, this model has a large number of exact and approximate dark states, whose structure and decay rates can be estimated analytically. We argue that this system may be used for the adiabatic preparation of boson sampling states encoded in the matter qubits.

## V-shape artificial atom based on superconducting quantum circuit

transmon qubits coupled via a large inductance [1]. The resulting

circuit exhibits a symmetric and an antisymmetric oscillation [2]

which we use as a transmon and ancilla qubit, respectively.We observe

a cross-anharmonicity between the two oscillations which is explained

by the Josephson nonlinearity [1].This coupling leads the artificial

atom to a have V-shape energy diagram.

We have predicted that such V-shape artificial atom, inside a circuit

cavity quantum electrodynamics architecture, allows to read out the

transmon qubit state by using the ancilla qubit frequency [3].

In comparison with the most widely employed readout scheme for

superconducting qubits, the dispersive readout, our approach promises

a quantum non-demolition measurement with a significantly stronger

measurement signal and without suffering from Purcell effect. In a

measurement chain based on a state-of-the-art Josephson parametric

amplifier, we predict a QND fidelity of up to 99.9% for a measurement

time down to 60 ns [3].

[1] É. Dumur, et al, “A V-shape superconducting artificial atom based

on two inductively coupled transmons”, Phys. Rev. B 92, 020515 (2015).

[2] F. Lecocq, et al, “Coherent Frequency Conversion in a

Superconducting Artificial Atom with Two Internal Degrees of

Freedom”, Physical Review Letters 108, 107001 (2012).

[3] I. Diniz, et al, “Ultrafast quantum nondemolition measurements

based on a diamond shaped artificial atom”, Physical Review A 87, 033837 (2013).

## Chiral Quantum Optics with Spins, Photons, and Phonons

fascinating perspectives for realizing directional quantum networks with photons as quantum information carriers, as well as novel many-body quantum phases of light and matter.

In this context, we study the implications of chiral interactions on the driven-dissipative dynamics of many quantum emitters coupled via a one-dimensional waveguide. In particular, we determine how the interplay between coherent drive, chiral interactions and collective

decay can lead a chain of two-level atoms to a steady state that is pure and multi-partite entangled, which can be interpreted as a novel non-equilibrium magnetic phase of matter. In addition, we propose various purely atomic realizations of this model, where not only the emitters but also the waveguides are realized with atomic degrees of freedom such as Rydberg atoms, trapped ions, or Bose condensed atoms. Therefore, instead of photons, phonons or spin excitations mediate the chiral interactions, giving a high degree of control over the resulting open many-body dynamics for the emitters. These engineered

atomic setups also provide a route to controllably access physics beyond the Markovian quantum optics paradigm. For instance, using modern many-body numerical methods, we include the full dynamics of the atomic waveguide on the same footing as the emitters and thereby describe non-Markovian effects such as retardation in the exchange of excitations between emitters and non-linear dispersive effects. On the other hand, the same framework allows for the realization of `on-chip’ chiral quantum networks, which we illustrate with basic building blocks for quantum information applications, such as state transfer protocols or time-reversal of wave-packets.

## Majorana bound states from exceptional points in non-topological superconductors

alternative route, which achieves zero-energy Majorana bound states when a topologically trivial superconductor is strongly coupled to a helical normal region. Such a junction can be experimentally realised by e.g. proximitizing a finite section of a nanowire with spin-orbit

coupling, and combining electrostatic depletion and a Zeeman field to drive the non-proximitized portion into a helical phase.

Majorana zero modes emerge in these junctions without fine-tuning as a result of charge-conjugation symmetry, and can be ultimately linked to the existence of `exceptional points’ (EPs) in parameter space (non-hermitian degeneracies extensively studied in photonics [1-3], but seldom discussed in electronic systems), where two quasibound Andreev levels bifurcate into two quasibound Majorana zero modes. After the EP, one of the latter becomes non-decaying and fully localised as the junction approaches perfect Andreev reflection. As I will show, these Majoranas generated through EPs exhibit the full range of properties associated to conventional closed-system Majorana bound states, while not requiring topological superconductivity [4].

[1]The physics of exceptional points, W. D. Heiss, J. Phys. A 45, 444016 (2012).

[2]Spawning rings of exceptional points out of Dirac cones, Bo Zhen et al, Nature 525, 354 (2015)

[3]Topologically protected defect states in open photonic systems with non-hermitian charge-conjugation and parity-time symmetry, Simon Malzard, Charles Poli, Henning Schomerus, Phys. Rev. Lett. 115, 200402 (2015)

[4]Majorana bound states from exceptional points in non-topological superconductors, P. San-Jose, J. Cayao, E. Prada and R. Aguado, Scientific Reports, 6, 21427 (2016)

## Quantum fluctuation relations for generalized Gibbs ensembles

The need for an accurate understanding of the non-equilibrium dynamics of such microscopic devices and their interactions with the environment has lead to a revival of the field of quantum thermodynamics, including the derivation of fluctuation relations for processes far from equilibrum [2].

Recent work has shown how single quantum systems can be used as probes to extract statistics of the work done in such processes [3], and how this can be used to measure their temperature with precision even in the strongly-interacting regime [4].

In this talk, I will present our extension of these proposals to measuring correlation functions, and on the derivation of more general quantum fluctuation relations for systems with conserved quantities whose equilibrium state is described by a generalized Gibbs ensemble.

[1] G. Kurizki, P. Bertet, Y. Kubo, K. Mølmer, D. Petrosyan, P. Rabl, and J. Schmiedmayer, “Quantum technologies with hybrid systems”, PNAS 110, 3866-3873 (2014).

[2] P. Hänggi and P. Talkner, “The other QFT”, Nature Phys. 11, 108 (2015).

[3] R. Dorner et al., Phys. Rev. Lett. 110, 230601 (2013); L. Mazzola et al., ibid. 110, 230602 (2013);

T. Batalhão et al., ibid. 113, 140601 (2013).

[4] T. H. Johnson et al., arXiv:1508.02992 (2015).

## Quantum Acoustics: Using surface acoustic waves as quantum bus between solid-state qubits

means to address, manipulate and couple different solid-state qubits.

We show that state-of-the-art SAW resonators allow to

reach the strong coupling regime for several well-studied qubits,

including quantum dots, trapped ions, nitrogen vacancy centers. In

combination with acoustic waveguides, this system can be utilized

efficiently as a quantum bus, serving as an on-chip, mechanical

cavityQED equivalent and enabling long-range coupling of a wide range

of qubits.

## Multi-Photon scattering and bound states

dimensional confined systems (e.g., the photonic waveguides and the photonic crystals) coupled to some quantum

emitters. Using the path integral formalism, we show the exact results for the transmission spectra and second

order correlation functions of scattered photons, which agree with the Markovian results from the quantum regression theorem. This formalism provides a systematical and convenient way to investigate the exotic photon statistics in the system with strong single-photon nonlinearities (e.g., Rydberg-EIT systems) and the generations of single photon (bundles) [4]. By considering the Markovian effects, we show that a single two-level impurity can bind multi-photons around it to form N -photon bound states [5] in the nonlinear photon bath.

[1] T. Shi and C. P. Sun, Phys. Rev. B 79, 205111 (2009).

[2] T. Caneva, M. T. Manzoni, T. Shi, J. S. Douglas, J. I. Cirac, and D. E. Chang, New J. of Phys. 17, 113001 (2015).

[3] T.Shi, D. E. Chang, J. I. Cirac, Phys. Rev. A 92, 053834 (2015).

[4] Y. Chang, A. González-Tudela, C. Sánchez-Muñoz, C. Navarrete-Benlloch, and T. Shi, arXiv:1510.07307 (2015).

[5] T. Shi, Y. H. Wu, A. Gonzalez-Tudela, J. I. Cirac, arXiv:1512.07238.

## Undecidability of the spectral gap

## Uniqueness of the Fock quantization of Dirac fields with unitary dynamics

## The complex Dirac Delta, Plemelj formula and integral representations

In quantum scattering theory the DD usually arises as an integral representation involving plane waves of real momenta.We shall deal with the complex extension of these representationsby using a Gaussian regularization. Their interpretation as distributions requires prescribing the integration path besides the space of test functions. An extension of the Sokhotski-Plemelj formula is obtained.

## Quantum versus Thermal annealing, the role of Temperature Chaos

We utilize our method to experimentally study the D-Wave Two chip on different temperature-chaotic problems and find, surprisingly, that its performance scales unfavorably as compared to several analogous classical algorithms. We detect, quantify and discuss several purely classical effects that possibly mask the quantum behavior of the chip.

## Unpaired Majorana modes in Josephson junctions arrays with gapless bulk excitations

[1] M. Pino, A. M. Tsvelik, and L. B. Ioffe, Phys. Rev. Lett. 115, 197001 (2015)