2D materials with an embedded colour center, that is an optically active electron spin, offer novel ways to control the motional dynamics of such membranes. Here we are exploring the potential of this set-up for simulating the dynamics of the eponymous spin-boson model in which a spin interacts with an environment composed of a large number of bosonic degrees of freedom. The action of the environment is characterised by its spectral density, essentially a product of mode density and coupling strength. When the spectral density is growing slower than linear, this is referred to as the sub-Ohmic spectral density which is particularly challenging to simulate classically. This motivated us to explore whether this model can be realised under realistic experimental conditions. This works answers this question in the positive.

Quantum simulators are devices that can emulate the dynamics of other physical systems in a highly controlled fashion, providing access to dynamics that cannot be simulated efficiently on a classical computer. Currently the most successful implementations of such quantum simulators are using trapped ions cooled to micro-Kelvin temperatures or atoms in optical lattices cooled to nano-Klevin regime where both are held in ultrahigh vacuum and are shielded from external influences. In 2013 we suggested a radical departure from this approach by using 2-dimensional layers of nuclear spins chemically bonded to a diamond and initialised, controlled and read-out by a close-by NV center to provide an architecture for a quantum simulator that can operate at room temperature and ambient conditions. In the present work we are taking some steps towards the realisation of this design with the construction of an ultrathin 13C layer inside of diamond and the exploration of the spin polarisation and control of this layer by means of nearby NV-centers.

Sideband cooling is an important technique that enables ground state cooling of trapped ions and other physical systems. It relies on the fact that a red detuned laser field does not have quite enough energy to excite the ion so that the systems has to borrow a phonon from its vibrational mode to make up for the energy difference. In a tight trap, the subsequent spontaneous decay will typically not generate a phonon. The principle in use here is energy conservation which prevents heating transitions to occur, i.e. those that excite the ion and create a phonon. However, this argument is true only if the driving field is weak thus limiting the cooling rate. In this work we extend earlier work of ours which instead uses the principle of destructive interference to prevent certain undesired transitions. As this interference is independent of the driving rate we can use it to achieve faster cooling. The more off-resonant transitions are excluded the better and our work provides such a scheme.

The goal of building a quantum simulator is to construct highly controlled quantum systems which allow us to probe the physics of many-particle systems that are not accessible to simulation on classical computers. Particularly promising in this respect are systems that are composed of both spin and harmonic oscillators as these are particularly hard to simulate numerically. Here we propose a method to simulate the dynamics of spin-boson models with small crystals of trapped ions where the electronic degree of freedom of one ion is used to encode the spin while the collective vibrational degrees of freedom are employed to form an effective harmonic environment. The key idea of our approach is that a single damped mode can be used to provide a harmonic environment with Lorentzian spectral density. More complex spectral functions can be tailored by combining several individually damped modes. The protocol is especially well-suited to simulate spin-boson models with structured environments which is a paradigmatic model for the system-environment interaction with applications for example to exciton dynamics in contact with their vibrational environment in biological systems.

Phenomenological models of quantum gravity predict that the usual canonical commutation relations of quantum mechanics have small corrections. Recently, it was proposed to use cavity-optomechanical systems to test for these corrections. In this work, we improve the achievable precision of such devices. More specifically, we develop sophisticated paths in phase space of such optomechanical systems to obtain significantly improved accuracy and precision under contributions from higher-order corrections to the optomechanical Hamiltonian. An accurate estimate of the required number of experimental runs is presented based on a rigorous error analysis that accounts for previously unaccounted mean photon-number uncertainty, which can arise from classical fluctuations or from quantum shot noise in measurement. Furthermore, we propose a method to increase precision by using squeezed states of light. Finally, we demonstrate the robustness of our scheme to experimental imperfection. This analysis improves the prospects of carrying out tests of quantum gravity with near-future optomechanical technology.

Nonlinear spectroscopy has revealed long-lasting oscillations in the optical response of a variety of photosynthetic complexes. Different theoretical models which involve the coherent coupling of electronic (excitonic) or electronic-vibrational (vibronic) degrees of freedom have been put forward to explain these observations. The ensuing debate concerning the relevance of either mechanism may have obscured their complementarity. To illustrate this balance, we quantify how the excitonic delocalization in the LH2 unit of purple bacterium provides adequate conditions for robust coherence in the vibronic dynamics, which helped us to explain  the long-lasting oscillations observed in fluorescent traces in LH2, and allowed us to comment on the role of this complementarity in the Fenna-Matthews-Olson complex of green sulphur bacteria.

Thanks to the advances in fabrication and control, two-dimensional (2D) materials are under intense investigation nowadays for their potential applications in quantum technology. The recent reports on the observation of single photon emission from few layers of hexagonal boron nitride (h-BN) samples has sparked a considerable interest. The origin of these emitters, nonetheless, is still unknown whilst a better understanding of their origins is required for their control, which cannot be achieved without theoretical investigations. The purpose of our work is to provide an analysis based on symmetry observations in a few of the point defects in h-BN and correlate the experimental observations and our ab-initio calculations to explain the electronic and magnetic properties of the optical emitters via group theory analysis. We have shown that the properties of some of the studied defects allow for their application in quantum information processing.

A quantum computer makes use of qubits and engineered interactions among them. Trapped ions offer both, qubits encoded in electronic levels and interactions mediated by their collective oscillation modes. Ideally, after a quantum gate, the qubits should be unentangled from the motional modes such that the quantum algorithm can go ahead. To this end, existing approaches continuously rotate the qubits close to the frequency of a specific mode in order to decouple them from the rest of modes, which oscillate at inconmensurable frequencies. However, this produces gates that are orders of magnitude slower than the oscillation period of the ions. Here, with the advent of microwave control of trapped ions, we propose a pulsed gate scheme that accommodates the dynamics of all the modes, bringing the gate time down to the order of the oscillation period of the ions.

In the pursuit of quantum computers, we require fast and accurate entangling quantum gates, generated with control elements growing in a tractable manner with the system size. We combine these three considerations, introducing a conceptually novel scheme for the generation of quantum gates that opens a new road to the scalable quantum control of trapped ions.

In particular, we numerically optimize the inter-pulse spacing for a sequence of microwave pi-pulses, such that at the end of the sequence the desired phase has been acquired by the qubits, while they are left unentangled from the motional modes. Moreover, the symmetry of our sequence makes it robust against the dominant sources of noise.

This work improves and compares methods for the reconstruction of many-particle quantum states. Reconstructing a quantum state of many particles represents a challenge because the Hilbert space dimension grows exponentially with the number of particles. However, under certain conditions on the correlations present in the state in question, the state can be reconstructed from a small number of measurements which grows only linearly with the number of particles. For example, a state with a suitable matrix product state structure may be reconstructed from reduced states of few neighbouring particles in a linear chain. As shown by previous work, the reconstruction may in this case be performed with the so-called Petz recovery map or a suitable, repeated application of a matrix pseudoinverse. We compare the conditions used by the two reconstructions methods. In addition, we improve the two methods by extending them to use long-range measurements in addition to local reduced states, enabling reconstruction of a larger set of states.

At the heart of useful quantum technologies such as simulators or computers lies the ability to create quantum correlations such as entanglement. One challenge is to establish and maintain this fragile property for large numbers of particles. Another challenge is to develop methods to detect entanglement. In this work, we present two methods for entanglement detection and use them to detect entanglement in a system of 20 qubits, the largest fully controllable entangled quantum system to date. In particular, we experimentally study the dynamical evolution of the multipartite entanglement structure of 20 qubits encoded in a quantum simulator built from trapped ions. We track the buildup of entanglement between neighboring qubit pairs and triples, and we even verify genuine quantum correlations for up to 5 neighbors. Most importantly, our evaluation only requires a number of measurement settings proportional to the number of qubits—just 27 out of the more than 3 billion possible measurements. This is in contrast to full state reconstruction, where this number grows exponentially with the system size, which is not viable for scalable entanglement detection.

General quantum states possess a complex entanglement structure that makes their description on a classical computer inefficient in the sense that the computational effort can grow exponentially with the number of subsystems. However, in a wide variety of important states, including the ground and thermal states of local Hamiltonians, the entanglement and correlations are typically more limited as they satisfy area laws. Such states can be approximated well in terms of tensor network (TN) states parametrisation, in which only a polynomial, in the number of subsystems, number of parameters is required to describe the state. Although TNs in one dimension can be efficiently manipulated on a classical computer, the treatment of TN states in higher spatial dimensions remains challenging because the computational effort, while polynomial, grows with a high power in the number of subsystems and bond dimension. Therefore, the experimental generation of TN states and their use for quantum simulation is of considerable interest. Furthermore, the generation of TNs is important as they include important resource states for quantum communication, teleportation and metrology. In our work, we propose an all-optical scheme for the generation of TN states in one and higher dimensions. Our all-optical scheme exploits well established parametric down-conversion methods to build entanglement in the generated state. The scheme also promises robustness against loss and mode mismatch and can be realised with current and near-future optical technology.

NV centers in diamond are one of the most promising platforms for the realization of quantum technologies. They possess numerous features that make out of them a powerful tool to access the quantum world. In this work we exploit their strong interaction with surrounding nuclei to build up a hyperpolarization protocol for organic molecules. That is, using the NV center we can efficiently increase the polarization of nuclear spins belonging to different organic molecules. This is of enormous help, for example, for bio-medicals routines such as NMR. Our contribution consists on an extensive theoretical analysis along with numerical simulations that explains and predict polarization transfer between one single NV center and a vast number of nuclei. Our predictions reproduce in a strong agreement the behavior observed experimentally. Together, these results set the route to novel NMR techniques and better medical diagnosis in the near future.

Many fundamental and practical problems in physics reduce to studying a network of coupled quantum systems driven out of equilibrium by external noise. This broad paradigm incorporates biological transport networks, critical phenomena in dissipative many-body systems, and next-generation mesoscopic electronic devices. However, the theoretical description of these systems is difficult and typically requires approximations of questionable validity. Here, we shed some light on this problem by studying a simple non-equilibrium network comprising just two coupled subsystems, each of which interacts with an external heat reservoir. This model has some very nice symmetry properties, which help us to solve its dynamics exactly. We show that, when the reservoirs are not in thermal equilibrium with each other, they +ACI-interfere+ACI with each other in the sense that their effect on the network cannot be described as a sum of independent contributions. This interference is absent from standard descriptions based on master equations and can even lead to highly non-Markovian dynamics. Remarkably, the effect persists even when the reservoirs are spectrally unstructured and weakly coupled to the open system. Our research contributes to a body of recent work highlighting the subtle interplay between multiple noise sources in dissipative quantum networks.

In open quantum systems, a delicate interplay between coherent interactions and dissipations can bring a system into a non-equilibrium steady state that undergoes an abrupt and non-analytical change, marked by a so-called dissipative phase transition (DPT). In this work, we provide arguably the simplest model system, consisting of a damped harmonic oscillator and a qubit described by the so-called open quantum Rabi model, that exhibits a DPT, by extending the notion of finite-component system phase transitions to an open quantum system. We propose a trapped-ion experiment where the predicted DPT can be realised. Our proposal allows one to control the strength of dissipation and even turn on and off the dissipation so that one can switch from realizing a quantum phase transition to dissipative phase transition; this unique feature of our proposal opens a door for exploring the dynamics of dissipative phase transition in a highly controlled fashion.