Organic solar cells have attracted considerable interest in recent years due to their potential as efficient and low-cost solar energy converters. For various conjugated polymers in solar cell applications, experimental studies have reported ultrafast charge separation at the interfaces between polymers and fullerenes on a sub-100 fs timescale. Theoretical studies based on time-dependent density functional theory suggested that strong electronic and vibronic couplings of organic photovoltaics may support ultrafast charge separation dynamics. However, the functional relevance of the coherent electronic and vibronic couplings under noise and disorder and their experimental verification remain open questions. In this work, we investigate the polaron pair formation in a reference conjugated polymer for solar cell applications by means of two-dimensional electronic spectroscopy. Here the excitons created by light absorption dissociate into polaron pairs that have been considered as the precursors of free charges. We show that experimentally observed polaron pair formation on a sub-100 fs timescale is governed by coherent electronic and vibronic couplings even in the presence of a high noise level at room temperature and a large energetic disorder of the conjugated polymer. We show that a coherent, vibronic model can reproduce the main features of experimental finding, including vibronic-level structures observed in experiments, shown in the figure, which cannot be accounted by an incoherent model where charge separation is governed by a classical rate equation. Based on model parameters estimated from experimental data, we show that non-equilibrium vibrational motions of the C=C stretch mode of the conjugated polymer underpin ultrafast polaron pair formation and that this mechanism is robust against noise and disorder due to the strong electronic coupling and large Huang-Rhys factors of the present system. Our results suggest the possibility that vibronic coupling, which was shown to play a central role in the excitonic dynamics of natural and artificial light-harvesting systems, also plays an important role in the charge separation dynamics in the organic photovoltaics.

At the heart of quantum mechanics lies the principle that states can be in a linear superposition even while only being observable in discrete quanta. Recently the notion of how much superposed a state is relative to a given basis has been formalized and named coherence theory. This theory hence allows to compare different states in their degree of superposition. In a similar spirit one might now wonder on how much superposing (and hence not classical) a map is and therefore ask how much coherence it can maximally produce; in other words, one might ask what its coherence power is. In this notes we discuss basic properties of the coherence power and show tools that are useful for calculating the coherence power of maps, explicitly giving a new proof for the two-dimensional unitary case and analysing some instructive examples.

Whenever a quantum device (e.g. the electron spin qubit of the NV center in diamond) is used as a sensor to measure parameters of interest, one needs to ensure that the collected data is interpreted correctly. In the case of remaining uncertainties, proper tools are required to eliminate them completely.

In this work, we address the emergence of spurious resonances in pulsed NMR sequences and develop a criterion to identify these fake signals. These resonances occur due to the specific design of the pulse sequences which itself are already meant to suppress errors, however the finite width of the pulses causes the accumulation of the undesired signals. The proposed criterion relays on the free choice of the initial phase relation of the equally weighted superposition of the sensing qubit’s energy eigenstates, which is generally used as the input state in sensing experiments. It turns out, that a variation of this phase relation causes an oscillating amplitude of spurious signals while real signals remain stable. Additionally, we find that the effect of a varying phase relation is equivalent to a variation in phase of the applied pulses, which qualifies the criterion for an easy implementation in existing experimental setups.

To gain a deep understanding of the criterion’s working principle, we motivate it with an approximated analytical calculation and verify its validity with an extensive numerical simulation.

While controlling a quantum system is a standard task nowadays, we are still far away from developing quantum computers, and one might wonder what is the difference between the two. Qualitatively the difference is that for quantum computing one needs to control quantum systems in a quantum way, using quantum systems instead of directly using the large apparata or (classical) electromagnetical fields that often are enough to control a quantum system directly. In this letter we make this idea precise by building a theory which allows us to quantify the usefulness of controlling a quantum system through a quantum system instead of using a classical one.

Cooling of atomic motion is an essential precursor for many interesting experiments and technologies, such as quantum computing and simulation using trapped atoms and ions. In most cases, this cooling is performed using lasers to create a kind of light-induced friction force which slows the atoms down. This process is often rather wasteful, because lasers use up a huge amount of energy relative to the tiny size of the atoms we want to cool. Here, we propose to solve this problem using a quantum absorption refrigerator: a machine that is powered only by readily available thermal energy, such as sunlight, as it flows through the device. We describe how to build such a refrigerator, and predict that sunlight could actually be used to cool an atom to nearly absolute zero temperature. The refrigerator works by trapping the sunlight between two mirrors, in such a way that every single photon makes a significant contribution to the friction force slowing the atom down. Similar schemes could eventually be important for reducing the energy cost of cooling in future quantum technologies.

To observe the quantum effect, we usually need to cool an object to nearly the ground state.  If the object is interacting with a two-level system that is dissipative, then we can apply laser cooling.  Although different kinds of object, ranging from singly trapped atom to visibly large mechanical oscillator, have been successfully cooled by laser cooling, it is believed to be efficient only under demanding conditions.  One such condition is that the background temperature is sufficiently low that satisfies the so-called Lamb-Dicke regime.  Surprisingly, it was recently found that laser cooling is sometimes possible even when the background temperature is higher than the Lamb-Dicke regime.  In our work, we study this phenomenon more closely.  We start from developing a technique to separate the classical part from the quantum part of the dynamics of the object, so the system is easier to study.  Then we extend previous analysis to three-level systems, which are more easily found in the real world.  We confirm previous observation of high temperature laser cooling, and provide a modified criterion to judge when laser cooling is efficient.  Surprisingly, we also find some situations that laser cooling can cool the object only for a limited time, after that the object would be heat up significantly.

In sensing and metrology noise is a fundamental problem because it randomises the sensor and therefore limits its precision. Quantum information science has found different ways to reduce the impact of noise and one of these is quantum error correction. Typically, quantum error correction requires a significant overhead in qubits, one needs at least three qubits to encode and protect a single qubit against phase noise. But this overhead can be reduced when combining stable qubits for encoding with error prone qubits for sensing. This has been achieved experimentally in our recent work where a sensing electron spin is combined with a encoding nuclear spin and repeated error correction has been achieved in combination with sensing.

The Institute of Theoretical Physics is Corresponding PI of the ERC Synergy Grant BioQ “Diamond Quantum Devices and Biology” which is looking in the development of quantum technologies for studying biology, the impact of quantum dynamics on biological function and the use of self-organised structures for creating quantum devices. This invited minireview is summarizing recent developments in the first and the third topic (for a review on the second topic see Contemp. Phys. 54, 181 (2013)).

This work derives the necessary and sufficient condition for quantum adiabatic evolution, that is, when a system remains in an eigenstate of a Hamiltonian even if this Hamiltonian changes over time. It settles a problem that had been identified in the conditions for adiabatic quantum evolution that had been formulated over the last 50 years or so. With the finding that the widely used quantum adiabatic quantitative condition is not always valid, various new necessary or sufficient adiabatic conditions were proposed for replacing the traditional one. However, none of these conditions has been successfully shown to be both necessary and sufficient. Our work settles the problem by providing a simple condition with proofs for both its necessity and sufficiency, by using a gauge invariant formalism to extract all the nonadiabatic transitions. Counterintuitively, the condition reveals that quantum adiabatic evolution allows rapid changes and/or arbitrary numbers of energy crossings in the system Hamiltonian. New ways to achieve quantum adiabatic evolution by pulse sequences or fast varying fields are demonstrated.

In this paper we present a method to detect, locate, and control individual nuclei in interacting clusters by using a single nitrogen-vacancy (NV) center and external control provided by microwave and radio-frequency radiation. The method overcomes the inability of previous schemes to address individually spins that are interacting. It also does not suffer from serious technical drawbacks related to the reorientation of the external alignment field, which affects the NV readout and initialization and is time consuming in current laboratory setups. Detailed numerical simulations demonstrate the precision that our method can achieve by resolving 3D structures of nuclear spins in complex ensembles with Angstrom fidelity. With this information at hand, different applications that include quantum information processing using clusters of carbon-13 nuclei in diamond or identification of the stereoisomeric forms of molecules are available. Note that the former paves the way to exploit the exceptional properties of nuclear spins as solid-state quantum registers while, the latter, corresponds to a question that could previously be answered only when billions of these molecules were available.

Sensing and metrology in the presence of environmental noise represents a considerable challenge because the noise limits the observation time and hence the achievable precision. In this work we explore the idea what may happen when we have limited access to the environment, e.g. we can measure whether the sensor has spontaneously emitted a photon. This information alone may improve the achievable precision quite considerably and the present work shows this at the hand of some examples.

Charged particles can interact with each other, or feel the presence of other charged particles, through the Coulomb interaction. While such an interaction mechanism does not naturally exist for photons, it is well-known that a single atom placed inside of a cavity can mediate an interaction among photons inside of the cavity. The magnitude of this induced photon-photon interaction is often thought to monotonically increase as a function of the atom-photon interaction strength. Contrary to this belief, we show in our work that beyond a certain threshold value, the stronger atom-photon interaction starts to reduce the induced photon-photon interaction. This rather counter-intuitive property of the induced photon-photon interaction is discussed in the context of the photon population dynamics in a coupled cavity array where there occurs a double dynamical transition from a delocalization to localization, back to delocalization, of the photon population. Moreover, it is found that the second delocalization dynamics shows a quasi-equilibration despite of being a closed, finite quantum system.

In a wide class of systems, sweeping back and forth the driving amplitude of a nonlinear resonator produces a hysteresis cycle, which for electromagnetic resonators is referred to as optical bistability. For weak nonlinearities, it can be described by a semiclassical approach neglecting quantum fluctuations. It is known that quantum fluctuations induce switching between two classically stable branches. The steady-state is then unique and consists of a statistical mixture of the two branches. in other words, in the quantum regime (for large nonlinearities), there is no static hysteresis.
We show in this work that there is nonetheless a dynamic hysteresis in the quantum regime. By sweeping the driving amplitude in a finite time we show that the area of the hysteresis cycle exhibits a rich temporal power-law behavior, qualitatively different from semiclassical predictions. We connect this behavior to a nonadiabatic response of the system and establish a link with the Kibble-Zurek mechanism for quenched phase transitions.

Strong entanglement, a quintessential feature of quantum mechanics, is typically a signature of a successfully implemented quantum simulator. Indeed, the aim of these experiments is the preparation and control of systems which show genuine quantum features. For example, recent experiments with trapped ions were designed to implement critical systems – a scenario which is known to become hard to simulate numerically with increasing number of spins due to the presence of large quantum correlations. In order to determine the entanglement in an experiment, one has to take into account that measurements to extract the required information are not only limited by the specifications of the different experimental platforms but, more generally, also by the large dimension of the Hilbert space. A familiar choice to overcome these issues are entanglement witnesses. In our work we determine entanglement witnesses that are quantitative in the sense that they determine lower bounds to the logarithmic negativity, a widely used entanglement measure, that can be obtained to reveal the behavior of entanglement across quantum phase transitions experimentally. In particular, we describe how these witnesses can be measured, e.g., using a short sequence of gate operations and we study the robustness of these measurements in the presence of noise.

The electronic spin of nitrogen-vacancy (NV) centers in diamond is extremely sensitive to local magnetic fields, which makes them a valuable tool for sensing nuclear and electronic spins in their vicinity. While the detected spin targets are static in most applications, the situation is entirely different for ambient thermal conditions as encountered, for example, in biological environments. In this case, the spin-carrying molecules and thus the spins undergo random diffusive motion. In our work we simulate the motion of diffusive spins for specific molecules and derive effective models for the interaction between NV centers and spin targets. We find in particular that spins undergoing rapid diffusion in a restricted domain behave as “averaged” spins, which can still be detected under realistic experimental conditions.

Nuclear spin hyperpolarization (DNP) is a key emerging method for increasing the sensitivity of nuclear magnetic resonance (NMR). Using DNP, a wide range of novel applications in biomedical sciences is made possible, such as metabolic MR imaging or the characterization of molecular chemical compositions. The prevalent methods for achieving DNP in solutions are typically most effective in the regime of small interaction correlation times between the electron and nuclear spins, limiting the size of accessible molecules. To solve this limitation, we design a mechanism for DNP in the liquid phase that is applicable for large interaction correlation times (e.g. slow-moving molecules). We combine this scheme with optically polarized nitrogen-vacancy (NV) center spins in diamonds which provides near perfect electron polarization source at room temperature. Considering the model in a flow cell containing nanodiamonds immobilized in a hydrogel, numerical illustration shows flowing water molecules can be polarized over 1000-fold, in sufficient volumes for detection by current NMR scanners.

The numerical simulation of the interaction of electronic degrees of freedom with their environment is highly challenging when we want to know precise answers and when the environment has a complicated structure so that the evolution cannot be approximated by master equations. This is just the regime in which biological excitation energy transfer in photosynthetic complexes is taking place. In our group we had developed earlier the TEDOPA method while Javier Cerrillo, a former PhD student in the group, had developed another approach, the memory kernel method. During a visit at MIT and a return visit at Ulm we decided to merge the two methods to combine their strengths and the present paper is the result.

The interaction of a quantum system with its environment is of fundamental importance for the development of quantum technologies. The spin-boson model in which a spin degree of freedom couples to a continuum of harmonic oscillators is one of the most fundamental models in this area. A continuum of harmonic oscillators is not easy to treat numerically and ideally one would like to reduce them to a finite number of harmonic oscillators, i.e. to discretize the harmonic environment. This will make us to commit errors that we need to bound. Our work uses a chain mapping that we had developed earlier together with Lieb-Robinson type methods which limit the speed at which perturbation travel through such chains to develop such error bounds.

When a physical system in an excited state is coupled to a cold environment, typically the direction of energy flow is from “hot” system to “cold” environment. This situation changes when the environment is non-Markovian, that is, when its spectral distribution is not flat and when the system exhibits quantum coherence. In this work we show that under these circumstance uphill energy transport is possible, i.e. for the system to absorb energy temporarily from the colder environment thus violating detailed balance.

Symmetry breaking phase transitions and their dynamics are at the heart of a broad range of physical phenomena ranging from the physics of the early universe to solid state physics. Of particular interest are dynamical properties when traversing such a symmetry-breaking second-order phase transition at a finite rate as then spatially separated parts of the system may chose symmetry-broken phases independently and where such choices are incompatible, defects form whose number dependence on the quench rate is given by simple power laws.

Here we study the dynamics of symmetry breaking phase transitions in the formation of helical ion strings that are being held in ion traps that are rotationally invariant around one trap axis. The structures that can occur can be visualised by taking a belt from your trouser and holding it front of you with both hands. Turning one end of the belt creates a helical structure with a winding number that depends on the number of turns. When you go through a phase transition in which the planar structure becomes unstable, we will end up with a finite winding number depending on how fast we traverse the phase transition. We derive analytical scaling laws and confirm them with extensive numerical simulations.

Magnetic resonance imaging (MRI) detects the tiny magnetic moment generated by the nuclear spin of hydrogen atoms in your body. Its sensitivity depends strongly on nuclear spin polarisation, i.e. the magnetisation of the body. Motivated by this observation, dynamical nuclear spin polarisation (DNP), the transfer of electron spin polarization to nuclear spins, has recently been applied to enhance MRI signal. Typically, large signal enhancements (hyperpolarization) require cryogenic temperatures (< 2 K) and bulky, specialized equipment for achieving first a high electron polarization. The Nitrogen-Vacancy defect (NV centre) in diamonds and diamond nanocrystals (nanodiamonds) provides a unique alternative for DNP as the NV centre electron spin can be optically polarized to over 90% polarization at room temperature by short laser pulses.
In our paper we demonstrate an efficient scheme that realises laser induced 13C nuclear spin hyperpolarization in a bulk diamond at room temperature and low ambient magnetic field. Importantly, our protocol is robust against an unknown alignment of the diamond crystal, making this a viable protocol for optical polarization of solutions of nanodiamond. This result gives rise to the possibility of using nanodiamonds, suitably treated to attach themselves to targets that we would like to detect, as MRI markers for molecular imaging applications.

Using laser light it is possible to polarize the electron spin of a colour center in diamond even at room temperature. Resonant microwave radiation can then be used to transfer this polarization to surrounding nuclear spins. Repeating this procedure many times over can lead to very strongly (hyper)-polarized nano-diamonds that may be useful as a contrast agent in MRI. One of the challenges is the random orientation of nanodiamonds in a solution which makes it difficult to achieve resonance with microwaves. In this work we explore the use of microwave frequency combs, i.e. the use of many microwave frequencies to achieve better performance.