QUROPE Quantum Information Processing and Communication in Europe

We consider stochastic and open quantum systems with a finite number of states, where a stochastic transition between two specific states is monitored by a detector. The long-time counting statistics of the observed realizations of the transition, parametrized by cumulants, is the only available information about the system. We present an analytical method for reconstructing generators of the time evolution of the system compatible with the observations.

Performing complex cryptographic tasks will be an essential element in future quantum communication networks. These tasks are based on a handful of fundamental primitives, such as coin flipping, where two distrustful parties wish to agree on a randomly generated bit. Although it is known that quantum versions of these primitives can offer information-theoretic security advantages with respect to classical protocols, a demonstration of such an advantage in a practical communication scenario has remained elusive.

We experimentally demonstrate that a non-classical state prepared in an atomic memory can be
efficiently transferred to a single mode of free-propagating light. By retrieving on demand a single
excitation from a cold atomic gas, we realize an efficient source of single photons prepared in a pure,
fully controlled quantum state. We characterize this source using two detection methods, one based
on photon-counting analysis, and the second using homodyne tomography to reconstruct the density

We experimentally demonstrate that a non-classical state prepared in an atomic memory can be
efficiently transferred to a single mode of free-propagating light. By retrieving on demand a single
excitation from a cold atomic gas, we realize an efficient source of single photons prepared in a pure,
fully controlled quantum state. We characterize this source using two detection methods, one based
on photon-counting analysis, and the second using homodyne tomography to reconstruct the density

The long-term promises of quantum simulators are far-reaching. The field, however, also needs clearly defined short-term goals.

We analyze the robustness of a quantum memory based on Majorana modes in a Kitaev chain. We identify the optimal recovery operation acting on the memory in the presence of perturbations and evaluate its fidelity in different scenarios. We show that for time-dependent Hamiltonian perturbations that preserve the topological features, the memory is robust even if the perturbation contains frequencies that lie well above the gap. We identify the condition that is responsible for this feature. At the same time we find that the memory is unstable with respect to particle losses.

We derive a criterion to determine when a translationally invariant matrix product state (MPS) has long-range localizable entanglement, where that quantity remains finite in the thermodynamic limit. We give examples fulfilling this criterion and eventually use it to obtain all such MPS with bond dimension 2 and 3.

Initialization and read-out of coupled quantum systems are essential ingredients for the implementation of quantum algorithms1, 2. Single-shot read-out of the state of a multi-quantum-bit (multi-qubit) register would allow direct investigation of quantum correlations (entanglement), and would give access to further key resources such as quantum error correction and deterministic quantum teleportation1. Although spins in solids are attractive candidates for scalable quantum information processing, their single-shot detection has been achieved only for isolated qubits3, 4, 5, 6.

Two-qubit interactions are at the heart of quantum information processing. For single-spin qubits in semiconductor quantum dots, the exchange gate has always been considered the natural two-qubit gate. The recent integration of a magnetic field or g-factor gradients in coupled quantum dot systems allows for a one-step, robust realization of the controlled-phase (C-phase) gate instead.

 We investigate theoretically the performance of resonant two-qubit gates in the crossover from the strong to the ultrastrong coupling regime of light–matter interaction in circuit quantum electrodynamics. Two controlled-phase (CPHASE) gate schemes—which work well within the rotating wave-approximation—are analysed while taking into account the effects of counter-rotating terms appearing in the Hamiltonian.