The coherence properties of an atom or superconducting qubit strongly depend on the electromagnetic environment. In waveguide QED the qubit is strongly coupled to a continuous mode spectrum, thus it decays rapidly. Collective effects between multiple qubits can be utilized to generate subradiant states that decouple from the dissipative waveguide environment.

In our experiment we strongly...

We introduce a previously unidentified paradigm for the direct experimental realization of excitation dynamics of three-dimensional networks by exploiting the hybrid action of spatial and polarization degrees of freedom of photon pairs propagating in coupled waveguide circuits with tailored birefringence. The photons exhibit Hong-Ou-Mandel-like interference simultaneously in both degrees of...

In the near future, molecular simulations will be aided by quantum computer-assisted calculations which have the potential to target systems intractable for classical computers. However, since the resources offered by near term quantum computers are still limited, it is necessary to investigate hybrid quantum-classical computational schemes. In our recent work (J. Chem. Phys. **154**, 114105,...

Experimental investigation of quantum mechanics with heavy objects (>m_planck~20µg) has not been achieved. It requires the combination of decoupling the quantum object from environmental influences, while remaining high control of it, a challenge increasing with the mass of the object.

To achieve these, we implement the approach of superconducting microspheres in a magnetic field trap,...

Advances in quantum information processing and technologies lead to promising developments towards a quantum network. The latter would feature local quantum processors exchanging information and entanglement via quantum links, enabling, for instance, long-distance quantum communication. The focus of this contribution is to investigate quantum correlations in networks from the point of view of...

We present progress on an experimental setup in which we aim to implement two-dimensional ion-crystals in a Paul trap. We will use novel optical microtraps to manipulate the phonon spectrum of the crystal. This in turn allows us to engineer the spin-spin interactions. In particular, the pinning of a single ion can be used to create short-range spin-spin interactions. In 2D crystals, this can...

This talk will discuss the stability of symmetry protected topological (SPT) order under noisy channels. It has been shown that SPT order is destabilized by simple models of noise, collapsing the classification of SPT phases into a single trivial phase, even if the noise is symmetric in the usual sense. We introduce a stronger symmetry condition on channels that ensures that they preserve SPT order.

We investigate scalable surface ion traps for quantum simulation and quantum computing.

We developed a micro-fabricated surface trap consisting of two parallel linear-trap arrays with 11

trapping sites each.

We demonstrate trapping and shuttling of multiple ions, and form square and triangular ion-lattice configurations with up to six ions.

We characterize stray electric fields and...

Quantum information experiments are advancing rapidly with realisations of real-time quantum error correction [1], demonstrations of variational quantum computing, for example, in metrology [2], and explorations of novel topological phases [3]. Underpinning these advancements is the requirement for high fidelity gates with low and uncorrelated errors between gates and qubits [4]. However, as...

This talk presents recent experimental demonstrations that use integrated nanophotonic processors for various quantum computations such as quantum machine learning and in particular reinforcement learning, where agents interact with environments by exchanging signals via a communication channel. We show that this exchange allows boosting the learning of the agent. Another experiment underlines...

In this talk, I will discuss applications of quantum networks going beyond quantum key distribution. I will show that quantum networks can be used to perform distributed computing, both classical and quantum. Furthermore, I will present a recent experiment that shows quantum communication in a multipartite network and allows – besides security in the communication – keeping the identities of...

Optically active spins in solids are often considered prime candidates for scalable and feasible quantum-optical devices. Numerous material platforms including diamond, semiconductors, and atomically thin 2d materials are investigated, where each platform brings their own advantages along with their challenges. Semiconductor quantum dots are the current state-of-the-art for optical properties...

Algorithms that run on near term quantum devices will be a decisive step towards applications of quantum computers. In this talk I will focus on optimization problems and how to encode and solve them on quantum devices. I will present the parity mapping and algorithms that make use of it. In particular, the talk will focus on encoding of optimization problems with higher-order terms and side...

We investigate the relationship between quantum correlations and the communication of quantum bits of information. We go beyond standard qubits and instead consider a more general notion of informational restriction which makes no reference to the dimension of Hilbert space. We show how to characterise such informationally restricted quantum correlations and how they qualitatively go beyond...

Developing novel quantum technology exhibits the challenge of their efficient characterisation. We introduce and experimentally demonstrate a methodology to automatically formulate and select Hamiltonian models, learning the most appropriate in reproducing the observed system’s dynamics. Here, we propose and experimentally demonstrate the quantum model learning agent (QMLA), a Bayesian...

In this talk I will describe a quantum-classical variational protocol for learning the structure of the Entanglement Hamiltonian (EH) in Quantum Simulation experiments. In this approach, spatial deformations of the many-body Hamiltonian, physically realized on the quantum device, serve as an efficient variational ansatz for a local EH. On-device spectroscopy of the learned Hamiltonian provides...

Ensembles of cold atoms and ions excell in metrology and quantum information processing. This opens the opportunity to utilize tailored, programmable entanglement generation to approach the 'optimal quantum sensor'. Here we report first quantum enhancement in metrology beyond squeezing through low-depth, variational quantum circuits searching for optimal input states and measurement operators....

Variational quantum algorithms (VQAs) have become an indispensable tool for noisy near-term quantum computation, enabling small-scale simulations on present-day hardware. So far, however, their success was mainly limited to optimization problems. The go-to method for dynamics remains Trotter-evolution, relying on deep circuits and thus hampered by the substantial limitations of available...

I will introduce a "coherence equality" that, in the spirit of Bell's inequalities, can be used to discriminate between classical and quantum resources. This equality is satisfied by any classical communication (localized carrier), but is violated when the carrier is in a quantum superposition of communication directions. This implies that the classical success probability of a certain...

It is usually believed that coarse-graining of quantum correlations leads to classical correlations in the macroscopic limit. Such a principle, known as macroscopic locality, has been proved for correlations arising from independent and identically distributed (IID) entangled pairs. In this work we consider the generic (non-IID) scenario. We find that the Hilbert space structure of quantum...

Just like their classical counterparts, quantum algorithms require a set of inputs, provided for example as real numbers, and a list of operations to be performed on some reference initial state. Unlike classical computers, however, information is stored in a quantum processor in the form of a wavefunction, thus requiring special procedures to read out the final results. In fact, it is in...

As quantum technologies advance, the ability to generate increasingly large quantum states has experienced rapid development. In this context, the verification of large entangled systems represents one of the main challenges in the employment of such systems for reliable quantum information processing. Though the most complete technique is undoubtedly full tomography, the inherent exponential...

Characteristic impedance in superconducting quantum circuits determines whether the ground state wavefunction is dominated by charge or phase fluctuations. The crossover occurs at RQ = 6.45 kΩ above which the charge fluctuations are suppressed below 2e-. Most interesting is the behavior of the Josephson junction (JJ), which acts as a non-linear inductor at low impedance and as a non-linear...

Physicists have been fascinated by the non-equilibrium dynamics of quantum systems for a long time. Microscopic details are relevant at the early stage of the time dynamics where the system has just begun thermalizing. However, at the onset of thermal equilibrium, the dynamics are governed by symmetries and topology, and at this stage, classical hydrodynamics is expected to emerge. In this...

In ultra-thin media, the phase-matching condition for nonlinear optical processes, such as four-wave mixing (FWM), relaxes. We characterize the resulting broadband biphoton states by stimulated emission tomography and present progress towards photon pair generation in ultra-thin carbon nanotube films. Our 200 nm thick single-walled carbon nanotube film (much smaller than the pump wavelength of...

I will discuss the recent proposal of a set of experimentally accessible conditions for detecting entanglement in mixed states based on comparing moments of the partially transposed density operator. The union of all inequalities reproduces the Peres-Horodecki criterion. Exploiting symmetries can help to further improve their detection capabilities and the estimation of the inequalities is...

The double slit experiment provides a demarcation between classical and quantum theory, while multi-slit experiments demarcate quantum and higher-order interference theories. In this work we show that these experiments pertain to a broader class of processes, which can be formulated as information-processing tasks. We provide a connection between the order of interference and the probabilities...

Bell's theorem shows that no hidden-variable model can explain the measurement statistics of a quantum system shared between two parties, thus ruling out a classical (local) understanding of nature. In this work we demonstrate that by relaxing the positivity restriction in the hidden-variable probability distribution it is possible to derive quasiprobabilistic Bell inequalities whose sharp...

Next-generation quantum sensors are expected to outperform classical sensors. Since the success of these quantum sensors depends on the efficient use of limited resources (such as probe states and coherence time), we introduce the paradigm of smart quantum sensors, i.e., quantum sensors which make autonomous adjustments in order to optimize the measurement precision and to save resources. A...

In this talk, I will discuss the so-called device independent quantum key distribution (DIQKD) protocols -- where all elements of the setup are analysed as black boxes. Contrary to standard QKD, the security of DIQDK does not rely on detailed quantum models of the devices and is proof against "quantum hacking". After a concise introduction I will present some ideas (noisy preprocessing,...

Deep neural networks have had a profound impact on the field of reinforcement learning by achieving unprecedented performance in challenging decision-making tasks. Almost in parallel, the idea that variational quantum circuits could be used in quantum-classical machine learning systems started gaining increasing traction. Such hybrid systems have already shown the potential to tackle...

We discuss the emergence of a moderate biased error in non-ideal integrated photonic circuits. We investigate its correlation with properties of the optical paths, revealing potential issues for high-precision tests and optical implementations of machine learning.

Given a quantum gate implementing a unitary operation U without any specific description but its dimension, we present a universal quantum circuit that implements its inverse by making k uses of the given operation. We consider probabilistic and deterministic scenarios, in both cases, the performance exponentially approaches to a perfect implementation. The protocols employ an adaptive...

In order to reject the local hidden variables hypothesis, the usefulness of a Bell inequality can be quantified by how small a p-value it will give for a physical experiment. Here we show that to obtain a small expected p-value it is sufficient to have a large gap between the local and Tsirelson bounds of the Bell inequality, when it is formulated as a nonlocal game. We develop an algorithm...

A fascinating fact about the collective behavior of indistinguishable quantum particles is the existence of only two types of statistics: bosonic and fermionic, characterized by the exchange symmetry of their associated quantum states. So far, all attempts to explain the origin of these symmetries resort on oblivious assumptions added to the abstract quantum formalism (e.g. dimensionality of...

Interference of single particles lies at the core of quantum mechanics. The most prominent demonstration of this effect is the double-slit experiment: a single experimental run indicates an experiment with single particles, however the statistics of repeated runs reassembles interference fringes. This is the source of the celebrated wave-particle duality. In this work we show that classical...

We consider the Unruh effect for a pointlike multilevel particle detector coupled to a massless real scalar field and moving in a quantum superposition of accelerated trajectories. The state of the detector excitations is, in general, not a mere mixture of the thermal spectrum characteristics of the Unruh effect for each trajectory with well-defined acceleration separately. For certain...

Quantum resource theories (QRTs) provide a unified framework for understanding quantum-mechanical properties, but physically well-motivated resources may possess structure whose analysis is mathematically intractable, such as **non-uniqueness of maximally resourceful states, non-convexity, and infinite-dimensionality**. We systematically study manipulation and quantification of resources in...

The large interest from the general public combined with the need to develop the next generation of quantum workforce, set a new challenge for the quantum computing experts: educating a vast public of not expert. Several national and international initiatives focus on education and outreach targeting a broad range of audiences, from high school pupils to developers. In this scenario, Quantum...

Quantum interference of indistinguishable bosons is indispensable for many quantum optical experiments. As in the famous Hong-Ou-Mandel effect, symmetry of the input state and symmetries in the scattering scenario can lead to destructive interference and the suppression of a large number of output events. The rules specifying which input-output combinations interfere totally destructively are...

One of the challenges of scaling up quantum processors is the optimization of the quantum gates, as each gate may require different control parameters. We developed and tested a fast protocol to automatically calibrate the entangling 2-qubit Mølmer Sørensen gate using Bayesian parameter estimation. Such a protocol promises to increase experimental uptime by decreasing the time needed for...

I will discuss the challenges and progress in building quantum computers and the potential impact scalable quantum computers would bring to science and engineering. I will then discuss a recent proposal for building fault-tolerant quantum computers using cat qubits.