Progress in experimental quantum error correction with trapped ions

Quantum error correction promises to enable quantum computation of arbitrary depth. The generally stated requirement here is to realize quantum gate operations operating better than the fault tolerance threshold. For currently preferred error correction codes, the threshold is at about 1% error rate - a threshold repetitively achieved in ion-trap based quantum computers by several teams. However, this threshold is based on the assumption that the sole error source is 'errors within the computational Hilbert space' , meaning the model that the physical implementation of a qubit really a two-level system. Experimentally, there are more possible errors, for instance qubit loss. In this presentation I will present experimental work by the team in Innsbruck on extending the toolbox for quantum error correction to deal with qubit loss.

Quantum Computing Applications

Following the demonstrated feasibility of building quantum computers in the laboratory, finding and establishing practical use case scenarios for them is currently a highly active field of research, in particular when taking NISQ-era limitations into account. A promising and natural direction, as put forward by Richard Feynman almost 40 years ago, is quantum simulation, i.e., using a quantum computer to simulate another quantum system, with applications to condensed matter systems or quantum chemistry. Beyond that, employment for "classical" problems, like high-dimensional optimization or machine learning, likewise attract large interest; nevertheless, there are still many open questions, e.g., how to feed large amounts of (classical) data through a quantum computer. This talk will provide an overview of recent developments regarding envisioned applications of (future) quantum computers.

Quantum supremacy using a programmable superconducting processor

The promise of quantum computers is that certain computational tasks might be executed exponentially faster on a quantum processor than on a classical processor. A fundamental challenge is to build a high-fidelity processor capable of running quantum algorithms in an exponentially large computational space. Here we report the use of a processor with programmable superconducting qubits to create quantum states on 53 qubits, corresponding to a computational state-space of dimension 2^53 (about 10^16). Measurements from repeated experiments sample the resulting probability distribution, which we verify using classical simulations. Our Sycamore processor takes about 200 seconds to sample one instance of a quantum circuit a million times—our benchmarks currently indicate that the equivalent task for a state-of-the-art classical supercomputer would take approximately 10,000 years. This dramatic increase in speed compared to all known classical algorithms is an experimental realization of quantum supremacy for this specific computational task, heralding a much-anticipated computing paradigm.

Tailored quantum gates for chemistry calculations on superconducting quantum devices

With the rapid development of quantum technology, quantum computers hold promise to eventually outperform conventional computers in certain types of problems such as the computation of complex molecules. A key requirement to perform computations on current and near-term quantum processors is the design of quantum algorithms with short circuit depth that finish within the coherence time of the qubits. To this end, it is essential to implement a set of quantum gates that is tailored to the problem at hand and that can be directly implemented in hardware. We utilize a parametrically driven tunable coupler to realize exchange-type gates that are configurable in amplitude and phase on two fixed-frequency superconducting qubits. Such gates are particularly well suited for quantum chemistry applications because they preserve the number of qubit excitations corresponding to the fixed number of electrons in the molecule. We analyse the origin of operational errors and are able to reduce the error per gate to 1.3%. Following the equation of motion (EOM) approach, we compute not only the ground state but also higher-excited states of molecular hydrogen with an average accuracy of 50 milli-Hartree, a good starting point for the simulation of larger molecular systems.

Magnons as Probes of Strongly Correlated Electron Physics

Scattering experiments have revolutionized our understanding of nature. Examples include the discovery of the nucleus, crystallography, and the discovery of the double helix structure of DNA. Scattering techniques differ by the type of the particles used, the interaction these particles have with target materials and the range of wavelengths used. Here, we demonstrate a new 2-dimensional table-top scattering platform for exploring magnetic properties of materials on mesoscopic length scales. Long lived, coherent magnonic excitations are generated in a thin film of YIG and scattered off a magnetic target deposited on its surface. The scattered waves are then recorded using a scanning NV center magnetometer that allows sub-wavelength imaging and operation under conditions ranging from cryogenic to ambient environment. While most scattering platforms measure only the intensity of the scattered waves, our imaging method allows for spatial determination of both amplitude and phase of the scattered waves thereby allowing for a systematic reconstruction of the target scattering potential. Our experimental results are consistent with theoretical predictions for such a geometry and reveal several unusual features of the magnetic response of the target, including suppression near the target edges and gradient in the direction perpendicular to the direction of surface wave propagation. Our results establish magnon scattering experiments as a new platform for studying correlated many-body systems.

Inductively coupled nano-electromechanics – a pathway towards strong coupling?

The field of nano-electromechanics challenges force sensitivity limits and has the potential to study quantum mechanical phenomena in the literal sense. Demonstrated achievements range from zN/√Hz force sensitivities, the preparation of mechanical elements in is ground state, squeezing of mechanical states, to microwave storage applications. However, all of these accomplishments are based on the linearization of the nano-electromechanical interaction in the limit of large photon numbers, and hence do not utilize the full wealth of the interaction Hamiltonian.
Most experimental implementations of nano-electromechanical devices combine superconducting microwave resonators with mechanically compliant elements employing a capacitive coupling scheme. However, it was realized early on that inductive coupling schemes based on SQUIDs represent an alternative route and promise a tunable interaction strength with the potential to reach the strong vacuum coupling regime.
In this presentation, we discuss the experimental implementation of an inductively coupled nano-electromechanical device based on a partly suspended SQUID shunting a coplanar microwave resonator to ground. We will explore the scaling of the flux tuning and demonstrate interaction rates exceeding 1 kHz. Moreover, we set the performance of the device in context with achievable force sensitivities, ground state cooling, and the potential to reach the strong coupling limit.

Quantum-Enhanced Sensing

Quantum sensors, such as the Nitrogen Vacancy color center in diamond, exploit the strong sensitivity of quantum systems to external disturbances to measure various signals in their environment with high precision.
These quantum sensors have the potential to be a revolutionary tool in material science, quantum information processing, and bioimaging. However, the same strong coupling to the environment also limits their sensitivity due to its decohering effects. Error correction strategies, including quantum error correction codes, robust many-body quantum phases, and dynamical decoupling, can help in fighting decoherence, but they incur the risk of also canceling the coupling to the signal to be measured.
Here I will show various schemes, focusing in particular on exploiting entanglement with ancillary systems, to achieve an advantageous compromise between noise and signal cancellation. This strategy can not only improve the sensitivity of the quantum sensor, but also yield new applications, via the transduction of the signals of interest into quantum perturbations.

Quantum sensing with (sub)-Hz frequency resolution: A new tool for chemical and life sciences

Recently, optically probed nitrogen-vacancy (NV) quantum defects in diamond have emerged as a new class of sensors allowing the measurement of magnetic fields on unprecedented length scales. However, the sensing scheme relies on the intrinsic coherence time of the NV-center which limits the spectral resolution in sensing of ac magnetic fields. Here, I would like to introduce a new pulse sequence which allows for the detection of magnetic fields with arbitrary frequency resolution. This technique is applied in my lab for sensing magnetic resonance signals with high spectral resolution on microscopic volumes, with applications from chemical to life sciences. I will discuss current progress in this field and provide a foresight of this rapidly advancing technology.

Continuous tensor networks for many body systems

Recent advances in simulating correlated quantum many body systems using continuous matrix product states will be discussed.

The electron correlation problem from a quantum information perspective

Describing strongly interacting electrons is one of the crucial challenges of modern quantum physics. A comprehensive solution to this electron correlation problem would simultaneously exploit both the pairwise interaction and its spatial decay. By taking a quantum information perspective, we explain how this structure of realistic Hamiltonians gives rise to two conceptually di?erent notions of correlation and entanglement. The ?rst one describes correlations between orbitals while the second one refers more to the particle picture. We illustrate those two concepts of orbital and particle correlation and present measures thereof. Intriguingly, our results for di?erent molecular systems reveal that the total correlation between orbitals is mainly classical, raising questions about the general signi?cance of entanglement in chemical bonding. Finally, we also speculate on a promising relation between orbital and particle correlation and explain why this may replace the obscure but widely used concept of static and dynamic correlation.

Observation of gauge invariance in a 71-site quantum simulator

The modern description of elementary particles is built on gauge theories. Such theories implement fundamental laws of physics by local symmetry constraints, such as Gauss's law in the interplay of charged matter and electromagnetic fields. Solving gauge theories by classical computers is an extremely arduous task, which has stimulated a vigorous effort to simulate gauge-theory dynamics in microscopically engineered quantum devices. Previous achievements used mappings onto effective models to integrate out either matter or electric fields, or were limited to very small systems. The essential gauge symmetry has not been observed experimentally. Here, we report the quantum simulation of an extended U(1) lattice gauge theory, and experimentally quantify the gauge invariance in a many-body system of 71 sites. Matter and gauge fields are realized in defect-free arrays of bosonic atoms in an optical superlattice. We demonstrate full tunability of the model parameters and benchmark the matter-gauge interactions by sweeping across a quantum phase transition. Enabled by high-fidelity manipulation techniques, we measure Gauss's law by extracting probabilities of locally gauge-invariant states from correlated atom occupations. Our work provides a way to explore gauge symmetry in the interplay of fundamental particles using controllable large-scale quantum simulators.

Investigation of long-range interactions between polar molecules and Rydberg atoms

Due to their large electric dipole moments, strong dipole-dipole interactions between individual polar molecules and Rydberg atoms extend to micrometer scale distances, offering exciting opportunities for applications in quantum science and technology. In the first part of my talk, I will provide an overview of some of the theoretical proposals which have been suggested for this hybrid system. Here, Förster resonant energy transfer offers a particularly powerful tool for controlling molecule-Rydberg-atom interactions. I will then present our experimental work on investigating molecule-Rydberg-atom interactions. In the past, we have investigated such interactions at room temperature, allowing Förster resonant energy transfer between molecules and Rydberg atoms to be studied in detail. This includes tuning the interactions via externally applied electric fields. Currently, we are setting up a new experiment to investigate molecule-Rydberg-atom interactions at low temperatures, providing new opportunities.

In the final part of my talk I will present a brief overview of our parallel work on cooling formaldehyde molecules to sub-millikelvin temperatures and discuss favorable properties of formaldehyde for QIP.