SP1: Entangling gates and quantum processors (leader: R. Blatt) SP1 deals with few-qubit quantum processing in the bottom-up approach from individually trapped particles to scalable systems, and will improve over the state-of-the-art of the AMO implementations of quantum processors (which has been set by the AQUTE predecessor SCALA) that lie at the core of the objective of developing scalable quantum computation. Further on, SP1 delivers the solid basis of scalable entanglement generation that will be used in the other SPs. SP1 is composed by four Work-Packages
WP1.1: Trapped ions for quantum processors (leader: R. Blatt)
WP1.2: Cavity-QED for entanglement operations (leader: D. Meschede)
WP1.3: Scalable neutral atom quantum processing (leader: Ph. Grangier)
WP1.4: Few-body quantum control (leader: K. Mølmer)
More about SP1
State-of-the-art . With elementary quantum processors operating on small qubit systems, we have seen single- and two-qubit gate operations in the recent past. However, even though AMO qubit systems appear to lead to scalable quantum processing in principle, we have identified two major challenges that have prevented so far the realization of a fully scalable quantum device for quantum information processing. The first issue is the limited fidelity of gate operations. Even though single- and two-qubit gates have been demonstrated with relatively high fidelity, no quantum operations on the level of logical qubits (i.e., encoded each on more than one physical qubit) has been possible. In other words, no decoherence free sub-spaces, and no quantum error correction schemes have been realized so far. As a consequence, already after a fairly limited number of gate operations the outcome of any quantum computing process is badly degraded, in even the best of state-of-the-art elementary processors. The second reason for a limited scalability so far has been the lack of systems containing more than only very few qubits that are under full control. The number of qubits has been quite small, and thus only rudimentary quantum algorithms have been possible. Here, we observe that the technologies that enable entanglement generation in atomic systems are not yet ready for scalable quantum devices.
Progress by AQUTE. In AQUTE’s SP1 we focus on few-body quantum systems, following a bottom-up approach for elementary quantum processors. Prominent systems, which have been investigated so far, are trapped ions and individually interacting atoms. These systems allow for a very high degree of controllability of all quantum degrees of freedom, achieving excellent control over single qubits and their mutual coupling. For the engineering of entangling quantum gates, our goal is to employ decoherence-free subspaces to realize long-lived logical qubits. We will investigate methods to increase the gate fidelity towards the fault-tolerance threshold. A second goal of the AQUTE project is to combine these atomic building blocks, and to integrate those technologically more mature qubits into e.g. novel micro chip structures, or multiple optical tweezers potentials. In this way, technological and scientific excellence are combined allowing for a significant step forward in scalability. The project will achieve systems of 10 individual accessible qubits in some implementations after the project period. While we follow those lines of QIFT implementations that have been successful so far, on the other hand the project encompasses also new, promising approaches that only came up recently. Extraordinarily fast quantum gates will be demonstrated using dipole-dipole interactions between Rydberg-excited states. The special structure of atomic energy levels in Ytterbium atoms shall be employed for qubits that exhibit ultra-long coherence times, orders of magnitude better than the current status. Theory is closely linked to these experimental efforts enabling high fidelity quantum operations. SP1 will increase significantly the number of qubits as well as the fidelity of individual operations. Already-established systems will prove scalability and the suitability of new candidates will be tested. At the end of the project we shall know which ones among the latter can effectively lead to new and better platforms.
SP2: Hybrid quantum systems and interconnects (leader: G. Rempe) SP2 proceeds from different individual qubit implementations and deals with interconnections between them, including hybrid systems, reaching out to recent successes in solid-state quantum information processing. SP2 is composed by three Work-Packages
WP2.1: Interconnects between ions and/or atoms (leader: G. Morigi)
WP2.2: Flying qubits and cavity-QED (leader: G. Rempe)
WP2.3: AMO connecting with solid state systems (leader: T. Hänsch)
More about SP2
State of the art. Paving the route towards a multi-qubit and scalable quantum processor needs certainly to be accompanied by some interface, similar to conventional, classical information processing where the optimal carrier for information is selected depending on the specific task and thus interconversion between different physical bits (e.g., from magnetic bits on a hard disk to electric bits in a CPU bus) is required. For quantum information devices, however, we have seen so far only very few interface proposals and even less experimental realizations. A prominent example is a cavity QED setting in the strong coupling regime between atoms and photons which has been employed for first investigations of quantum interfaces. However, interconnects between different static qubit carriers are largely missing as well as between solid-state and atomic qubits.
Progress by AQUTE. The AQUTE project will go, in its work, far beyond the current state of the art and it will provide various interfaces, such that for a future quantum processor or quantum repeater qubits may be converted from one to the other form and that we will benefit from the advantages of those various qubit carriers, either for fast transport, or for long and coherent storage, or for error protected processing. We are going to demonstrate e.g. interconnects between neutral atoms and ions, and between qubits at even a meter distance. All these systems will fulfil the basic requirements for a quantum network, namely, the individual nodes can perform quantum information storage and retrieval in a deterministic way, and addressing of individual nodes is possible to read-out intermediate and final results. The goals of the AQUTE project start from physical systems from the atomic, molecular and optical (AMO) quantum world. However, we will build hybrid systems that consist of an AMO part coupled to a component from the solid state quantum world. For example, micro- and nano-mechanically oscillating devices and high Q superconducting strip-line resonators will be combined with AMO elements. Semiconductor devices such as a quantum dot emitting single photons – a genuine solid state object – will be coupled to a single trapped ion. In conclusion, the AQUTE project goals will address a series of different interconnects and hybrid systems, such that for future quantum processors, quantum repeaters or quantum sensors the road blocks can be lifted.
SP3 Quantum simulators (leader: M. Lewenstein) SP3 addresses the top-down side of AQUTE objective A by investigating implementations of quantum simulators, including new systems that have recently become experimentally accessible for quantum manipulations: polar molecules, alkaline earths and Rydberg atoms. SP3 is composed by four Work-Packages
WP3.1: Optical lattices quantum simulation (leader: I. Bloch)
WP3.2: Dipolar system interactions and entanglement ( leader: J-I. Cirac)
WP3.3: Alkaline earths (leader: M. Inguscio)
WP3.4: Quantum state and reservoir engineering (leader: P. Zoller)
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State of the art. Quantum computers of special purpose – quantum simulators – represent the top-down approach to scalability, where many-particle systems are employed directly. R. Feynman suggested that in view of the complexity of classical simulations of quantum systems, it would be desirable to consider quantum simulators using quantum systems. In fact, a quantum simulator can be defined as an experimental system that mimics complex models in quantum science coming from condensed matter, high energy, cosmology, quantum field theories, especially in the case of dynamical and non-equilibrium problems. In practice one aims simulating models that, on the one hand, are of high relevance for applications and/or our understanding of the physics and, on the other hand, are computationally very hard to solve for classical computers. Nowadays this concept is being realized in many body ultracold atomic, molecular and ionic systems, representing first instances of scalable quantum computers. Very importantly, quantum simulation relaxes the experimental requirements as here only a restricted set of quantum logic operations is necessary, and these operations may be performed in parallel on many qubits in one go. A second ingredient of quantum simulators is that the parameters of the simulated model should be varied in a broad range. In the past, ultracold quantum gases in optical lattices have proven to be versatile and highly controllable systems for quantum simulation with impact on many fundamental model Hamiltonians in condensed matter physics. They also offer the possibility to explore ground-state as well as non-equilibrium behaviour of strongly interacting spin systems. We will extend this research significantly beyond the state-of-the-art in our research program.
Progress by AQUTE. The core competence of the AQUTE consortium in many-qubit entanglement is directed towards novel quantum simulators (SP3). From the experimental side, we will employ various kinds of optical lattices and interactions, investigate alkaline-earth atoms or mixtures of species, fermions and bosons, including studies of anyons and topological systems. A close integration of theory and experiment in common tasks is required because the analysis of strongly correlated quantum systems requires new mathematical tools. The quantum simulators we want to develop and study do not necessarily simulate other physical systems but rather ideal theoretical models that are used to explain various physics phenomena. A perfect example is the Hubbard model for spin-1/2 fermions describing high-Tc superconductivity. Quantum simulation of this model system will allow understanding much better the nature of the phenomenon, and possibly paving the way to novel technologies. Well within the scope of quantum simulation and among our goals is the engineering of quantum reservoirs leading to desired multi-qubit entangled state via dissipation. The performance of quantum simulators can be validated by comparison to reference problems, which can be handled exactly (albeit numerically). This procedure provides both a measure of the quantum simulator performance through the comparison of the time it takes to run the classical algorithm compared with that for quantum simulation and a validation of the quantum simulator itself, through the comparison of the results obtained with the classical calculations. We will reach within the 3 year period a status where quantum simulators truly deliver insight in solid state problems which remained unsolved regardless of their high importance for more than a decade.
SP4 Quantum technologies (leader: J. Schmiedmayer) SP4 looks for applications of controlling and manipulating entanglement other than information processing, including quantum technologies such as entanglement-enhanced metrology and sensing, thereby addressing the objective of developing novel technological applications of entanglement. SP4 is composed by three Work-Packages
WP4.1: Chip clocks and entangled ion frequency measurements (leader: J. Reichel)
WP4.2: Quantum enhanced measurements (leader: J. Schmiedmayer)
WP4.3: Novel experimental techniques for quantum devices (leader: S. Haroche)
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State of the art. Various quantum devices for real-world applications of entanglement have been proposed, but so far we have seen applications mostly limited to the transmission of qubits, to generate secure quantum communication channels. To improve the measurement of distances of squeezed light is starting to be applied in the next generation gravitational wave antennas. However, there is a wealth of possible other applications where a true quantum gain could be realized. These future applications could include the ultra precise determination of physical constants, where the measurement relies on quantum processing techniques and entanglement, but also the measurement of quantities such as gravity, acceleration, electric and magnetic fields or surface properties.
Progress by AQUTE. The research programme of AQUTE answers the above open questions within SP4, such that a whole wealth of mid- and short-term applications are targeted, leading to significant advances within the 3 year project timeline. The engineering of quantum properties for reaching the measurement goal is expected to deliver many applications. First of all, we put forward atomic clocks based on QIFT, where entanglement created in ion crystals allows forming a decoherence-free sub-space, and therein ultra precise measurements that are shielded from outside systematic errors. A second key ingredient is the enhanced measurement sensitivity of interferometers by squeezing and the related entanglement. Such squeezing requires interactions; the latter then cause phase diffusion and limited accuracy in measurements. Controlling the interactions and employing optimal control strategies in squeezing and measurement will allow for creating large scale entanglement and use it optimally, paving the way towards quantum limited sensing. As an example application, a Rubidium clock will be operated with a spin-squeezed sample. The tasks in SP4 include interferometer devices using entangled matter and based on microscopic structures, highly integrated electronic and optical elements on chip. Bose condensates are split and recombined, the interferometer signal allows probing surface properties, and it can be a high-sensitivity magnetometer and electric field sensor. Central to all these technologies are methods to measure and control many body quantum systems, their quantum states and the dynamics. A third important development direction is the delivery of a pre-determined stream of single atoms or ions on demand, a technique uniquely present in the AQUTE consortium. This type of deterministic sources is breaking new scientific ground and delivers well suited atoms for cavity-QED interfaces of micro-cavities with atomic qubits almost at rest. Very long light-atom interaction times allow for e.g. Schrödinger cat states of truly mesoscopic photon number. In conclusion, we will see quantum technologies and entanglement based procedures reaching well to the level of application after the three year founding period of AQUTE research.
SP0: Management and Communication (leaders: T. Calarco and F. Schmidt-Kaler) SP0 will set up an appropriate organization and an optimal workflow that will ensure the smooth running of all the projects components. SP0 is composed by two Work-Packages
WP0.1: Financial, contractual and project management (leader F. Schmidt-Kaler)
WP0.2: Internal and external communication ( leader: T. Calarco)
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SP0 will monitor all the project activities, and be responsible for
Reaching the objectives of the project within the agreed budget and time scales;
Coordinating the work of the partners and ensure effective communication among them;
Ensuring the quality of the work performed as well as of the deliverables;
Ensuring that informed decisions are taken;
Informing the governing bodies about any problem or conflicting situation;
Organizing the dissemination of results both internally and to the external world.
