Quantum simulation with 20 qubits and integrated self-check

In collaboration with the theory group led by P. Zoller, we have implemented a novel method to simulate, for the first time, particle physics phenomena on up to 20 quantum bits. Furthermore, we demonstrated how the quantum simulator was able to check the quality of the result itself, which represents an important milestone in the field of quantum simulation. The here applied technique of “Variational Quantum Simulation” enables simulation of a large class of quantum phenomena on todays available quantum hardware, beyond the limits of previous quantum simulation methods. The work has now been published in Nature and opens the door to the simulation of previously intractable problems in chemistry, materials research or high-energy physics.

Properties and behavior of many interesting systems in all fields of research are well explained by quantum mechanical theories. Such systems can be difficult to access and manipulate directly, such that efficient modeling and simulation has become a great cornerstone during the past two decades. However, the required computer resources increase exponentially with the number of qubits to simulate, such that already the simulation of tens of particles become extremely demanding for classical computers. This is when the concept of a Quantum Simulator comes into play. The idea is to employ a highly controllable, well accessible system which itself obeys quantum mechanical laws, to simulate a complex quantum system of interest.

Quantum-classical feedback loop. IQOQI Innsbruck/Harald Ritsch

In the now published work, we implemented a feedback loop between a classical computer and a quantum simulator based on 20 trapped calcium ions: The quantum simulator generates a complex quantum state, by using the available entanglement and single-qubit rotation gates. The state is then measured and the results are handed over to a classical computer. The classical device interprets the received data, using a specifically developed optimization algorithm, and instructs the quantum simulator with new settings for its gates. This loop is repeated thousands of times until the intended quantum state is reached. This way, the computationally extremely demanding task of preparing a complex quantum state is outsourced to a quantum device. At the same time, the quantum simulator does not have to exhibit the exact same properties as the system under investigation, because it is guided by the classical computer. The sophisticated optimization algorithm, coupled with the fast and automized measurement cycles of the quantum experiment, form an enormously powerful and efficient quantum simulator. It allowed us to simulated the spontaneous creation and destruction of pairs of elementary particles in a vacuum on 20 quantum bits, for the first time. As soon as quantum simulators start to solve classically intractable problems, an important question arises: Can we trust the result or how can we verify it? In our work, we demonstrated for the first time how a quantum simulator assesses the quality of the results on its own.

Read the full article: Christian Kokail, Christine Maier, Rick van Bijnen, Tiff Brydges, Manoj K. Joshi, Petar Jurcevic, Christine A. Muschik, Pietro Silvi, Rainer Blatt, Christian F. Roos and Peter Zoller, Nature 569, 355-360 (2019)