Discovering Quantum Circuit Components with Program Synthesis

Leopoldo Sarra, Kevin Ellis, Florian Marquardt

Despite rapid progress in the field, it is still challenging to discover new<br>ways to take advantage of quantum computation: all quantum algorithms need to<br>be designed by hand, and quantum mechanics is notoriously counterintuitive. In<br>this paper, we study how artificial intelligence, in the form of program<br>synthesis, may help to overcome some of these difficulties, by showing how a<br>computer can incrementally learn concepts relevant for quantum circuit<br>synthesis with experience, and reuse them in unseen tasks. In particular, we<br>focus on the decomposition of unitary matrices into quantum circuits, and we<br>show how, starting from a set of elementary gates, we can automatically<br>discover a library of new useful composite gates and use them to decompose more<br>and more complicated unitaries.<br>

Quantum interference between distant creation processes

Johannes Pseiner, Manuel Erhard, Mario Krenn

The search for macroscopic quantum phenomena is a fundamental pursuit in<br>quantum mechanics. It allows us to test the limits quantum physics and provides new avenues for exploring the interplay between quantum mechanics and relativity. In this work, we introduce a novel approach to generate macroscopic quantum systems by demonstrating that the creation process of a quantum system can span a macroscopic distance. Specifically, we generate photon pairs in a coherent superposition of two origins separated by up to 70 meters. This new<br>approach not only provides an exciting opportunity for foundational experiments<br>in quantum physics, but also has practical applications for high-precision measurements of distributed properties such as pressure and humidity of air or gases.

Multiphoton non-local quantum interference controlled by an undetected photon

Kaiyi Qian, Kai Wang, Leizhen Chen, Hou Zhaohua, Mario Krenn, Shining Zhu, Xiao-Song Ma

The interference of quanta lies at the heart of quantum physics. The multipartite generalization<br>of single-quanta interference creates entanglement, the coherent superposition of states shared by several quanta. Entanglement allows non-local correlations between many quanta and hence is a key resource for quantum information technology. Entanglement is typically considered to be essential for creating non-local correlations, manifested by multipartite interference. Here, we show that this is not the case and demonstrate multiphoton non-local quantum interference without entanglement of any intrinsic properties of the photons. We harness the superposition of the physical origin of a four-photon product state, which leads to constructive and destructive interference of the photons’ mere existence. With the intrinsic indistinguishability in the generation process of photons, we realize four-photon frustrated quantum interference. We furthermore establish non-local control of multipartite quantum interference, in which we tune the phase of one undetected photon and observe the interference of the other three photons. Our work paves the way for fundamental studies of non-locality and potential applications in quantum technologies.