Chiral ground-state currents of interacting photons in a synthetic magnetic field

Abstract

The intriguing many-body phases of quantum matter arise from the interplay of particle interactions, spatial symmetries, and external fields [1]. Generating these phases in an engineered system could provide deeper insight into their nature and the potential for harnessing their unique properties [2][3][4][5][6][7]. However, concurrently bringing together the main ingredients for realizing manybody phenomena in a single experimental platform is a major challenge. Using superconducting qubits, we simultaneously realize synthetic magnetic fields and strong particle interactions, which are among the essential elements for studying quantum magnetism and fractional quantum Hall (FQH) phenomena [8, 9]. The artificial magnetic fields are synthesized by sinusoidally modulating the qubit couplings. In a closed loop formed by the three qubits, we observe the directional circulation of photons, a signature of broken time-reversal symmetry. We demonstrate strong interactions via the creation of photonvacancies, or "holes", which circulate in the opposite direction. The combination of these key elements results in chiral groundstate currents, the first direct measurement of persistent currents in low-lying eigenstates of strongly interacting bosons. The observation of chiral currents at such a small scale is interesting and suggests that the rich many-body physics could survive to smaller scales. We also motivate the feasibility of creating FQH states with near future superconducting technologies. Our work introduces an experimental platform for engineering quantum phases of strongly interacting photons and highlight a path toward realization of bosonic FQH states.It is commonly observed that when the number of particles in a system increases, complex phases can emerge which were absent in the system when it had fewer particles, i.e. the "more is different" [10]. This observation drives experimental efforts in synthetic quantum systems, where the primary goal is to engineer and utilize these emerging phases. However, it has generally been overlooked that these sought-after phases can only emerge from simultaneous realization and control of particle numbers, real-space arrangements, external fields, particle interactions, state preparation, and quantum measurement. The simultaneous realization of all these ingredients makes synthesizing many-body phases a holistic task, and hence constitutes a major experimental challenge. Engineering these factors, in particular synthesizing magnetic fields, have been performed in several platforms [4,[11][12][13][14][15][16][17][18][19]. However, these ingredients have not been jointly realized in any system thus far. To provide a tangible framework, we discuss realization of these key elements in the context of quantum Hall physics, and show when these ingredients come together they can construct a basic building block for creating FQH states.The FQH states are commonly studied in 2-dimensional electron gases, a fermionic condensed matter system [8, 9]. However, many of the recent advancem...

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