Sharing Quantum States

Abstract

Quantum engineering requires controllable artificial systems with quantum coherence exceeding the device size and operation time. This can be achieved with geometrically confined low-dimensional electronic structures embedded within ultraclean materials, with prominent examples being artificial atoms (quantum dots) and quantum corrals (electronic cavities). Combining the two structures, we implement a mesoscopic coupled dot-cavity system in a high-mobility two-dimensional electron gas, and obtain an extended spin-singlet state in the regime of strong dot-cavity coupling. Engineering such extended quantum states presents a viable route for nonlocal spin coupling that is applicable for quantum information processing. DOI: 10.1103/PhysRevLett.115.166603 PACS numbers: 73.23.-b, 73.21.La, 85.75.-d Quantum physics has profited enormously from combining optical cavities and atoms into a quantum engineering platform, where atoms mediate interactions among photons and photons communicate information between atoms [1]. In mesoscopic physics, the constituents of the optical success story have been independently realized: (i) prototypes of electronic cavities were implemented through structuring of a two-dimensional electron gas (2DEG) [2,3], yielding extended fermionic modes akin to quantum corrals on metal surfaces [4,5], and (ii) artificial atoms in the form of quantum dots have been studied, unraveling numerous interesting phenomena such as the Coulomb blockade [6] and the Kondo effect [7,8]. The combination of several dots into controlled quantum bits (qubits) has been demonstrated [9,10], thus promoting the next challenge of introducing coherent coupling between distant qubits without relying on nearest-neighbor exchange. Analogously to cavity quantum optics, such coherent coupling could be provided by a suitably engineered cavity mode. Here, we report on transport spectroscopy and control of a coherently coupled mesoscopic dot-cavity system, where the cavity is embedded in the drain of the dot. In the weak-coupling regime, we find signatures of standard dot transport enhanced by a tunnel coupling to drain which is modulated by electrostatic control of the cavity. In stark contrast, the strong-coupling regime exhibits the formation of a dotcavity singlet state testifying to the coherent dynamics of the hybrid system. This singlet competes with and blocks the formation of a Kondo resonance, signaling the presence of a many-body quantum phase transition. Conjoining our measurements with the large spatial scale of the cavity modes alludes to the dot-cavity hybrid being an original realization of a Kondo box setup [11][12][13]. Focusing on applications, it constitutes a tunable and purely electrical component that may serve as a quantum bus for coherently coupling spatially separated qubits.The electronic dot-cavity device is shown in Fig. 1(a). It resides within a 2DEG, 90 nm underneath the surface of a GaAs=AlGaAs heterostructure. Applying negative voltages to Schottky top gates depletes the underlying 2DEG and defi...