Remote Controlled Entanglement

Abstract

The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separated by more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory. DOI: 10.1103/PhysRevLett.112.170501 PACS numbers: 03.67.Bg, 42.50.Dv, 42.50.Lc, 85.25.Dq Entanglement-the property that binds two independent objects into a single, highly correlated, nonseparable system-is a hallmark of quantum theory. Entanglement schemes for superconducting qubits have traditionally relied on direct qubit-qubit coupling [1,2], cavity-mediated interactions [3], photon-mediated interactions [4], or autonomous cooling [5]. Measurement, in contrast, has traditionally been viewed as a means to restore classical behavior: a quantum system, once observed, is projected onto a single measurement basis state. However, in certain cases, it is possible to design [6-11] a measurement that projects onto an entangled state, thereby purifying, rather than destroying, quantum correlations. Such a measurement has recently been used to entangle two superconducting qubits coupled to the same microwave resonator [12].Measurement-induced entanglement is a particularly important resource in spatially separated quantum systems, for which no local interactions and therefore no direct methods of creating entanglement exist. Such remote entanglement has been demonstrated using optical photons in several atomic systems [13][14][15] and nitrogen vacancy centers [16], but has remained elusive for superconducting qubits, which operate in the microwave regime. In this Letter, we demonstrate measurement-induced entanglement between two superconducting qubits, each dispersively [17] coupled to a separate cavity for readout and separated by 1.3 meters of ordinary coaxial cable, by engineering a continuous measurement for which one of the three outcomes is a Bell state [18]. Unlike previous experiments in spatially separated quantum systems, in which the detection of individual spontaneous fluorescence events reveals whether or not entanglement has been generated, we employ time-continuous measurements [19]. This allows us to access the ensemble-averaged dynamics of entanglement generation, which are well described by a statistical model and by a full masterequation treatment. Furthermore, our measurement efficiency is sufficiently high to resolve the individual quantum trajectories in the ensemble [20], thus enabling the observation of the stochastic evolution of a joint two-qubit state under measurement. This functionality sheds new light on the fundamental interplay b...

Loss-tolerant parity measurement for distant quantum bits

“…Those probe states play the role of parity meter. Since the quantum channel is the unequivocal bottleneck for remote entanglement in state-of-the-art technology [14], we focus on this issue and assume in this letter that all (reasonable) local actions (i.e. U A , U B , generating |ψ p , detection at B) are implemented perfectly.…”

Loss-tolerant parity measurement for distant quantum bits

“…Those probe states play the role of parity meter. Since the quantum channel is the unequivocal bottleneck for remote entanglement in state-of-the-art technology [28], we focus on this issue and assume in this paper that the local actions (i.e., U A , U B , generating |ψ p , detection at B) are implemented perfectly. As we will show later, imperfections on such operations will have a small effect on the steady state when we stabilize quantum entanglement via feedback.…”