Abstract Graph states are a broad family of entangled quantum states, each defined by a graph composed of edges representing the correlations between subsystems. Such states constitute versatile resources for quantum computation and quantum-enhanced measurement. Their generation and engineering require a high level of control over entanglement. Here we report on the generation of continuous-variable graph states of atomic spin ensembles, which form the nodes of the graph. We program the entanglement structure encoded in the graph edges by combining global photon-mediated interactions in an optical cavity with local spin rotations. By tuning the entanglement between two subsystems, we either localize correlations within each subsystem or enable Einstein–Podolsky–Rosen steering—a strong form of entanglement that enables the extraction of precise information from one subsystem through measurements on the other. We further engineer a four-mode square graph state, highlighting the flexibility of our approach. Our method is scalable to larger and more complex graphs, laying groundwork for measurement-based quantum computation and advanced protocols in quantum metrology.
Abstract Graph states are a broad family of entangled quantum states, each defined by a graph composed of edges representing the correlations between subsystems. Such states constitute versatile resources for quantum computation and quantum-enhanced measurement. Their generation and engineering require a high level of control over entanglement. Here we report on the generation of continuous-variable graph states of atomic spin ensembles, which form the nodes of the graph. We program the entanglement structure encoded in the graph edges by combining global photon-mediated interactions in an optical cavity with local spin rotations. By tuning the entanglement between two subsystems, we either localize correlations within each subsystem or enable Einstein–Podolsky–Rosen steering—a strong form of entanglement that enables the extraction of precise information from one subsystem through measurements on the other. We further engineer a four-mode square graph state, highlighting the flexibility of our approach. Our method is scalable to larger and more complex graphs, laying groundwork for measurement-based quantum computation and advanced protocols in quantum metrology.
Quantum networks are composed of quantum nodes which coherently interact by way of quantum channels. They offer powerful capabilities for quantum computation, communication, and metrology. A generic requirement for these realizations is the capability to generate and store quantum states among multiple quantum nodes, and to disseminate these resources throughout the network via the quantum channels. In this thesis, I describe a series of experiments whereby single excitations in atomic ensembles are strongly coupled to optical modes and provide efficient means for the coherent control of entangled states between matter and light. By following the seminal proposal by Duan et al., we have generated measurement-induced entanglement of an excitation between two cold atomic ensembles. Using this system, we investigated the relationship for the global bipartite entanglement and local correlations in its subsystems. In addition, we achieved functional quantum nodes for entanglement distribution. Two pairs of remote ensembles at two quantum nodes were prepared into entangled states in a heralded and asynchronous fashion by the conditional controls of the entanglement. The quantum states of the ensembles were then distributed into polarization entangled states of photons. We also prepared an analogous quantum state and transferred the nonlocal coherence between two pairs of heralded entangled atomic ensembles, providing a step towards entanglement connection. Beyond such probabilistic approaches, we demonstrated an experiment where entanglement between two quantum memories is created by the reversible and deterministic mapping of an entangled state of light via dynamic electromagnetically induced transparency. This experiment opens novel prospects of integrating hybrid quantum systems by way of reversible quantum interfaces between light and matter. Then, we extended our work to multipartite quantum systems. We theoretically investigated the characterization of multipartite mode-entangled states by way of quantum uncertainty relations, and introduced theoretical tools to verify the entanglement orders in multipartite systems. In particular, we achieved entanglement for one delocalized photon among multiple optical modes (N > 2). Finally, we have achieved measurement-induced entanglement of spin waves among four quantum memories. The individual atomic components for the entangled W state of the four ensembles were then coherently converted into four propagating entangled beams of light via superradiant emissions. We observed the statistical and dynamic transitions for the multipartite entangled spin waves. Experiments described in this thesis thereby represent significant advances of experimental and theoretical capabilities to generate, store, transfer, and characterize entanglement of matter and light over quantum networks.
Highly entangled multiphoton graph states are a crucial resource in photonic quantum computation and communication. Yet, the lack of photon-photon interactions makes the construction of such graph states especially challenging. Typically, these states are produced through probabilistic single-photon sources and linear-optics entangling operations that require indistinguishable photons. The resulting inefficiency of these methods necessitates a large overhead in the number of sources and operations, creating a major bottleneck in the photonic approach. Here, we show how harnessing single-atom-based photonic operations can enable deterministic generation of photonic graph states, while also lifting the requirement for photon indistinguishability. To this end, we introduce a multigate quantum node comprising a single atom in a W -type level scheme coupled to an optical resonator. This configuration provides a versatile toolbox for generating graph states, allowing the operation of both the controlled- Z and photon-atom gates, as well as the deterministic generation of single photons. Furthermore, the ability to deterministically entangle photonic qubits enables the expansion of the generated state by stitching together graph states produced by different nodes. We investigate the implementation of this gate-based approach using 87 Rb atoms and evaluate its performance through numerical simulations.
Abstract Entanglement has evolved from an enigmatic concept of quantum physics to a key ingredient of quantum technology. It explains correlations between measurement outcomes that contradict classical physics and has been widely explored with small sets of individual qubits. Multi-partite entangled states build up in gate-based quantum-computing protocols and—from a broader perspective—were proposed as the main resource for measurement-based quantum-information processing1,2. The latter requires the ex-ante generation of a multi-qubit entangled state described by a graph3–6. Small graph states such as Bell or linear cluster states have been produced with photons7–16, but the proposed quantum-computing and quantum-networking applications require fusion of such states into larger and more powerful states in a programmable fashion17–21. Here we achieve this goal by using an optical resonator22containing two individually addressable atoms23,24. Ring25and tree26graph states with up to eight qubits, with the names reflecting the entanglement topology, are efficiently fused from the photonic states emitted by the individual atoms. The fusion process itself uses a cavity-assisted gate between the two atoms. Our technique is, in principle, scalable to even larger numbers of qubits and is the decisive step towards, for instance, a memory-less quantum repeater in a future quantum internet27–29.
Future quantum computers require a scalable architecture on a scalable technology-one that supports millions of high-performance components. Measurement-based protocols, using graph states, represent the state of the art in architectures for optical quantum computing. Silicon photonics technology offers enormous scale and proven quantum optical functionality. Here we produce and encode photonic graph states on a mass-manufactured chip, using four on-chip-generated photons. We programmably generate all types of four-photon graph state, implementing a basic measurement-based protocol, and measure high-visibility heralded interference of the chip's four photons. We develop a model of the device and bound the dominant sources of error using Bayesian inference. The combination of measurement-based quantum computation, silicon photonics technology, and on-chip multi-pair sources will be a useful one for future scalable quantum information processing with photons.
Quantum computation using photons offers one possible route towards the long-standing goal of scalable, fault-tolerant quantum computing. Within this realm, measurement-based quantum computing offers a promising approach exploiting many of the advantages of photons. Implementing photonic measurement-based architectures relies upon the deterministic generation of graph states composed of many entangled photons. Semiconductor quantum dots (QDs) are sources of indistinguishable single photons and hosts of solid-state spins, marking them desirable platforms for controlled entanglement generation. Such a platform consisting of a single charged QD coupled to a photonic cavity has been proven for the generation of multipartite spin-photon graph states. This dissertation will present advancements in the versatility and coherence of such entanglement generation. First, background on quantum dots, single photon generation, spin control, and entanglement will be introduced. Then, sources of error and decoherence will be addressed using control of the hole g-factor and dynamical decoupling techniques. The aim of versatility is advanced through results on the generation of four-partite graph states with on-demand reconfigurability of the type of state produced. Finally, a different paradigm using quantum dot molecules will be introduced, and novel estimation of important physical parameters in this system is developed.
We show that it is possible to generate a quadripartite square graph-state entanglement in two-photon resonant systems. Every vertex of the square represents an optical field and every side stands for the entanglement of the two vertices. Four optical fields of different frequencies are coupled to coherently driven three-level ladder atoms. Through such a system, quantum beats and two-photon resonant interactions combine to cause the collective-mode parametric interactions. This results in the collective-mode entanglement, which corresponds to the square graph-state entanglement of the four individual fields.