Measurement-based quantum computation offers exponential computational speed-up via simple measurements on a large entangled cluster state. We propose and demonstrate a scalable scheme for the generation of photonic cluster states suitable for universal measurement-based quantum computation. We exploit temporal multiplexing of squeezed light modes, delay loops, and beam-splitter transformations to deterministically generate a cylindrical cluster state with a two-dimensional (2D) topological structure as required for universal quantum information processing. The generated state consists of more than 30 000 entangled modes arranged in a cylindrical lattice with 24 modes on the circumference, defining the input register, and a length of 1250 modes, defining the computation depth. Our demonstrated source of 2D cluster states can be combined with quantum error correction based on the Gottesman-Kitaev-Preskill qubit encoding to enable fault-tolerant quantum computation.arXiv:1906.08709v1 [quant-ph]

Networking plays a ubiquitous role in quantum technology [1, 2]. It is an integral part of quantum communication and has significant potential for upscaling quantum computer technologies that are otherwise not scalable [3]. Recently, it was realized that sensing of multiple spatially distributed parameters may also benefit from an entangled quantum network [4][5][6][7][8][9]. Here we experimentally demonstrate how sensing of an averaged phase shift among four distributed nodes benefits from an entangled quantum network. Using a four-mode entangled continuous variable (CV) state, we demonstrate deterministic quantum phase sensing with a precision beyond what is attainable with separable probes. The techniques behind this result can have direct applications in a number of primitives ranging from biological imaging to quantum networks of atomic clocks.Quantum noise associated with quantum states of light and matter ultimately limits the precision by which measurements can be carried out [10][11][12]. However, by carefully designing the coherence of this quantum noise to exhibit properties such as entanglement and squeezing, it is possible to measure various physical parameters with significantly improved sensitivity compared to classical sensing schemes. Numerous realizations of quantum sensing utilizing non-classical states of light [2,13,15] and matter [16] have been reported, while only a few applications have been explored. Examples are quantum-enhanced gravitational waves interferometry [17], detection of magnetic fields [18][19][20] and sensing of the viscous-elasticity parameter of yeast cells [21]. All these implementations are, however, restricted to the sensing of a single parameter at a single location.Spatially distributed sensing of parameters at multiple locations in a network is relevant for applications from local beam tracking [22] to global scale clock synchronization [4]. The development of quantum networks [1, 2,23,24] enables new strategies for enhanced performance in such scenarios. Theoretical works [5][6][7][8][25][26][27][28] have shown that entanglement can improve sensing capabilities in a network using either twin-photons * or Greenberger-Horne-Zeilinger (GHZ) states combined with photon number resolving detectors [6,7] or using CV entanglement for the detection of distributed phase space displacements [8]. In this Letter, we experimentally demonstrate an entangled CV network for sensing of multiple phase shifts inspired by the theoretical proposal of Ref. [8]. Moreover, for the first time in any system, we demonstrate deterministic distributed sensing in a network of four nodes with a sensitivity beyond that achievable with a separable approach using similar quantum states. BSN … vaccum vaccum S a b c d FIG. 1.Distributed phase sensing scheme. The task is to estimate the average value of M spatially distributed phase shifts φ1, . . . , φM . (a) Without a network, the average phase shift must be estimated by probing each sample individually. This can be done with homodyne detection of the...

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