Entanglement is the key resource for measurement-based quantum computing. It is stored in quantum states known as cluster states, which are prepared offline and enable quantum computing by means of purely local measurements. Universal quantum computing requires cluster states that are both large and possess (at least) a two-dimensional topology. Continuous-variable cluster states—based on bosonic modes rather than qubits—have previously been generated on a scale exceeding one million modes, but only in one dimension. Here, we report generation of a large-scale two-dimensional continuous-variable cluster state. Its structure consists of a 5- by 1240-site square lattice that was tailored to our highly scalable time-multiplexed experimental platform. It is compatible with Bosonic error-correcting codes that, with higher squeezing, enable fault-tolerant quantum computation.

Tracking a randomly varying optical phase is a key task in metrology, with applications in optical communication. The best precision for optical-phase tracking has until now been limited by the quantum vacuum fluctuations of coherent light. Here, we surpass this coherent-state limit by using a continuous-wave beam in a phase-squeezed quantum state. Unlike in previous squeezing-enhanced metrology, restricted to phases with very small variation, the best tracking precision (for a fixed light intensity) is achieved for a finite degree of squeezing because of Heisenberg's uncertainty principle. By optimizing the squeezing, we track the phase with a mean square error 15 ± 4% below the coherent-state limit.

Since its original proposal by Bennett et al. [1], the concept of quantum teleportation has attracted a lot of attention and has even become one of the central elements for advanced and practical realizations of quantum information protocols. It is essential for long-distance quantum communication by means of quantum repeaters [2] and it has also been shown to be a useful tool for realizing universal quantum logic gates in a measurement-based fashion [3].Many proposals and models for quantum computation rely upon quantum teleportation, such as the efficient linear-optics quantum computing scheme by Knill, Laflamme, and Milburn [4] and the so-called one-way quantum computer using cluster states [5].Although much progress has been made in demonstrating quantum teleportation of photonic qubits [6][7][8][9][10][11], most of these schemes shared one fundamental restriction: an unambiguous two-qubit Bell-state measurement (BSM), as needed for teleporting a qubit using two-qubit entanglement, is always probabilistic when linear optics is employed, even if photon-numberresolving detectors (PNRDs) had been available [12,13]. There are two experiments avoiding this constraint, however, in these, either a qubit can no longer be teleported when it is coming independently from the outside [7] or an extra nonlinear element leads to extremely low measurement efficiencies of the order of 10 −10 [8]. A further experimental limitation, rendering these schemes fairly inefficient, is the probabilistic nature of the entangled resource states [13]. Efficient, near-deterministic quantum teleportation, however, is of great benefit in quantum communication in order to save quantum memories in a quantum repeater; and it is a necessity in teleportation-based quantum computation. An additional drawback of the previ-2 ous experiments, due to the lack of PNRDs, was the need for either destroying the teleported qubit [20] or attenuating the input qubit [10], thus further decreasing the success rate of teleportation.We overcome all the above limitations through a totally distinct approach: continuousvariable (CV) quantum teleportation of a photonic qubit. The strength of CV teleportation lies in the on-demand availability of the quadrature-entangled states and the completeness of a BSM in the quadrature bases using linear optics and homodyne detections [15]. So far, these tools have been employed to unconditionally teleport CV quantum states such as coherent states [16,21]. However, their application to qubits [18,22] has long been out of reach, since typical pulsed-laser-based qubits (like those in Refs. [6][7][8][9][10][11]) have a broad frequency bandwidth, incompatible with the original continuous-wave-based CV teleporter that only works on narrow sidebands [16,21]. We overcome this incompatibility by utilizing a very recent, advanced technology: a broadband CV teleporter [23] and a narrow-band time-bin qubit compatible with that teleporter [24]. Importantly, this qubit uses two temporal modes to represent a so-called dual-rail encoded q...

In the early development of quantum information processing (QIP), a communication protocol called quantum teleportation was discovered (1) that involves 1

Photonic quantum computing is one of the leading approaches to universal quantum computation. However, large-scale implementation of photonic quantum computing has been hindered by its intrinsic difficulties, such as probabilistic entangling gates for photonic qubits and lack of scalable ways to build photonic circuits. Here we discuss how to overcome these limitations by taking advantage of two key ideas which have recently emerged. One is a hybrid qubit-continuous variable approach for realizing a deterministic universal gate set for photonic qubits. The other is time-domain multiplexing technique to perform arbitrarily large-scale quantum computing without changing the configuration of photonic circuits. These ideas together will enable scalable implementation of universal photonic quantum computers in which hardware-efficient error correcting codes can be incorporated. Furthermore, all-optical implementation of such systems can increase the operational bandwidth beyond THz in principle, utimately enabling large-scale fault-tolerant universal quantum computers with ultra-high operation frequency. arXiv:1904.07390v1 [quant-ph]