Shota Yokoyama

Ultra-large-scale continuous-variable cluster states multiplexed in the time domain

A continuous-variable cluster state containing more than 10,000 entangled modes is deterministically generated and fully characterized. The developed time-domain multiplexing method allows each quantum mode to be manipulated by the same optical components at different times. An efficient scheme for measurement-based quantum computation on this cluster state is presented.

Demonstration of a Controlled-Phase Gate for Continuous-Variable One-Way Quantum Computation

We experimentally demonstrate a controlled-phase gate for continuous variables using a cluster-state resource of four optical modes. The two independent input states of the gate are coupled with the cluster in a teleportation-based fashion. As a result, one of the entanglement links present in the initial cluster state appears in the two unmeasured output modes as the corresponding entangling gate acting on the input states. The genuine quantum character of this gate becomes manifest and is verified through the presence of entanglement at the output for a product two-mode coherent input state. By combining our gate with the recently reported module for single-mode Gaussian operations [R. Ukai et al., Phys. Rev. Lett. 106, 240504 (2011)], it is possible to implement any multimode Gaussian operation as a fully measurement-based one-way quantum computation.

Invited Article: Generation of one-million-mode continuous-variable cluster state by unlimited time-domain multiplexing

In recent quantum optical continuous-variable experiments, the number of fully inseparable light modes has drastically increased by introducing a multiplexing scheme either in the time domain or in the frequency domain. Here, modifying the time-domain multiplexing experiment reported in Nature Photonics 7, 982 (2013), we demonstrate successive generation of fully inseparable light modes for more than one million modes. The resulting multi-mode state is useful as a dual-rail CV cluster state. We circumvent the previous problem of optical phase drifts, which has limited the number of fully inseparable light modes to around ten thousands, by continuous feedback control of the optical system.

Demonstration of a fully tunable entangling gate for continuous-variable one-way quantum computation

We introduce a fully tuneable entangling gate for continuous-variable one-way quantum computation. We present a proof-of-principle demonstration by propagating two independent optical inputs through a three-mode linear cluster state and applying the gate in various regimes. The genuine quantum nature of the gate is confirmed by verifying the entanglement strength in the output state. Our protocol can be readily incorporated into efficient multi-mode interaction operations in the context of large-scale one-way quantum computation, as our tuning process is the generalisation of cluster state shaping.

Nonlocal quantum gate on quantum continuous variables with minimal resources

We experimentally demonstrate, with an all-optical setup, a nonlocal deterministic quantum non-demolition interaction gate applicable to quantum states at nodes separated by a physical distance and connected by classical communication channels. The gate implementation, based on entangled states shared in advance, local operations, and classical communication, runs completely in parallel fashion at both the local nodes, requiring minimum resource. The nondemolition character of the gate up to the local unitary squeezing is veri?ed by the analysis using several coherent states. A genuine quantum nature of the gate is con?rmed by the capability of deterministically producing an entangled state at the output from two separable input states. The all-optical nonlocal gate operation can be potentially incorporated into distributed quantum computing with atomic or solid state systems as a cross-processor unitary operation.

Optical quantum information processing and storage

Here we report our recent experimental progresses in optical quantum information processing. In particular, the following topics are included. First, we extend the heralding scheme to multi-mode states and demonstrate heralded creation of qutrit states. Next, we demonstrate storage of single-photon states and synchronized release of them. Then, we demonstrate real-time acquisition of quadrature values of heralded states by making use of an exponentially rising shape of wave-packets. Finally, we demonstrate cluster states in an arbitrarily long chain in the longitudinal direction.

Continuous-variable quantum optical experiments in the time domain using squeezed states and heralded non-Gaussian states

Continuous-variable quantum information processing with optical field quadrature amplitudes is advantageous in deterministic creation of Gaussian entanglement. On the other hand, non-Gaussian state preparation and operation are currently limited, but heralding schemes potentially overcome this difficulty. Here, we summarize our recent progress in continuous-variable quantum optical experiments. In particular, we have recently succeeded in creation of ultra-large-scale cluster-type entanglement with full inseparability, multiplexed in the time domain; storage and on-demand release of heralded single-photon states, which is applied to synchronization of two heralded single-photon states; real-time quadrature measurements regarding non-Gaussian single-photon states with exponentially rising wavepackets; squeezing with relatively broader bandwidth by using triangle optical parametric oscillator.

Experimental generation of 2000-mode entangled graph states

Expanding the size of spatial multi-partite entangled states is extremely challenging and inefficient. Recently, N. C. Menicucci et al. proposed two solutions for this problem whereby modes are encoded in an optical frequency modes or temporal modes. Although the generated states are equivalent mathematically, experimentally they are quite different. In both schemes, arbitrarily large entangled states can in principle be generated with an experimentally feasible setup. Here, we present the experimental generation of very large entangled graph states by encoding the modes temporally.

Demonstration of a controlled-phase gate for continuous-variable cluster computation

We demonstrate a controlled-phase gate for continuous variables using a cluster-state resource of four optical modes. Its nonclassicality is verified through the presence of entanglement at the outputs for product-state inputs of two coherent states.