Yoshichika Miwa

Parallel generation of quadripartite cluster entanglement in the optical frequency comb.

Scalability and coherence are two essential requirements for the experimental implementation of quantum information and quantum computing. Here, we report a breakthrough toward scalability: the simultaneous generation of a record 15 quadripartite entangled cluster states over 60 consecutive cavity modes (Q modes), in the optical frequency comb of a single optical parametric oscillator. The amount of observed entanglement was constant over the 60 Q modes, thereby proving the intrinsic scalability of this system. The number of observable Q modes was restricted by technical limitations, and we conservatively estimate the actual number of similar clusters to be at least 3 times larger. This result paves the way to the realization of large entangled states for scalable quantum information and quantum computing.

Demonstration of a Quantum Nondemolition Sum Gate

The sum gate is the canonical two-mode gate for universal quantum computation based on continuous quantum variables. It represents the natural analogue to a qubit C-NOT gate. In addition, the continuous-variable gate describes a quantum nondemolition (QND) interaction between the quadrature components of two light modes. We experimentally demonstrate a QND sum gate, employing the scheme by R. Filip, P. Marek, and U. L. Andersen [Phys. Rev. A 71, 042308 (2005)10.1103/PhysRevA.71.042308], solely based on off-line squeezed states, homodyne measurements, and feedforward. The results are verified by simultaneously satisfying the criteria for QND measurements in both conjugate quadratures.

Demonstration of a universal one-way quantum quadratic phase gate

We demonstrate a quadratic phase gate for one-way quantum computation in the continuous-variable regime. This canonical gate, together with phase-space displacements and Fourier rotations, completes the set of universal gates for realizing any single-mode Gaussian transformation such as arbitrary squeezing. As opposed to previous implementations of measurement-based squeezers, the current gate is fully controlled by the local oscillator phase of the homodyne detector. Verifying this controllability, we give an experimental demonstration of the principles of one-way quantum computation over continuous variables. Moreover, we can observe sub-shot-noise quadrature variances in the output states, confirming that nonclassical states are created through cluster computation.

Demonstration of cluster-state shaping and quantum erasure for continuous variables

We demonstrate experimentally how to remove an arbitrary node from a continuous-variable cluster state and how to shorten any quantum wires of such a state. These two basic operations, performed in an unconditional fashion, are a manifestation of quantum erasure and can be employed to obtain various graph states from an initial cluster state. Starting with a sufficiently large cluster, the resulting graph states can then be used for universal quantum information processing. In the experiment, all variations of this cluster shaping are demonstrated on a four-mode linear cluster state through homodyne measurements and feedforward.

Demonstration of a reversible phase-insensitive optical amplifier

We experimentally demonstrate phase-insensitive linear amplification of a continuous variable system in the optical regime, preserving the ancilla system at the output. Since our amplification operation is unitary up to small excess noise, it is reversible beyond the classical limit. Here, entanglement between the amplified output system and the ancilla system is the resource for the reversibility, and the amplification gain is G=2.0. In addition, combining this amplifier with a beamsplitter, we also demonstrate approximate cloning of coherent states where an anticlone is present. We investigate the reversibility by reconstructing the initial state from the output correlations, and the results are slightly beyond the cloning limit. Furthermore, full characterization of the amplifier and cloner is given by using coherent states with several different mean values as inputs. Our amplifier is based on linear optics, offline-prepared additional ancillas in nonclassical states, and homodyne measurements followed by feedforward. Squeezed states are used as the additional ancillas, and nonlinear optical effects are exploited only for their generation. They introduce nonclassicality into the amplifying operation, making entanglement at the output.

Unconditional conversion between a single-photon state and a coherent-state superposition via squeezing operation

We experimentally demonstrate a conversion of a single-photon state into a superposition of two weak coherent states and its inverse, via squeezing operation based on offline-prepared squeezed states, measurement and feedforward.

Toward quantum computing with oscillators

Toward the implementation of universal quantum computing in the optical frequency comb, we recently demonstrated the entanglement of 60 cavity modes of a single optical parametric oscillator into 15 independent quadripartite ring cluster states.

A new method for locking the signal-field phase difference in a type-II optical parametric oscillator above threshold

We propose and demonstrate a new method for phaselocking the signal fields emitted above threshold by a nondegenerate, type-II optical parametric oscillator (OPO). This method is based on the observation that amplitude modulation of the pump beam produces a related modulation of the frequency difference of the OPO signals via the temperature-tuning of the index of refraction in the nonlinear crystal. We successfully use pump modulation as a correction for phase-difference locking of the OPO signals and observe a 1 kHz beat note stable over more than 10 s, both figures solely limited by the measurement time. This method eliminates the need for applying electronic phase-correction signals directly to the nonlinear crystal which caused crystal damage in a previous phaselocking technique.

One-way quantum computation using a quantum nondemolition entangling gate

We demonstrate a Û = exp(iκx̂²) gate as an example of one-way quantum computation. The coefficient κ is controlled via the local oscillator phase of a homodyne measurement. The squeezing below the shot noise limit is observed.

Experimental Demonstration of a Quantum Nondemolition Gate

We experimentally demonstrate a quantum nondemolition (QND) gate which is a continuous variable analogue of a qubit controlled‐NOT gate. The QND gate is shown to entangle two initially separable coherent states, which will find uses in cluster state computation.