Purification of photon subtraction from continuous squeezed light by filtering

Photon subtraction from squeezed states is a powerful scheme to create good approximation of so-called Schr\"odinger cat states. However, conventional continuous-wave-based methods actually involve some impurity in squeezing of localized wavepackets, even in the ideal case of no optical losses. Here we theoretically discuss this impurity, by introducing mode-match of squeezing. Furthermore, here we propose a method to remove this impurity by filtering the photon-subtraction field. Our method in principle enables creation of pure photon-subtracted squeezed states, which was not possible with conventional methods.Comment: 10 pages, 6 figure

Quantum mode filtering of non-Gaussian states for teleportation-based quantum information processing

We propose and demonstrate an effective mode-filtering technique of non-Gaussian states generated by photon-subtraction. More robust non-Gaussian states have been obtained by removing noisy low frequencies from the original mode spectrum. We show that non-Gaussian states preserve their non-classicality after quantum teleportation to a higher degree, when they have been mode-filtered. This is indicated by a stronger negativity $-0.033 \pm 0.005$ of the Wigner function at the origin, compared to $-0.018 \pm 0.007$ for states that have not been mode-filtered. This technique can be straightforwardly applied to various kinds of photon-subtraction protocols, and can be a key ingredient in a variety of applications of non-Gaussian states, especially teleportation-based protocols towards universal quantum information processing

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.Comment: 5 pages, 4 figure

Experimental demonstration of entanglement assisted coding using a two-mode squeezed vacuum state

We have experimentally realized the scheme initially proposed as quantum dense coding with continuous variables [Ban, J. Opt. B \textbf{1}, L9 (1999), and Braunstein and Kimble, \pra\textbf{61}, 042302 (2000)]. In our experiment, a pair of EPR (Einstein-Podolski-Rosen) beams is generated from two independent squeezed vacua. After adding two-quadrature signal to one of the EPR beams, two squeezed beams that contain the signal were recovered. Although our squeezing level is not sufficient to demonstrate the channel capacity gain over the Holevo limit of a single-mode channel without entanglement, our channel is superior to conventional channels such as coherent and squeezing channels. In addition, optical addition and subtraction processes demonstrated are elementary operations of universal quantum information processing on continuous variables.Comment: 4 pages, 4 figures, submitted to Phys. Rev.

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

We experimentally demonstrate a controlled-phase gate for continuous variables in a fully measurement-based fashion. In our scheme, the two independent input states of the gate, encoded in two optical modes, are teleported into a four-mode Gaussian cluster state. 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 controlled-phase gate with the recently reported module for universal single-mode Gaussian operations [R. Ukai et al., Phys. Rev. Lett. 106, 240504 (2011)], it is possible to implement universal Gaussian operations on arbitrary multi-mode quantum optical states in form of a fully measurement-based one-way quantum computation.Comment: 4 pages, 3 figure

Universal linear Bogoliubov transformations through one-way quantum computation

We show explicitly how to realize an arbitrary linear unitary Bogoliubov transformation (LUBO) on a multi-mode quantum state through homodyne-based one-way quantum computation. Any LUBO can be approximated by means of a fixed, finite-sized, sufficiently squeezed Gaussian cluster state that allows for the implementation of beam splitters (in form of three-mode connection gates) and general one-mode LUBOs. In particular, we demonstrate that a linear four-mode cluster state is a sufficient resource for an arbitrary one-mode LUBO. Arbitrary input quantum states including non-Gaussian states could be efficiently attached to the cluster through quantum teleportation.Comment: 10 pages, 6 figure

Creation, storage, and on-demand release of optical quantum states with a negative Wigner function

Highly nonclassical quantum states of light, characterized by Wigner functions with negative values, have been created so far only in a heralded fashion. In this case, the desired output emerges rarely and randomly from a quantum-state generator. An important example is the heralded production of high-purity single-photon states, typically based on some nonlinear optical interaction. In contrast, on-demand single-photon sources were also reported, exploiting the quantized level structure of matter systems. These sources, however, lead to highly impure output states, composed mostly of vacuum. While such impure states may still exhibit certain single-photon-like features such as anti-bunching, they are not enough nonclassical for advanced quantum information processing. On the other hand, the intrinsic randomness of pure, heralded states can be circumvented by first storing and then releasing them on demand. Here we propose such a controlled release, and we experimentally demonstrate it for heralded single photons. We employ two optical cavities, where the photons are both created and stored inside one cavity, and finally released through a dynamical tuning of the other cavity. We demonstrate storage times of up to 300 ns, while keeping the single-photon purity around 50% after storage. This is the first demonstration of a negative Wigner function at the output of an on-demand photon source or a quantum memory. In principle, our storage system is compatible with all kinds of nonclassical states, including those known to be essential for many advanced quantum information protocols.Comment: 14 pages, 5 figure