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arXiv:1804.08905 · Physical Review Letters

Generation of a Cat State in an Optical Sideband

Takahiro Serikawa, Jun–ichi Yoshikawa, Shuntaro Takeda, Hidehiro Yonezawa, Timothy C. Ralph, Elanor H. Huntington, Akira Furusawa

Published Oct 4, 2018• 29 citations
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Abstract

We propose a method to subtract a photon from a double sideband mode of continuous-wave light. The central idea is to use phase modulation as a frequency sideband beam splitter in the heralding photon subtraction scheme, where a small portion of the sideband mode is down-converted to 0 Hz to provide a trigger photon. An optical cat state is created by applying the proposed method to a squeezed state at 500 MHz sideband, which is generated by an optical parametric oscillator. The Wigner function of the cat state reconstructed from a direct homodyne measurement of the 500 MHz sideband modes shows the negativity of W(0,0)=-0.088±0.001 without any loss corrections.

DOI
10.1103/physrevlett.121.143602

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We propose a method to subtract a photon from a double sideband mode of continuous-wave light. The central idea is to use phase modulation as a frequency sideband beam splitter in the heralding photon subtraction scheme, where a small portion of the sideband mode is down-converted to 0 Hz to provide a trigger photon. An optical cat state is created by applying the proposed method to a squeezed state at 500 MHz sideband, which is generated by an optical parametric oscillator. The Wigner function of the cat state reconstructed from a direct homodyne measurement of the 500 MHz sideband modes shows the negativity of W(0,0)=-0.088±0.001 without any loss corrections.
We have generated and measured an approximation of an optical Schrödinger cat state by photon subtraction from a squeezed state. Using single-photon avalanche photodiode detectors and photon-number-resolving transition edge sensors, we were able to extract Wigner distributions for one or two photons subtracted from the squeezed state, resulting in both an odd and even cat state. The one-photon-subtracted Wigner distribution shows measured negative features (without any correction), indicating quantum character. The Wigner distribution of the two-photon-subtracted state is negative after correcting for homodyne losses. A full statistical analysis is under way to quantify the magnitude and fidelity of the cat states generated.
Continuous-wave (CW) squeezed light is used in the generation of various optical quantum states, and thus is a fundamental resource of fault-tolerant universal quantum computation using optical continuous variables. To realize a practical quantum computer, a waveguide optical parametric amplifier (OPA) is an attractive CW squeezed light source in terms of its THz-order bandwidth and suitability for modularization. The usages of a waveguide OPA in quantum applications thus far, however, are limited due to the difficulty of the generation of the squeezed light with a high purity. In this paper, we report the first observation of Wigner negativity of the states generated by a heralding method using a waveguide OPA. We generate Schrödinger cat states at the wavelength of 1545 nm with Wigner negativity using a quasi-single-mode ZnO-doped periodically poled LiNbO3 waveguide module we developed. Wigner negativity is regarded as an important indicator of the usefulness of the quantum states as it is essential in the fault-tolerant universal quantum computation. Our result shows that our waveguide OPA can be used in wide range of quantum applications leading to a THz-clock optical quantum computer.
Quantum states of light having a Wigner function with negative values represent a key resource in quantum communication and quantum information processing. Here we present the generation of such a state at the telecommunication wavelength of 1550 nm. The state is generated by means of photon subtraction from a weakly squeezed vacuum state and is heralded by the ``click'' of a single photon counter. Balanced homodyne detection is applied to reconstruct the Wigner function, also yielding the state's photon-number distribution. The heralding photons are frequency up-converted to 532 nm to allow for the use of a room-temperature (silicon) avalanche photodiode. The Wigner function reads $W(0,0)=\ensuremath{-}0.063\ifmmode\pm\else\textpm\fi{}0.004$ at the origin of phase space, which certifies negativity with more than 15 standard deviations.
Until now, Schrödinger's cat states are generated by subtracting single photons from the whole bandwidth of squeezed vacua. However, it was pointed out recently that the achievable purities are limited in such method (J. Yoshikawa, W. Asavanant, and A. Furusawa, arXiv:1707.08146 [quant-ph] (2017)). In this paper, we used our new photon subtraction method with a narrowband filtering cavity and generated a highly pure Schrödinger's cat state with the value of −0.184-0.184−0.184 at the origin of the Wigner function. To our knowledge, this is the highest value ever reported without any loss corrections. The temporal mode also becomes exponentially rising in our method, which allows us to make a real-time quadrature measurement on Schrödinger's cat states, and we obtained the value of −0.162-0.162−0.162 at the origin of the Wigner function.
We present generation of photon-subtracted squeezed state at 860nm, from continuous-wave squeezed vacua generated with a periodically-poled KTiOPO4 crystal as a nonlinear medium of a subthreshold optical parametric oscillator. We observe various kinds of photon-subtracted squeezed states, including “non-Gaussian” states similar to the single-photon state and superposition states of coherent states, simply by changing the pump power. Nonclassicality of the generated states clearly emerges as its negative region around the origin of the phase-space distributions, i. e., the Wigner functions. We obtain the value,-0.083 at the origin of the Wigner function, which is largest ever observed without any correction for experimental imperfections.