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arXiv:0806.2965 · Physical Review Letters

Generation of Large-Amplitude Coherent-State Superposition via Ancilla-Assisted Photon Subtraction

Hiroki Takahashi, Kentaro Wakui, Shigenari Suzuki, Masahiro Takeoka, Kazuhiro Hayasaka, Akira Furusawa, Masahide Sasaki

Published Dec 4, 2008• 252 citations
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Abstract

We propose and demonstrate a novel method to generate a large-amplitude coherent-state superposition (CSS) via ancilla-assisted photon subtraction. The ancillary mode induces quantum interference of indistinguishable processes in an extended space, widening the controllability of quantum superposition at the conditional output. We demonstrate this by a simple time-separated two-photon subtraction from continuous wave squeezed light. We observe the largest CSS of traveling light ever reported without correcting any imperfections, which will enable various quantum information applications with CSS states.

DOI
10.1103/physrevlett.101.233605

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We propose and demonstrate a novel method to generate a large-amplitude coherent-state superposition (CSS) via ancilla-assisted photon subtraction. The ancillary mode induces quantum interference of indistinguishable processes in an extended space, widening the controllability of quantum superposition at the conditional output. We demonstrate this by a simple time-separated two-photon subtraction from continuous wave squeezed light. We observe the largest CSS of traveling light ever reported without correcting any imperfections, which will enable various quantum information applications with CSS states.
A theoretical analysis is given for a two-photon subtraction from a continuous-wave squeezed vacuum with finite time separation between two detection events. In the cw photon subtraction process, generated states are inevitably described by temporal multimode states. Our approach is based on analytical formulas that are mathematically simple and provide an intuitive understanding of the multimode structure of the system. We show that, in our process, the photon-subtracted squeezed vacuum is generated in two temporal modes and one of these modes acts as an ancillary mode to make the other one a large-amplitude coherent-state superposition.
We study how photon addition and subtraction can be used to generate squeezed superpositions of coherent states (SSCSs) in free-traveling fields with high fidelities and large amplitudes. It is shown that an arbitrary NNN-photon subtraction results in the generation of a SSCS with nearly the perfect fidelity (F>0.999)(F>0.999)(F>0.999) regardless of the number of photons subtracted. In this case, the amplitude of the SSCS increases as the number of the subtracted photons gets larger. For example, two-photon subtraction from a squeezed vacuum state of 6.1dB6.1\phantom{\rule{0.3em}{0ex}}\mathrm{dB}6.1dB can generate a SSCS of $\ensuremath{\alpha}=1.26$, while in the case of the four-photon subtraction a SSCS of a larger amplitude $\ensuremath{\alpha}=1.65$ is obtained under the same condition. When a photon is subtracted from a squeezed vacuum state and another photon is added subsequently, a SSCS with a lower fidelity $(F\ensuremath{\approx}0.96)$ yet higher amplitude $(\ensuremath{\alpha}\ensuremath{\approx}2)$ can be generated. We analyze some experimental imperfections including inefficiency of the detector used for the photon subtraction.
We have created heralded coherent-state superpositions (CSSs) by subtracting up to three photons from a pulse of squeezed vacuum light. To produce such CSSs at a sufficient rate, we used our high-efficiency photon-number-resolving transition edge sensor to detect the subtracted photons. This experiment is enabled by and utilizes the full photon-number-resolving capabilities of this detector. The CSS produced by three-photon subtraction had a mean-photon number of $2.{75}_{\ensuremath{-}0.24}^{+0.06}$ and a fidelity of $0.{59}_{\ensuremath{-}0.14}^{+0.04}$ with an ideal CSS. This confirms that subtracting more photons results in higher-amplitude CSSs.
We develop an all-optical scheme to generate superpositions of macroscopically distinguishable coherent states in traveling optical fields. It nondeterministically distills coherent-state superpositions (CSS's) with large amplitudes out of CSS's with small amplitudes using inefficient photon detection. The small CSS's required to produce CSS's with larger amplitudes are extremely well approximated by squeezed single photons. We discuss some remarkable features of this scheme: it effectively purifies mixed initial states emitted from inefficient single-photon sources and boosts negativity of Wigner functions of quantum states.