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

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.

Figure 1

(a) The time-separated two-photon subtraction and the temporal mode functions ${\Psi }_{\pm }(t)$. OPO: optical parametric oscillator, APD: avalanche photo diode. (b) Schematic of the ancilla-assisted photon subtraction. (c) $\Delta$ dependence of the squeezing ${ϵ}_{+}$ (solid line) and ${ϵ}_{-}$ (dashed line). $ϵ=0.3$.

Figure 2

Theoretical Wigner functions of the CSS state generated by the time-separated two-photon subtraction and their photon-number distributions. $ϵ=0.3$. (a) ${\zeta }_0\Delta =0$ and (b) ${\zeta }_0\Delta =1.4$ correspond to approximate $|C_{+}⟩$ of $|\alpha {|}^2=1.2$ and 2.6, respectively.

Figure 3

Experimental setup of the time-separated two-photon subtraction. SHG: second harmonic generation cavity, OPO: optical parametric oscillator, AOM: acousto-optic modulator, FC: fiber coupler, TBS: tapping beam splitter ($R=10\%$) FBS: $50/50$ fiber beam splitter, and APDs: Si avalanche photo diodes (Perkin-Elmer SPCM-AQR-14).

Figure 4

(a) Wigner functions of the state ${\widehat{\rho }}_{+}$ in mode ${\Psi }_{+}$ for $\Delta =0\mathrm{ns}$ (${\zeta }_0\Delta =0$, left) and $\Delta =32\mathrm{ns}$ (${\zeta }_0\Delta =0.90$, right). Upper row: experimental Wigner functions ($W(x,p)$), their contours, and photon-number distributions ($p(n)$). Lower row: model calculation of the Wigner functions. (b) Average photon numbers in modes ${\Psi }_{\pm }$. Upper (filled) plots: experimental $⟨{\widehat{n}}_{+}⟩$ with $\Delta =0$, 32, 48, 64, 96 ns, from left to right. Lower (unfilled) plots: experimental $⟨{\widehat{n}}_{-}⟩$ with $\Delta =32$, 48, 64, 96 ns, from left to right. Solid ($⟨{\widehat{n}}_{+}⟩$) and dashed ($⟨{\widehat{n}}_{-}⟩$) lines are the theoretical predictions including 15% optical loss.