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Physical Review A

Experimental generation of broadband quadrature entanglement using laser pulses

Yun Zhang, Tatsuya Furuta, Ryuhi Okubo, Kosuke Takahashi, Takuya Hirano

Published Jul 16, 2007• 23 citations
Publisher

Abstract

We report on the generation of broadband pulsed quadrature entanglement by combining two squeezed vacua, which are generated from two degenerate optical parametric amplifiers (OPAs), on a beam splitter. With a single pass through OPA, in which a periodically poled lithium niobate waveguide is used as a nonlinear material, the noise reduction of $3.4\ifmmode\pm\else\textpm\fi{}0.2$ dB below the shot noise limit is observed with a bandwidth of more than 200 MHz. The entanglement correlation or EPR correlation is confirmed with a sufficient criterion $⟨{\ensuremath{\Delta}}^{2}({X}_{a}+{X}_{b})⟩+⟨{\ensuremath{\Delta}}^{2}({Y}_{a}\ensuremath{-}{Y}_{b})⟩=1.28<2$.

DOI
10.1103/physreva.76.012314

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We report on the generation of broadband pulsed quadrature entanglement by combining two squeezed vacua, which are generated from two degenerate optical parametric amplifiers (OPAs), on a beam splitter. With a single pass through OPA, in which a periodically poled lithium niobate waveguide is used as a nonlinear material, the noise reduction of 3.4\ifmmode±\else\textpm\fi0.23.4\ifmmode\pm\else\textpm\fi{}0.23.4\ifmmode±\else\textpm\fi0.2 dB below the shot noise limit is observed with a bandwidth of more than 200 MHz. The entanglement correlation or EPR correlation is confirmed with a sufficient criterion ⟨\ensuremathΔ2(Xa+Xb)⟩+⟨\ensuremathΔ2(Ya\ensuremath−Yb)⟩=1.28<2⟨{\ensuremath{\Delta}}^{2}({X}_{a}+{X}_{b})⟩+⟨{\ensuremath{\Delta}}^{2}({Y}_{a}\ensuremath{-}{Y}_{b})⟩=1.28<2⟨\ensuremathΔ2(Xa​+Xb​)⟩+⟨\ensuremathΔ2(Ya​\ensuremath−Yb​)⟩=1.28<2.
Continuous-wave light beams with broadband quantum entanglement are created with two independent squeezed light beams generated by two periodically poled lithium niobate waveguides and a symmentric beam splitter. The quantum entanglement is confirmed with a sufficient criterion ΔA,B2=⟨[Δ(x̂A−x̂B)]2⟩+⟨[Δ(p̂A+p̂B)]2⟩<1 and the observed ΔA,B2 is 0.75 over the bandwidth of 30MHz. Although the bandwidth is limited by that of the detector so far, it would be broadened up to 10THz which would be only limited by the bandwidth of phase matching for the second-order nonlinear process.
We report generation of 10.1\pm0.2-dB squeezed light from a broadband periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). Based on our previous report where a similar PPLN waveguide shows 8.3-dB squeezing [T. Kashiwazaki et al., Appl. Phys. Lett. 122, 234003 (2023)], we reduce phase fluctuations and overall optical losses in the measurement system. In particular, we introduce a novel phase detection technique that does not require tapping a part of the squeezed light to get a phase locking signal. We use a phase-detection OPA seeded by a tapped probe and pump light before a squeezer OPA. This configuration breaks the conventional trade-off between generating a phase-locking signal with high-signal-to-noise ratio and suppressing degradation of squeezing level caused by optical tapping. With all these improvements, the phase fluctuation angle is reduced from 14 mrad to 9 mrad, and the total optical loss from 12\% to 8\%. Achieving more than 10 dB of squeezing by the broadband waveguide OPA is a significant step towards realization of fault-tolerant ultra-fast universal optical quantum computation.
We demonstrate the experimental generation of quadrature amplitude entanglement in the 1550 nm band by using a fiber optical parametric amplifier. The measured noise variances of the difference and sum of the quadrature amplitudes of the pulsed signal and idler twin beams fall below the shot noise limit by about 1 and 0.8 dB (4.2 and 3.6 dB after the correction for efficiency), respectively, showing that the inseparability criterion of Einstein-Podolsky-Rosen entanglement I<2 is satisfied. Our investigation reveals that the quality of the measured entanglement can be further improved by increasing the transmission efficiency of the twin beams and by optimizing the temporal mode matching of the two sets of homodyne detection systems.
We report detection of 10.1 ± 0.2-dB squeezed light from a broadband periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). Based on our previous report, where a similar PPLN waveguide shows 8.3-dB squeezing [T. Kashiwazaki et al ., Appl. Phys. Lett. 122 , 234003 ( 2023 ) 10.1063/5.0144385 ], we reduce phase fluctuations and overall optical losses in the measurement system. In particular, we introduce a phase detection technique that does not require tapping a part of the squeezed light to get a phase locking signal. We use a phase-detection OPA seeded by a tapped probe and pump light before a squeezer OPA. This configuration breaks the conventional trade-off between generating a phase-locking signal with a high signal-to-noise ratio and suppressing the degradation of the squeezing level caused by optical tapping. With all these improvements, the phase fluctuation angle is reduced from 14 mrad to 9 mrad, and the total optical loss from 12% to 8%. Achieving more than 10 dB of squeezing by the broadband waveguide OPA is a significant step towards the realization of fault-tolerant ultra-fast universal optical quantum computation.