Photon number squeezing with a noisy fiber amplifier source by balanced detection technique

Summary form only given. Squeezed light at telecommunication wavelength is important source in the field of quantum information and communication technology, especially for realization of entanglement state with continuous variables. However, erbium-doped fiber amplifier (EDFA) used in classical communication systems contains substantially high intensity noise which is much higher than the shot noise level (SNL) due to beat noise of amplified spontaneous emission. Therefore, alternatively, optical parametric oscillators and Cr:YAG lasers have been employed for generation of squeezed light. In this work, we achieved photon number squeezing at 1.55 μm using a noisy EDFA laser as a light source, making use of collinear balanced detection technique, demonstrated by Nose et al.[1]. This technique compensates intensity noise at a specific radio-frequency by means of pulse splitting and recombination with a relative time delay τ. The photocurrent contributed by pulse trains is given by I(t) = I1(t) + I2(t) = I1(t) + I1(t-τ), where I1(t) and I2(t) is the photocurrent contributed by the first and second pules, respectively. The equation does not contain an interference term because any pulse does not temporally overlap each other. In the Fourier domain,we can obtain I(ω) = I1(ω) + I2(ω) = I1(ω) + I1(ω)exp(iωτ) and confirm the photocurrents are added up destructively at ω = π/τ. In our experiment, laser pulses, of which intensity noise is higher than 10dB from SNL, were divided into two parts and each of them experienced self-phase modulation during propagation through two different single-mode optical fibers. We detected the two pulse trains with a relative time delay and observed photon number squeezing a specific frequency. We used a femtosecond fiber laser (Femtolite, IMRA) as a light source. The center wavelength was 1560 nm and the repetition rate was 47.5 MHz. Figur- 1 shows the experimental setup. We added time delay τ=1/frep, where frep is the repetition frequency, to one of the two pulse trains and added another delay deltaτ for preventing optical interference. We confirmed that intensity noise of the two pulse trains colineared at a polarization beam splitter (PBS) was coincident to the SNL both before and after fiber propagation at 22MHz. SNL was measured by balanced homodyne detection. We employed nonlinear polarization interferometer to generate squeezed light[2]. For measuring photon number squeezing, a branch ratio at PBS1 and PBS3,4 was adjusted to 90:10 and 95:5, respectively. We constructed another Michelson interferometer after a beam splitter to compensate the difference of fiber length. As a result, a maximum photon-number squeezing of 2.6 dB was obtained when average power was 4mW.