Experimental generation of broadband quadrature entanglement using laser pulses

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 $3.4\pm 0.2$ 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 $⟨{\Delta }^2(X_a+X_b)⟩+⟨{\Delta }^2(Y_a-Y_b)⟩=1.28<2$.

Figure 1

(Color online) Experimental setup. SHG: second harmonic generator, WG: waveguide, BS: 50:50 beam splitter, L: lens, PBS: polarizing beam splitter, Ds: photodiodes, PZTs: piezoelectric transducers, $\lambda$/2: half-wave plate, and LO: local oscillator.

Figure 2

(Color online) Homodyne system. PBS: polarizing beam splitter, $\lambda ∕2$: half-wave plate.

Figure 3

(Color online) Frequency response of the broadband low noise detector. Trace a: electronical noise, trace b: spectral power of the detected signal when both photodiodes are illuminated by laser pulse with power of $2\mathrm{mW}$. Two peaks are due to first and second harmonic frequency of the laser repetition rate. Inset: oscilloscope traces of the homodyne detector at a LO power of $1.5\mathrm{mW}$. Trace c: output of homodyne detector, trace d: trigger signal by the $Q$-switch synchronization from the laser power supply.

Figure 4

(Color online) (a) Output probe versus the relative phase between the input probe and pump beams. (b) Classical parametric gain versus average pump power.

Figure 5

(Color online) Estimated squeezing parameter and mode matching efficiency versus the square root of the average pump power.

Figure 6

(Color online) Noise measurement results on a squeezed vacuum generated by a PPLN waveguide. Trace (a) is electronical noise of homodyne detector, trace (b) is shot noise level with LO power of $2.5\mathrm{mW}$ and trace (c) is the noise power of squeezed vacuum. The two peaks appear at $76\mathrm{MHz}$ corresponds to the repetition frequency of the mode-locked laser and its second harmonic. Resolution bandwidth is $1\mathrm{MHz}$ and video bandwidth is $10\mathrm{Hz}$.

Figure 7

(Color online) Normalized noise power of squeezed vacuum at fixed analyzer frequency of $10\mathrm{MHz}$. The black curve is measured result and the red dashed curve is a fitted result using parameters,which are calculated from the classical amplification and deamplification gain. Resolution bandwidth is $300\mathrm{kHz}$ and video bandwidth is $100\mathrm{Hz}$.

Figure 8

(Color online) Comparison between the measured normalized noise power of output squeezed state from OPA at different pump power and the predicted noise power from the measured classical gain. The measured noise power were obtained at fixed analyzer frequency of $10\mathrm{MHz}$.

Figure 9

(Color online) Normalized noise of one beam of the entangled beams obtained by scanning the mutual phase difference between the two squeezed vacua in addition to a rapid sweep of the LO in the homodyne detector.

Figure 10

(Color online) Entanglement correlation between two entangled beams. It is obtained by adding signal of two homodyne detectors measuring the two beams while scanning one of the LO phase.