A Photonic System for Generating Unconditional Polarization-Entangled Photons Based on Multiple Quantum Interference

We present a high-performance photonic system that takes advantage of multiple quantum interference effect to produce polarization-entangled, degenerate, post-selection free photons at the high-emission rate with a large broadband distribution. Our approach uses a multiple reverse Hong-Ou-Mandel interference process to produce polarization-entangled photons with a high-generation efficiency and reliable separation of degenerate photon pairs into different optical modes without post-selection. To begin, turn on a laser diode, and set the power to a few milliwatts.

Please a holographic grating at about a 45-degree angle with respect to the laser diode surface, and adjust the angle until the beam intensity appears to be maximized. Next, couple the laser to a polarization-maintaining optical fiber. Direct the fiber to a power meter, and adjust the coupler screws to maximize the output power.

Direct the laser through a free-space isolator. Then, place a half-wave plate and a quarter-wave plate for 405-nanometer light in the path of the beam. Set the plate angles to achieve the desired beam polarization state.

Next, place a short-pass dichroic mirror and a polarizing beam-splitter cube in the path of the beam. Use a regular mirror to direct the reflected s-polarized beam parallel to the transmitted p-polarized beam. Place a type-zero ppKTP crystal on a temperature-controlled platform, mounted in the path of the beam.

Adjust the platform until the split beams pass through the crystal. Then, adjust the beam splitter and mirrors until both the s-and p-polarized beams are parallel for a few meters. Use both the 405-nanometer pump laser and an 810-nanometer reference laser for this adjustment.

Next, mount a dual-wave half-wave plate on either side of the ppKTP crystal, perpendicular to the incident light. The half-wave plate between the beam splitter and the crystal has been set to 22.5 degrees and the other plate to 45 degrees in advance. Then, place a retroreflector at the end of the setup to direct the down-converted beams back through the ppKTP crystal and the 22.5-degree half-wave plate.

Position the 45-degree half-wave plate so that only the inbound beam reflected from the beam splitter and the outbound beam from the other side pass through it. Ensure that both outbound beams are directed into the beam splitter to generate the clockwise and counterclockwise photon beams. Place CCD camera beam profilers inline with the output photon beams.

Adjust the mirrors and retroreflector so that the clockwise and counterclockwise beam pairs are in the same spatial modes. Then, mount a 300-millimeter focus lens between the quarter-wave plate and the dichroic mirror. Position the lens so that the focal point of the pump laser beam is around the generation position of the second photon down-conversion in the ppKTP crystal.

Remove the beam profilers, and place a quarter-wave plate, a wire grid polarizer, and an interference filter in the path of each output beam. Couple the beams to multimode fibers with a collimator lens. Place a 300-millimeter focus lens before each quarter-wave plate, and focus the output beams on the collimators.

Then, connect the multimode fibers to single-photon counting modules that use silicon avalanche photodiodes. Once the setup has been fully assembled, turn off the reference laser, and reconnect the diode laser. Turn off the room lights, and exclude all external light.

Then, turn on the counting modules, and count the down-converted photons. Then, adjust the temperature of the ppKTP crystal and the tilting angle of the 45-degree half-wave plate to improve the count rate of the down-converted photons. Repeat the measurements and adjustments until the count rate is maximized.

Before the measurement, set the angles of the quarter-wave plates and polarizers for the output beams to attain the desired polarization base for the measurement. Then, connect the single-photon counting module of the output beam reflected off the dichroic mirror to the start signal input of a time-to-amplitude converter. Connect the other beam to the stop signal input via an electrical delay line.

Set the delay time to 50 nanoseconds and the displayed time range to 100 nanoseconds. Open the instrument software, set the measurement time to 30 seconds, and start the measurement. When the measurement finishes, record the pulse height distribution.

Repeat the measurement with several polarization base combinations, and identify a coincidence time window based on the temporal resolution of the counting modules. For each measurement, integrate the area under the peak within the coincidence time window to estimate the coincidence counts. Calculate the fidelity and Bell parameters to confirm that the system is generating polarization-entangled photons.

Analysis of coincidence detection measurements from six combinations of polarization bases confirmed that the system could generate and detect polarization-entangled photons. The entanglement fidelity was 0.85, exceeding the classical local correlation limit of 0.5. The correlations from the bases of polarization all exceeded the classical parameter limit of two, violating the Bell inequality.

Our method allows post-selection free separation of degenerate photon pairs into different optical modes characteristic of type-two spontaneous parametric down-conversion, while maintaining the large bandwidth and high efficiency of type-zero spontaneous parametric down-conversion. This method of using multiple quantum interference processes is also useful for the application of entangled photons through the stimulated emission of spontaneous parametric down-conversion. Owing to the simplicity of our scheme, we can further improve the polarization-entangled photon generation efficiency by modifying the pulse laser pumping and the wave guide structures in nonlinear crystals.

We can also generate photons in the theorical wavelengths band by changing the poling period of the crystal. Our technique improves the total photon-pair production rate per unit pump power by two to three orders of magnitude, owing to the large bandwidth of type-zero spontaneous parametric down-conversion. A large bandwidth of correlated photon pairs gives a very short coincidence time, which has attracted considerable attention for use in quantum optical coherence tomography and in many other applications.