Case Study CS-TC03 Rev. A

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With this system, both lasers can be stabilized to the Cs D2

line and independently tuned to specific frequencies.

The OPO has been designed in a linear, semihemispherical

configuration. This allows for non-degeneracy in transverse

modes with asymmetric geometry. This simplifies alignment

and coupling the light to the single-mode (SM) fibers. Inside

the cavity there are two crystals. The first is a 30 mm-

long Periodically Poled Potassium Titanyl Phosphate

(PPKTP) Type-II crystal used for the SPDC. The second

crystal is added specifically to tune the cluster separation

independently of the SPDC phase matching, solving a

critical issue with previous methods. It is 15 mm-long

Barium Borate (BBO). Tuning the cluster effect allows a

broad range of frequencies of photon generation. The

temperature stability of these crystals is critical to the

success of the photon source. The crystal temperature

needs to be regulated to a high level of precision for control

of the characteristics of the beam passing through it given

the temperature-dependent birefringent properties of

the crystal. Once the two beams (pump and probe) pass

through the cavity, the residual pump beam is separated

from the single photons using two dichroic mirrors (DM),

two long pass filters, and one bandpass filter. This way, only

the single photons are coupled into the single-mode fibers.

To couple the photons into the SM fibers, a Polarizing

Beam Splitter (PBS) is used to separate the signal and

idler photons. This is possible due to the SPDC crystal

properties. The PPKTP is a Type-II crystal, meaning the

output photons (signal and idler) have perpendicular

polarizations. In this way, when separated using the PBS,

one photon in the pair can be heralded by the detection of

the other in the pair using Avalanche Photodiodes (APD).

The photons are then counted for detection and analysis.

RESULTS

With this novel design, the researchers have accomplished

the goal of a single-mode, narrow-band photon source using

an OPO for SPDC. The cross-correlation function, G(1,1) (τ)

and the autocorrelation function, G(2)(τ) characterize the

spectral properties of this photon source. These functions

are related to the bandwidth and number of modes of the

photon source. The cross-correlation function is related to

the probability of detecting a signal photon at time t and an

idler photon at time t + τ.

The FWHM of the cross correlation temporal profile

is defined as the cross correlation time, τ

c,

and is

inversely proportional to the bandwidth of emitted photons.1

Figure 4 shows the measured cross correlation.

From the data, the effective bandwidth of the photons

is found to be 10.9 ± 0.3 MHz for the photon pairs. This

is close to the few megahertz linewidth of typical atomic

transitions.

The autocorrelation function is related to the statistics of the

single SPDC fields and the number of modes in the system.

Figure 3. Narrow-band single photon source. Two

tunable diode lasers operating at 852 nm—called probe,

stabilized to Cs D2 line via polarization spectroscopy—

and 426 nm—called pump—locked to the probe by

frequency-offset lock constitute the laser system. The

OPO cavity’s geometry is semihemispherical, and it

contains two crystals: A 30-mm-long PPKTP (periodically

poled potassium titanyl phosphate) for collinear, type-II

phase-matched SPDC of 426 nm–852 nm and a 15-mm-

long BBO (barium borate) for tuning the cluster effect.

Two optical shutters switch the probe beam and block

the APDs to iterate between the cavity lock and the

measurement mode with 60% duty cycle. The photons are

coupled into single-mode fibers and routed to the APDs

and the TTM for analyzing the temporal correlations.1

Figure 4. Experimental Cross Correlation with Curve

Fit1