Case Study CS-TC03 Rev. A

Page 2

© 2020 • Sales & Technical Support: (406) 587-4910 • email: sales@teamWavelength.com • web: www.teamWavelength.com

If the SPDC method is used, it can be very difficult to

obtain narrow photon bandwidth. Generally, SPDC creates

photons with very broad bandwidth, orders of magnitude

larger than atomic transitions. In order for these hybrid

quantum systems to be successful, the bandwidth of the

generated photons needs to be narrow enough to interact

with and match the linewidths of the atomic transitions. A

simple solution is to introduce passive spectral filtering into

the system. However, this creates a large amount of loss in

the design. An alternate solution is using an OPO to utilize

the advantages of cluster effects.

An OPO, combined with SPDC, however, can generate

multi-mode spectral characteristics which are not ideal for

efficient interaction with the atomic transition. Two ways to

suppress the unwanted modes are filter cavities and atomic

line filters. These methods of mode-filtering work but at the

cost of photon loss and increased complexity in the setup.1

Using doubly resonant OPOs, it can be difficult to reach the

doubly resonant condition as the tuning behavior is heavily

dependent on stable crystal temperature. Once attained,

cluster effects can enable a narrow band photon source

with fewer modes. However, the attempts at achieving this

condition have been unsuccessful at highly non-degenerate

(photon pair with different energies) SPDC.

UNIVERSITY OF VIENNA'S SOLUTION

Researchers at the University of Vienna, Austria have

developed a single-mode, narrow-bandwidth photon

source for quantum applications. They accomplish this

through a novel design utilizing a doubly resonant Optical

Parametric Oscillator (OPO) with Spontaneous Parametric

Down-Conversion (SPDC). In order to achieve the doubly

resonant condition, an additional birefringent crystal is

inserted into the Type-II OPO. This allows for narrow band

photons with fewer modes. A complete setup, including a

pump laser, probe laser, OPO cavity with two crystals, and

other components can be seen in

Figure 3.

The two lasers shown are used as a pump laser (426 nm,

30 mW) and a probe laser (852 nm, 50 mW). The probe

laser is tuned to the Cs D2 line by passing some of the light

through Cs vapor, performing polarization spectroscopy

on the Cs vapor and allowing frequency tuning. The

pump laser is locked to the probe laser by combining

a portion of the pump laser beam with the frequency-

doubled probe laser. The probe laser is frequency-

doubled using a single-pass Type-I PPKTP crystal to

426 nm. Another portion of the probe laser is sent to a

double-pass Acousto-Optic Modulator (AOM) system.

The combination of Equation (1) and Equation (2) is

called the "phase-matching conditions." This allows

some control of the characteristics (wavelength and

bandwidth) of the signal and idler photons to produce

a desired result.

There are two types of OPO: Singly Resonant OPOs

(SRO) and Doubly Resonant OPOs (DRO). An SRO

provides feedback to only one of the signal or idler

photons, while a DRO provides feedback at both the

signal and the idler frequencies.

The DRO is designed such that both frequencies

coincide with resonator modes to allow SPDC emission

but exhibit different free spectral ranges (FSR).1 This is

the OPO that is used for single-mode photon sources.

DROs have lower threshold pump power, and the

tuning behavior of the photons can be affected

by changing crystal temperature.3 DROs also can

leverage cluster effects. When signal and idler modes

overlap, the spectrum of the OPO is made of clusters

of a few modes. This can be seen in

Figure 2.

SPDC

Signal

modes

Idler

FSRs

FSR i

Clusters

modes

Designers change the properties of the SPDC crystal

to reach this doubly resonant condition where both the

signal and idler modes are resonant to the cavity at the

same time.

Figure 2. Cluster effect. The longitudinal modes for

signal are shown in orange and the modes for idler

are in green. The two arrows indicate the direction

in which frequency increases. The overlap condition

is periodical, but because of the difference in the

FSRs for signal and idler, the modes that are next

to the fully overlapping ones coincide only partially.

This leads to a spectrum that is made of clusters of

modes, separated by Δνc.1