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Case Study CS-TC03 Rev. A

Single-Mode, Narrow Band Photon

Source Using SPDC for Hybrid

Quantum Systems

April, 2020

Page 1

ABSTRACT

Generating single-mode, narrow-band photons with high purity and brightness has proven to be difficult.

Researchers at the University of Vienna have designed a photon source using Spontaneous Parametric

Down-Conversion (SPDC) with a doubly resonant Optical Parametric Oscillator (OPO) utilizing an

additional birefringent crystal to tune clusters independent of SPDC phase-matching. Achieving the doubly

resonant condition in the OPO that enables a single mode is the challenge when creating a photon source

with both single-mode and narrow-bandwidth. This is resolved here, as the resulting photon source has

a bandwidth of 10.9 MHz to match the linewidths of atomic transitions. The source also produces single-

mode photon pairs at a rate of 47.5 Hz without mode filters reducing loss in the system. This novel design

enables research and applications in the quantum realm using hybrid light-matter interactions.

HYBRID QUANTUM SYSTEMS

Many quantum systems utilize single photons and their

spectral properties. Hybrid quantum systems combine

single photon-based and matter-based designs to

implement two-qubit gates, the foundation for quantum

computers and information processing.1 Applications for

this type of photon source range from observing quantum

phenomena to quantum communication.

Spontaneous Parametric Down-Conversion (SPDC) is one

hybrid photon-matter based system used to generate single

photons at a high purity level. This method also allows

some control of the wavelength and bandwidth of the output

photons.

An Optical Parametric Oscillator (OPO) can be used to

generate coherent light.2 The novel use of a birefringent

crystal in the OPO combined with SPDC achieves photon

selection with minimal losses.

METHODS & PROBLEMS

To implement two-qubit gates for many quantum

applications, hybrid quantum systems need to be created

with photon-matter interactions. Several characteristics

of the generated photon must be achieved to establish a

quality photon source for quantum applications.

One challenge is obtaining high-purity single photons.

Successful methods include: quantum dots, single nitrogen

vacancy center in nanodiamonds, and SPDC.1 The first two

methods do not allow for photon heralding. That is, after

a photon is down-converted to two lower energy photons,

one of the signal or idler photons is detected or measured,

ensuring the other will also be detected with accurate

knowledge of its state.

What is sPDC & OPO?

SPDC is, essentially, a "down-conversion" of a single

photon instantaneously.

Figure 1 shows how SPDC

works.

Crystal

Input Beam γ

1

Signal γ

2

Idler γ

3

One photon (γ

1) enters the nonlinear crystal and is

down-converted into two separate photons: signal and

idler (γ

2 and γ3). Down-converted means that the two

output photons will have lower frequencies (and also

energies) than the input photon. Because of the laws of

conservation of energy and momentum, the two output

photons must have lower energies and momenta than

that of the initial photon such that:

Eγ

1 = Eγ2 + Eγ3

(1)

k

1 = k2 + k3

(2)

where E is the energy of the appropriate photon, γ

1,2,3

distinguishes each photon, and k is the momentum

of the photon. The frequency of each photon can

be related in the same way, as energy is related to

frequency by either h or ħ (planck constant or reduced

planck constant depending on using frequency or

angular frequency).

Figure 1. Spontaneous Parametric Down-

Conversion Process