Single Photon Source – My First Hand Experience

Updated on May 13, 2024

Quantum computing is one of the most discussed technologies today and it deserves attention for its potential to revolutionize computational methods. This interest & hype have taken us over at Techglimpse, leading us to create a few articles on Quantum computing simulators such as Qiskit. Well, I know that it is slightly off-topic, but trust me I’m here to share my experience in trying my hands with the Single Photon Source deployed at one of the Quantum computing labs in India.

What is a Single Photon Source first of all?

The idea is that information processing takes place in physical devices, hence the components & materials used in the devices should have quantum behaviors rather than classical behavior, which is then exploited to process information in potentially advantageous ways.

Building these physical devices involves mainly two approaches: matter-based approach and light-based approach. The matter-based approaches rely on superconducting qubits, cold atoms, or trapped ions and the light-based approach uses photons as information carriers. Quantum mechanics describes light as a stream of discrete packets known as photons and these photons are massless bosons with both particle & wave behaviors.

In quantum computing, the information is encoded in qubits. In photonic quantum computers, single photon properties such as polarization or path are used to represent the qubit state (0 or 1).

A single photon? Yes, you heard it right! In a light containing a stream of photons, it’s difficult to determine the individual state of each photon, making it difficult to manipulate the qubits effectively. That’s where the Single Photon Sources play a key role in photonic-based quantum computing systems.

A single photon source is a special device that emits light as individual photons, one at a time, making it different from general light sources such as lasers & bulbs. There are multiple approaches for generating single photons: Quantum dots, diamond-based nitrogen-vacancy centers, single molecules, and parametric down-conversion. However, I was fortunate to have hands-on with an SPDC (Spontaneous Parametric Down Conversion) based single photon source.

Experimental setup of the Heralded Single Photon Source

Well, here’s the experimental setup of the Single Photon Source deployed at one of the research labs in India.

Single Photon Source
Experimental setup of Single Photon Source

As discussed earlier, the SPDC (Spontaneous Parametric Down Conversion) process is used to generate the high purity of single photons; which involves creating a pair of photons simultaneously in two spatially separate modes. Now, by detecting one photon signals that the other photon is present, this approach is called ‘Heralded Single Photon‘ generation. Just assume that we label the pair of photons: the photon that arrives in one path as ‘Signal Photon’ and the other as ‘Idler Photon’.

Now coming back to the above diagram, a stream of photons is pumped through a 405 nm laser, which is a continuous wave, single mode, diode laser coupled to a single mode fiber. This stream is passed through an ND (Neutral Density) filter, which controls the intensity of the pump radiation without changing the polarization state of the photons. The pump is reflected using two half-inch mirrors after the ND filter to make it straight and it travels through Half Wave Plate (HWP) which selects the required polarization and the plano-convex lens of focal length 75mm is used to focus the pump into Periodically Polled Pottasium Titanyl Phosphate (PPKTP) crystal of 10mm length (specifically cut for Type 0 & Type II SPDC).

Inside the PPKTP crystal, the photons get down-converted into two beams: the signal and idler beams with double the wavelength of the pump controlled using a temperature controller. Since the polarization state of the incoming photon is ‘Horizontal polarization (H); the output state after down conversion is a Type II state as shown below.

What are various Non-Linear Crystals?

  • PPKTP – Periodically Polled Potassium Titanyl Phosphate
  • KTP Crystal – Potassium Titanyl Phosphate (KTP)
  • BBO Crystal – Beta Barium Borate
  • MgO:LN – MgO doped lithium niobate

The output photons emerge as cone-shaped downconverted photons and are non-degenerate (i.e., of different wavelengths) and can be made the same wavelength by varying the temperature controller connected to the PPKTP crystal. At this point, there exist two co-related photons (which are time tagged using another device called Time tagger) and these are collimated into a cylinder via a plano-convex lens of 30 mm.

Using the Polarized Beam Splitter (PBS), the co-related photons are separated into two paths (signal photon in one path & idler photon in another path) while preserving entanglement. The signal photon is typically detected using the detector, which ensures that the idler photon is present because they are co-related photons (Learn about the entangled particles). In the above schematic diagram, we detect both photons just to ensure that they have arrived. But in the ideal case, the idler photon will not be detected & will be used for computational purposes.

For the demonstration purpose, the photons on both the paths were detected and the photon counts were recorded using the specialized time tagging software. For instance, the software recorded 1.2 million single photon counts and close to 10k co-related photon counts per second. (These counts are specific to the setup and might vary depending on the configurations & components used).

Applications of Single Photon Source

Single photon sources are key to the building blocks of various applications in Quantum Computing, Communication & Metrology. In Quantum computing, the single photons act as qubits, the basic unit of information, and their properties such as path, and polarization are used to represent the qubit state 0 or 1. In Quantum communication, single photons are used in Quantum Key Distribution (QKD), a secured method for information transmission. In metrology, they are used for precise measurements in sensing and imaging applications.


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