Investigation of Nano- and Quantum-Structures Based on theSingle-photon Counting Method
Toward the realization of higher performance colloidal-dot quantum emitter

esearch and development intoR quantum technology is accelerating

Toshiyuki IHARA
Advanced ICT Research Institute
Frontier Research Laboratory
Researcher(Tenure-Track)
After getting his Ph.D. in Physics, heworked at Institute of Industrial Sciencein The University of Tokyo and Institutefor Chemical Research in Kyoto University. He then joined NICT in 2017. His
primary research interests are in developing advanced technologies of microscopic spectroscopy for understanding
the physics in nanostructures and quantum emitters. Ph. D. (Science).

throughout the world in preparation for arevolution in info-communications and information processing. Using microscopic,
nano-sized materials allows easy manufacturing of "quantum emitters" that emit
special light with non-classical properties.
Quantum emitters can be used for a varietyof purposes, including use in info-communications. This has led to research being carried out all over the world with the aim ofmaking them brighter, more stable, capable of room temperature operation, etc. In
this article, we introduce the characteristics
of colloidal-dot quantum emitters madefrom a semiconductor nanoparticle material known as colloidal quantum dots, andan original measurement system we developed for accurately evaluating the emitter
performance.

■ Nano-Sized Quantum Emitter,
the Key to Quantum ICT
In recent years, researches are being carried out all over the world on a variety ofemitters capable of generating non-classicallight, which cannot be explained by concepts of classical waves. Examples include

Figure 1 A: How a colloidal quantum dot (CQD) dispersed sample is made. It is diluted
to a low concentration using organic solvent, then dropcast onto a cover slip. B: How
single photons are generated using the CQD dispersed sample. Irradiating it with ashort wavelength light results in the emission of single photons with long wavelengths.
This system is characterized by its ability to be operated at room temperature.

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emitters that generate a series of single photons, which cannot be split any further, andemitters that generate quantum-correlated
entangled photon pairs and indistinguishable
photons. These emitters have the potential
to realize tap-proof communication throughquantum cryptography, or be used in, highspeed information processing technologythat is overwhelmingly superior to existing
computers. Among emitters capable of generating non-classical light, the nano-sized
quantum emitter is one of the most expecting candidates to achieve outstanding performance. Nano-sized quantum emitters areemitters made of microscopic, nano-sizedmaterials, and are characterized under thelaws of quantum mechanics. As competition
heats up around the world to develop cutting-edge "quantum ICT technology," the development of nano-sized quantum emitters,which are efficient at generating non-classical light, is also accelerating dramatically.

■ The Characteristics of Colloidal
Quantum Emitters
Our research focused on a material known
as the colloidal quantum dot (CQD), and weare carrying out experiments to investigate its
performance as easily quantum emitters.

Figure 2 Examples of physical phenomena that affect theperformance of the colloidal-dot quantum emitter.

The CQD is a semiconductor nanoparticle that exhibits high luminescence efficiency at room temperature, and it is a material
that allows the low cost manufacturing ofquantum emitters. The particles are around2 – 10 nanometers in diameter, and they arecharacterized by dramatic changes in theirproperties, such as the color of the emitted
light (wavelength), depending on their size,
shape, material quality, etc. In this research,
we set up our colloidal-dot quantum emitter(a quantum emitter made from CQD) according to the procedures shown in Figure 1.
Irradiating dispersed, low-concentration
CQD with a short wavelength light causes
the CQD to emit single photons with longwavelengths. The single photons emittedfrom the CQD have low directivity, but amicroscope component known as an "oil-immersion objective lens" can be used to reduce
loss, realizing greater brightness.
Recent researches have revealed thatvarious physical phenomena affect the performance of the colloidal-dot quantum emitter as shown in Figure 2. Figure 2A shows
the neutralization and ionization of a single
CQD, which is one of the causes of temporalfluctuation (photoluminescence blinking) in
the strength of emissions. Figure 2B shows
how the presence or absence of an electricfield inside the CQD results in a phenomenon in which the wave function of electronschanges, leading to temporal fluctuation in
the color of emissions. Figure 2C shows theprocess in which two photons are emitted sequentially from a single CQD in a phenomenon known as "two-photon cascade emission." It is not just the case of CQD, but theperformance of many nano-sized quantum
emitters is affected strongly by these physicalphenomena. The development of a high-per-formance nano-sized quantum emitter will
require a deeper understanding of the mechanisms behind how these phenomena occur,and advanced technologies to control them.

■ Analyzing Emitters Using theSingle-Photon Counting Method
To observe and analyze the variety ofphysical phenomena as shown in Figure 2, thisresaerch utilizes the "single-photon counting
method." The use of this counting method
enables us to record the times of single-photon detection, at a high temporal resolution ofaround 10 picoseconds, in a text file. Figure 3
shows a schematic diagram of the measurement system developed for this research. A
low-noise, high-sensitivity super-conducting nanowire single-photon detector is used,to analyze the characteristics of temporalfluctuation in the strength of emissions andtwo-photon cascade emissions with extreme
precision.
In this system, the generated photons arepassed through an optical element (beamsplitter) to split the beam into half before
they are passed through two optical fibers
to measure what is known as their intensity
correlation. If the single photons are being
generated in an orderly manner at regularintervals, the probability of the two split photon beams entering the optical fibers at thesame time will be 0%. So, the signal indicating the delay time between two photons
being zero will be eliminated when using a
two-channel single-photon detector to measure the coincidence count. Figure 4 shows
an example of actual data of the intensity correlation obtained through experiments. The
data shows that the ratio between the maximum coincidence count value and the value

Figure 3 Schematic diagram of the counting system developed for this research. The generated
single photons are detected by a super-conducting nanowire single-photon detector, and the signals
are recorded by a single-photon counting board.

when the delay time between two photons iszero is around 0.01. The lower the value, thehigher the performance, and tha value of 0.01
indicates its top-class performance as a single-photon emitter among all emitters aroundthe world operable at room temperature.
Data obtained through recent experimentssuggests that performance can be improved
even further by making use of the characteristics of super-conducting nanowire single-photon detectors. We expect that thefurther experiments will prove the limitationof performance improvement of nano-sized
quantum emitters at room temperature.

■ Future Prospects
This research proved how easy it is tomake a high-performance single-photon
emitter (colloidal-dot quantum emitter) bymaking use of CQD. If technology could be
developed in the future to efficiently generate
not only single photons, but also entangled
photon pairs and indistinguishable photons,
we can expect a dramatic spread in the practical application of ICT technology using
non-classical light throughout society. Meanwhile, experiments using the advanced single-photon counting method have also madeit clear that it is possible to quantitatively
evaluate the non-classical properties andoptical characteristics of a variety of nanomaterials, including CQD. In the next stageof research we will involve R&D into further
improving the performance of nano-sized
quantum emitters, while also aiming to ensure the advanced use of the single-photon
counting method, and the extensive application of non-classical properties seen in nanoand quantum structures.

Figure 4 Example data on the coincidence count of
single colloidal quantum dot emissions. It shows thatthe ratio between the maximum coincidence countvalue and the value when the delay time between two
photons is zero is around 0.01, which is indicative of its
ability to achieve top-class performance in the world.

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