4.3 Quantum Optics
113
Efficient Single-Photon Sources
In addition to conventional concepts incorporating epitaxially-grown quantum dots,
scientists recently demonstrated electrically-pumped room-temperature singlephoton sources based on different quantum emitter platforms (cf. low- and hightemperature current-driven platforms such as [31, 33, 167], respectively). In fact,
different approaches lead to efficient sources, but for practical applications compactness, scalability, room-temperature operation and the possibility to electrically excite
and control the emission have been particularly desirable [22].
For instance, single neutral nitrogen-vacancy centers in a novel diamond diode
structure were used to achieve a stable, room-temperature, electrically-driven singlephoton source [168]. Similarly, the development of a room-temperature device was
pursued with the help of organic molecules, which feature strong exciton binding
energies on the order of 1 eV. From such material platform, stable single-molecule
emission was achieved by incorporating carefully chosen molecular emitters in a
solid-state matrix in the form of a specially designed organic light-emitting diode
structure [169]. Alternatively, single defect systems in silicon carbide were exploited
to obtain bright single-photon emitting diodes at room temperature [170].
However, to bring the demonstrated antibunching behaviour that characterises
single-photon emission closer to an ideal result, solution-processed devices based on
CdSe/CdS core/shell quantum dots were recently reported, from which high-purity
single photons with g
(2)
(0) < 0.05 was obtained under electrical pumping at room
temperature [167]. Without carefully tailoring hole-transport and electron-transport
layers in such devices, in which isolated quantum dots were embedded in a PMMA
layer, a high quantum yield would be impossible from electrical injection of opposite
charge-carriers from two sides of the sample.
While many practical platforms for single-photon generation have evolved in
recent years, even defects in the host lattice of atomically-thin 2D materials have
emerged as promising room-temperature quantum emitters and moved into the focus
of single-photon source research based on novel material systems (see [171–176]).
Advances in the Field of Quantum Emitters
Still, quantum emitters based on conventional III/V semiconductors are heavily
developed towards higher device fidelity, pulse repetition rate and photon indistinguishability. Numerous studies were devoted to the generation, guiding and detection
of triggered single photons, for instance from resonantly-excited quantum dots in a
photonic circuit [38, 39].
On the one hand, the advantages of nonclassical light emission from advanced
devices such as quantum-dot–microlense structures have led to the deterministic
fabrication of bright quantum-dot-based single-photon sources. On the other hand,
on-chip waveguide structures defined by means of in-situ electron-beam lithography
(EBL) have promised better yield and faster implementation of such structures in
quantum cryptography schemes with semiconductor quantum emitters. The recently
demonstrated integration of pre-selected quantum dots into photonic devices such as
microlenses and waveguide systems with nm-accuracy has offered a flexible device
design and high process yield [37, 177, 178]. Similarly, molecule-based quantum
113
Efficient Single-Photon Sources
In addition to conventional concepts incorporating epitaxially-grown quantum dots,
scientists recently demonstrated electrically-pumped room-temperature singlephoton sources based on different quantum emitter platforms (cf. low- and hightemperature current-driven platforms such as [31, 33, 167], respectively). In fact,
different approaches lead to efficient sources, but for practical applications compactness, scalability, room-temperature operation and the possibility to electrically excite
and control the emission have been particularly desirable [22].
For instance, single neutral nitrogen-vacancy centers in a novel diamond diode
structure were used to achieve a stable, room-temperature, electrically-driven singlephoton source [168]. Similarly, the development of a room-temperature device was
pursued with the help of organic molecules, which feature strong exciton binding
energies on the order of 1 eV. From such material platform, stable single-molecule
emission was achieved by incorporating carefully chosen molecular emitters in a
solid-state matrix in the form of a specially designed organic light-emitting diode
structure [169]. Alternatively, single defect systems in silicon carbide were exploited
to obtain bright single-photon emitting diodes at room temperature [170].
However, to bring the demonstrated antibunching behaviour that characterises
single-photon emission closer to an ideal result, solution-processed devices based on
CdSe/CdS core/shell quantum dots were recently reported, from which high-purity
single photons with g
(2)
(0) < 0.05 was obtained under electrical pumping at room
temperature [167]. Without carefully tailoring hole-transport and electron-transport
layers in such devices, in which isolated quantum dots were embedded in a PMMA
layer, a high quantum yield would be impossible from electrical injection of opposite
charge-carriers from two sides of the sample.
While many practical platforms for single-photon generation have evolved in
recent years, even defects in the host lattice of atomically-thin 2D materials have
emerged as promising room-temperature quantum emitters and moved into the focus
of single-photon source research based on novel material systems (see [171–176]).
Advances in the Field of Quantum Emitters
Still, quantum emitters based on conventional III/V semiconductors are heavily
developed towards higher device fidelity, pulse repetition rate and photon indistinguishability. Numerous studies were devoted to the generation, guiding and detection
of triggered single photons, for instance from resonantly-excited quantum dots in a
photonic circuit [38, 39].
On the one hand, the advantages of nonclassical light emission from advanced
devices such as quantum-dot–microlense structures have led to the deterministic
fabrication of bright quantum-dot-based single-photon sources. On the other hand,
on-chip waveguide structures defined by means of in-situ electron-beam lithography
(EBL) have promised better yield and faster implementation of such structures in
quantum cryptography schemes with semiconductor quantum emitters. The recently
demonstrated integration of pre-selected quantum dots into photonic devices such as
microlenses and waveguide systems with nm-accuracy has offered a flexible device
design and high process yield [37, 177, 178]. Similarly, molecule-based quantum