4.3 Quantum Optics
115
Prospects of Quantum Channels with Record-High Single-Photon Flux
In addition, in terms of overcoming the limitations of optical pulsed excitation
with common Titanium-Sapphire oscillators running at a repetition rate of 80 MHz,
the combination of ultrafast mode-locked VECSELs with quantum-dot–microlens
single-photon sources by the author and co-workers has resulted in a record-high
single-photon flux with effective duty cycle of currently as high as 143 MHz [34]
(Fig. 4.7). To combine the self-built laser source and the emitter in one quantum
device in Berlin, the picosecond-short pulses from the (breadboard-mounted and
thereby mobile) mode-locked VECSEL had to be frequency-doubled externally in a
nonlinear BBO crystal (by second-harmonic generation) to enable optical pumping
12
of the microlens-embedded quantum emitter (see Fig. 4.2), which was addressed in a
μ-PL setup for photon statistics measurements (Fig. 4.7c). Thereby, the combination
of quantum technologies of the first and of the second generation in an overarching
quantum-technological system has become a natural success story which will be
further exploited in different device schemes.
4.3.2 Strong Light–Matter Coupling for Polariton Research
Strong light–matter coupling as described above, although mainly for quantum-wellbased systems, is at the verge of being utilised. Many proposals and experiments
regarding application-oriented polariton systems have emerged in recent years (cf.
[185, 186]).
Different approaches were suggested in the literature to employ polariton systems for optoelectronic or quantum optical applications, such as switches based on
polaritons for use in all-optical/integrated photonic circuits [187, 188], spin switches
[189], optically-imprinted polaritonic logic circuits [190], and transistors based on
polariton spin [191]. For some, condensates of polaritons with their long-range spatial coherence had become attractive candidates for a polariton transistor based on
ultrafast coherent switches [192], whereas others discussed a two-fluid polariton
switch [193]. As part of the evolution of polaritonics as a new field for light-based
devices, resonant-tunneling diodes [194], polariton transistors [195], routers [196],
and interferometers [197] were recently introduced, as summarised for instance in
[185].
The subject of polariton lasing has been raised in the previous section and remains
an exciting playground for device development and the delivery of condensates on
demand (also see [119, 120]).
12 Here, strongly off-resonant optical pumping with few-ps pulses at 507 nm with 500 MHz repetition
rate was achieved due to device limitations. However, resonant p-shell excitation and repetition rates
better adjusted to the quantum-dot exciton (or trion, here X + , see Fig. 4.7a) lifetime would be more
desired for future high-repetition rate single-photon sources, in order to provide for the best possible
photon indistinguishability and optimised single-photon duty cycle, respectively.
115
Prospects of Quantum Channels with Record-High Single-Photon Flux
In addition, in terms of overcoming the limitations of optical pulsed excitation
with common Titanium-Sapphire oscillators running at a repetition rate of 80 MHz,
the combination of ultrafast mode-locked VECSELs with quantum-dot–microlens
single-photon sources by the author and co-workers has resulted in a record-high
single-photon flux with effective duty cycle of currently as high as 143 MHz [34]
(Fig. 4.7). To combine the self-built laser source and the emitter in one quantum
device in Berlin, the picosecond-short pulses from the (breadboard-mounted and
thereby mobile) mode-locked VECSEL had to be frequency-doubled externally in a
nonlinear BBO crystal (by second-harmonic generation) to enable optical pumping
12
of the microlens-embedded quantum emitter (see Fig. 4.2), which was addressed in a
μ-PL setup for photon statistics measurements (Fig. 4.7c). Thereby, the combination
of quantum technologies of the first and of the second generation in an overarching
quantum-technological system has become a natural success story which will be
further exploited in different device schemes.
4.3.2 Strong Light–Matter Coupling for Polariton Research
Strong light–matter coupling as described above, although mainly for quantum-wellbased systems, is at the verge of being utilised. Many proposals and experiments
regarding application-oriented polariton systems have emerged in recent years (cf.
[185, 186]).
Different approaches were suggested in the literature to employ polariton systems for optoelectronic or quantum optical applications, such as switches based on
polaritons for use in all-optical/integrated photonic circuits [187, 188], spin switches
[189], optically-imprinted polaritonic logic circuits [190], and transistors based on
polariton spin [191]. For some, condensates of polaritons with their long-range spatial coherence had become attractive candidates for a polariton transistor based on
ultrafast coherent switches [192], whereas others discussed a two-fluid polariton
switch [193]. As part of the evolution of polaritonics as a new field for light-based
devices, resonant-tunneling diodes [194], polariton transistors [195], routers [196],
and interferometers [197] were recently introduced, as summarised for instance in
[185].
The subject of polariton lasing has been raised in the previous section and remains
an exciting playground for device development and the delivery of condensates on
demand (also see [119, 120]).
12 Here, strongly off-resonant optical pumping with few-ps pulses at 507 nm with 500 MHz repetition
rate was achieved due to device limitations. However, resonant p-shell excitation and repetition rates
better adjusted to the quantum-dot exciton (or trion, here X + , see Fig. 4.7a) lifetime would be more
desired for future high-repetition rate single-photon sources, in order to provide for the best possible
photon indistinguishability and optimised single-photon duty cycle, respectively.