4.1 Into the Quantum Realm
101
pulsed lasers, which contain with very high probability no photons per pulse and
sometimes one single photon, whereas the probability to obtain a second photon is
non-zero due to the underlying Poissonian photon statistics for the coherent state.
Quantum emitters in microcavities (making use of controlled spontaneous emission
by tailored low-mode-volume light fields [31–33]), on the other hand, can provide
single photons with record-high repetition rates [34] and become more and more
practical platforms for bright sources of indistinguishable single photons due to
recent technological improvements (see above references on quantum light sources
and [35–41], to name but a few).
Concerning the generation of entangled photons (e.g. for quantum communication or ghost imaging applications), typically spontaneous parametric down conversion (SPDC) is used. This technique requires high-repetition rate laser pulses (for
ultrafast communication schemes) and femtoseconds pulse durations (for high pulse
peak powers) to make the low-probability conversion effect as efficient and frequent
as possible, linking first-generation and second-generation quantum technologies.
Recently, also other sources of entangled photons based on quantum dots have been
investigated (see for instance [42]).
4.2 Coherent Light Sources
The development of semiconductor lasers
1 heavily relied on concepts of the early
quantum mechanics world and fundamental theory of light–matter interactions.
Among the members of the laser family, such as solid-state lasers (see for instance
[8]), semiconductor lasers (e.g. [43, 44]) are widely used in our modern world for
various applications and in different frequency ranges. They became increasingly
reliable, mass-producible and, therefore, cheap owing to the well-matured field of
semiconductor technology.
Coherence in Advantageous Light
Another reason to pay attention to coherent light sources is motivated by the very
nature of their light output: A directional beam of phase-coherent, polarised and
typically monochromatic light can be conveniently and precisely used to deliver high
intensities, ultrashort pulses or spectrally well-defined irradiances to experiments and
applications. Moreover, the coherent state of light is a very prominent example of
a light state, which represents the vacuum state displaced by a coherent amplitude
(certain mean number of photons).
2 The photon statistics for an ideal coherent state is
totally random
3 and centred around the expectation value of the photon number (N ,
1 Laser is the acronym for Light Amplification by Stimulated Emission of Radiation.
2 See vacuum state |0 displaced by the displacement operator D(α) representing the coherent state
|α = D(α) |0, e.g., in [5].
3 The photon number of the coherent state is Poisson distributed. Correspondingly, the second-order
temporal autocorrelation function is g (2) (0) = 1.
101
pulsed lasers, which contain with very high probability no photons per pulse and
sometimes one single photon, whereas the probability to obtain a second photon is
non-zero due to the underlying Poissonian photon statistics for the coherent state.
Quantum emitters in microcavities (making use of controlled spontaneous emission
by tailored low-mode-volume light fields [31–33]), on the other hand, can provide
single photons with record-high repetition rates [34] and become more and more
practical platforms for bright sources of indistinguishable single photons due to
recent technological improvements (see above references on quantum light sources
and [35–41], to name but a few).
Concerning the generation of entangled photons (e.g. for quantum communication or ghost imaging applications), typically spontaneous parametric down conversion (SPDC) is used. This technique requires high-repetition rate laser pulses (for
ultrafast communication schemes) and femtoseconds pulse durations (for high pulse
peak powers) to make the low-probability conversion effect as efficient and frequent
as possible, linking first-generation and second-generation quantum technologies.
Recently, also other sources of entangled photons based on quantum dots have been
investigated (see for instance [42]).
4.2 Coherent Light Sources
The development of semiconductor lasers
1 heavily relied on concepts of the early
quantum mechanics world and fundamental theory of light–matter interactions.
Among the members of the laser family, such as solid-state lasers (see for instance
[8]), semiconductor lasers (e.g. [43, 44]) are widely used in our modern world for
various applications and in different frequency ranges. They became increasingly
reliable, mass-producible and, therefore, cheap owing to the well-matured field of
semiconductor technology.
Coherence in Advantageous Light
Another reason to pay attention to coherent light sources is motivated by the very
nature of their light output: A directional beam of phase-coherent, polarised and
typically monochromatic light can be conveniently and precisely used to deliver high
intensities, ultrashort pulses or spectrally well-defined irradiances to experiments and
applications. Moreover, the coherent state of light is a very prominent example of
a light state, which represents the vacuum state displaced by a coherent amplitude
(certain mean number of photons).
2 The photon statistics for an ideal coherent state is
totally random
3 and centred around the expectation value of the photon number (N ,
1 Laser is the acronym for Light Amplification by Stimulated Emission of Radiation.
2 See vacuum state |0 displaced by the displacement operator D(α) representing the coherent state
|α = D(α) |0, e.g., in [5].
3 The photon number of the coherent state is Poisson distributed. Correspondingly, the second-order
temporal autocorrelation function is g (2) (0) = 1.