112
4 In the Field of Quantum Technologies
coupling regime, showed sub-Poissonian photon statistics [162]. Interesting experiments with quantum-dot micropillar systems had furthermore addressed photon–
photon coupling mediated by the exciton spin state [163] or the climbing of the
Jaynes–Cummings ladder [164], to name but a few examples in this section.
4.3.1 Tailored Light–Matter Interactions for Quantum Light
Generation
In the past decades, quantum-dot-based photonic structures have been widely utilised
for the development of quantum-light sources, which are particularly characterised
by their nonclassical light output.
11 To achieve the Purcell effect (resulting in directional output, i.e. a 1D quantum emitter, and a fast and highly likely recombination),
(various) optical microcavities have for many years remained highly attractive for
quantum emitter studies [26, 165].
Two-Photon N00N States for Sensing
Moreover, using quantum dots for photonic quantum technologies enabled the development of real-world quantum sensors. For instance, a super-resolving phase measurement based on two-photon N00N states generated by quantum-dot single-photon
sources was recently demonstrated and first efforts to perform on-chip quantum sensing were indicated [166]. Remarkably, the classical barrier for the achievable resolution phase ΔΦ = 1/
√
N in interferometric measurements of a phase Φ can be
undercut by entangled light conditions, whereas N is the number of photons used
in a measurement. By pushing the boundaries to the fundamental limit ΔΦ = 1/N ,
referred to as Heisenberg limit, multi-photon entangled states such as N00N states
could become very useful for quantum-enhanced phase determination.
Exploiting Nonclassical Light
The development of nonclassical light sources has remained crucial for photonic
quantum information technologies, among others for secure communication schemes.
While single-photon emitters were generally highly attractive for these purposes,
entangled photons and squeezed light gain more and more importance with a growing demand for applications in optical quantum metrology and sensing, such as ghost
imaging (see use in quantum radars, similarly LIDARs), noise-reduced interferometry (for instance for improved gravitational wave detection), or in quantum cryptography, for which long-haul transmission of optical qubits with so-called quantum
repeaters have been targeted.
11 Single-photon sources exhibit light output being in a Fock state with photon number N = 1,
which can be conveniently probed by the second-order temporal autocorrelation function g (2) , which
features a zero probability at (delay) time zero (g (2) (τ = 0)) of obtaining more than one photon
per pulse/time period. This is typically characterised by the famous antibunching phenomenon,
which accompanies systems that exhibit a sub-Poissonian probability distribution, but it doesn’t
necessarily mean the same (see [5]).
4 In the Field of Quantum Technologies
coupling regime, showed sub-Poissonian photon statistics [162]. Interesting experiments with quantum-dot micropillar systems had furthermore addressed photon–
photon coupling mediated by the exciton spin state [163] or the climbing of the
Jaynes–Cummings ladder [164], to name but a few examples in this section.
4.3.1 Tailored Light–Matter Interactions for Quantum Light
Generation
In the past decades, quantum-dot-based photonic structures have been widely utilised
for the development of quantum-light sources, which are particularly characterised
by their nonclassical light output.
11 To achieve the Purcell effect (resulting in directional output, i.e. a 1D quantum emitter, and a fast and highly likely recombination),
(various) optical microcavities have for many years remained highly attractive for
quantum emitter studies [26, 165].
Two-Photon N00N States for Sensing
Moreover, using quantum dots for photonic quantum technologies enabled the development of real-world quantum sensors. For instance, a super-resolving phase measurement based on two-photon N00N states generated by quantum-dot single-photon
sources was recently demonstrated and first efforts to perform on-chip quantum sensing were indicated [166]. Remarkably, the classical barrier for the achievable resolution phase ΔΦ = 1/
√
N in interferometric measurements of a phase Φ can be
undercut by entangled light conditions, whereas N is the number of photons used
in a measurement. By pushing the boundaries to the fundamental limit ΔΦ = 1/N ,
referred to as Heisenberg limit, multi-photon entangled states such as N00N states
could become very useful for quantum-enhanced phase determination.
Exploiting Nonclassical Light
The development of nonclassical light sources has remained crucial for photonic
quantum information technologies, among others for secure communication schemes.
While single-photon emitters were generally highly attractive for these purposes,
entangled photons and squeezed light gain more and more importance with a growing demand for applications in optical quantum metrology and sensing, such as ghost
imaging (see use in quantum radars, similarly LIDARs), noise-reduced interferometry (for instance for improved gravitational wave detection), or in quantum cryptography, for which long-haul transmission of optical qubits with so-called quantum
repeaters have been targeted.
11 Single-photon sources exhibit light output being in a Fock state with photon number N = 1,
which can be conveniently probed by the second-order temporal autocorrelation function g (2) , which
features a zero probability at (delay) time zero (g (2) (τ = 0)) of obtaining more than one photon
per pulse/time period. This is typically characterised by the famous antibunching phenomenon,
which accompanies systems that exhibit a sub-Poissonian probability distribution, but it doesn’t
necessarily mean the same (see [5]).