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4 In the Field of Quantum Technologies
Quantum Technology Basics
These quantum technologies of the second generation were preceded by quantum
technologies of the first generation, such as semiconductor light source technologies
(i.e. LEDs and lasers) or micro-/nano-electronics (i.e. circuitry with classical gate
structures), which have remained attractive due to their maturity and good integrability in existing technologies and have been subject of further development. Numerous
textbooks discuss fundamentals and advances in this important field, such as [2, 6–9],
and their underlying theoretical framework [10] and semiconductor optics [11–14].
Quantum Computing on the Verge
Likely, quantum computing is one of the most prominent subjects, as the news
coverage on the latest achievements by major information-technology companies
worldwide shows, such as on quantum supremacy [15]. So far, different complex
techniques are at play, such as those employing superconducting qubits (such as in
[16]). Trapping of ions for quantum control and logic operations is also a possible
path, while the use of photons and solid-state quantum/photonic structures becomes
increasingly popular [17–21].
Quantum Light for Quantum Applications
In recent years, major leaps have been evidenced in the field of quantum light source
development [22–26], which is important for future optical quantum computation
[20, 21] and communication schemes. This has set free a quantum space race in
which labs around the world try to bridge farther and farther distances using quantum
teleportation for photons (see for instance the news feature from 2012, [27]). A record
distance for ‘spooky action’ was for instance in recent years achieved via satellitebased transmission over more than thousand kilometers [28].
Entanglement for Communications
Typically, heralded photons from entangled photon pairs or nonclassical states of
light are utilised to enable secure quantum channels for quantum key distribution.
Future long-haul transmission schemes may rely on devices sending out polarisationentangled photons (see for instance [29]) bidirectionally from a quantum repeater
station, whereas optical microcavity devices with embedded quantum emitters at the
ends of the transmission line provide indistinguishable single photons on demand,
or may be used as quantum storage. Also, microcavities have been proposed for
memory-based quantum repeaters [30]. Moreover, quantum measurements (Bell state
measurements) could swap the entanglement between individual photons from different quantum repeaters. Thereby, two parties (famously named) Alice and Bob can
exchange flying quantum bits (qubits) with the chance to discover any eavesdropping
event (by person Eve) via the evaluation of the error rate—as the no-cloning theorem
renders the mere copying and replacing of photons in the quantum channel technically impossible, apart from measuring the quantum state of the photon without
manipulating it.
Advances in Quantum Light Generation
The demand for the fundamental equipment for such schemes is rapidly growing.
However, most commercial products are based on conventional heavily attenuated
4 In the Field of Quantum Technologies
Quantum Technology Basics
These quantum technologies of the second generation were preceded by quantum
technologies of the first generation, such as semiconductor light source technologies
(i.e. LEDs and lasers) or micro-/nano-electronics (i.e. circuitry with classical gate
structures), which have remained attractive due to their maturity and good integrability in existing technologies and have been subject of further development. Numerous
textbooks discuss fundamentals and advances in this important field, such as [2, 6–9],
and their underlying theoretical framework [10] and semiconductor optics [11–14].
Quantum Computing on the Verge
Likely, quantum computing is one of the most prominent subjects, as the news
coverage on the latest achievements by major information-technology companies
worldwide shows, such as on quantum supremacy [15]. So far, different complex
techniques are at play, such as those employing superconducting qubits (such as in
[16]). Trapping of ions for quantum control and logic operations is also a possible
path, while the use of photons and solid-state quantum/photonic structures becomes
increasingly popular [17–21].
Quantum Light for Quantum Applications
In recent years, major leaps have been evidenced in the field of quantum light source
development [22–26], which is important for future optical quantum computation
[20, 21] and communication schemes. This has set free a quantum space race in
which labs around the world try to bridge farther and farther distances using quantum
teleportation for photons (see for instance the news feature from 2012, [27]). A record
distance for ‘spooky action’ was for instance in recent years achieved via satellitebased transmission over more than thousand kilometers [28].
Entanglement for Communications
Typically, heralded photons from entangled photon pairs or nonclassical states of
light are utilised to enable secure quantum channels for quantum key distribution.
Future long-haul transmission schemes may rely on devices sending out polarisationentangled photons (see for instance [29]) bidirectionally from a quantum repeater
station, whereas optical microcavity devices with embedded quantum emitters at the
ends of the transmission line provide indistinguishable single photons on demand,
or may be used as quantum storage. Also, microcavities have been proposed for
memory-based quantum repeaters [30]. Moreover, quantum measurements (Bell state
measurements) could swap the entanglement between individual photons from different quantum repeaters. Thereby, two parties (famously named) Alice and Bob can
exchange flying quantum bits (qubits) with the chance to discover any eavesdropping
event (by person Eve) via the evaluation of the error rate—as the no-cloning theorem
renders the mere copying and replacing of photons in the quantum channel technically impossible, apart from measuring the quantum state of the photon without
manipulating it.
Advances in Quantum Light Generation
The demand for the fundamental equipment for such schemes is rapidly growing.
However, most commercial products are based on conventional heavily attenuated