http://spie.org/newsroom/opto-2017-plenary-show-daily
Quantum dots for encryption
photo of Dieter BimbergOPTO plenary speaker Dieter Bimberg of Technische University in Berlin described the benefits and potential of quantum dots in a variety of applications - and how they are vital for quantum cryptography and energy-efficient nanophotonics.
Quantum dots can be fabricated via self-organizing processes. For example, indium arsenide dots are grown on and embedded in gallium arsenide. The dots, appearing as pyramids with a diameter of a few nanometers - 1010 times smaller than those in Egypt - act like individual atoms, with completely quantized energy levels. This allows them to emit light at discrete wavelengths. By embedding quantum dots in a waveguide, for example, you can create a nanophotonic device, like a laser or amplifier.
A single quantum dot, Bimberg explained, can have important uses in quantum cryptography and communication. Within a quantum dot, a hole and an electron, bound together as a quasiparticle called an exciton, can recombine and emit one or at most two polarized photons. One photon can serve as a qubit for sending encrypted signals; two are useful for entanglement.
Unlike in classical encryption, quantum encryption enables the sender and receiver to know immediately if an interloper has broken the coded signal. Quantum dot technology, Bimberg said, is also relatively simple and inexpensive, since it is based on classical semiconductor technology. A single qubit emitter is just a LED with one single quantum dot inside.
A single quantum dot behaves like an individual atom. But in a large assembly of them - say, several million in a semiconductor device - their discrete properties are hidden. But that's what makes them advantageous for creating or transmitting optical signals through communication networks.
Not every quantum dot is identical, so a big collection of them will have a range of sizes and shapes, which means they emit light with a Gaussian distribution of wavelengths. A laser based on quantum dots exploits this broad emission. The broader the emission, the faster the laser can fire pulses - pulses narrower than one picosecond, Bimberg said.
Because quantum dots have both a ground state and an excited state, quantum-dot lasers can generate pulses at different wavelengths. Lastly, such lasers allow for quantum techniques to suppress the slight fluctuations in arrival times of signals called jitter, down to as little as 200 femtoseconds, which would otherwise be very difficult with conventional lasers.
Using quantum-dot technology for other network devices like amplifiers will reduce energy consumption and cost, he said. For instance, devices like an erbium-doped fiber amplifier (EDFA) compensate for the intensity loss of a signal that travels through kilometers of fiber-optic cables. But these amplifiers are complex and expensive and do not operate in the O-band around 1310 nm, which is the range where local and metropolitan area networks operate. Instead, amplifiers based on quantum-dot technology are a cheaper and simpler solution.
Quantum-dot amplifiers have several other advantages. A single device can amplify multiple signals with different wavelengths and does this wavelength division multiplexing without crosstalk between signals. These devices can even change the wavelength of signals, which is sometimes necessary in a network because signals of the same wavelength can interfere with one another.
In general, quantum-dot technology is more energy efficient, which is increasingly important given the rising energy demands of the internet. "We really have to work on energy-efficient devices," Bimberg said. "Quantum-dot-based lasers and amplifiers are absolutely essential."
A quantum-dot laser, for instance, has a threshold current density three to four times lower than that of a conventional quantum well laser, which means it requires much less electricity. Such a device is also more thermally stable, not changing its threshold current density up to 70-80 degrees C, and thus doesn't need an energy-consuming cooling system.
Because a quantum-dot amplifier works for both upstream and downstream signals, a network can cut the number of devices — and thus energy demand — by half. Conventional technology would require a separate amplifier for upstream and downstream signals.
Quantum dots for encryption
photo of Dieter BimbergOPTO plenary speaker Dieter Bimberg of Technische University in Berlin described the benefits and potential of quantum dots in a variety of applications - and how they are vital for quantum cryptography and energy-efficient nanophotonics.
Quantum dots can be fabricated via self-organizing processes. For example, indium arsenide dots are grown on and embedded in gallium arsenide. The dots, appearing as pyramids with a diameter of a few nanometers - 1010 times smaller than those in Egypt - act like individual atoms, with completely quantized energy levels. This allows them to emit light at discrete wavelengths. By embedding quantum dots in a waveguide, for example, you can create a nanophotonic device, like a laser or amplifier.
A single quantum dot, Bimberg explained, can have important uses in quantum cryptography and communication. Within a quantum dot, a hole and an electron, bound together as a quasiparticle called an exciton, can recombine and emit one or at most two polarized photons. One photon can serve as a qubit for sending encrypted signals; two are useful for entanglement.
Unlike in classical encryption, quantum encryption enables the sender and receiver to know immediately if an interloper has broken the coded signal. Quantum dot technology, Bimberg said, is also relatively simple and inexpensive, since it is based on classical semiconductor technology. A single qubit emitter is just a LED with one single quantum dot inside.
A single quantum dot behaves like an individual atom. But in a large assembly of them - say, several million in a semiconductor device - their discrete properties are hidden. But that's what makes them advantageous for creating or transmitting optical signals through communication networks.
Not every quantum dot is identical, so a big collection of them will have a range of sizes and shapes, which means they emit light with a Gaussian distribution of wavelengths. A laser based on quantum dots exploits this broad emission. The broader the emission, the faster the laser can fire pulses - pulses narrower than one picosecond, Bimberg said.
Because quantum dots have both a ground state and an excited state, quantum-dot lasers can generate pulses at different wavelengths. Lastly, such lasers allow for quantum techniques to suppress the slight fluctuations in arrival times of signals called jitter, down to as little as 200 femtoseconds, which would otherwise be very difficult with conventional lasers.
Using quantum-dot technology for other network devices like amplifiers will reduce energy consumption and cost, he said. For instance, devices like an erbium-doped fiber amplifier (EDFA) compensate for the intensity loss of a signal that travels through kilometers of fiber-optic cables. But these amplifiers are complex and expensive and do not operate in the O-band around 1310 nm, which is the range where local and metropolitan area networks operate. Instead, amplifiers based on quantum-dot technology are a cheaper and simpler solution.
Quantum-dot amplifiers have several other advantages. A single device can amplify multiple signals with different wavelengths and does this wavelength division multiplexing without crosstalk between signals. These devices can even change the wavelength of signals, which is sometimes necessary in a network because signals of the same wavelength can interfere with one another.
In general, quantum-dot technology is more energy efficient, which is increasingly important given the rising energy demands of the internet. "We really have to work on energy-efficient devices," Bimberg said. "Quantum-dot-based lasers and amplifiers are absolutely essential."
A quantum-dot laser, for instance, has a threshold current density three to four times lower than that of a conventional quantum well laser, which means it requires much less electricity. Such a device is also more thermally stable, not changing its threshold current density up to 70-80 degrees C, and thus doesn't need an energy-consuming cooling system.
Because a quantum-dot amplifier works for both upstream and downstream signals, a network can cut the number of devices — and thus energy demand — by half. Conventional technology would require a separate amplifier for upstream and downstream signals.
