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Quantum Dots For Quantum Information Technologies

This book highlights the most recent developments in quantum dot spin physics and the generation of deterministic superior non-classical light states with quantum dots. In particular, it addresses single quantum dot spin manipulation, spin-photon entanglement and the generation of single-photon and entangled photon pair states with nearly ideal properties. The role of semiconductor microcavities, nanophotonic interfaces as well as quantum photonic integrated circuits is emphasized. The latest theoretical and experimental studies of phonon-dressed light matter interaction, single-dot lasing and resonance fluorescence in QD cavity systems are also provided. The book is written by the leading experts in the field.

Quantum Dots for Quantum Information Technologies


Peter Michler got his Physics Diploma and his PhD degree from the University of Stuttgart in 1990 and 1994, respectively. He worked as post-doc at the Max-Planck Institute for Solid State Research in Stuttgart from 1994 to 1995. From 1995 to 1999, he was a research group leader at the University of Bremen and from 1999 until 2000 he spent a one year research stay at the University of California, Santa Barbara. In 2001, he performed his habilitation at the University of Bremen and he became a professor in 2003 at the University of Stuttgart. Since May 2006, he has headed the Institute for Semiconductor Optics and Functional Interfaces at the University of Stuttgart, concentrating research on quantum dots, quantum optics, non-classical light sources and semiconductor lasers.

"There has been a lot of work to develop this system for quantum information science, but we've been missing an understanding of what the electrons look like in these quantum dots," said corresponding author Jairo Velasco Jr., assistant professor of physics at UC Santa Cruz.

While conventional digital technologies encode information in bits represented as either 0 or 1, a quantum bit, or qubit, can represent both states at the same time due to quantum superposition. In theory, technologies based on qubits will enable a massive increase in computing speed and capacity for certain types of calculations.

A variety of systems, based on materials ranging from diamond to gallium arsenide, are being explored as platforms for creating and manipulating qubits. Bilayer graphene (two layers of graphene, which is a two-dimensional arrangement of carbon atoms in a honeycomb lattice) is an attractive material because it is easy to produce and work with, and quantum dots in bilayer graphene have desirable properties.

"These quantum dots are an emergent and promising platform for quantum information technology because of their suppressed spin decoherence, controllable quantum degrees of freedom, and tunability with external control voltages," Velasco said.

Understanding the nature of the quantum dot wave function in bilayer graphene is important because this basic property determines several relevant features for quantum information processing, such as the electron energy spectrum, the interactions between electrons, and the coupling of electrons to their environment.

Velasco's team used a method he had developed previously to create quantum dots in monolayer graphene using a scanning tunneling microscope (STM). With the graphene resting on an insulating hexagonal boron nitride crystal, a large voltage applied with the STM tip creates charges in the boron nitride that serve to electrostatically confine electrons in the bilayer graphene.

"We see circularly symmetric rings in monolayer graphene, but in bilayer graphene the quantum dot states have a three-fold symmetry," Velasco said. "The peaks represent sites of high amplitude in the wave function. Electrons have a dual wave-particle nature, and we are visualizing the wave properties of the electron in the quantum dot."

This work provides crucial information, such as the energy spectrum of the electrons, needed to develop quantum devices based on this system. "It is advancing the fundamental understanding of the system and its potential for quantum information technologies," Velasco said. "It's a missing piece of the puzzle, and taken together with the work of others, I think we're moving toward making this a useful system."

Electrons in graphene (an atomically thin form of carbon) behave as if they were massless, like photons, which are massless particles of light. Although graphene electrons do not move at the speed of light, they exhibit the same energy-momentum relationship as photons and can be described as "ultra-relativistic." When these electrons are confined in a quantum dot, they travel at high velocity in circular loops around the edge of the dot.

Velasco is a corresponding author of a paper on the new findings, published March 6 in Nature Nanotechnology. His group at UC Santa Cruz used a scanning tunneling microscope (STM) to create the quantum dots in graphene and study their properties. His collaborators on the project include scientists at the University of Manchester, U.K., and the National Institute for Materials Science in Japan.

The unique optical and electrical properties of quantum dots -- which are often made of semiconductor nanocrystals -- are due to electrons being confined within a nanoscale structure such that their behavior is governed by quantum mechanics. Because the resulting electronic structure is like that of atoms, quantum dots are often called "artificial atoms." Velasco's approach creates quantum dots in different forms of graphene using an electrostatic "corral" to confine graphene's speeding electrons.

"Part of what makes this interesting is the fundamental physics of this system and the opportunity to study atomic physics in the ultra-relativistic regime," he said. "At the same time, there are interesting potential applications for this as a new type of quantum sensor that can detect magnetic fields at the nano scale with high spatial resolution."

Additional applications are also possible, according to co-first author Zhehao Ge, a UCSC graduate student in physics. "The findings in our work also indicate that graphene quantum dots can potentially host a giant persistent current (a perpetual electric current without the need of an external power source) in a small magnetic field," Ge said. "Such current can potentially be used for quantum simulation and quantum computation."

The study looked at quantum dots in both monolayer graphene and twisted bilayer graphene. The graphene rests on an insulating layer of hexagonal boron nitride, and a voltage applied with the STM tip creates charges in the boron nitride that serve to electrostatically confine electrons in the graphene.

Although Velasco's lab uses STM to create and study graphene quantum dots, a simpler system using metal electrodes in a cross-bar array could be used in a magnetic sensor device. Because graphene is highly flexible, the sensor could be integrated with flexible substrates to enable magnetic field sensing of curved objects.

"You could have many quantum dots in an array, and this could be used to measure magnetic fields in living organisms, or to understand how the magnetic field is distributed in a material or a device," Velasco said.

In the global quest to develop practical computing and communications devices based on the principles of quantum physics, one potentially useful component has proved elusive: a source of individual particles of light with perfectly constant, predictable, and steady characteristics. Now, researchers at MIT and in Switzerland say they have made major steps toward such a single photon source.

The study, which involves using a family of materials known as perovskites to make light-emitting particles called quantum dots, appears today in the journal Science. The paper is by MIT graduate student in chemistry Hendrik Utzat, professor of chemistry Moungi Bawendi, and nine others at MIT and at ETH in Zurich, Switzerland.

The ability to produce individual photons with precisely known and persistent properties, including a wavelength, or color, that does not fluctuate at all, could be useful for many kinds of proposed quantum devices. Because each photon would be indistinguishable from the others in terms of its quantum-mechanical properties, it could be possible, for example, to delay one of them and then get the pair to interact with each other, in a phenomenon called interference.

Previous chemically-made colloidal quantum dot materials had impractically short coherence times, but this team found that making the quantum dots from perovskites, a family of materials defined by their crystal structure, produced coherence levels that were more than a thousand times better than previous versions. The coherence properties of these colloidal perovskite quantum dots are now approaching the levels of established emitters, such as atom-like defects in diamond or quantum dots grown by physicists using gas-phase beam epitaxy.

One of the big advantages of perovskites, they found, was that they emit photons very quickly after being stimulated by a laser beam. This high speed could be a crucial characteristic for potential quantum computing applications. They also have very little interaction with their surroundings, greatly improving their coherence properties and stability.

Such coherent photons could also be used for quantum-encrypted communications applications, Bawendi says. A particular kind of entanglement, called polarization entanglement, can be the basis for secure quantum communications that defies attempts at interception.

The quantum devices in which quantum bits are stored and processed will form the lowest layer of a complex multi-layer system.1,2,3 The system also includes classical electronics to measure and control the qubits, and a conventional computer to control and program these electronics. Increasingly, some of the important challenges involved in these intermediate layers and how they interact have become clear, and there is a strong need for forming a picture of how these challenges can be addressed. 041b061a72


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