The Experts below are selected from a list of 5133 Experts worldwide ranked by ideXlab platform
Daniel Loss - One of the best experts on this subject based on the ideXlab platform.
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Molecular Magnets for Quantum Information Processing
NanoScience and Technology, 2020Co-Authors: Kevin A. Van Hoogdalem, Dimitrije Stepanenko, Daniel LossAbstract:In this chapter we will examine the possibility of utilizing molecular magnets for quantum information processing purposes. We start by giving a brief introduction into quantum computing, and highlight the fundamental differences between classical- and quantum computing. We will introduce the five Divincenzo criteria for successful physical implementation of a quantum computer, and will use these criteria as a guideline for the remainder of the chapter. We will discuss how one can utilize the spin degrees of freedom in molecular magnets for quantum computation, and introduce the associated ways of controlling the state of the qubit. In this part we will focus mainly on the spin-electric effect, which makes it possible to control the quantum states of spin in molecular magnets by electric means. We will discuss ways to couple the quantum state of two molecular magnets. Next, we will identify and discuss the different decoherence mechanisms that play a role in molecular magnets. We will show that one of the advantages of using molecular magnets as qubits is that it is possible to use degrees of freedom that are more robust against decoherence than those in more traditional qubits. We briefly discuss preparation and read-out of qubit states. Finally, we discuss a proposal to implement Grover’s algorithm using molecular magnets.
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Quantum phenomena in Nanotechnology
Nanotechnology, 2009Co-Authors: Daniel LossAbstract:Twenty years ago the Institute of Physics launched the journal Nanotechnology from its publishing house based in the home town of Paul Dirac, a legendary figure in the development of quantum mechanics at the turn of the last century. At the beginning of the 20th century, the adoption of quantum mechanical descriptions of events transformed the existing deterministic world view. But in many ways it also revolutionised the progress of research itself. For the first time since the 17th century when Francis Bacon established inductive reasoning as the means of advancing science from fact to axiom to law, theory was progressing ahead of experiments instead of providing explanations for observations that had already been made. Dirac's postulation of antimatter through purely theoretical investigation before its observation is the archetypal example of theory leading the way for experiment. The progress of nanotechnology and the development of tools and techniques that enabled the investigation of systems at the nanoscale brought with them many fascinating observations of phenomena that could only be explained through quantum mechanics, first theoretically deduced decades previously. At the nanoscale, quantum confinement effects dominate the electrical and optical properties of systems. They also render new opportunities for manipulating the response of systems. For example, a better understanding of these systems has enabled the rapid development of quantum dots with precisely determined properties, which can be exploited in a range of applications from medical imaging and photovoltaic solar cells to quantum computation, a radically new information technology being currently developed in many labs worldwide. As the first ever academic journal in nanotechnology, {\it Nanotechnology} has been the forum for papers detailing progress of the science through extremely exciting times. In the early years of the journal, the investigation of electron spin led to the formulation of quantum cellular automata, a new paradigm for computing as reported by Craig S Lent and colleagues (Lent C S, Tougaw P D, Porod W and Bernstein G H 1993 Nanotechnology 4 49–57). The increasingly sophisticated manipulation of spin has been an enduring theme of research throughout this decade, providing a number of interesting developments such as spin pumping (Cota E, Aguado R, Creffield C E and Platero G 2003 Nanotechnology 14 152–6). The idea of spin qubits, proposed by D Loss and D P Divincenzo (1998 Phys. Rev. A 57 120), developed into an established option for advancing research in quantum computing and continues to drive fruitful avenues of research, such as the integrated superconductive magnetic nanosensor recently devised by researchers in Italy (Granata C, Esposito E, Vettoliere A, Petti L and Russo M 2008 Nanotechnology 19 275501). The device has a spin sensitivity in units of the Bohr magneton of 100 spin Hz-1/2 and has large potential for applications in the measurement of nanoscale magnetization and quantum computing. The advance of science and technology at the nanoscale is inextricably enmeshed with advances in our understanding of quantum effects. As Nanotechnology celebrates its 20th volume, research into fundamental quantum phenomena continues to be an active field of research, providing fertile pasture for developing nanotechnologies.
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Handbook of Magnetism and Advanced Magnetic Materials - Quantum Computing with Spins in Solids
Handbook of Magnetism and Advanced Magnetic Materials, 2007Co-Authors: W. A. Coish, Daniel LossAbstract:The ability to perform high-precision one- and two-qubit operations is sufficient for universal quantum computation. For the Loss–Divincenzo proposal to use single-electron spins confined to quantum dots as qubits, it is therefore sufficient to analyze only single- and coupled double-dot structures, since the strong Heisenberg exchange coupling between spins in this proposal falls off exponentially with distance and long-ranged dipolar coupling mechanisms can be made significantly weaker. A scaled-up quantum computer could then be constructed from many single- and double-dot elements. This scalability of the Loss–Divincenzo design is both a practical necessity for eventual applications of multiqubit quantum computing and a great conceptual advantage, making analysis of the relevant components relatively transparent and systematic. We review the Loss–Divincenzo proposal for quantum-dot-confined electron-spin qubits, and survey the current state of experiment and theory regarding the relevant single- and double-quantum dots, with a brief look at some related alternative schemes for quantum computing. Keywords: quantum dots; quantum computing; single dot; double dot; decoherence; entanglement; quantum information processing; Coulomb blockade; stability diagram; encoded qubits; hyperfine interaction; spin-orbit interaction; relaxation; spin echo
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Quantum computing with spins in solids
arXiv: Mesoscale and Nanoscale Physics, 2006Co-Authors: W. A. Coish, Daniel LossAbstract:The ability to perform high-precision one- and two-qubit operations is sufficient for universal quantum computation. For the Loss-Divincenzo proposal to use single electron spins confned to quantum dots as qubits, it is therefore sufficient to analyze only single- and coupled double-dot structures, since the strong Heisenberg exchange coupling between spins in this proposal falls off exponentially with distance and long-ranged dipolar coupling mechanisms can be made significantly weaker. This scalability of the Loss-Divincenzo design is both a practical necessity for eventual applications of multi-qubit quantum computing and a great conceptual advantage, making analysis of the relevant components relatively transparent and systematic. We review the Loss-Divincenzo proposal for quantum-dot-confned electron spin qubits, and survey the current state of experiment and theory regarding the relevant single- and double- quantum dots, with a brief look at some related alternative schemes for quantum computing.
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TUTORIAL: Recipes for spin-based quantum computing
Nanotechnology, 2005Co-Authors: Veronica Cerletti, W. A. Coish, Oliver Gywat, Daniel LossAbstract:Technological growth in the electronics industry has historically been measured by the number of transistors that can be crammed onto a single microchip. Unfortunately, all good things must come to an end; spectacular growth in the number of transistors on a chip requires spectacular reduction of the transistor size. For electrons in semiconductors, the laws of quantum mechanics take over at the nanometre scale, and the conventional wisdom for progress (transistor cramming) must be abandoned. This realization has stimulated extensive research on ways to exploit the spin (in addition to the orbital) degree of freedom of the electron, giving birth to the field of spintronics. Perhaps the most ambitious goal of spintronics is to realize complete control over the quantum mechanical nature of the relevant spins. This prospect has motivated a race to design and build a spintronic device capable of complete control over its quantum mechanical state, and ultimately, performing computations: a quantum computer. In this tutorial we summarize past and very recent developments which point the way to spin-based quantum computing in the solid state. After introducing a set of basic requirements for any quantum computer proposal, we offer a brief summary of some of the many theoretical proposals for solid-state quantum computers. We then focus on the Loss-Divincenzo proposal for quantum computing with the spins of electrons confined to quantum dots. There are many obstacles to building such a quantum device. We address these, and survey recent theoretical, and then experimental progress in the field. To conclude the tutorial, we list some as-yet unrealized experiments, which would be crucial for the development of a quantum dot quantum computer.
Hoi-kwong Lo - One of the best experts on this subject based on the ideXlab platform.
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Proof of unconditional security of six-state quatum key distribution scheme
Quantum Information & Computation, 2001Co-Authors: Hoi-kwong LoAbstract:We prove the unconditional security of the standard six-state scheme for quantum key distribution (QKD). We demonstrate its unconditional security up to a bit error rate of 12.7 percents, by allowing only one-way classical communications in the error correction/privacy amplification procedure between Alice and Bob. This shows a clear advantage of the six-state scheme over another standard scheme--BB84, which has been proven to be secure up to only about 11 percents, if only one-way classical communications are allowed. Our proof technique is a generalization of that of Shor-Preskill's proof of security of BB84. We show that a advantage of the six-state scheme lies in the Alice and Bob's ability to establish rigorously from their test sample the non-trivial mutual information between the bit-flip and phase error patterns. A modified version of the degenerate quantum codes studied by Divincenzo, Shor and Smolin is employed in our proof.
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Proof of unconditional security of six-state quantum key distribution scheme
arXiv: Quantum Physics, 2001Co-Authors: Hoi-kwong LoAbstract:We prove the unconditional security of the standard six-state scheme for quantum key distribution (QKD). We demonstrate its unconditional security up to a bit error rate of 12.7 percents, by allowing only one-way classical communications in the error correction/privacy amplification procedure between Alice and Bob. This shows a clear advantage of the six-state scheme over another standard scheme---BB84, which has been proven to be secure up to only about 11 percents, if only one-way classical communications are allowed. Our proof technique is a generalization of that of Shor-Preskill's proof of security of BB84. We show that a advantage of the six-state scheme lies in the Alice and Bob's ability to establish rigorously from their test sample the non-trivial mutual information between the bit-flip and phase error patterns. A modified version of the degenerate quantum codes studied by Divincenzo, Shor and Smolin is employed in our proof.
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A quantum analog of Huffman coding
IEEE Transactions on Information Theory, 2000Co-Authors: Samuel L. Braunstein, Christopher A. Fuchs, Daniel Gottesman, Hoi-kwong LoAbstract:We analyze a generalization of Huffman coding to the quantum case. In particular, we notice various difficulties in using instantaneous codes for quantum communication. Nevertheless, for the storage of quantum information, we have succeeded in constructing a Huffman coding inspired quantum scheme. The number of computational steps in the encoding and decoding processes of N quantum signals can be made to be of polylogarithmic depth by a massively parallel implementation of a quantum gate array. This is to be compared with the O(N/sup 3/) computational steps required in the sequential implementation by Cleve and Divincenzo (see Phys. Rev., vol.A54, p.2636, 1996) of the well-known quantum noiseless block-coding scheme of Schumacher. We also show that O(N/sup 2/(log N)/sup a/) sequential computational steps are needed for the communication of quantum information using another Huffman coding inspired scheme where the sender must disentangle her encoding device before the receiver can perform any measurements on his signals.
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A quantum analog of Huffman coding
arXiv: Quantum Physics, 1998Co-Authors: Samuel L. Braunstein, Christopher A. Fuchs, Daniel Gottesman, Hoi-kwong LoAbstract:We analyze a generalization of Huffman coding to the quantum case. In particular, we notice various difficulties in using instantaneous codes for quantum communication. Nevertheless, for the storage of quantum information, we have succeeded in constructing a Huffman-coding inspired quantum scheme. The number of computational steps in the encoding and decoding processes of N quantum signals can be made to be of polylogarithmic depth by a massively parallel implementation of a quantum gate array. This is to be compared with the O (N^3) computational steps required in the sequential implementation by Cleve and Divincenzo of the well-known quantum noiseless block coding scheme of Schumacher. We also show that O(N^2(log N)^a) computational steps are needed for the communication of quantum information using another Huffman-coding inspired scheme where the sender must disentangle her encoding device before the receiver can perform any measurements on his signals.
Andreas Winter - One of the best experts on this subject based on the ideXlab platform.
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On the Chernoff Distance for Asymptotic LOCC Discrimination of Bipartite Quantum States
Communications in Mathematical Physics, 2008Co-Authors: William Matthews, Andreas WinterAbstract:Motivated by the recent discovery of a quantum Chernoff theorem for asymptotic state discrimination, we investigate the distinguishability of two bipartite mixed states under the constraint of local operations and classical communication (LOCC), in the limit of many copies. While for two pure states a result of Walgate et al. shows that LOCC is just as powerful as global measurements, data hiding states (Divincenzo et al.) show that locality can impose severe restrictions on the distinguishability of even orthogonal states. Here we determine the optimal error probability and measurement to discriminate many copies of particular data hiding states (extremal d × d Werner states) by a linear programming approach. Surprisingly, the single-copy optimal measurement remains optimal for n copies, in the sense that the best strategy is measuring each copy separately, followed by a simple classical decision rule. We also put a lower bound on the bias with which states can be distinguished by separable operations.
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On the Chernoff distance for asymptotic LOCC discrimination of bipartite quantum states
2008 IEEE Information Theory Workshop, 2008Co-Authors: William Matthews, Andreas WinterAbstract:Motivated by the recent discovery of a quantum Chernoff theorem for asymptotic state discrimination, we investigate the distinguishability of two bipartite mixed states under the constraint of local operations and classical communication (LOCC), in the limit of many copies. While for two pure states a result of Walgate et al. shows that LOCC is just as powerful as global measurements, data hiding states (Divincenzo et al.) show that locality can impose severe restrictions on the distinguishability of even orthogonal states. Here we determine the optimal error probability and measurement to discriminate many copies of particular data hiding states (extremal d times d Werner states) by a linear programming approach. Surprisingly, the single-copy optimal measurement remains optimal for n copies, in the sense that the best strategy is measuring each copy separately, followed by a simple classical decision rule. We also put a lower bound on the bias with which states can be distinguished by separable operations. This is a shortened version of a paper [1] recently submitted to Communications in Mathematical Physics; here the proofs have been omitted.
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ITW - On the Chernoff distance for asymptotic LOCC discrimination of bipartite quantum states
2008 IEEE Information Theory Workshop, 2008Co-Authors: William Matthews, Andreas WinterAbstract:Motivated by the recent discovery of a quantum Chernoff theorem for asymptotic state discrimination, we investigate the distinguishability of two bipartite mixed states under the constraint of local operations and classical communication (LOCC), in the limit of many copies. While for two pure states a result of Walgate et al. shows that LOCC is just as powerful as global measurements, data hiding states (Divincenzo et al.) show that locality can impose severe restrictions on the distinguishability of even orthogonal states. Here we determine the optimal error probability and measurement to discriminate many copies of particular data hiding states (extremal d times d Werner states) by a linear programming approach. Surprisingly, the single-copy optimal measurement remains optimal for n copies, in the sense that the best strategy is measuring each copy separately, followed by a simple classical decision rule. We also put a lower bound on the bias with which states can be distinguished by separable operations. This is a shortened version of a paper [1] recently submitted to Communications in Mathematical Physics; here the proofs have been omitted.
Mikhail D. Lukin - One of the best experts on this subject based on the ideXlab platform.
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Fault-tolerant architecture for quantum computation using electrically controlled semiconductor spins
Nature Physics, 2005Co-Authors: J M Taylor, Hans-andreas Engel, A. Yacoby, W. Dür, C M Marcus, Peter Zoller, Mikhail D. LukinAbstract:Information processing using quantum systems provides new paradigms for computation and communication and may yield insights into our understanding of the limits of quantum mechanics. However, realistic systems are never perfectly isolated from their environment, hence all quantum operations are subject to errors. Realization of a physical system for processing of quantum information that is tolerant of errors is a fundamental problem in quantum science and engineering. Here, we develop an architecture for quantum computation using electrically controlled semiconductor spins by extending the Loss–Divincenzo scheme and by combining actively protected quantum memory and long-distance coupling mechanisms. Our approach is based on a demonstrated encoding of qubits in long-lived two-electron states, which immunizes qubits against the dominant error from hyperfine interactions. We develop a universal set of quantum gates compatible with active error suppression for these encoded qubits and an effective long-range interaction between the qubits by controlled electron transport. This approach yields a scalable architecture with favourable error thresholds for fault-tolerant operation, consistent with present experimental parameters.
J. Ignacio Cirac - One of the best experts on this subject based on the ideXlab platform.
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Quantum computation and quantum-state engineering driven by dissipation
Nature Physics, 2009Co-Authors: Frank Verstraete, Michael M. Wolf, J. Ignacio CiracAbstract:We investigate the computational power of creating steady-states of quantum dissipative systems whose evolution is governed by time-independent and local couplings to a memoryless environment. We show that such a model allows for efficient universal quantum computation with the result of the computation encoded in the steady state. Due to the purely dissipative nature of the process, this way of doing quantum computation exhibits some inherent robustness and defies some of the Divincenzo criteria for quantum computation. We show that there is a natural class of problems that can be solved with such a model - the preparation of ground states of frustration free quantum Hamiltonians. This allows for robust and efficient creation of exotic states that exhibit features like topological quantum order and the creation of PEPS and it proves the existence of novel dissipative phase transitions. In particular the latter can in principle be verified experimentally with present day technology such as with optical lattices.