1,051 research outputs found
Towards Quantum Repeaters with Solid-State Qubits: Spin-Photon Entanglement Generation using Self-Assembled Quantum Dots
In this chapter we review the use of spins in optically-active InAs quantum
dots as the key physical building block for constructing a quantum repeater,
with a particular focus on recent results demonstrating entanglement between a
quantum memory (electron spin qubit) and a flying qubit (polarization- or
frequency-encoded photonic qubit). This is a first step towards demonstrating
entanglement between distant quantum memories (realized with quantum dots),
which in turn is a milestone in the roadmap for building a functional quantum
repeater. We also place this experimental work in context by providing an
overview of quantum repeaters, their potential uses, and the challenges in
implementing them.Comment: 51 pages. Expanded version of a chapter to appear in "Engineering the
Atom-Photon Interaction" (Springer-Verlag, 2015; eds. A. Predojevic and M. W.
Mitchell
Unforgeable Noise-Tolerant Quantum Tokens
The realization of devices which harness the laws of quantum mechanics
represents an exciting challenge at the interface of modern technology and
fundamental science. An exemplary paragon of the power of such quantum
primitives is the concept of "quantum money". A dishonest holder of a quantum
bank-note will invariably fail in any forging attempts; indeed, under
assumptions of ideal measurements and decoherence-free memories such security
is guaranteed by the no-cloning theorem. In any practical situation, however,
noise, decoherence and operational imperfections abound. Thus, the development
of secure "quantum money"-type primitives capable of tolerating realistic
infidelities is of both practical and fundamental importance. Here, we propose
a novel class of such protocols and demonstrate their tolerance to noise;
moreover, we prove their rigorous security by determining tight fidelity
thresholds. Our proposed protocols require only the ability to prepare, store
and measure single qubit quantum memories, making their experimental
realization accessible with current technologies.Comment: 18 pages, 5 figure
Quantum Cryptography Beyond Quantum Key Distribution
Quantum cryptography is the art and science of exploiting quantum mechanical
effects in order to perform cryptographic tasks. While the most well-known
example of this discipline is quantum key distribution (QKD), there exist many
other applications such as quantum money, randomness generation, secure two-
and multi-party computation and delegated quantum computation. Quantum
cryptography also studies the limitations and challenges resulting from quantum
adversaries---including the impossibility of quantum bit commitment, the
difficulty of quantum rewinding and the definition of quantum security models
for classical primitives. In this review article, aimed primarily at
cryptographers unfamiliar with the quantum world, we survey the area of
theoretical quantum cryptography, with an emphasis on the constructions and
limitations beyond the realm of QKD.Comment: 45 pages, over 245 reference
Applications of atomic ensembles in distributed quantum computing
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes - stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium - an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step Q-qubit GHZ states with success probability p(success) similar to eta(Q/2), where eta is the combined detection and source efficiency. This is signifcantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer
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