Toward measurement-based quantum computing using solid state spins (Proceedings Paper) (original) (raw)

Quantum register based on coupled electron spins in a room-temperature solid

Nature Physics, 2010

Devices that harness the laws of quantum physics hold the promise for information processing that outperforms their classical counterparts, and for unconditionally secure communication 1 . However, in particular, implementations based on condensed-matter systems face the challenge of short coherence times. Carbon materials 2,3 , particularly diamond 4-6 , however, are suitable for hosting robust solid-state quantum registers, owing to their spin-free lattice and weak spin-orbit coupling. Here we show that quantum logic elements can be realized by exploring long-range magnetic dipolar coupling between individually addressable single electron spins associated with separate colour centres in diamond. The strong distance dependence of this coupling was used to characterize the separation of single qubits (98±3 Å) with an accuracy close to the value of the crystal-lattice spacing. Our demonstration of coherent control over both electron spins, conditional dynamics, selective readout as well as switchable interaction should open the way towards a viable room-temperature solid-state quantum register. As both electron spins are optically addressable, this solid-state quantum device operating at ambient conditions provides a degree of control that is at present available only for a few systems at low temperature.

Spin qubits in solid-state structures

2002

It is remarkable that today's computers, after the tremendous development during the last 50 years, are still essentially described by the mathematical model formulated by Alan Turing in the 1930's. Turing's model describes computers which operate according to the laws of classical physics. What would happen if a computer was operating according to the quantum laws? Physicists and computer scientists have been interested in this question since the early 1980's, but research in quantum computation really started to flourish after 1994 when Peter Shor discovered a quantum algorithm to find prime factors of large integers efficiently, a problem which is intrinsically hard for any classical computer (see for an introduction into quantum computation). The lack of an algorithm for efficient factoring on a classical machine is actually the basis of the widely used RSA encryption scheme. Phase coherence needs to be maintained for a sufficiently long time in the memory of a quantum computer. This may sound like a harmless requirement, but in fact it is the main reason why the physical implementation of quantum computation is so difficult. Usually, a quantum memory is thought of as a set of two-level systems, named quantum bits, or qubits for short. In analogy to the classical bit, two orthogonal computational basis states |0 and |1 are defined. The textbook example of a quantum two-level system is the spin 1/2 of, say, an electron, where one can identify the "spin up" state with |0 and the "spin down" state with |1 . While several other two-level systems have been proposed for quantum computing, we will devote the majority of our discussion to the potential use of electron spins in nanostructures (such as quantum dots) as qubits.

Measurements of spin-coherence in NV centers for diamond-based quantum sensors

2021 SBFoton International Optics and Photonics Conference (SBFoton IOPC)

One of the biggest challenges to implement quantum protocols and quantum information processing (QIP) is achieving long coherence times, usually requiring systems at ultra-low temperatures. The nitrogen-vacancy (NV) center in diamond is a promising alternative to this problem. Due to its spin properties, easy manipulation, and the possibility of doing optical state initialization and readout, it quickly became one of the best solid-state spin systems for QIP at room temperature. Here*, we present the characterization of the spin-coherence of an ensemble of NV centers in an engineered sample of ultrapure diamond as a testbed for quantum protocols for quantum metrology.

Quantum control and quantum measurement in solid state qubits

2010

In the past two decades significant theoretical and experimental efforts have been devoted to the study and development of mesoscopic devices, that exploit quantum coherence at the nanoscale. Quantum computing represents an emerging promising field of science and technology and is currently subject of extensive investigation. One of the fundamental issues, that still represents a major obstacle to the realization of a quantum computer, is certainly decoherence. The interaction of a quantum system with the external environment is what ultimately limits the efficiency of a quantum device. On the other hand, in order to perform precise tasks and implement quantum algorithms, it is necessary to address the quantum devices from the lab. It is therefore desirable to achieve full control and to minimize the detrimental residual interaction. Control protocols and read out schemes are still performed on a basic level and many aspects of the communication between the quantum systems and the external environment need still to be investigated from a fundamental point of view.

Introduction to quantum dot in quantum information

In principle, quantum computer can be made by any physical system, however there are several properties we want a system own in order to realize an effective quantum computer. For this reason much effort is being putted in the physical implementation of them in the last decades. The list of the properties a system should have, collected under the name of DiVincenzo criteria, is the following • identification of well-defined qubits; • reliable state preparation; • low decoherence; • accurate quantum gate operations and • strong quantum measurements. Examples of systems took into account by scientist are: nuclear spin in liquids, trapped ions and trapped atoms, atoms in optical lattices, photons, superconducting circuits, electron suspended over liquid helium surfaces, molecular magnets, nuclear spins in solid, electron spins in semiconductor quantum dots (QD), hole spins in QD, electron spins in impurity centres in semiconductors (such as, phosphorus donors in silicon and nitrogen-vacancy centres in diamond) and non-Abelian anyon excitations in quantum matters with topological orders. Solid-state systems have the advantage of stability and integrability, however they often have short coherence time due to the interaction with the complex environment. On the other hand, a complication of quantum computing with single atoms in vacuum is the necessity of cooling and trapping them. Large arrays of qubits may be easier to assemble and cool if the atoms are integrated into a solid-state host. In this paper we want to focus on QD, a promising candidate to use in solid-state quan- tum computation. With several entangled QDs, which are directly related to qubits, plus a way of performing operations, quantum calculations and computers might be possible. The qubit carriers is the electron spin. Compared to superconducting qubit where the carrier may be lost during the measurement or the control process, spin is an elementary degree of freedom, which always exists until the particle (the electron) disappear, thus this kind of qubit is very stable.

Quantum information processing using frequency control of impurity spins in diamond

Physical Review B, 2007

Spin degrees of freedom of charged nitrogen-vacancy (NV −) centers in diamond have large decoherence times even at room temperature, can be initialized and read out using optical fields, and are therefore a promising candidate for solid state qubits. Recently, quantum manipulations of NV −-centers using RF fields were experimentally realized. In this paper we show; first, that such operations can be controlled by varying the frequency of the signal, instead of its amplitude, and NV −-centers can be selectively addressed even with spacially uniform RF signals; second, that when several NV −-centers are placed in an off-resonance optical cavity, a similar application of classical optical fields provides a controlled coupling and enables a universal two-qubit gate (CPHASE). RF and optical control together promise a scalable quantum computing architecture.

Measurement, control, and decay of quantum-dot spins

physica status solidi (b), 2006

In this review we discuss a recent proposal to perform partial Bell-state (parity) measurements on twoelectron spin states for electrons confined to quantum dots. The realization of this proposal would allow for a physical implementation of measurement-based quantum computing. In addition, we consider the primary sources of energy relaxation and decoherence which provide the ultimate limit to all proposals for quantum information processing using electron spins in quantum dots. We give an account of the Hamiltonians used for the most important interactions (spin -orbit and hyperfine) and survey some of the recent work done to understand dynamics, control, and decoherence under the action of these Hamiltonians. We conclude the review with a table of important decay times found in experiment, and relate these time scales to the potential viability of measurement-based quantum computing.

Single-Qubit Operations with the Nitrogen-Vacancy Center in Diamond

physica status solidi (b), 2002

A concept combining optics and microwave pulses with the negative charge-state of the nitrogenvacancy (NV-) center in diamond is demonstrated through experiments that are equivalent to single-qubit gates, and decoherence for this qubit is examined. The spin levels of the ground state provide the two-level system. Optical excitation provides polarization of these states. The polarized state is operated coherently by 35 GHz microwave pulses. The final state is read out through the photoluminescence intensity. Decoherence arises from different sources for different samples. For high-pressure, high-temperature synthetic diamonds, the high concentration of substitutional N limits the phase-memory to a few ms. In a single-crystal CVD diamond, the phase memory time is at least 32 ms at 100 K. 14 N is tightly coupled to the electronic spin and produces modulation of the electron-spin echo decay under certain conditions. A two-qubit gate is proposed using this nuclear spin. This demonstration provides a great deal of insight into quantum devices in the solid state with some possibility for real application.

Quantum computers and quantum coherence

Journal of Magnetism and Magnetic Materials, 1999

If the states of spins in solids can be created, manipulated, and measured at the single-quantum level, an entirely new form of information processing, quantum computing, will be possible. We first give an overview of quantum information processing, showing that the famous Shor speedup of integer factoring is just one of a host of important applications for qubits, including cryptography, counterfeit protection, channel capacity enhancement, distributed computing, and others. We review our proposed spin-quantum dot architecture for a quantum computer, and we indicate a variety of first generation materials, optical, and electrical measurements which should be considered. We analyze the efficiency of a two-dot device as a transmitter of quantum information via the propagation of qubit carriers (i.e. electrons) in a Fermi sea.