Mark S. Everitt

Coherent coupling of a superconducting flux qubit to an electron spin ensemble in diamond

During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress. Many foundational experiments have been performed, and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are likely to be insufficient for future large-scale quantum computation. A natural solution to this problem is a dedicated engineered quantum memory based on atomic and molecular systems. The question of whether coherent quantum coupling is possible between such natural systems and a single macroscopic artificial atom has attracted considerable attention since the first demonstration of macroscopic quantum coherence in Josephson junction circuits. Here we report evidence of coherent strong coupling between a single macroscopic superconducting artificial atom (a flux qubit) and an ensemble of electron spins in the form of nitrogen–vacancy colour centres in diamond. Furthermore, we have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3 × 107 such colour centres. This provides a foundation for future quantum memories and hybrid devices coupling microwave and optical systems.

Photonic Architecture for Scalable Quantum Information Processing in Diamond

Building a quantum computer has long been thought to require futuristic technologies. New calculations reveal that physical qubits can be assembled that are scalable and function at the readily accessible temperature of 4 K.

Creating and observing N-partite entanglement with atoms

The Mermin inequality provides a criterion for experimentally ruling out local-realistic descriptions of multiparticle systems. A violation of this inequality means that the particles must be entangled, but does not, in general, indicate whether N-partite entanglement is present. For this, a stricter bound is required. Here we discuss this bound and use it to propose two different schemes for demonstrating N-partite entanglement with atoms. The first scheme involves Bose–Einstein condensates trapped in an optical lattice and the second uses Rydberg atoms in microwave cavities.

Multiphoton resonances for all-optical quantum logic with multiple cavities

We develop a theory for the interaction of multilevel atoms with multimode cavities yielding cavity-enhanced multiphoton resonances. The locations of the resonances are predicted from the use of effective two- and three-level Hamiltonians. As an application we show that quantum gates can be realized when photonic qubits are encoded on the cavity modes in arrangements where ancilla atoms transit the cavity. The fidelity of operations is increased by conditional measurements on the atom and by the use of a selected, dual-rail, Hilbert space. A universal set of gates is proposed, including the Fredkin gate and iSWAP operation; the system seems promising for scalability.

High-fidelity gate operations with the coupled nuclear and electron spins of a nitrogen-vacancy center in diamond

In this article we investigate the dynamics of a single negatively charged nitrogen-vacancy center (NV-) coupled to the spin of the nucleus of a 15-nitrogen atom and show that high fidelity gate operations are possible without the need for complicated composite pulse sequences. These operations include both the electron and nuclear spin rotations, as well as an entangling gate between them. These are experimentally realizable gates with current technology of sufficiently high fidelities that they can be used to build graph states for quantum information processing tasks.

QUANTUM MEASUREMENTS OF ATOMS USING CAVITY QED

Generalized quantum measurements are an important extension of projective or von Neumann measurements in that they can be used to describe any measurement that can be implemented on a quantum system. We describe how to realize two nonstandard quantum measurements using cavity QED. The first measurement optimally and unambiguously distinguishes between two nonorthogonal quantum states. The second example is a measurement that demonstrates superadditive quantum coding gain. The experimental tools used are single-atom unitary operations effected by Ramsey pulses and two-atom Tavis-Cummings interactions. We show how the superadditive quantum coding gain is affected by errors in the field-ionization detection of atoms and that even with rather high levels of experimental imperfections, a reasonable amount of superadditivity can still be seen. To date, these types of measurements have been realized only on photons. It would be of great interest to have realizations using other physical systems. This is for fundamental reasons but also since quantum coding gain in general increases with code word length, and a realization using atoms could be more easily scaled than existing realizations using photons.

Universal continuous variable quantum computation in the micromaser

We present universal continuous variable quantum computation (CVQC) in the micromaser. With a brief history as motivation we present the background theory and define universal CVQC. We then show how to generate a set of operations in the micromaser which can be used to achieve universal CVQC. It then follows that the micromaser is a potential architecture for CVQC but our proof is easily adaptable to other potential physical systems.

Multimode Quantum Optical Logic

We embed qubits within simple two-mode states and effect a Fredkin gate by a resonant multi-photon interaction with an atom.

Quantum communication utilizing cavity-based quantum devices

Photons play a central role in performing general communication tasks. For long distance communications, photons will be lost on the way to receiver, and naturally a quantum communication system then needs to be equipped with functions to overcome the photon losses and to correct errors. We present several quantum repeater schemes and their potential implementation, and compare the advantages and disadvantages of each.

Quantum device and architecture based on NV centers for quantum networks

In this paper, we have developed a design of a quantum device which is capable of distributed quantum information processing. An negatively charged NV center is imbedded in a optical cavity, of which resonance is tuned to realize dipole induced transparency dependent on the electron state in the NV center. This quantum device is then connected via optical fiber, and is assembled to perform arbitrary large quantum information tasks in a distributed manner.In this presentation, we discuss these physical imperfections and their effects to the performance of the quantum information processing. Then we show how we can control them to achieve the accuracy for the information task at hand.