PhD Candidate, University of Pennsylvania
Utilizing multi-color illumination, single spins in diamond can be set to precise quantum states.
Nitrogen-vacancy (NV) centers in diamond are optically active spins locked in place by the surrounding crystal lattice. These spins can be controlled and measured by illuminating them with the proper color of light, ranging from the microwave to visible spectrum. These properties allow researchers to perform experiments on single spins, which are inherently quantum mechanical, at room temperature. This allows for studies exploring how the quantum world interacts with the usual classical world we are familiar with. The study of this spin-environment interaction has also opened the possibility of new types of sensors embedded in a solid state material. A current thrust in the field is to utilize the above properties to build devices that solve currently outstanding problems in engineering and biology by leveraging the underlying quantum technology.
One current hurdle to overcome is that of quantum state preparation, or how well we can prepare our spin state before allowing it to interact with its environment or other spins. Due to the probabilistic nature of quantum mechanics, we only know our spin's initial state (say up or down) to some probability. For NV centers in diamond, we can typically initialize into the down state 80% of the time. The other 20% we had the wrong initial condition, and our results will add noise to our final signal. In addition to this, there are charge fluctuations that limit our ability to maintain our single-spin quality to 75% of the time. Thus in reality, we are only in the desired initial state 60% of the time. The research I have been conducting has shown that we can improve upon these spin and charge initialization percentages, commonly referred to as fidelity, by combining multiple colors of excitation light.
Abstract: The intersystem crossing (ISC) is an important process in many solid-state atomlike impurities. For example, it allows the electronic spin state of the nitrogen-vacancy (NV) center in diamond to be initialized and read out using optical fields at ambient temperatures. This capability has enabled a wide array of applications in metrology and quantum information science. Here, we develop a microscopic model of the state-selective ISC from the optical excited state manifold of the NV center. By correlating the electron-phonon interactions that mediate the ISC with those that induce population dynamics within the NV center's excited state manifold and those that produce the phonon sidebands of its optical transitions, we quantitatively demonstrate that our model is consistent with recent ISC measurements. Furthermore, our model constrains the unknown energy spacings between the center's spin-singlet and spin-triplet levels. Finally, we discuss prospects to engineer the ISC in order to improve the spin initialization and readout fidelities of NV centers.
Pub.: 03 Mar '15, Pinned: 17 Aug '17
Abstract: A negatively charged nitrogen vacancy (NV) center in diamond has been recognized as a good solid-state qubit. A system consisting of the electronic spin of the NV center and hyperfine-coupled nitrogen and additionally nearby carbon nuclear spins can form a quantum register of several qubits for quantum information processing or as a node in a quantum repeater. Several impressive experiments on the hybrid electron and nuclear spin register have been reported, but fidelities achieved so far are not yet at or below the thresholds required for fault-tolerant quantum computation (FTQC). Using quantum optimal control theory based on the Krotov method, we show here that fast and high-fidelity single-qubit and two-qubit gates in the universal quantum gate set for FTQC, taking into account the effects of the leakage state, nearby noise qubits and distant bath spins, can be achieved with errors less than those required by the threshold theorem of FTQC.
Pub.: 08 May '15, Pinned: 17 Aug '17
Abstract: Nitrogen-vacancy (NV) centers in diamond are versatile candidates for many quantum information processing tasks, ranging from quantum imaging and sensing through to quantum communication and fault-tolerant quantum computers. Critical to almost every potential application is an efficient mechanism for the high fidelity readout of the state of the electronic and nuclear spins. Typically such readout has been achieved through an optically resonant fluorescence measurement, but the presence of decay through a meta-stable state will limit its efficiency to the order of 99%. While this is good enough for many applications, it is insufficient for large scale quantum networks and fault-tolerant computational tasks. Here we explore an alternative approach based on dipole induced transparency (state-dependent reflection) in an NV center cavity QED system, using the most recent knowledge of the NV center's parameters to determine its feasibility, including the decay channels through the meta-stable subspace and photon ionization. We find that single-shot measurements above fault-tolerant thresholds should be available in the strong coupling regime for a wide range of cavity-center cooperativities, using a majority voting approach utilizing single photon detection. Furthermore, extremely high fidelity measurements are possible using weak optical pulses.
Pub.: 29 Apr '17, Pinned: 17 Aug '17