Abstract
In this thesis we experimentally investigate quantum nonlocality: entangled states of spatially separated objects. Entanglement is one of the most striking consequences of the quantum formalism developed in the 1920's; the predicted outcomes of independent measurements on entangled objects reveal strong correlations that cannot be explained by classical physics. Early on, such predictions led physicists to doubt the validity and completeness of quantum theory. At the same time, entanglement is a key resource for applications in quantum information processing and a prerequisite for many tasks in quantum communication and computation.
This thesis attempts to answer two application driven questions: Firstly, can we generate useful entangled states of solid state spins for applications in quantum information processing? Secondly, can we use such entangled states as a resource to teleport an unknown quantum state?
Finally, we ask a foundational question: Are our entangled states indeed inconsistent with the classical notions of free choice, locality and realism? Can we prove this experimentally, under the minimal assumptions of a loopholefree Bell test?
To answer these questions we use single spins in ultrapure diamonds. In particular, we use the electronic and nuclear spins associated with single nitrogenvacancy (NV) defects. The NV centre is a point defect in diamond, consisting of a substitutional nitrogen (N) atom and a neighbouring missing carbon atom (vacancy, V). The NV centre possesses bound electronic states, whose energy levels lie well within the bandgap of the diamond host and whose spin degree of freedom can be used as a quantum bit (qubit). Because of the large diamond bandgap and the 99% spin free carbon12 environment, the electronic spin qubit has exceptional coherence properties even at room temperature. Optical and microwave fields allow control of the electronic spin, which in turn allows control of nearby nuclear spins (the host nitrogen nuclear spin, and nearby carbon13 spins). At liquid helium temperatures, spinpreserving optical transitions provide a powerful optical interface to the electronic spin, allowing, for example, projective readout of the spin state.
By employing a protocol where entanglement is heralded by the detection of a single photon from each of two NV centres in diamonds separated by three metres, we find we can answer the first question in the affirmative. We show for the first time heralded entanglement between solid state quantum systems separated by a humanscale distance.
Then, by combining the heralded entanglement with a deterministic local Bell state measurement and fast feedforward, we show for the first time unconditional quantum teleportation over humanscale distances. We teleport an unknown quantum state from a nuclear spin in one diamond to an electronic spin in a diamond three meters away.
Finally, by employing techniques from the previous experiments, we implement the first loopholefree Bell test. We separate two diamonds by 1.3 kilometres and optimize all operational fidelities, collection efficiencies and rates. This allows us to generate heralded entanglement between them approximately once an hour. The distance provides us with time to read out the electronic spin state in each diamond, faster than any lightlike signal could travel between them. The highfidelity entangled state preparation and spin readout are sufficient to violate the ClauserHorneShimonyHolt Bellinequality. Combined with fast random number generators and a robust statistical analysis, we find a significant rejection of the localrealist hypothesis, without requiring additional experimental assumptions.
The results in this thesis open the door to various applications in quantum information processing. In particular, a remote photonic entangling operation may enable future quantum networks. In such a network the nodes would be formed by the NV centre's combined electronic and nuclear spin register. The nodes would be linked by photonic entanglement operations. Such a network could be used for long distance secure communication, provide a connection between separated quantum computers, or form the basis of a fault tolerant quantum computer by itself. Furthermore, a loopholefree Bell test demonstrates the possibility to do device independent randomness generation and key distribution, that could form the basis for future secure communication channels.
This thesis attempts to answer two application driven questions: Firstly, can we generate useful entangled states of solid state spins for applications in quantum information processing? Secondly, can we use such entangled states as a resource to teleport an unknown quantum state?
Finally, we ask a foundational question: Are our entangled states indeed inconsistent with the classical notions of free choice, locality and realism? Can we prove this experimentally, under the minimal assumptions of a loopholefree Bell test?
To answer these questions we use single spins in ultrapure diamonds. In particular, we use the electronic and nuclear spins associated with single nitrogenvacancy (NV) defects. The NV centre is a point defect in diamond, consisting of a substitutional nitrogen (N) atom and a neighbouring missing carbon atom (vacancy, V). The NV centre possesses bound electronic states, whose energy levels lie well within the bandgap of the diamond host and whose spin degree of freedom can be used as a quantum bit (qubit). Because of the large diamond bandgap and the 99% spin free carbon12 environment, the electronic spin qubit has exceptional coherence properties even at room temperature. Optical and microwave fields allow control of the electronic spin, which in turn allows control of nearby nuclear spins (the host nitrogen nuclear spin, and nearby carbon13 spins). At liquid helium temperatures, spinpreserving optical transitions provide a powerful optical interface to the electronic spin, allowing, for example, projective readout of the spin state.
By employing a protocol where entanglement is heralded by the detection of a single photon from each of two NV centres in diamonds separated by three metres, we find we can answer the first question in the affirmative. We show for the first time heralded entanglement between solid state quantum systems separated by a humanscale distance.
Then, by combining the heralded entanglement with a deterministic local Bell state measurement and fast feedforward, we show for the first time unconditional quantum teleportation over humanscale distances. We teleport an unknown quantum state from a nuclear spin in one diamond to an electronic spin in a diamond three meters away.
Finally, by employing techniques from the previous experiments, we implement the first loopholefree Bell test. We separate two diamonds by 1.3 kilometres and optimize all operational fidelities, collection efficiencies and rates. This allows us to generate heralded entanglement between them approximately once an hour. The distance provides us with time to read out the electronic spin state in each diamond, faster than any lightlike signal could travel between them. The highfidelity entangled state preparation and spin readout are sufficient to violate the ClauserHorneShimonyHolt Bellinequality. Combined with fast random number generators and a robust statistical analysis, we find a significant rejection of the localrealist hypothesis, without requiring additional experimental assumptions.
The results in this thesis open the door to various applications in quantum information processing. In particular, a remote photonic entangling operation may enable future quantum networks. In such a network the nodes would be formed by the NV centre's combined electronic and nuclear spin register. The nodes would be linked by photonic entanglement operations. Such a network could be used for long distance secure communication, provide a connection between separated quantum computers, or form the basis of a fault tolerant quantum computer by itself. Furthermore, a loopholefree Bell test demonstrates the possibility to do device independent randomness generation and key distribution, that could form the basis for future secure communication channels.
Original language  English 

Qualification  Doctor of Philosophy 
Awarding Institution 

Supervisors/Advisors 

Award date  29 Apr 2016 
Edition  Casimir PhD Series DelftLeiden 201611 
Print ISBNs  9789085932536 
DOIs  
Publication status  Published  29 Apr 2016 