The scaling of controlled quantum systems to large numbers of degrees of freedom is one of the long term goals of experimental quantum information science. Trapped-ion systems are one of the most promising platforms for building a quantum information processor with enough complexity to enable novel computational power, but face serious challenges in scaling up to the necessary numbers of qubits. In this thesis, I present both technical and operational advancements in the control of trapped-ion systems and their juxtaposition with photonic modes used for quantum networking. After reviewing the basic physics behind ion trapping, I then describe in detail a new method of implementing Raman transitions in atomic systems using optical frequency combs. Several dierent experimental setups along with simple theoretical models are reviewed and the system is shown to be capable of full control of the qubit-oscillator system. Two-ion entangling operations using optical frequency combs are demonstrated along with an extension of the operation designed to suppress certain experimental errors. I then give an overview of how spatially separated ions can be entangled using a photonic interconnect. Experimental results show that pulsed excitation of trapped ions provide an excellent single photon source that can be used as a heralded entangling gate between macroscopically separated systems. This heralded entangling gate is used to show a violation of a Bell inequality while keeping the detection loophole closed and can be used a source private random numbers. Finally, the coherent Coulomb force-based gates are combined with the probabilistic photon-based gates in a proof of concept experiment that shows the feasibility of a distributed ion-photon network.
The scaling of controlled quantum systems to large numbers of degrees of freedom is one of the long term goals of experimental quantum information science. Trapped-ion systems are one of the most promising platforms for building a quantum information processor with enough complexity to enable novel computational power, but face serious challenges in scaling up to the necessary numbers of qubits. In this thesis, I present both technical and operational advancements in the control of trapped-ion systems and their juxtaposition with photonic modes used for quantum networking. After reviewing the basic physics behind ion trapping, I then describe in detail a new method of implementing Raman transitions in atomic systems using optical frequency combs. Several dierent experimental setups along with simple theoretical models are reviewed and the system is shown to be capable of full control of the qubit-oscillator system. Two-ion entangling operations using optical frequency combs are demonstrated along with an extension of the operation designed to suppress certain experimental errors. I then give an overview of how spatially separated ions can be entangled using a photonic interconnect. Experimental results show that pulsed excitation of trapped ions provide an excellent single photon source that can be used as a heralded entangling gate between macroscopically separated systems. This heralded entangling gate is used to show a violation of a Bell inequality while keeping the detection loophole closed and can be used a source private random numbers. Finally, the coherent Coulomb force-based gates are combined with the probabilistic photon-based gates in a proof of concept experiment that shows the feasibility of a distributed ion-photon network.
Scaling up controlled quantum systems to involve large numbers of qubits remains one of the outstanding challenges of quantum information science. One path toward scalability is the use of a modular architecture where adjacent qubits may be entangled with applied electromagnetic fields, and remote qubits may be entangled using photon interference. Trapped atomic ion qubits are one of the most promising platforms for scaling up quantum systems by combining long coherence times with high fidelity entangling operations between proximate and remote qubits. In this thesis, I present experimental progress on combining entanglement between remote atomic ions separated by 1 meter with near-eld entanglement between atomic ions in the same ion trap. I describe the experimental improvements to increase the remote entanglement rate by orders of magnitude to nearly 5 per second. This is the first experimental demonstration where the remote entanglement rate exceeds the decoherence rate of the entangled qubits. The flexibility of creating remote entanglement through photon interference is demonstrated by using the interference of distinguishable photons without sacrificing remote entanglement rate or fidelity. Next I describe the use of master clock in combination with a frequency comb to lock the phases of all laser-induced interactions between remote ion traps while removing optical phase stability requirements. The combination of both types of entanglement gates to create a small quantum network are described. Finally, I present ways to mitigate cross talk between photonic and memory qubits by using different trapped ion species. I show preliminary work on performing state detection of nuclear spin 0 ions by using entanglement between atomic ion spin and photon polarization. These control techniques may be important for building a large-scale modular quantum system.
Trapped atomic ions are a promising medium for quantum computing, due to their long coherence times and potential for scalability. Current methods of entangling ions rely on addressing individual modes of motion within the trap and applying qubit state dependent forces with external fields. This approach can limit the speed of entangling gates and make them vulnerable to decoherence due to coupling to unwanted modes or ion heating. This thesis is directed towards demonstrating novel entanglement schemes which are not limited by the trap frequency, and can be made almost arbitrarily fast. Towards this goal, I report here on the first experiments using ultrafast laser pulses to control the internal and external states of a single trapped ion. I begin with experiments in ultrafast spin control, showing how a single laser pulse can be used to completely control both spin degrees of freedom of the ion qubit in tens of picoseconds. I also show how a train of weak pulses can be used to drive Raman transitions based on a frequency comb. I then discuss experiments using pulses to rapidly entangle the spin with the motion, and how careful spectral redistribution allows a single pulse to execute a spin-dependent momentum kick. Finally, I explain how these spin-dependent momentum kicks can be used in the future to create an ultrafast entangling gate. I go over how such a gate would work, and present experimentally realizable timing sequences which would create a maximally entangled state of two ions in a time faster than the period of motion in the trap.
Irhe field of quantum information processing (QIP) has evolved rapidly in the last decade. Strings of single rapped ions have proven to be one of the most promising candidates for the implementation of quantum information processing tasks such as universal gate operations or eleportation. Besides the challenge of scaling up these quantum processors to a larger number of ions, another important problem is to interconnect different processors on a macroscopic scale and integrate them into a quantum network. Irhis thesis describes the construction of a new experiment dedicated to the study of quantum communication and quantum networking with laser cooled single ions and first experiments carried out with it. Two linear Paul traps for 4DCa+ have been set up at a distance of one meter. Two-photon interference of the fluorescence light of two remotely trapped single ions could be observed with a contrast of 80%. This proves the suitability of the setup for the creation of remote entanglement, an important building block for quantum networking. Irhe first part of the thesis describes the experimental setup that ~as constructed from scratch. It consists of a newly designed trap apparatus and the laser setup. The apparatus was designed to ulfill the requirement of interferometric stability, which is a pre requisite for one remote entanglement protocol. Another important feature are two high numerical aperture in-vacuum lenses hat optimize the collection efficiency of photons scattered by he ions. In order to frequency-stabilize the laser systems, a ransfer stabilization scheme referenced to a transition in atomic ~s has been developed. The stabilization chain comprises low-cost open transfer cavities and offers very good long term stability. Irhe second part of the thesis presents the experimental results starting with the calibration of the two traps through excitation spectra. After that, the realization of two-photon interference rom single ions under continuous excitation is presented. The model analysis of the data usinq eiqht level Bloch equations reveals the individual coherence properties of the interfering ~ingle photons. The coherence time of the fluorescence light does not exceed 20- ns, indicating that incoherent scattering is ~ominant. fA,n analysis of the efficiency of methods for remote entanglement relying on two-photon and single-photon interference, focussing on he role of the detection efficiency is discussed. It is revealed hat Single-photon schemes are more efficient if the detection ~fficiency is low (like in present experiments). The created setup is suitable for the realization of both types of protocols, based pn single- and two-photon interference. Jhe conditioned dynamics of photon emission of a single ion are studied using correlation functions. The engineering of second ~rder correlation functions used as a tool which relies on conditional dynamics, might have interesting applications in probabilistic quantum information processing. pingle-atom single-photon interfaces are an important application in quantum networks. As opposed to the emission of single photons by single atoms, the absorption of Single photons by single atoms cannot be controlled in a similar way. With the future goal of realizing a heralded absorption of a single photon by a single atom, one of the ion traps was linked to a Spontaneous Parametric Down Conversion source. Quantum jump spectroscopy on a single ion using single photons from this source is shown. Finally the reported two-photon interference could be extended also to pulsed excitation. This marks an important technological step towards remote entanglement.
Trapped atomic ions are a promising medium for quantum computing, due to their long coherence times and potential for scalability. Current methods of en-tangling ions rely on addressing individual modes of motion within the trap and applying qubit state dependent forces with external fields. This approach can limit the speed of entangling gates and make them vulnerable to decoherence due to coupling to unwanted modes or ion heating. This thesis is directed towards demon-strating novel entanglement schemes which are not limited by the trap frequency, and can be made almost arbitrarily fast. Towards this goal, I report here on the first experiments using ultrafast laser pulses to control the internal and external states of a single trapped ion. I begin with experiments in ultrafast spin control, showing how a single laser pulse can be used to completely control both spin degrees of freedom of the ion qubit in tens of picoseconds. I also show how a train of weak pulses can be used to drive Raman transitions based on a frequency comb. I then discuss exper-iments using pulses to rapidly entangle the spin with the motion, and how careful spectral redistribution allows a single pulse to execute a spin-dependent momentum kick. Finally, I explain how these spin-dependent momentum kicks can be used in the future to create an ultrafast entangling gate. I go over how such a gate would work, and present experimentally realizable timing sequences which would create a maximally entangled state of two ions in a time faster than the period of motion in the trap.
Harnessing quantum mechanics to solve problems intractable on classical computers is one of the foremost goals of applied physics. Trapped ions make an excellent physical platform for such a quantum computer, as they have demonstrated unparalleled qubit operation fidelities and memory times, but significant challenges remain in scaling up these systems. A promising scalable architecture is a photonically connected network of mixed element trapped ion nodes, each containing a few ions of two species. Poor quality remote entanglement links between nodes can be tolerated, if entanglement between ions in the same node can be achieved with high fidelity. While local gates have been repeatedly demonstrated between ions of the same species, the fidelity of mixed element quantum gates have not previously surpassed the fault tolerant threshold, above which errors can in principle be corrected. In this thesis we demonstrate an entangling operation between 43Ca+, useful for its long lived qubits, and 88Sr+, well suited for remote entanglement, using a novel mechanism requiring only one laser to manipulate both species. This relatively simple scheme, which is independent of either qubit frequency, allows us to achieve Bell state fidelities of F = 99.8(1)%. We assess the operation of the gate further with state and process tomography, which achieve Bell state fidelity F = 98.1% and average gate fidelity F̅ = 99.05(6)% respectively, though they are hampered by experimental drifts during the long data collection. Finally, we use interleaved randomised benchmarking to get the best estimate of our gate errors, finding an average gate fidelity of F̅ = 99.62(3)% for a sequence of 60 interleaved gates. Along with a demonstration of entanglement between two 88Sr+ ions, achieved with fidelity F = 95.9(4)%, this work paves the way towards demonstrating an elementary mixed species quantum network in future experiments.
Quantum devices are rapidly gaining momentum as a technology that will induce a paradigm shift in computing, communication and cryptography. Trapped ion qubits are one of the leading candidates for implementing a quantum computer, having previously demonstrated all of the required criteria. Local gate fidelities between ions exceed those for all other platforms, but the total number of ions in a trap is limited by unavoidable issues – one possibility for scaling the ion trap quantum processor is to create entanglement between ions in separate traps via single photons. The work in this thesis demonstrates the generation of remote entanglement between ions at high fidelity and rate, paving the way towards protocols using multiple entangled pairs for computations. We describe the construction from scratch of a twin-trap apparatus designed to entangle separated ions. We demonstrate the entanglement of strontium ions in traps separated by ∼ 2 m by swapping entanglement from single photons emitted by the 88Sr+ ions. A novel photon collection geometry is used, maximising the entanglement between the ion and photon without impeding optical access for standard ion trap laser beam geometries. The Bell state fidelity of the ion-photon state, which is itself a valuable entanglement resource for blind computation, is at least 97.70(12) % in either trap, with entangled pairs detected at a rate of at least 3.98 × 103 s−1. The remote ion-ion Bell state fidelity is 94.0(5) %, and is generated at a rate of 182 s−1, representing the highest fidelity remote entanglement reported in ions by a large margin, at a rate more than an order of magnitude faster than previous experiments. The two identical trap systems are designed with capabilities beyond those demonstrated here. We can co-trap 88Sr+ and 43Ca+ in a microfabricated trap suitable for local ion transport operations, opening up the possibility of performing distillation of remote entanglement in the same apparatus. Entanglement distillation can be used to make remote entanglement with similar fidelity to local operations, and would represent a significant step towards a fully scalable ion trap quantum computer architecture.