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.
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.
Experiments in quantum optics are motivated by fundamental questions about the quantum mechanical description of matter and light as well as by applications in quantum technologies such as quantum information processing. Exploiting the quantum character of both matter and light has turned out to be extremely rewarding for novel technological applications in the emerging fields of quantum communication and computation. Here we consider the implementation of quantum information technology with trapped ions and single photons, focusing on two key issues: the entanglement of distant atoms and the controlled interaction of a single photon with a single atom.
This thesis describes the construction of a double ion trap apparatus including the necessary frequency-stabilized laser systems, and first experiments to investigate the generation of distant atomic entanglement, the implementation of a simple quantum network and controlled single-atom single-photon interaction. The experimental setup comprises two ion traps at a distance of one meter, which are independent but identical in construction. We introduce novel realizations of a photoionization method for calcium atoms, a laser frequency stabilization scheme and a radio frequency pulse sequencer. Linear Paul traps are used to load individual or strings of calcium ions. High-numerical aperture objectives mounted on sub-micrometer positioning systems allow us to address and manipulate a single ion with strongly focused light fields and collect the emitted fluorescence photons from a large solid angle. With these bidirectional single-atom single-photon interfaces the essential building blocks of a quantum network with atoms and photons are available. One of the main objectives we pursue with the double-trap apparatus is the generation of entanglement between ions in different traps, which is one of the key tasks in a quantum network. We compare single- and two-photon schemes for probabilistic entanglement creation and analyze how the performance of the schemes depends on photon detection efficiency. As a first step towards the experimental realization of such schemes we measure the second order correlation function of fluorescence photons emitted from two distant trapped ions. We observe high-contrast quantum (Hong-Ou-Mandel) interference, proving the indistinguishability of photons emitted from different ions, which is a prerequisite for the considered entanglement schemes. To investigate the interaction of single photons and a single atom in free space we combine the ion trap apparatus with a tunable, narrowband spontaneous parametric down-conversion source. The high-numerical aperture objectives allow us to efficiently couple bandwidth-tunable photons to a single trapped ion. Such atom-photon interaction at the single-particle level is the most fundamental process in quantum optics and lies literally at the focus of many experimental implementations of quantum information processing protocols and of studies of the (quantum) limits of matter -light interaction, which were so far restricted to weak coherent light fields. Here we observe individual atomic absorption events of one photon of an entangled pair by employing a quantum jump detection scheme. To characterize and control the interaction process we modify the spectral properties of the down conversion photons and perform single-photon spectroscopy of a single ion. To go beyond experiments with cohe rent fields we study temporal correlations between the absorption of photons from the spontaneous parametric down conversion source by a single ion and the detection of the corresponding partner photon.
For the applied scheme we observe promising indications that the quantum correlations of photons produced in spontaneous parametric down conversion are preserved in the resonant interaction with a single atom. The implementation of more efficient atom-light coupling and entanglement transfer schemes with our apparatus is outlined.
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.
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.
The interaction of single photons with single atoms is a fundamental process in quantum optics which manifests the quantum mechanical nature of light and matter. As an experimentally controlled process it enables the implementation of quantum technological applications, such as quantum information processing and quantum metrology. In the last decades, substantial progress has been attained to experimentally control and manipulate single atoms.
For example, single or multiple ions can be stored in an ion trap and their quantum states can be controlled with great precision. At the same time, experimental techniques to generate and control single photons and entangled photon pairs have been widely developed. However, only modest progress has been attained in controlling the interaction of single atoms with sin~le photons. The aim of this thesis project was to experimentally study he absorption of single photons and entangled photons pairs by single trapped DCa· ions. To this end, a source of polarization-entangled photon pairs based on the process of spontaneous parametric down-conversion (SPDC) was designed and constructed . Generating photon pairs whose interaction with an atom can be controlled imposed stringent requirements on the bandwidth, tunability, stability and brightness of the source, making its setup much more complex than usual SPDC sources. In a parallel project, a set of two separate ion traps were built, capable of storing a single or a set of a few 4DCa· ions.
he first part of the thesis documents the design, construction , operation and characterization of the entangled photon pair source. The characterization proves all the necessary requirements for the photons to interact with either of two transitions, D312 P312 and D5i2 P312 at 850 and 854 nm respectively, in 4DCa· One of the crucial requirements was to make the source bandwidth match the bandwidth of these transitions (22MHz). This was not straightforward since the typical emission bandwidths of the SPDC process is several orders of magnitude higher (1 05MHz). The ion trap setup is also briefly described. One of its crucial features is the inclusion of two high numerical-aperture objectives very close to the ion trap, allowing to enhance the interaction efficiency of the SPDC photons with the ion, and to efficiently collect the fluorescence light they emit. In a second part of the thesis, a concise description of the schemes and techniques used to detect the absorption of single photons by a single trapped ion is given, including a heoretical treatment of the methods used to analyze the data.
he core part of the thesis describes the methods used to couple the entangled photon source to one of the ion traps, and explains the results obtained with the combined setup. First it shows how the source is configured to produce heralded single photons. It then presents experiments in which the absorption by a single ion of photons produced by the source is detected, and how these absorption events are time-correlated with the detection of the heralding photon. These experiments are performed under several conditions, with a growing level of control over the interaction process. Finally, two proof-of-principle experiments are presented. In the first one, the ion is pumped to polarization-sensitive Zeeman substates and the polarization dependence of the correlated absorption rate is observed. This is a first step toward more advanced experiments in which the polarization entanglemen of the photon pairs is transferred to the entanglement of one photon with one atom or to two distant atoms. A second experiment performs heralded single photon spectroscopy on a single ion, showing the expected frequency dependence of the absorption process.
Finally, methods to extend these experiments to implement entanglement transfer from photons to atoms are outlined. In particular, it is shown how the efficiency of the source can be improved to make these experiments feasible.
Long-range transport of quantum information across a network is most readily achieved through the use of photons as the information carriers. The nodes of such a quantum network are most naturally a quantum memory or information processor, which typically do not rely upon photonic qubits. Realizing a large-scale distributed quantum network therefore requires an efficient interface between the two physical manifestations of the quantum information. The most promising platform for realizing a quantum computer is the manipulation of trapped atomic ions, as this system has demonstrated all the fundamental requirements to realize a quantum computer. Additionally, trapped ions posses long coherence times necessary for quantum memories. The interface between atomic and photonic qubits relies on a high photon emission rate into the collected solid angle. Most methods of photon collection for quantum networks use a high numerical aperture microscope objective, which only collects on the order of 1% of all the emitted photons. This results in a small probability of successfully linking two quantum nodes. In this work, I present the design, simulation, and implementation of a trapped ion cavity QED system capable of improving the photon collection efficiency. A single ytterbium ion is coupled to a high-finesse optical cavity. The photon scatter rate into the cavity mode is enhanced by a factor of 130 over the free-space scatter rate into the cavity solid angle. This represents the first step towards realizing a protocol to entangle the polarization of an emitted photon to the Zeeman sublevels of the ion with high photon collection efficiency. Simulations of the experimental system indicate that with an optical cavity up to 4% of the light emitted by the ion can be collected.