Abstract : The ability to manipulate the carrier frequency of quantum states of light, through a process called quantum frequency conversion (QFC), has numerous applications for both technology and basic science. For example, one can upconvert a single-photonlevel signal in the 1.5- m telecommunications band (where single-photon detection has been challenging) to a visible wavelength to take advantage of well-developed single-photon detectors based on silicon avalanche photodiodes. On the more fundamental side, the manipulation of a single photon's frequency may enable the construction of networks of dissimilar quantum systems, whereby one can imagine generating many-body entangled quantum states over vast distances.
Abstract : The ability to manipulate the carrier frequency of quantum states of light, through a process called quantum frequency conversion (QFC), has numerous applications for both technology and basic science. For example, one can upconvert a single-photonlevel signal in the 1.5- m telecommunications band (where single-photon detection has been challenging) to a visible wavelength to take advantage of well-developed single-photon detectors based on silicon avalanche photodiodes. On the more fundamental side, the manipulation of a single photon's frequency may enable the construction of networks of dissimilar quantum systems, whereby one can imagine generating many-body entangled quantum states over vast distances.
We demonstrate the utility of frequency upconversion in a non-linear crystal for quantum information applications. By combining a bright “escort” beam with a signal photon inside of a periodically poled lithium niobate (PPLN) crystal, we are able to achieve near unit conversion efficiency. We use this conversion process to create high-efficiency infrared single-photon detectors by upconverting infrared photons to visible ones and using efficient silicon avalanche photo diodes to detect the upconverted photon. We have achieved a net system detection efficiency of 56%, far superior to previous methods for infrared photon detection. We also demonstrate the phase coherence of the upconversion process, which enables it to convert photonic qubits from one wavelength to another, potentially allowing quantum networks of disparate operating wavelengths to communicate efficiently. This quantum “transduction” feature is observed by upconverting photons passed between two unbalanced Michelson interferometers that create and measure time-bin qubits, similar to those used in many quantum cryptography systems, and verifying that the phase of the qubits is preserved. The Rabi oscillation-like behavior of upconversion is also demonstrated, allowing for the creation and manipulation of highly non-degenerate frequency qubits. These states may be used in conjunction with other degrees of freedom such as polarization, time-bin, or orbital angular momentum states to enlarge the usable single-photon Hilbert space and create a rich vocabulary for transmission of quantum information.
Quantum frequency up-conversion (QFC) of non-classical states of light allows the\nintegration of different quantum systems working at different energies. This process takes\nadvantage of telecommunication wavelengths photons for optical fiber transmission of\nquantum information, and near visible wavelengths for data manipulation and storage.\nThe key objective of QFC is to guarantee that the input photon number probability\ndistribution is maintained after the conversion process. With this, we will be able to\nefficiently study the single-photon emission properties of epitaxial InAs/GaAs quantum\ndots embedded in a photonic crystal nanocavities without the problems that inefficient IR\nwavelengths detectors imply. The up-conversion process consists on combining two\noptical fields, in a nonlinear medium, to generate a third field that is equal to the two inputs\nfrequency sum. To fulfill the momentum conservation, required by this process, we need\nto compensate the wave vector mismatch between the output and input beams. This is\nachieved by using a grating in the nonlinear medium, a process named quasi-phase\nmatching (QPM). In this work we use a Zinc doped periodically poled LiNbO3 (Zn:PPLN\nWG) waveguide as the nonlinear material. To achieve the quantum frequency conversion\nof a semiconductor quantum dot (QD) single photons emission, embedded in a photonic\ncrystal nanocavity, we started by characterizing the performance of the PPLN-WG in the\noptical power macro-regime. Signal photons, produced by an 1175nm laser, simulating\nour QD emission line, are combined with pump photons, produced by a C-band laser,\nwith a dichroic mirror before entering the PPLN waveguide. In order to achieve the phase\nmatched wavelength in the PPLN, its temperature must be finely tuned. The signal\ncoming out of the PPLN waveguide is filtered and analyzed.
Establishing a network of quantum systems will be based on single photons as flying qubits. For a long range transmission, wavelengths in the low-loss telecom bands will be necessary. As such a network may consist of dissimilar systems emitting at different wavelengths, an interface will be required converting the photons to the telecom band without disturbing the single photon quantum properties. We here demonstrate frequency down-conversion of single photons from a quantum dot and prove the preservation of the single photon character [1].
Optical frequency conversion is an indispensable technology in modern science and engineering. In particular, recent demand in the field of quantum information processing requires an efficient frequency conversion of a single photon which has quantum information [1], [2] for realizing quantum internet to interconnect various quantum systems. In conventional experiments, such a quantum frequency conversion (QFC) assumes an input of photons at a single frequency mode. However, when we look at frequency-multiplexed quantum communication [3], [4], the QFC will need to accommodate the frequency-multiplexed manner such that it can convert only photons in a target frequency mode from densely frequency-multiplexed input to any desired frequency mode. For this purpose, we construct an optical frequency converter inside a cavity only for the converted mode, which we call optical frequency tweezers [5].
In a typical quantum repeater scenario, remote quantum memories have to be interconnected by flying qubits transmitted in quantum channels. e.g. single photons in photonic fiber links. To establish long-distance communication, photons in the low-loss telecom wavlength regime are desirable. However, the majority of the candidates for quantum memories such as trapped atoms or ions, rare earth ensembles, color centers in diamond, atomic ensembles etc. possess optical transitions merely in the visible or near-infrared spectral regime below 1000 nm. One possible solution is the transduction of these wavelengths to the telecom region via quantum frequency conversion (QFC) based on three-wave-mixing processes in a nonlinear χ(2)-medium. Technical achievements in recent years enabled high conversion efficiencies and low conversion-induced noise contributions resulting in signal-to-background ratios high enough to demonstrate the preservation of several classical and quantum properties of the photons, e.g. first- and second-order coherence [1], time-bin entanglement [2], polarization entanglement [3] or indistinguishability [4]. QFC thus provides a powerful toolbox to interface quantum systems to the telecom bands and beyond that yet to interconnect dissimilar quantum systems.
The ability to control the optical frequency of quantum state carriers (i.e. photons) is an important functionality for future quantum networks. It allows all matter quantum systems - nodes of the network - to be compatible with the telecommunication C-band, therefore enabling long distance fiber quantum communication between them. It also allows dissimilar nodes to be connected with each other, thus resulting in heterogeneous networks that can take advantage of the different capabilities offered by the diversity of its constituents. Quantum memories are one of the building blocks of a quantum network, enabling the storage of quantum states of light and the entanglement distribution over long distances. In our group, two different types of memories are investigated: a cold atomic ensemble and an ion-doped crystal. In this thesis I investigate the quantum frequency conversion of narrow-band photons, emitted or absorbed by optical quantum memories, with two different objectives: the first one is to connect quantum memories emitting or absorbing visible single photons with the telecommunication wavelengths, where fiber transmission loss is minimum. The second and main goal is to study the compatibility between disparate quantum nodes, emitting or absorbing photons at different wavelengths. More precisely the objective is to achieve a quantum connection between the two optical memories studied using quantum frequency conversion techniques. The main core of this work is the quantum frequency conversion interface that bridges the gap between the cold ensemble of Rubidium atoms, emitting photons at 780nm, and the Praseodymium ion doped crystal, absorbing photons at 606nm. This interface is composed of two different frequency conversion devices, where a cascaded conversions takes place: the first one converts 780nm photons to the telecommunication C-band, and the second one converts them back to visible, at 606nm. This comes with several challenges such as conversion efficiency, phase stability and parasitic noise reduction, which are important considerations to show the conservation of quantum behaviors through the conversion process. This work can be divided in three parts. In a first one, we built a quantum frequency conversion interface between 606nm and the C-band wavelength, capable of both up and down-conversion of single photon level light. We also characterized the noise processes involved in this specific conversion. In the down-conversion case we showed that memory compatible heralded single photons emitted from a photon pair source preserve their non-classical properties through the conversion process. In the up-conversion case, we showed the storage of converted telecom photons in the praseodymium doped crystal, and their retrieval with high signal to noise ratio. The second part of the work was devoted to the conversion of photons from an emissive Rubidium atomic quantum memory to the telecom C band. In this work we converted the heralding photons from the atomic ensemble and measured non-classical correlations between a stored excitation and a C-band photon, necessary for quantum repeater applications. In the last part of the thesis, we setup the full frequency conversion interface and showed that heralded photons emitted by the atomic ensemble are converted, stored in the solid state memory and retrieved with high signal to noise ratio. We demonstrated that a single collective excitation stored in the atomic ensemble is transfered to the crystal by mean of a single photon at telecom wavelength. We also showed time-bin qubit transfer between the two quantum memories. This work represents the first proof of principle of a photonic quantum connection between disparate quantum memory nodes. The results presented in this thesis pave the way towards the realization of modular and hybrid quantum networks.