Advances in Photonics of Quantum Computing, Memory, and Communication
Engineering integrated photon pair sources and multiplexed detectors (Conference Presentation)
Evan Meyer-Scott, Nidhin Prasannan, Nicola Montaut, Johannes Tiedau, Christof Eigner, Georg Harder, Linda Sansoni, Thomas Nitsche, Harald Herrmann, Raimund Ricken, Viktor Quiring, Tim J. Bartley, Sonja Barkhofen, Christine Silberhorn
Two important challenges in quantum photonics are to generate useful states with high fidelity, and to detect them and verify their properties. Particularly valuable states are single photons and entangled photon pairs in well-defined optical modes, as they can be used in many quantum information protocols or used to build up more complex states. For sources, we employ integrated nonlinear optics (waveguides in lithium niobite and potassium titanyl phosphate) to maximize brightness and go beyond what is possible in bulk optics, showing simultaneously high state fidelity, heralding efficiency, and spectral purity across three experiments: first we show record heralding efficiency in a fully-fibered heralded single-photon source, and use it to probe the tradeoff between spectral purity and heralding efficiency in non-engineered sources. With an engineered source, we then herald up to 50 photons in a nonclassical state. The last source is for polarization-entangled photon pairs, with brightness of 3.5 million pairs/s·mW, fidelity to a Bell state of 96%, heralding efficiency of 43%, and HOM interference visibility of 82%. Once a complex state is constructed, it must also be verified. For this we employ a time-multiplexed detector consisting of a fibre loop and a single-photon detector. Surprisingly, we are able to extract information even in the saturation regime of the detector. We use the click statistics of the time-multiplexed detector to verify the non-classicality of quantum light, and we use its extremely high dynamic range (123 dB) to measure a macroscopic power level with a single-photon detector. Eliminating calibrated attenuators with this approach will allow direct standardization of quantum and classical optical power levels.
Two important challenges in quantum photonics are to generate useful states with high fidelity, and to detect them and verify their properties. Particularly valuable states are single photons and entangled photon pairs in well-defined optical modes, as they can be used in many quantum information protocols or used to build up more complex states. For sources, we employ integrated nonlinear optics (waveguides in lithium niobite and potassium titanyl phosphate) to maximize brightness and go beyond what is possible in bulk optics, showing simultaneously high state fidelity, heralding efficiency, and spectral purity across three experiments: first we show record heralding efficiency in a fully-fibered heralded single-photon source, and use it to probe the tradeoff between spectral purity and heralding efficiency in non-engineered sources. With an engineered source, we then herald up to 50 photons in a nonclassical state. The last source is for polarization-entangled photon pairs, with brightness of 3.5 million pairs/s·mW, fidelity to a Bell state of 96%, heralding efficiency of 43%, and HOM interference visibility of 82%. Once a complex state is constructed, it must also be verified. For this we employ a time-multiplexed detector consisting of a fibre loop and a single-photon detector. Surprisingly, we are able to extract information even in the saturation regime of the detector. We use the click statistics of the time-multiplexed detector to verify the non-classicality of quantum light, and we use its extremely high dynamic range (123 dB) to measure a macroscopic power level with a single-photon detector. Eliminating calibrated attenuators with this approach will allow direct standardization of quantum and classical optical power levels.
Integrated micro- and nano-structures allows for the efficient generation of photon pairs via parametric fluorescence, thanks to the enhancement of the light-matter interaction associated with light confinement in small volumes. For instance, the efficiency of spontaneous four wave mixing in a silicon micro ring resonator can range up to 10 orders of magnitude larger than in bulk silicon. Yet the advantages of using integrated devices go well beyond the sole efficiency improvement, for micro structures grant an unprecedented control over the properties of generated non-classical light. For instance, in a single photonic integrated circuit, one can exploit the interferences of many optical elements to construct complex multipartite states in a compact, stable, and scalable system. In this talk I will focus on the generation of photon pairs and show how their spectral properties can be engineered in an integrated device for use as building blocks in the construction of a whole variety of states, from heralded single-photons to multipartite states.
Light, which is composed of discrete quanta, or photons, is one of the most fundamental concepts in physics. Being an elementary entity, the behaviour of photons is governed by the rules of quantum mechanics. The ability to create, manipulate and measure quantum states of light is not only useful in foundational tests of quantum theory, but also in a wide range of quantum technologies – which aim to utilize non-classical properties of quantum systems to perform tasks not possible with classical resources. Only recently has it been possible to control the properties of number states of light, which have a fixed photon-number. Two-photon states are central to testing fundamental physical theories (such as locality and reality) and the implementation of quantum information technologies. The versatility of photon-pair states is en- abled by the potential entanglement properties it can posses. Thus controlling the correlations between photons is crucial to both pure and applied physics. To produce a single photon, a photon-pair state can be used. Detection of one photon indicates its twin’s existence. Many applications, such as optical quantum computation, require pure indistinguishable single photons. Heralding single pho- tons from a photon-pair will, in general, produce single photons in a mixed quantum state due to correlations within the pair. A common approach to creating photon-pairs is through the nonlinear sponta- neous four-wave mixing interaction in optical fibres. This thesis presents a theoreti- cal and experimental implementation of a scheme to tailor the spectral correlations within the pairs. Emphasis is placed on engineering the two-photon state such that they are completely uncorrelated. Spatial entanglement is naturally avoided due to the discrete nature of the optical fibre modes. Spectral correlations are eliminated by careful choice of dispersion characteristics and conditions. The purity of the photons generated by this scheme is demonstrated by means of two-photon inter- ference from independent sources. We measure a purity of (85.9 ± 1.6)% with no spectral filtering, exhibiting the usefulness of this source for quantum technologies and applications.
Over the last sixty years, classical information theory has revolutionized the understanding of the nature of information, and how it can be quantified and manipulated. Quantum information processing extends these lessons to quantum systems, where the properties of intrinsic uncertainty and entanglement fundamentally defy classical explanation. This growing field has many potential applications, including computing, cryptography, communication, and metrology. As inherently mobile quantum particles, photons are likely to play an important role in any mature large-scale quantum information processing system. However, the available methods for producing and detecting complex multi-photon states place practical limits on the feasibility of sophisticated optical quantum information processing experiments. In a typical quantum information protocol, a source first produces an interesting or useful quantum state (or set of states), perhaps involving superposition or entanglement. Then, some manipulations are performed on this state, perhaps involving quantum logic gates which further manipulate or entangle the intial state. Finally, the state must be detected, obtaining some desired measurement result, e.g., for secure communication or computationally efficient factoring. The work presented here concerns the first and last stages of this process as they relate to photons: sources and detectors. Our work on sources is based on the need for optimized non-classical states of light delivered at high rates, particularly of single photons in a pure quantum state. We seek to better understand the properties of spontaneous parameteric downconversion (SPDC) sources of photon pairs, and in doing so, produce such an optimized source. We report an SPDC source which produces pure heralded single photons with little or no spectral filtering, allowing a significant rate enhancement. Our work on detectors is based on the need to reliably measure single-photon states. We have focused on optimizing the detection efficiency of visible light photon counters (VLPCs), a single-photon detection technology that is also capable of resolving photon number states. We report a record-breaking quantum efficiency of 91 +/- 3% observed with our detection system. Both sources and detectors are independently interesting physical systems worthy of study, but together they promise to enable entire new classes and applications of information based on quantum mechanics.
The generation of high purity single photons and entangled photon pairs is a crucial first step in many quantum optics experiments, including those that require quantum teleportation [1], entanglement swapping [2], and the generation of multi-photon entanglement [3]. The most widely used and established approach for creating photon pairs is spontaneous parametric down conversion (SPDC) - where pump photons undergo a non-linear optical process to create a down-converted photon pair [4]. Traditionally, the generation of high-purity heralded single photons required severe spectral filtering, introducing additional loss. However, through careful engineering of spectral correlations of the down-converted photon pairs one can circumvent the need for filtering and avoid the accompanying reduction in heralding efficiency.
Abstract : Single photons sources are desired for many potential quantum information applications. One common method to produce single photons is based on a heralding protocol, which involves generating a pair of photons in an optical nonlinear medium and then detecting one photon in the pair to announce the existence of the other photon of the pair. Compared to other schemes, such a method offers several significant advantages. A major challenge to overcome which is inherent to this method, however, is that the photon pairs are created stochastically in the optical nonlinear medium. Thus, in order to ensure high purity levels for the single photons generated via heralding, sources of this type must be operated at low photon-pair production rates in order to keep the probability for simultaneous creation of multiple photon-pairs to an acceptably low level. A direct result of this restriction is a low heralding rate of single photons. A potential solution for achieving high-rate, high-purity, single-photon generation combines several heralding single-photon sources which are actively multiplexed to act as one source. The overall goal of this project is to develop methods to perform such multiplexing based on the use of a low loss high speed fiber optical switch.
This thesis reports an advanced monolithic integrated quantum photonic realization on lithium niobate, where nonlinear optics and electro-optics properties have been harnessed simultaneously for generating heralded configurable, two-photon states. Taking advantage of a picosecond pump laser and telecom components, we demonstrated the capability of producing various path-coded heralded two-photon, showing 94% raw visibility for Hong-Ou-Mandel (HOM) interference. The versatility and performance of such a highly integrated photonic entanglement source enable exploring more complex quantum information processing protocols finding application in communication, metrology and processing tasks. To enable on-chip two-photon interference, synchronization of two pump pulses entering the chip is crucial. Inspired by Young's double-slit experiment, we synchronize two pulses with a precision of 100 µm by investigating the interference-fringe contrast when varying the relative delay of the pulses. Using an autocorrelator and spectrometer, we determined the time-bandwidth product of the pump pulse and calculated the optimal filter bandwidth to obtain pure quantum states. Since the visibility of two-photon interference relates to the pump power, a theoretical investigation figuring out suitable pump power level was performed as well. In addition, the functionality of generating high quality two-photon path-entangled state was demonstrated by fourfold coincidence measurement relying on a four-channel superconducting single photon detector (SNSPD) and a programmable time-digital converter. In this thesis, the SNSPDs were calibrated and a logical counting algorithm based on Labview was developed to efficiently and accurately record the fourfold coincidences for N00N and product states.