This PDF file contains the front matter associated with SPIE Proceedings Volume 11167 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Quantum communication networks based on fiber optics are restricted in length since efficient quantum repeaters are not yet available. A free-space channel between a satellite in orbit and Earth can circumvent this problem. We have constructed a system to demonstrate the feasibility of quantum communication between space and earth using photons hyperentangled in their polarization and time-bin degrees of freedom. With this system, we have implemented superdense teleportation (SDT) with a fidelity of 0.94±0.02. To increase the efficiency of SDT, we have developed an active, polarization-independent switch compatible with SDT. We characterized the performance of its switching efficiency. Finally, we have constructed a novel two-level interferometer for time-bin qubit creation and analysis in orbit, and bounded its stability.

Designing a laser communications payload integral to a nano-satellite, capable of fine pointing, is a difficult task. Integrating this with a quantum key distribution payload for inter-satellite secure communication adds significant complexity, as does ground based communication with the moving objects in low Earth orbit. The advantages of low path length, secure, global communication make this an opportunity not to be missed. We report on developments of such a system. Beginning with a satellite demonstrator with a Pointing, Acquisition-and Tracking system that involves fine precision star-tracker, plasma and aerodynamic force interactions for steering, and an inter-satellite laser link demonstration on orbit using LED beacon for acquisition, and laser for communications link closure. Combining design studies for a nano-satellite quantum payload for transferring entangled photon-pairs between the satellites at long-range in Low Earth Orbit, and incorporating ground to space and space to ground laser communications, are described in the context of quantum optical ground station efforts within Australia.

Recent experimental advances in preparing regular defect-free 2D arrays of atoms with optical traps have offered new opportunities to engineer strong collective couplings between light and atoms as quantum optical dipoles. Applications of such collective light-matter interactions include the realization of atom-thick mirrors, retrieval of quantum memories with high efficiency, or the implementation of topological quantum optics. I will present novel applications in the context of quantum networking. First, I will discuss the design of a chiral photonic quantum link, where distant single atoms interact by exchanging photons propagating in a unique direction in free space. This is achieved by coupling both single atoms, representing qubits implemented using a pair of Rydberg states, to bi-layer atomic arrays which act as quantum phased-array antennas. Exploiting the combination of a “Rydberg-dressing” laser and a control laser driving the atomic arrays, an effective Rydberg dipole-dipole interaction can be engineered between atoms and atomic arrays as quantum emitters, allowing to match the spatial modes of spontaneously emitted and absorbed photons to a Gaussian mode of interest. In this way, we realize a chiral quantum interface, i.e., where atoms couple to photonic modes with a unique direction of propagation. This setup provides a basic building block of a novel platform for quantum networks in free space, i.e., without requiring coupling atoms to modes of cavities or nanostructures, which I will illustrate with the deterministic coherent transfer of quantum states between single atoms with high fidelity. Second, I will discuss the collective radiation properties of two distant single-layer atomic arrays acting as quantum memories. This system can support a long-lived non-local superposition state of atomic excitations exhibiting strong subradiance. This “dark” mode consists of an atomic excitation which is delocalized between the two distant atomic arrays, in such a way that the overall radiation is strongly suppressed by quantum interference. I will describe the preparation of these states and their application as resource of non-local entanglement. In particular, by weakly driving the arrays, the state of the quantum memories, localized in each array, can be transferred to this “non-local” subradiant state, allowing again for a deterministic coherent quantum state transfer with high fidelity between the memories. Finally, I will discuss experimental realizations using cold atoms in optical trap arrays, and the effect of experimental imperfections in the arrays, such as finite defect probability, temperature, and depth of the optical traps. 

Simulations of satellite downlinks have previously shown adaptive optics (AO) can enable daytime satellite QKD by allowing strong spatial filtering of sky noise while preserving the quantum signal. In the downlink scenario, the light from the satellite beacon samples the same atmospheric path as the QKD photons. In an uplink scenario however, the satellite changes position after launching the beacon light and the QKD photons are directed upward through a different atmospheric path in order to intercept the satellite at its new location. As a consequence, the quantum channel and beacon channel deviate from one another by more than 50 urad in angle for a LEO engagement. This angular separation exceeds the isoplanatic angle by several multiples. Yet, this simulation suggests that AO in this scenario will still provide sufficient wavefront compensation to justify its inclusion in a QKD uplink.

The precise quantum control of single photons, together with the intrinsic advantage of being mobile make optical quantum system ideally suited for delegated quantum information tasks, reaching from well-established quantum cryptography to quantum clouds and quantum computer networks. Here I will present that the exploit of quantum photonics allows for a variety of quantum-enhanced data security for quantum and classical computers. First, I will present a homomorphic-encrypted quantum random walk using single-photon states and non-birefringent integrated optics. The client encrypts their input state in the photons’ polarization degree of freedom, while the server performs the computation using the path degree of freedom. Then I will briefly discuss the realization of a feasible hybrid classical-quantum technology, which shows promising new applications of readily available quantum photonics technology for secure classical computing by enabling probabilistic one-time programs.

Photons play a central role in many areas of quantum information science, either as qubit themselves or to mediate interactions between long-lived matter based qubits. Techniques for (1) high-fidelity generation, (2) precise manipulation and (3) ultra-efficient detection of quantum states of light are therefore a prerequisite for virtually all quantum technologies. A quantum photonics processor is the union of these three core technologies into a single system, and, bolstered by advances in integrated photonics, promises to be a versatile platform for quantum information science. In this talk we present recent progress towards large-scale quantum photonic processors, leveraging the platform of silicon photonics. We demonstrate how quantum photonic processors can accelerate both quantum and classical machine learning, and how optimization techniques can enhance large-scale quantum control and provide a new path towards efficient verification of near-term quantum processors.

Running general quantum algorithms on quantum computers is hard, especially in the early stage of development of the quantum computer that we are in today. Many resources are required to transform a general problem to be run on a quantum computer, for instance to satisfy the topology constraints of the quantum hardware. Furthermore, quantum computers need to operate at temperatures close to absolute zero, and hence resources are required to keep the quantum hardware at that level. Therefore, simulating small instances of a quantum algorithm is often preferred over running it on actual quantum hardware. This is both cheaper and gives debugging capabilities which are unavailable on actual quantum hardware, such as the evaluation of the full quantum state, at intermediate points in the algorithm as well as at the end of the algorithm. By simulating small instances of quantum algorithms, the quantum algorithm can be checked for errors and be debugged before implementing and running it on actual quantum hardware for larger instances. There are multiple initiatives to create quantum simulators and while looking alike, there are difference among them. In this work we compare seven often used quantum simulators offered by various parties by implementing the Shor-code, an error-correcting technique. The Shor-code can detect and correct all single qubit errors in a quantum circuit. For most multi-qubit errors, correct detection and correction is not possible. We compare the seven quantum simulators on different aspects, such as how easy it is to implement the Shor-code, what its capabilities are regarding translation to actual quantum hardware and what the possibilities of simulating noise are. We also discuss aspects such as topology restrictions and the programming interface.

Engineering-inspired filter functions are a powerful heuristic for the development of noise-robust quantum logic operations. We expand on existing single-qubit approaches and present a generalized, computationally efficient framework to calculate filter functions for operations performed on D-dimensional manifolds such as pairs of interacting qubits (including spectator levels), and qudits. We describe two applications of the generalized filter functions to superconducting qubits: 1. The design of robust controls for parametric gates. 2. Using entangled states as a probe for noise characterization.

The quantum information community is entering a period in which scientists and engineers are transitioning from ‘understanding’ to ‘control’ by harnessing aspects of quantum mechanics to realize capabilities in computing, sensing, and communication that are not possible in the classical world. Although many of these quantum technologies are still in their very early stages, it is clear that our future world will include many quantum devices. The existence of multiple devices will, in turn, necessitate the transfer of quantum information between devices—if isolated devices are useful, it follows that connected devices will be even more useful. Therefore, it is imperative to develop the resources needed for connecting quantum devices at different locations. One such resource is quantum entanglement, a physical phenomenon in which two or more quantum systems cannot be described independently, even when separated by large distances. Entanglement is an important resource for quantum computing and is critical for overcoming loss constraints that limit the transfer of quantum states across large distances. The latter can be accomplished via ‘quantum repeaters,’ which propose to connect multiple shorter entangled photon links to form a longer entangled photon link. Much of our understanding of entanglement comes from experiments involving entangled photon pairs, which are still the best candidates for the exchange of quantum information across any appreciable distance. Since the 1990s, spontaneous parametric downconversion (SPDC) has been established in the academic and research community as a reliable source of entangled photon pairs. During this time, source brightness and stability has improved considerably over the first demonstrations. Yet, entangled photon sources are still built primarily by graduate students, and there is significant variability among sources. Moreover, SPDC entangled photon sources lack flexibility, typically designed to output a single type of entangled state. In this paper we report on the efforts to develop a ‘user-programmable’ entangled photon source capable of producing any type of two-photon entangled state. The goal is a source capable of emitting photon pairs with user-defined polarization states. That is, the user will be able to adjust the probability amplitudes {𝛼,𝛽,𝛾,𝛿} in the general expression for a two-photon polarization state, |𝜓⟩=𝛼|𝐻𝐻⟩+𝛽|𝐻𝑉⟩+𝛾|𝑉𝐻⟩+𝛿|𝑉𝑉⟩, thereby accessing any point in the two-photon polarization entanglement Hilbert space. This requires a mapping between the experimentally controllable parameters and the probability amplitudes listed above. We will describe this mapping for our particular experimental setup and will share results showing the states that our system is able to access.

Quantum Illumination (QI)¹ is a proposed remote targeting protocol using entangled states in which the experimenter sends a signal towards an expected target in a noisy environment that has a probability of reflecting off the surface or, in the case of no surface, being lost. Upon return, the noisy signal is jointly measured with the idler, which has been held in local memory, to determine if the surface has been detected. The idler effectively increases the brightness of the noisy signal to help distinguish it from the surrounding noise. Even though the returned mixed state is in the un-entangled regime, it has been shown that QI outperforms a conventional protocol that only sends separable states as the signal. We analyzed QI as a quantum channel discrimination protocol and circumvented computational issues that rely on diagonalization of the quantum states by using the normalized Hilbert-Schmidt (NH-S) inner product as a measure of state distinguishability.² Because the NH-S inner product is simple to compute, for a choice of entangled pure state and bipartition, we were able to rank the performance of QI entirely in terms of the dimension of the composite Hilbert space and the purity of the idler subsystem. We also showed that the greatest advantage gained by quantum illumination over conventional illumination occurs when one uses a Bell state, and for a fixed dimension d, the optimal performance of QI is achieved when the purity of the idler subsystem is minimal. In this talk, we review the results of,² and present our progress on extending this analysis to a broader class of quantum information protocols beyond QI.

Quantum key distribution (QKD) is one of the most mature quantum technologies and can provide quantum-safe security in future communication networks. Since QKD in fiber is limited to a range of few hundred kilometers, one approach to bridge continental scale distances may be the use of high altitude pseudo satellites (HAPS) as mobile trusted nodes in the stratosphere. In parallel, free-space laser communication for high rate data transmission has been a subject of research and development for several decades and its commercialization is progressing rapidly. Important synergies exist between classical free-space communication and QKD systems since the quantum states are often implemented using the same degrees of freedom such as polarization or field amplitude and phase. These synergies can be used to benefit from the progress in classical free-space laser communication in QKD applications. In this paper, the use case of QKD in a stratospheric environment is described wherein HAPS may serve as relay station of secret keys and encrypted data. The mission scenario and HAPS capabilities are analyzed to derive special requirements on the stratospheric laser terminal, the link geometry and the ground segment with respect to a feasibility demonstration. To obtain a flexible and compatible system, discrete variable and continuous variable QKD protocols are considered to be implemented side by side in the HAPS payload. Depending on the system parameters, it can be beneficial to use the one or the other kind of protocol. Thus, a direct comparison of both in one and the same system is of scientific interest. Each of the protocols has particular requirements on coupling efficiency and implementation. Link budget calculations are performed to analyze possible distances, key rates and data transmission rates for the different schemes. In case of the QKD system, the mean coupling efficiency is of main interest, i.e. signal fluctuations arising from atmospheric turbulence must be taken into account in the security proof, but the buffered key generation relaxes real-time requirements. This is different to classical communications, where the corresponding fading loss must be assessed. A system architecture is presented that comprises the optical aircraft terminal, the optical ground terminal and the most important subsystems that enable implementation of the considered QKD protocols. The aircraft terminal is interfaced with the dedicated quantum transmitter module (Alice) and the ground station with the dedicated quantum receiver module (Bob). The optical interfaces are SMF couplings which put high requirements on the receiving optics, in particular the need for wave-front correction with adaptive optics. The findings of the system study are reviewed and necessary next steps pointed out.

Quantum light is a key resource for the development of quantum-enhanced technologies such as secure communications, quantum networks, distributed quantum computing, metrology, etc. The development of these technologies requires light sources emitting single photons in a pure quantum state as well as efficient photon-photon gates. Such sources and gates can be obtained making use of the single-photon sensitivity of an atomic transition. We study artificial atoms in the form of semiconductor quantum dots to develop building blocks for optical quantum technologies. We use the tools of opto-electronics and nanotechnology to fabricate close to ideal atom-photon interfaces, where a single artificial atom interacts with a single mode of the optical field. We show that cavity quantum electrodynamics allows to largely isolate the artificial atoms from all sources of decoherence such as charge noise and crystal vibrations. We obtain bright solid-state sources of single photons with very high quantum purity, not only in the frequency basis, but for the first time also in the photon number basis. Finally, we have made progresses toward the development of efficient two-photon gates, with devices performing as nonlinear switches at the single-photon level. Some references: High-performances quantum light sources Somaschi et al., Nature Photonics 10, 340 (2016) Grange et al, Phys. Rev. Lett. 118, 253602 (2017) Senellart, Solomon and White, Nature Nanotechnology 12, 1026 (2017) J. C. Loredo, C. Anton, et. al, arXiv:1810.05170 Scaling-up optical quantum computing Loredo et al., Optica 3, 433 (2016) Loredo et al., Phys. Rev. Lett. 118, 130503 (2017) Toward efficient photon-photon gates Giesz et al., Nat. Comm 7, 11986 (2016) De Santis et al, Nature Nanotechnology 12, 663 (2017)

Thanks to their unmatched specificity, nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy – jointly referred to as spin-based analytics – are tools of major importance in biology, chemistry, medicine and physics because they allow for the use of a spin (nuclear or electron) as an extremely sensitive, nanoscopic quantum probe of its electronic and magnetic environment inside a molecule. However, their main limitations are high equipment complexity and cost as well as a relatively poor sensitivity due to the very small thermal polarisation of the spin ensembles at room temperature. This poor sensitivity in turn severely compromises the required measurement time, the achievable signal-to-noise ratio and the minimum sample size. In the proposed talk, we will first introduce the so-called ESR-on-a-chip approach as a new tool in ESR spectroscopy that allows for a CMOS integrated manipulation and detection of electron spins up to very high frequencies in the hundreds of Gigahertz range. We will then discuss the use of nitrogen vacancy (NV) centers in diamond as a potential tool for hyperpolarizing a nuclear spin ensemble at ambient conditions using a laser and the abovementioned ESR-on-a-chip sensors as a compact and cheap, yet high-performance microwave source. Finally, we will introduce the NMR-on-a-chip approach, which integrates an entire NMR spectrometer into a tiny CMOS application specific integrated circuit (ASIC), as a very promising path towards miniaturizing the entire NMR spectrometer including the NV-based hyperpolarization into a compact portable system, which can extend the application range of NMR into entirely new areas including personalized medicine.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and cold-atom interferometry systems. This includes the [5s²S_1/2-5p²P_1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s₂ ¹S₀-5p¹P₁] cooling transition in neutral strontium clocks at 461 nm and the [5s² s_1/2 – 6p²P_3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

Quantum Radar is a promising technology that could have a strong impact on the civilian and military realms. In this study we introduce a new concept design for implementing a Quantum Radar, based on the time and polarization correlations of the entangled photons for detection and identification and tracking of high-speed targets. The design is focused on extracting high resolution details of the target with precision timing of entangled photons that provides important operational capabilities like distinguishing a target from a decoy. The quantum entanglement properties guarantee the legitimacy of the photons captured by the search telescope. Time correlations of the photon detection events can be extracted via cross-correlation operation between two sets of photon detection time-tags for the entangled photons. The fact that the wavelengths of the entangled photons can be tuned also makes the Quantum Radar concept an enticing candidate for tracking stealth objects. We present the proof-of-principle test results of the Quantum Radar and discuss the technical challenges and limitations of the design.