Optical quantum memory is a device that can store the quantum state of photons and retrieve it with high fidelity on demand. Many approaches to quantum memory have been proposed and demonstrated. Quantum memory can be used to enhance performance in many quantum communication systems and processes such as deterministic single photon sources, photon interference, measurement device independent (MDI) quantum key distribution (QKD), quantum teleportation and quantum repeaters.

Quantum key distribution is now a mature quantum communication protocol which allows the verifiably secure sharing of encryption keys between two communicating parties. It seeks to address potential vulnerabilities of data transmission and storage, offering a realistic possibility to share encryption keys which are robust to eavesdropping attacks and future-proof against hacking. Fibre-optic implementations of quantum key distribution currently have a limited practical transmission distance, of less than 400 km, making commercial applications limited. Quantum-specific amplifier/repeater technology is not yet mature enough to increase the transmission distance to achieve global capabilities. Optical fiber is also impractical and expensive for applications where a remote area or moving platform are involved. In recent years, long-distance free-space quantum communications using low-Earth orbit satellites has seen an increase in interest from the academic community as well as from industrial organisations and national research institutes. The source of this new interest was a series of proof-of-principle demonstrations of satellite-based quantum key distribution in 2017. The use of free-space channels implementing airborne or satellite platforms also opens a range of new applications for quantum communications, as they allow coverage of remote areas, moving platforms, and also avoid the requirement of spooling fibre through volatile regions. This presentation will give a general introduction to satellite-based quantum communications, an overview of the field, and discuss future endeavours. The talk will also include an overview of our research into novel photonic technology, such as the use of detector array technology, for the optical ground station receiver.

This paper discusses high-performance quantum optical circuits for processing polarization entanglement between correlated photon pairs. The primary application of this technology is secure communication links with quantum encryption that rely on the inherent properties of laser light and physical-layer processes in optical components. It can be used for generating identical pairs of encryption keys in quantum key distribution applications, for a variety of encryption protocols ranging from single-photon to multiple-photon, keyed communication in quantum noise, but we see a potential spectrum extending further to include multi-access systems. Success in these applications can be achieved by introducing additional degrees-of-freedom in each single-particle quantum state. We see hyper-entanglement as a very promising approach to do this. The concept is visualized by having multiple entangled photons with specifically assigned frequencies in the 100 GHz ITU grid and arriving simultaneously from multiple sources. Processing of such signals requires hyperspectral optical circuits capable of responding to non-classical features of quantum states. The function of these circuits is to direct the arriving signals along different processing paths to single photon detectors, which can be realized by a combination of different technologies, such as highly selective Lyot filters, dense wavelength division multiplexing, and others. Processing capabilities of these quantum circuits, while seemingly straight-forward in theory, still present a great implementation challenge. Practical operating conditions and characteristics of optical components must be taken into consideration to address the underlying design problems and make this technology feasible. This constitutes the main focus of our paper.

Since its first demonstration in 1995, ghost imaging has provided amazing insights into both classical and quantum physics as well as having found application in, for example, microscopy and imaging under low light conditions. Traditional ghost imaging uses correlations between two photons to reconstruct an image of an object from two systems which each individually know nothing about the object. In the quantum case, the state of the two photons is typically a symmetric, entangled state. Here we investigate the effect that changing the two-photon state's symmetry has on the reconstructed object, by using Dove prisms and a Hong-Ou-Mandel filter. Interestingly, it appears that post-selecting on the anti-symmetric Bell state results in a `double image': a juxtaposition of the original image rotated both clockwise and anti-clockwise. Furthermore, we consider a 4-photon experiment in which two photons, which originate from different entanglement sources and are hence completely independent initially, acquire correlations by way of entanglement swapping via appropriate post-selection on the remaining two photons. In such a setup, post-selecting on the symmetric Bell states results in the original object, but post-selecting on the anti-symmetric Bell state results in a contrast-reversed image of the object. These studies highlight the fundamental importance that state symmetry plays in quantum imaging.

It is now widely believed that in the near future quantum sensing devices will be realized to outperform some of our current sensing devices. As quantum metrology protocols are comprised of three stages: probe state preparation, sensing followed by readout, it is important that the time for all three steps are accounted when the performance is determined. Historically the time required for the first and last stages has usually been neglected, which could be unrealistic when entangled resources are used. Entangled resources lose their sensitivity in time under noise, and hence the sensing time with these resources is limited. To avoid this degradation of sensitivity, we could repeat the protocol to refresh the entanglement in the resources. However, as the process of the preparation and the readout of the entangled probe state costs us a certain period of time, we need to optimize the sensing protocol throughout the three stages. We find that an entangled state probe can give an advantage over separable ones only if the entangled state preparation and readout times are lower than a certain threshold and illustrate this result in an optical setting.

Placement, routing, and scheduling are essential tasks for near-optimal performance of programs for noisy quantum processors. Reliable execution of an arbitrary quantum circuit on current devices requires routing methods that overcome connectivity limitations while meeting data locality requirements. However, current devices also express highly variable noise levels in both the quantum gates and quantum registers. This requires any routing algorithm to be adaptive to both the circuit and the operating conditions. We demonstrate near-optimal routing methods of noisy quantum states that minimize the overall error of data movement while also limiting the computational complexity of routing decisions. We evaluate our methods against the noise characteristics of a 20-qubit superconducting quantum processor.

Randomness is essential ingredient for applications in cryptography, stochastic simulation, and fundamental science experiments [American Scientist 89, 4 (2001); PRL 115, 25 (2015)]. Common practice is to generate random numbers from mathematical algorithm using Pseudo-random number generators. We are interested in tackling this problem with quantum technology. Phase diffusion in spontaneous emission events is a quantum phenomena with inherent randomness [Opt. Express 19, 21 (2011)]. Implementations of this scheme using pulsed lasers can yield high-speed quantum random number generation (QRNG) [Opt. Express 22, 2 (2014)]. The general interest in the laser phase diffusion QRNG setup has been mainly focused on and motivated by the speed of the random number generations. Little has been stated about the performance of quantum phase noise as a randomness source in QRNG, from the perspective of the physics involved. We reanalyze the process of phase diffusion based QRNG and give an intuitive explaining picture of the underlying physics. Our findings show that a pulsed process is beneficial over the continuous-wave approach and give a upper bound of maximum random bit rate for a given experimental setting. In detail, the output of a semiconductor laser contains fluctuations in intensity and phase. This was originally studied first by Charles Henry in 1982 [J. Lightwave Techn. 4, 3 (1986)]. Based on that one can identify two phenomena contributing to the phase noise, which takes the form of a random walk. First, there will be phase changes due to the carrier-induced change in refractive index in the semiconductor laser. Second, spontaneous emission events take place in the active medium [Opt. Express 19, 21 (2011)]. Naturally, taking spontaneous emission events into account necessitates the application of stochastic laser rate equations for theoretical description. We solve these equations numerically be a Monte-Carlo simulation and investigate several random walk scenarios. One of our essential findings is that the phase fluctuation becomes much larger in the pulsed laser regime than in the continuous-wave mode. Additionally, there is a natural limit on the maximum pulse repetition rate for a given pulse width, since the phase jumps become smaller for shorter pulse distances. Since phase measurement is not a feasible procedure for optical signals, the phase fluctuation needs to be converted to an observable macroscopic parameter such as the optical intensity. This can be done by using an interferometer setup. The whole experimental setting is shown in Fig. 2 (left). An DFB laser diode is modulated using gain-switching technique to generate uniform amplitude signals with random phases. Each signal interferes with neighboring pulses through an unbalanced Mach-Zehnder interferometer. The delay in one arm was set to equal to the pulse repetition rate. With this setup we investigate the influence of several parameters on the shape of the probability distribution determining the quality of QRNG. The theoretical as well as experimental findings can help to find physical standards for QRNG verification rather than the ones based on classical statistical information theory.

The optical beam splitter is one of the main building blocks in photonics-based quantum information processing. Traditionally, beam splitters are used in feed-forward configurations that reflect their natural directional-bias, meaning a photon cannot leave the scattering element through the entrance port. However, this directional-bias constraint can be circumvented by designing new linear-optical configurations that include mirrors in addition to the beam splitter. This directionality removal restores a full symmetry in the scattering element and allows the input photon to leave the system also from the input port. Such a system can be seen as a scattering center, enabling execution of quantum walks on rather complex optical graph networks with great savings in hardware resources when compared with existing approaches utilizing directionally-biased devices. The directionallyunbiased configuration can be realized using different optical systems. Analysis of some originally directional optical devices and the basic principles of their conversion into directionally-unbiased systems form the base of this paper. Several quantum walk procedures executed on graph networks constructed using directionallyunbiased nodes are discussed.

The search for Planck scale effects is one of holy grains of physics. At Fermilab, a system of two Michelson interferometers (MIs) was built for this purpose: the holometer. This device operates using classical light, and, therefore, its sensitivity is shot-noise limited. In collaboration with the Danish Technical University, we built a proof of principle experiment devoted to experimentally demonstrate how quantum light could improve the holometer sensitivity below the shot noise limit. It is the first time that quantum light is used in a correlated interferometric system. In particular the injection of two single mode squeezed state (one in each interferometer) and of a twin-beam state is considered, and the system performance compared in the two cases. In this proceeding, after a general introduction to the holometer purposes and to our experimental set-up, we present some characterization measurements concerning the quantum light injection.

Quantum state engineering and state characterization is a key task in quantum information processing in both discrete and continuous variable systems in the optical domain. In particular, quantum states with non-Gaussian (i.e., non-positive) Wigner quasiprobability distribution functions are crucial to universal, fault-tolerant quantum computing with continuous variables. In this talk, we present our recent results on single-photon Fock state tomography using Photon-Number-Resolving (PNR) measurements. We generated a highly pure narrow-band single-photon Fock state by heralding cavity-enhanced spontaneous-parametric-downconversion from a PPKTP optical parametric oscillator. The Wigner function was reconstructed with photon statistics obtained using superconducting transition-edge sensors with an overall system efficiency of 58(2)%. We then discuss quantum state engineering for pure displaced single-photon Fock states, optical cat states, and approximate GKP states using coherent states and single-photon states along with linear optics and PNR measurements. We report our experimental progress for the same.

It is usually assumed that the energy dependent velocity of a photon moving in a dispersive medium is given by the group velocity at the frequency corresponding to the energy of the photon. This assumption corresponds to the notion that the velocity of a massive particle is determined by its momentum. However, a direct verification of this assumption for a single photon is impossible, since the velocity can only be obtained by measuring the position at two different times, and time-resolved measurements cannot also resolve photon energies. In previous work, I have shown how this limitation can be circumvented for position and momentum, demonstrating that quantum particles do not obey newton's first law in free space. Here, I apply a similar strategy to construct a quantum state in which a non-vanishing percentage of the photons travel a distance x in time t even though the probability of finding any photons with a group velocity of v = x/t is close to zero. Specifically, the suppression of frequencies with group velocities in the vicinity of v = x/t is achieved by destructive interference, while the probabilities of detecting the photons in the initial time window or in the final time window are simultaneously enhanced by constructive interferences between the mutually overlapping wavefunctions. Based on the statistical evidence obtained from separate measurements of single photon arrival times and frequencies, it is then possible to show that the group velocity does not represent the actual velocity at which individual photons propagate through the dispersive medium.

Directionally-unbiased multiports and topological states. The goal is to entangle states associated with distinct topological sectors, and to do so in a way that allows this entangled topology to be readily available for information processing and detection. Specifically, linear optics will be used to produce: (i) winding-number-entangled bulk states, and (ii) an entangled pair of error-protected memory registers. To create the states, a source of initial polarization-entangled light is necessary, specifically type-II spontaneous parametric down conversion (SPDC) in a nonlinear crystal. All further processing requires only linear optical elements. Topological invariants characterize global properties of systems and cannot be easily distinguished by localized measurements. This difficulty in measurement traditionally limits their use in many applications. That problem is solved here by linking topology to a more easily-measured variable, polarization. Polarization and winding number will be tightly correlated (and in fact, jointly entangled with each other), but will serve distinct purposes: winding number provides stability against perturbations, while polarization allows easy access and measurement. Jointly-entangled topologically-protected bulk states. Start with a polarization-entangled photon source, type-II spontaneous parametric down conversion in a nonlinear crystal allowing the two-photon output to be taken as an entangled Bell state. The goal is to convert this into a state of entangled winding number, while keeping polarization entanglement intact to use for control and measurement purposes.

The performance of diamond color centers for quantum information and sensing depends strongly on the quality of the material. Specifically, ultra-pure, low-strain diamond is preferred. However, in order to create color centers, non-carbon atoms must be incorporated into the lattice either by rapid growth or implantation. Both these techniques cause defects and lattice damage that degrades ultimate performance. To overcome these problems, we show a molecule-seeded growth technique that decouples the doping and growth processes. The result is near-deterministic creation of specific color centers in chemically pure, low-strain diamond. Data showing selective growth of H3 and NV centers will be presented, where the NV is used to verify that the crystals are low-strain. Future application to other color centers like silicon, germanium, and tin vacancies will be discussed. Also the possibility of direct growth of more complex quantum registers.

For fiber-based polarization entangled photon pairs generated by four-wave mixing nonlinear effect, a common method to process coincidence counts and reconstruct density matrix of original quantum states is maximum likelihood estimation. Pump power is an essential parameter to investigate throughout the process of entanglement generation, correlation detection and quantum state tomography. Defined as the optical power of input laser pulses that enter dispersion-shifted fiber and generate entangled pairs, pump power directly affects single counts rates for both signal and idler. As noise rate changes accordingly, coincidence to accidental counts ratio does not necessarily increase with more detected counts. We derive relation between pump power and entangled correlation. Because different power is associated with different order of susceptibility, we also study its effect on entangled photon generation rate. Transmission rate through fibers, filters, polarizing beam splitters and other optical components as well as the detection efficiency at each avalanche photodiode are taken into consideration because they contribute to the reliability of photon counting statistics. System error such as measuring basis error is studied whether it is amplified, suppressed or remain invariant with pump power modification. Many parameters’ relations with pump power cannot be simply described as a one-line equation. Therefore, we explain those relations in detail and propose a method of finding a suitable pump power within given circumstances that would serve reconstructing the most accurate quantum state.

We have seen remarkable progress made over the last decade in the realization and exploitation of small-scale quantum communication technologies. The community is now at the stage where we need to consider how quantum repeaters and their associated networks are efficiently designed and implemented. This first requires us to define what a quantum repeater actually is and what functionality it requires. Associated with this is also the issue of its performance and when it is better than non-repeater approaches. In this talk, we explore such issues by taking incites from the conventional telecommunication world and use them to establish a framework for what quantum repeaters are and how their performance can be established. As part of this we clearly show that non-repeater approaches can exceed the direct transmission bounds. We extend these concepts to quantum networks and show one must be extremely careful when establishing the optimal communication bounds within them.

Quantum state discrimination is a fundamental task in quantum information theory and also an important building block in practical quantum information applications. We here consider optimal state discrimination in a noisy environment in which quantum states may be corrupted due to the intervention of an environment. We show that even if there exists an unwanted interaction with an environment and the channel describing the interaction is not yet identified, a measurement prepared in the beginning for optimal state discrimination can be preserved as an optimal measurement ever after a channel use. This means that in a practical realization of quantum communication, verification of a channel can be circumvented and the cost of quantum tomography is saved. We also show a protocol of preserving an optimal measurement for state discrimination.

Information Reconciliation (IR) in QKD is a fundamental step in ensuring Alice and Bob share identical set of bits (reconciled key). IR could be done by one-way or two-way channel coding using an auxiliary public authenticated channel to send parities to correct the actual labels so that the sample labels at Alice and Bob match. We assume that communication is performed through an Optical Wireless (OW) or Free Space Optics (FSO) channel, which effects the received signal by a stochastic fading due to jitter in pointing. The effect is that the received samples do not match with the transmitted ones, this is the reason why IR is necessary in such a system. In a previous work, we analyzed the system performance over FSO channel, uncovering the dependence between performance and system parameters such as fading variance or the telescope gain. In this paper we want to study the overall performance and try to obtain optimal values for the parameters that influence the sign error probability.

Since the second quantum revolution, the growing exploitation of quantum states led to many sophisticated and novel applications [Phil. Trans. R. Soc. Lond. A 361, 1655 (2003). Former, mainly academic research is more and more transferred into real-world quantum technology ready to serve practical tasks. Today, the quantum computer is within reach, satellite based quantum communication already started, and the field of sensing and imaging was revolutionized, too. Non-classical states of light promise a phase sensitivity beyond any classical possibility [PRL 112, 103604 (2014)] and super-resolution capability [Nature Commun. 8, 14786 (2017)]. Moreover, based on quantum correlations, which are rooted in the very heart of quantum mechanics, the imaging of samples with photons that have never interacted with the object is feasible. This science fiction like phenomenon was first investigated by Mandel [PRA 44, 4614 (1991)] and later implemented for actual imaging purposes in the Zeilinger group [Nature 512, 409 (2014)]. We are going to present the transfer of this approach into applicable quantum technology within the realm of the Fraunhofer Key Research Initiative Quantum Methods for Advanced Imaging Solutions (QUILT). The method itself is based on induced coherence without induced emission [PRA 44, 4614 (1991)]. We built a novel compact implementation of this scheme based on only one nonlinear crystal. The crystal is coherently pumped by a laser from two sides such that signal and idler photon pairs can be collinearly emitted in either of two opposite directions (to the right or to the left). On the right side the idler beam will be separated by a dichroic mirror (DM) and interacts with an object (directly placed in front of a mirror or reflective object). On the left side the signal beam will be separated by another dichroic mirror and hit the camera, which is either a sCMOS or an EMCCD. Additional lenses provide the particular imaging. Sine the two possibilities for the idler beam – going right and interact with the object or going left from the nonlinear crystal – interfere the object could be observed by idler detection. However, the same interference can be observed for the signal beams although the never interacted with the object. Hence, the object can be imaged by detecting the signal light only. The crucial point here is the lack of wich-path information of the idler beams in this type of Michelson interferometric setup. The following points should be emphasized: (i) the signal channels never interact with the object, (ii) there is no induced emission due to the idler beam that interacted with the object, and (iii) there is no coincidence detection involved in this scheme. The obvious advantage of this technique is that the wavelength of the idler photons can be tailored to match the interesting spectral range for interaction with the object. At the same time, the signal photons, which are actually detected, can stay in the VIS range where, e.g., Si-based detectors are optimized. Besides the application for life science imaging, we are comparing the quantum imaging properties utilizing momentum correlations or spatial correlations, where the ladder is a modus that was never investigated before. As an outlook we work on significantly enhance SPDC sources in scope of quantum performance and wavelength separation. Obviously, pushing the short-wavelength idler photons further into the deep UV or XUV broadens the range of applications, but must be implemented by non-SPDC photon pair sources.

Ghost imaging gives the possibility of imaging objects with extremely low levels of light, which could be particularly useful for light-sensitive objects. In this study, we varied different important experimental parameters of our all-digital set-up, that condition both the acquisition time and quality of the reconstructed image, with the idea of finding the optimal ones. In addition to this, we introduced machine-learning techniques to include a recognition algorithm that further reduces the time necessary to identify the imaged object. This improvement in efficiency paves the way to use ghost imaging for living specimens.

Trapped ions coupled to optical cavities can be used to build up quantum interfaces between stationary and flying qubits in a quantum network. Shared entangled states between different network nodes have proven to be an essential resource for various applications of a quantum network, such as distributed quantum computation. At a first quantum network node, we have trapped ions in a linear Paul trap and coupled them to an optical cavity two centimeter in length. We have demonstrated entanglement of a single ion with a single photon, and used this high-fidelity operation to entangle two ions in a heralded fashion [1,2]. However, the speed of these operations is intrinsically limited by the ion-cavity coupling strength, which is predetermined by the length of the optical cavity. Fiber-based optical cavities have been coupled to single ions and it has been shown that these microscopic cavities allow access to the strong coupling regime [3]. Operating in this regime would enable quantum communication protocols to be carried out over long distances with enhanced fidelity and efficiency. With this goal, we have designed and constructed a novel ion-cavity system which incorporates a fiber cavity. In my talk, I will introduce basic building blocks of quantum networks based on trapped ions coupled to optical cavities and will present recent results, including simulation and characterization of our fiber-based ion-cavity system. [1] A. Stute et al., Nature. 485, 482 (2012) [2] B. Casabone et al., Phys. Rev. Lett. 111, 100505 (2013) [4] H. Takahashi et al., arXiv:1808.04031 (2018)

We present an inverse weak value amplification (IWVA) scheme to perform precision frequency measurements in an integrated optics environment. The IWVA technique allows us to amplify small signals by introducing a weak perturbation to the system and performing a post-selection on the data. A Bragg grating with two band gaps is used to convert the optical frequency into a phase, and a perturbation is applied to the mode coefficients. We demonstrate the advantages of a Bragg grating with two band gaps for obtaining high transmission and low group velocity. We numerically model the interferometer, and demonstrate that we obtain the desired amplification effect. By using an on-chip device instead of a free space implementation, precision measurements can be carried out in a small volume with reliable performance.

The most common approach to create orbital angular momentum (OAM) carrying vortex beams is by modulating a Gaussian beam with an azimuthal phase profile. This results in a beam with many radial modes and hence reduced power in the desired mode. Here we show that it is possible to select the mode size of the Laguerre-Gaussian basis so that the power in the detected p = 0 mode is maximised. We outline the idea theoretically and confirm it experimentally with both classical and quantum tests, demonstrating a power increase by as much as 50x. The consequence is higher signal-to- noise in optical communication and larger available dimensions in quantum entanglement experiments when using OAM modes as a basis.

We propose an advanced quantum ghost imaging setup for remote sensing applications, which does not rely on preliminary information on the distances of objects and investigate the quantum benefits of this setup for remote sensing applications. The photon source of the setup will be suited for standoff detection with illumination of remote objects in the infrared, while retrieving the spatial information in the visible regime with matured silicon technology. The setup utilizes single photon avalanche detectors with integrated time to digital converters for detection of both photons to capture the full 3D information of an unknown scene and is suited for imaging over large distances, without the necessity of an optical delay line

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