In this talk, we present designs of photonic integrated circuits and components for applications in quantum communications. The goal is to miniaturize the instrumentation while optimizing the security of various quantum key distribution protocols, such as BB84 with different qubit encodings, and measurement-device-independent protocols (MDI-QKD). The proposed components include diffraction gratings for polarization coupling, on-chip path-polarization interconverters, nanoantennas, and graphene nanocavities for coupling individual photon emitters. We simulate the behavior of the proposed circuits and components, and analyze the quantum key error for different parameters, comparing the advantages of single photons and weak coherent states.

Quantum Key Distribution (QKD) is a cutting-edge communication method that enables secure communication between two parties. Continuous-variable QKD (CV-QKD) is a promising approach to QKD that has several advantages over traditional discrete-variable systems. Despite its potential, CV-QKD systems are highly sensitive to optical and electronic component impairments, which can significantly reduce the secret key rate. In this research, we address this challenge by modeling a CV-QKD system to simulate the impact of individual impairments on the secret key rate. The results show that laser frequency drifts and small imperfections in electro-optical devices such as the beam splitter and the balanced detector have a negative impact on the secret key rate. This provides valuable insights into strategies for optimizing the performance of CV-QKD systems and overcome limitations caused by component impairments. By offering a method to analyze them, the study enables the establishment of quality standards for the components of CV-QKD systems, driving the development of advanced technologies for secure communication in the future.

The present study analyzes the efficiency of employing quantum error correction codes (QECC) to encode quantum information states in the context of Quantum Key Distribution (QKD). Specifically, the possibility of enhancing the security and reliability of QKD systems by adding a secondary layer of quantum coding to the states emitted by Alice in the \textit{Prepare-and-Measure} protocols is exhaustively quantified. Such an encoding scheme would be expected to be achievable by means of quantum hardware potentially available in the mid-term. This last statement refers to the assumed reasonable interconnectivity and scalability requirements that may be imposed on the physical encoding capabilities of a quantum processor for the case here considered, since only 1-qubit states are used in QKD. The model for quantum states transmission here considered does not impose any restrictions on the quantum channel, but does assume that the noise and errors to which qubits may be subject in QKD links can be characterized by discrete transformations. That is, depending on the physical encoding scheme chosen for photon's polarization, errors such as bit-flip or phase-shift errors (among others) can be corrected through logical gates derived from Pauli operators, which, along with identity, form the basis $\{I,X,Y,Z\}$ for 1-qubit discrete error operators of the form: \begin{equation} E = \left(\begin{array}{cc} \alpha_0 & \alpha_1\\ \alpha_2 & \alpha_3 \end{array}\right) \end{equation} \medskip Such a consideration imposes the need to be able to identify and correct up to a total of $k = 1 + 3n$ different types of errors (including no error at all, bit-flip, phase-shift, and combinations of the previous) that may affect a QKD state (encoded in an $n-$qubit physical state). In line with previous scalability arguments, that requires for the number of physical qubits needed to achieve such encoding to be lower bounded by the product of the previous magnitude and the dimension of the quantum code $C$ used (which, in the context of QKD, shall be $\mathrm{dim}(C) = 2$). Therefore, if $m=1$ is the number of qubits to be encoded for each transmitted state in QKD, the condition: \begin{equation} 2^n \geq dim(C)(1+3n) \end{equation} imposes a a minimum of $n=5$ physical qubits in a quantum algorithm to carry out encoding and correction of a 1-qubit quantum state. However, it should be noted that, beyond the anticipated error types, the efficiency of identifying errors in a key distilled through QKD (i.e., for all purposes, the correctable QBER associated with each transmission) will be all the more efficient the greater the number n of physical qubits available in a processor for such encoding (of the order of $2^{n-1}$). Thus, the minimum requirements of the quantum hardware topology for the feasibility of this type of encoding are specified, as well as the optimal trade-off in terms of the assumable QBER against different types of attacks, supported by future advances in quantum processor scalability. In this sense, beyond the security considerations associated with QKD implementations of this nature, the goal of this analysis is to parallelly discern the potential speed-up of employing quantum algorithms to carry out error correction of QKD keys and their potential superiority over classical error correction processes in the future. In this regard, two types of QECC are tested in this work. On the one hand, the widespread use of low-density parity check (LDPC)-type linear codes (being linearity a requirement that quantum error correction codes must necessarily satisfy) naturally leads to considering their use in Quantum CSS (Calderbank, Shor \& Steane) codes. The efficiency and performance benefits of LDPC codes applied to QKD are therefore as well transferable to a quantum processor in this context. The performance of these codes applied in QKD is contrasted, secondly, with stabilizer codes. It can be anticipated that the latter may present challenges in the initial algorithm for the encoding of the states emitted by Alice, however the decoding circuit algorithms can be implemented with relative simplicity -albeit scalability limitations- through 1-qubit logical gates (such is also the case with CSS codes once the parity matrix of the LDPC code is known, whose speed advantages over the classical use of belief-propagation algorithms are showed here). On the previous precepts, this study focuses on carrying out a comparative analysis of the convenience of potentially benefitting from the performance of either type of code, while analyzing technical considerations derived from the experimental implementation of QECC protocols in this QKD hybrid approach. The most important considerations are the following: -Complexity. From the point of view of reliability of these types of implementations, potential disadvantages are analyzed in terms of complexity added to real physical systems. Not only is the experimental complexity increase of combining quantum hardware with QKD optical transmissions estimated, but also the anticipation of additional error sources, considering the acceptable threshold values of decoding techniques and calibration errors for real applications and security proofs. - Efficiency. In terms of efficiency and overall code performance, estimated times (for different prospective states of quantum processor advancement) for quantum key generation through these techniques are simulated, and the circumstances under which each may be most convenient are identified. - Components demands. Increased demand for quality of the optics involved in the QKD protocol is expected. Protocols of this nature further increase the demand for high-quality transmissions, especially regarding photon sources, which may have a significant impact on both implementability and its associated costs. - Overhead. Additional overhead needs are projected in terms of code design, number of qubits required depending on the use case, as well as measurement operators necessary for error detection and correction. Consequently, partial limits have been found on the amount of data that can be transmitted in a QKD system that integrates this methodology, which is projected to be overcome when widely available quantum hardware reaches sufficient maturity. - Side channel attacks. Possible vulnerabilities to quantum hacking are preliminarily identified, and a testing method is suggested for this type of QECC-based QKD systems. In addition to the previous analyses, the authors note that one of the most significant features of -both of- the codes here analyzed is that they carry out the identification of errors that affect quantum states at the time of reception, while preserving the encoded quantum information in photons. In this sense, a protocol of these characteristics allows to anticipate, in some applications, the error correction process to the security analysis (although the syndromes of each of the states can be stored classically and the correction processed once the QBER estimation is finished). This can constitute a significant disadvantage in unnecessary computational energy costs when the transmission is not considered secure, but may also be exploited for beneficial applications on certain use cases. With all of the above, the work here presented collects the results on the aforementioned considerations, quantitative cost analysis and future feasibility prospects of this QECC-QKD proposal, as well as details on design and integration considerations, and requirements of both the QKD and quantum hardware components that support this type of implementation.

Practical implementations of quantum key distribution (QKD) protocols can introduce additional degrees of freedom in the quantum states that may render them distinguishable to an eavesdropper. This is the case of QKD systems using a different laser source to generate each quantum state, which can lead to temporal, spectral and/or spatial differences among them that can be exploited by a malicious party to extract information of the key. In this work we characterize, and experimentally verify, a side-channel attack on spatially distinguishable states against free-space QKD systems with misaligned laser sources. Specifically, for those emitting Gaussian beams, which is the most common case in free-space QKD. The attack makes theoretically unsafe any QKD system with any angular misalignment between the laser sources. Finally, we propose two countermeasures to eliminate the spatial distinguishability and secure the key exchange.

Regarding the security concerns arising from the threat that quantum computing algorithms pose on some today’s most efficient and widely implemented public-key cryptographic systems, it has become necessary to find alternative cryptographic techniques to support private (or symmetric) key cryptographic schemes by means of key distribution. In this article we survey the emerging technology of quantum key distribution (QKD), which enables two separate parties to distill a random secret key only known by them and whose security relies on the principles of quantum physics, with the advantage of also enabling the detection of potential eavesdroppers. Among the different existing approaches to QKD, a distinction is made between discrete and continuous variable protocols. We focus here on continuous variable (CV-QKD) protocols, which consist of encoding information in optical coherent states that can be transmitted by fiber by means of off-the-shelf devices widely used in today’s telecommunications industry. An important advantage is that these components have a significantly lower cost in comparison to those used in other QKD protocols. The present study highlights some information-theoretic aspects of CV-QKD while putting special efforts into the security analysis of these protocols (such as parameter estimation or error correction techniques) derived from the practical implementation, as well as the challenges this technology faces in its still ongoing development.

This paper investigates the generation of quantum entanglement by means of conditional stimulated Raman adiabatic passage (STIRAP) based on Rydberg blockade. The paper compares the entanglement fidelities in three-level and four-level schemes and analyzes the adiabatic conditions in both cases. In particular, Green–Horne–Zeilinger states can be deterministically generated in an atomic ensemble interacting with a single control atom.

This work proposes to investigate the photon statistics of the light transmitted and reflected by a two-dimensional array of interacting atoms. The reflected beam is characterized by photon antibunching. On the other hand, in the transmitted beam the indistinguishability between the driving laser photons and the photons re-emitted by the atoms results in photon bunching. The overlap between the driving and scattered fields is enhanced by the cooperative optical response of the atomic array. In the examples used in this paper, up to 25% of the transmitted photons are grouped in pairs. The simulations are carried out using the stochastic method of quantum trajectories.

A method is proposed to produce atomic ensembles with sub-Poissonian atom number distributions. The method consists of removing the excess atoms using the interatomic interactions induced by Rydberg dressing. The selective removal of atoms occurs via spontaneous decay into untrapped states using an electromagnetically induced transparency scheme. Ensembles with the desired number of atoms can be produced almost deterministically. Numerical simulations predict a strong reduction of the atom number fluctuations, with the variance twenty times less than the Poisson noise level (the predicted Fano factor is F = 0.05). Strikingly, the method is suitable for both fermions and bosons. It solves the problem of the atom-number fluctuations in bosons, whose weak interactions have usually been an obstacle to controlling the number of atoms.

Combining the quantum optical properties of single-photon emitters with the strong near-field interactions available in nanophotonic and plasmonic systems is a powerful way of creating quantum manipulation and metrological functionalities. The ability to actively and dynamically modulate emitter-environment interactions is of particular interest in this regard. While thermal, mechanical and optical modulation have been demonstrated, electrical modulation has remained an outstanding challenge. Here we realize fast, all-electrical modulation of the near-field interactions between a nanolayer of erbium emitters and graphene, by in-situ tuning the Fermi energy of graphene. We demonstrate strong interactions with a >1000-fold increased decay rate for ~25% of the emitters, and electrically modulate these interactions with frequencies up to 300 kHz – orders of magnitude faster than the emitter’s radiative decay (~100 Hz). This constitutes an enabling platform for integrated quantum technologies, opening routes to quantum entanglement generation by collective plasmon emission or photon emission with controlled waveform.

Atomic layer deposited (ALD) Y2O3 thin films have been thoroughly investigated for optical or electronic applications. The coherent spectroscopy of lanthanide ions doped into this material has also...

We describe a novel method for producing Greenberger-Horne-Zeilinger states in cold atoms coupled to a superconducting coplanar cavity. In the proposed scheme, atoms interact between each other by virtual photon exchange via a cavity mode. These interactions cause an asymmetric Rydberg blockade mechanism that suppresses simultaneous excitations into different atomic Rydberg levels, thus forcing all atoms to occupy the same Rydberg state. This mechanism has effect even if the atoms are separated by distances of the order of millimeters. The atomic populations are transferred adiabatically from the ground state into the entangled state by following a collective dark state with low dissipation rates.

We present an efficient method for generating maximum entanglement in one-dimensional atomic lattices. The proposed method relies on adiabatic rapid transfer into two Rydberg states with strongly asymmetric interactions. The method is suitable for Rydberg S states in the absence of applied electrostatic fields. We show numerical simulations of entanglement generation in rubidium atoms using calculated van der Waals potentials under realistic experimental conditions. We study the effect of the chosen Rydberg states on the final entanglement.

Superconducting circuits in the microwave regime are one of the most promising candidates for quantum information processing. However, while they can process quantum information on very short timescales, they lack the ability to store information for extended periods of time. A possible alternative would therefore be a hybrid quantum system, in which the quantum information is processed by superconducting circuits and stored in a different quantum

The coherence of quantum systems is crucial to quantum information processing. While it has been demonstrated that superconducting qubits can process quantum information at microelectronics rates, it remains a challenge to preserve the coherence and therefore the quantum character of the information in these systems. An alternative is to share the tasks between different quantum platforms, e.g. cold atoms storing the quantum information processed by superconducting circuits. In our experiment, we characterize the coherence of superposition states of 87Rb atoms magnetically trapped on a superconducting atom-chip. We load atoms into a persistent-current trap engineered in the vicinity of an off-resonance coplanar resonator, and observe that the coherence of hyperfine ground states is preserved for several seconds. We show that large ensembles of a million of thermal atoms below 350 nK temperature and pure Bose-Einstein condensates with 3.5 x 10^5 atoms can be prepared and manipulated at the superconducting interface. This opens the path towards the rich dynamics of strong collective coupling regimes.

We describe the preparation of ultracold atomic clouds in a dilution refrigerator. The closed-cycle 3 He/ 4 He cryostat was custom made to provide optical access for laser cooling, optical manipulation and detection of atoms. We show that the cryostat meets the requirements for cold atom experiments, specifically in terms of operating a magneto-optical trap, magnetic traps and magnetic transport under ultrahigh vacuum conditions. The presented system is a step toward the creation of a quantum hybrid system combining ultracold atoms and solid-state quantum devices.

We describe the preparation of ultra cold atomic clouds in a dilution refrigerator. The closed cycle 3He/4He cryostat was custom made to provide optical access for laser cooling, optical manipulation and detection of atoms. We show that the cryostat meets the requirements for cold atom experiments, specifically in terms of operating a magneto-optical trap, magnetic traps and magnetic transport under ultra high vacuum conditions. The presented system is a step towards the creation of a quantum hybrid system combining ultra cold atoms and solid state quantum devices.

We calculate interaction potentials between three rubidium Rydberg atoms in the absence of external fields. Several three-atom states are considered: vertical bar nS, nS, n'S > and vertical bar nD, nD, nD > (36 < n < 75). Among the considered states, nonadditive potentials appear in the S states if n not equal n' or if vertical bar n - n'vertical bar is small, and also in D states. Interatomic potentials are nonadditive when they induce intensive mixing between three-atom states of the same zero-order energy. The consideration of nonadditive potentials may add a new dimension to the research into Rydberg atoms.

We observe the shift of Rydberg levels of rubidium close to a copper surface when atomic clouds are repeatedly deposited on it. We measure transition frequencies of rubidium to S and D Rydberg states with principal quantum numbers n between 31 and 48 using the technique of electromagnetically induced transparency. The spectroscopic measurement shows a strong increase of electric fields towards the surface that evolves with the deposition of atoms. Starting with a clean surface, we measure the evolution of electrostatic fields in the range between 30 and 300 mu m from the surface. We find that after the deposition of a few hundred atomic clouds, each containing similar to 10(6) atoms, the field of adsorbates reaches 1 V/cm for a distance of 30 mu m from the surface. This evolution of the electrostatic field sets serious limitations on cavity QED experiments proposed for Rydberg atoms on atom chips.

We describe an experimental system that integrates the techniques for producing ultracoldatomic gases with the techniques for cooling solid bodies to cryogenic temperatures.Ultracold clouds of 87Rb are prepared in a trap setup based on room-temperaturecoils and subsequently transported to a superconducting microstructure by means of opticaltweezers. The superconducting microstructure generates a magnetic microtrap and is cooledby a helium-flow cryostat that can achieve temperatures down to 2 K. Both theroom-temperature trap setup and the superconducting microtrap are installed in the sameultra-high-vacuum chamber. The presented system is well suited to create hybrid quantumsystems by combining ultracold atomic gases and superconducting devices.