Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Quantum information is a rapidly-growing interdisciplinary field at the intersection of information science, computer science, mathematics, philosophy, and quantum science. This fruitful field of research is at the core of our developments of quantum technologies, while widening the frontiers of our fundamental knowledge, and has achieved remarkable progress in the last few decades. Regardless of its scientific success, quantum information is not exempt from the intrinsic features that come from the fact that scientists are humans and members of society: both the good and the bad of our social practices leak into the scientific activity. In our scientific community, diversity and equal opportunity problems are particularly difficult to observe due to social, economic, or cultural barriers, often remaining invisible. How can our lack of awareness negatively influence the progress of science in the long term? How can our community grow into a better version of itself? This article reflects on how research events—such as conferences—can contribute to a shift in our culture. This reflection draws on what we learn from Q-turn: an initiative triggered by postdoctoral researchers to discuss these questions, and by doing so raise awareness about diversity issues and equal opportunities in quantum science. In addition to the high calibre of science, one of Q-turn’s main missions is to foster an inclusive community and highlight outstanding research that may be under-appreciated in other high-impact venues due to systemic biases. As well as a scientific programme, Q-turn features talks and discussions on issues that affect the quantum information community, ranging from diversity and inclusion, health and mental health, to workers’ rights. In this perspective article, we will consider Q-turn as an example of how a research community can work to tackle systematic biases, review the successes, and identify further points for development.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We propose and study ways speeding up of the entangling operations in the trapped ions system with high fidelity. First, we find a scheme to increase the speed of a two-qubit gate without the limitation of trap frequency, which was considered as the fundamental limit. Second, we study the fast gate scheme for entangling more than two qubits simultaneously. We apply the method of applying multiple frequency components on laser beams for the gate operations. In particular, in order to avoid infinite terms from the coupling to carrier transition, we focus on the phase-insensitive gate scheme here. We carefully study the effect of large excitation of motional mode beyond the limit of Lamb–Dicke approximation by including up to second order terms of the Lamb–Dicke parameter. We study the speed limit of multi-qubit global entangling gates without individual addressing requirements. Furthermore, our gates can be made insensitive to the fluctuation of initial motional phases which are difficult to stabilise in the phase-insensitive gate scheme.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Simulating quantum systems is believed to be one of the most important applications of quantum computers. On noisy intermediate-scale quantum (NISQ) devices, the high-level circuit designed by quantum algorithms for Hamiltonian simulation needs to consider hardware limitations such as gate errors and circuit depth before it can be efficiently executed. In this work, we develop a hardware-agnostic circuit optimization algorithm to reduce the overall circuit cost for Hamiltonian simulation problems. Our method employ a novel sub-circuit synthesis in intermediate representation and propose a greedy ordering scheme for gate cancellation to minimize the gate count and circuit depth. To quantify the benefits of this approach, we benchmark proposed algorithm on different Hamiltonian models. Compared with state-of-the-art generic quantum compilers and specific quantum simulation compiler, the benchmarking results of our algorithm show an average reduction in circuit depth by 16.5× (up to 64.1×) and in gate count by 7.8× (up to 23.7×). This significant improvement helps enhance the performance of Hamiltonian simulation in the NISQ era.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quantifying how far the output of a learning algorithm is from its target is an essential task in machine learning. However, in quantum settings, the loss landscapes of commonly used distance metrics often produce undesirable outcomes such as poor local minima and exponentially decaying gradients. To overcome these obstacles, we consider here the recently proposed quantum earth mover’s (EM) or Wasserstein-1 distance as a quantum analog to the classical EM distance. We show that the quantum EM distance possesses unique properties, not found in other commonly used quantum distance metrics, that make quantum learning more stable and efficient. We propose a quantum Wasserstein generative adversarial network (qWGAN) which takes advantage of the quantum EM distance and provides an efficient means of performing learning on quantum data. We provide examples where our qWGAN is capable of learning a diverse set of quantum data with only resources polynomial in the number of qubits.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We demonstrate the generation, 40 km fiber transmission, and homodyne detection of single-mode squeezed states of light at 1550 nm using real-time phase control of a locally generated local oscillator (LO), often called a ‘real LO’ or ‘local LO’. The system was able to stably measure up to around 3.7 dB of noise suppression with a phase noise uncertainty of around 2.5°, using only standard telecom-compatible components and a field-programmable gate array. The compactness, low degree of complexity and efficacy of the implemented scheme makes it a relevant candidate for long distance quantum communication in future photonic quantum networks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>An atomics package is the heart of any atom based quantum sensing device. Here we report on the realisation of a field deployable atomics package for alkaline earth atoms, e.g. Sr or Yb. In terms of size (∼121 L), weight (&lt;75 kg) and power (∼320 W), it is the <jats:italic>smallest package to date</jats:italic> which is designed to load Sr atoms into an optical lattice. It consists of an ultra-high vacuum assembly (&lt;4 L), lasers, magnetic field coils &amp; optics required for cooling &amp; trapping as well as a module for imaging &amp; detection. The package can routinely produce ultra cold and dense samples of 1.6 × 10<jats:sup>5</jats:sup> <jats:sup>88</jats:sup>Sr atoms trapped in a 1D optical lattice in less than a second. Its robustness has been demonstrated by conducting two transportation campaigns within <jats:italic>out-of-the-lab</jats:italic> environments. This advancement will have impact not only on transportable optical clock development but also will influence the wider areas of quantum science and technologies, particularly requiring field deployable cold atom based quantum sensors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Microwave control of trapped ions can provide an implementation of high-fidelity two-qubit gates free from errors induced by photon scattering. Furthermore, microwave conductors may be embedded into a scalable trap structure, providing the chip-level integration of control that is desirable for scaling. Recent developments have demonstrated how amplitude modulation of the gate drive can permit a two-qubit entangling operation to become robust against motional mode noise and other experimental imperfections. Here, we discuss a method for the numerical optimization of the microwave pulse envelope to produce gate pulses with noise resilience, considerably faster operation and high energy efficiency.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Continuous variable quantum key distribution (CV-QKD) is a promising emerging technology for the distribution of secure keys for symmetric encryption. It can be readily implemented using commercial off-the-shelf optical telecommunications components. A key requirement of the CV-QKD receiver is the ability to measure the quantum states at the correct time instance and rate using the correct orthogonal observables, referred to as synchronization. We propose a digital synchronization procedure for a modern CV-QKD system with locally generated local oscillator for coherent reception. Our proposed method is modulation format independent allowing it to be used in a variety of CV-QKD systems. We experimentally investigate its performance with a Gaussian modulated CV-QKD system operating over a 10–20 km span of standard single mode fibre. Since the procedure does not require hardware modifications it paves the way for cost-effective QKD solutions that can adapt rapidly to changing environmental conditions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quantum computing opens exciting opportunities for kernel-based machine learning methods, which have broad applications in data analysis. Recent works show that quantum computers can efficiently construct a model of a classifier by engineering the quantum interference effect to carry out the kernel evaluation in parallel. For practical applications of these quantum machine learning methods, an important issue is to minimize the size of quantum circuits. We present the simplest quantum circuit for constructing a kernel-based binary classifier. This is achieved by generalizing the interference circuit to encode data labels in the relative phases of the quantum state and by introducing compact amplitude encoding, which encodes two training data vectors into one quantum register. When compared to the simplest known quantum binary classifier, the number of qubits is reduced by two and the number of steps is reduced linearly with respect to the number of training data. The two-qubit measurement with post-selection required in the previous method is simplified to single-qubit measurement. Furthermore, the final quantum state has a smaller amount of entanglement than that of the previous method, which advocates the cost-effectiveness of our method. Our design also provides a straightforward way to handle an imbalanced data set, which is often encountered in many machine learning problems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quantum key distribution (QKD) allows remote parties to share secret keys with proven security. To guarantee the security of practical QKD, the imperfections in realistic devices need to be characterized and considered in practical security analysis. Particularly, a standard QKD system normally uses optical isolator or optical circulator in the transmitter to prevent the injection from external light. Here we find that the optical isolators and circulators, commonly based on the Faraday effect, are subject to the perturbation from external magnetic fields. With this, we provide a comprehensive analysis of the security for practical QKD due to external magnetic effect. We experimentally demonstrate the influence of magnetic fields on both the transmittance and the isolation of standard optical isolators and circulators. We analyze the security risks caused by the potential magnetic attacks, together with the corresponding countermeasures.</jats:p>