New article by group’s member

PhD student Carlos Bravo-Prieto posted a pre-print article as the result of his summer stay at Los Álamos (NM, USA):

“Variational Quantum Linear Solver: A Hybrid Algorithm for Linear Systems”, by C. Bravo-Prieto (), R. LaRose (), M. Cerezo (), Y. Subasi, L. Cincio and P. J. Coles (). preprint:

In this work, they presented a variational quantum algorithm for solving the quantum linear system problem. On the analytical side, they derived efficient quantum circuits to estimate faithful cost functions, while showing that they are difficult to estimate classically.

Schematic of the quantum linear solver algorithm.

On the numerical side, they studied the scaling of the algorithm run time and found it to be efficient with respect to the condition number and the desired precision:

Furthermore, they implemented the variational algorithm in ‘s quantum computer, for particular problems up to a size of 32×32, which is the largest implementation of a linear system on quantum hardware:

New publication by Dr. Forn-Díaz

The review article by Dr. Forn-Díaz and co-workers from Bilbao and Huston titled “Ultrastrong coupling regimes of light-matter interaction” has finally been published in the prestigious journal Reviews of Modern Physics.

This article reviews the state of the field in the regime in which light and matter interact so strongly that the whole system becomes a new entity with exotic properties. The review particularly focuses on the experimental progress in the last decade on the fields of superconducting qubits coupled to microwave photons and polaritons in semiconductor quantum wells coupled to infrared radiation. This field keeps gathering interest due to its fundamental intricacies (recent works studying the gauge invariance is just one more example), and the potential to find applications in quantum technologies. In fact, Dr. Forn-Díaz is leading a proposal for a European call to fund a project on ultrastrong couplings and quantum technologies.

The landmark of the review is the evolution of the coupling strength normalized to the bosonic mode frequency over time and for many fields. Clearly, experiments have finally managed to enter the USC regime just very recently, and a whole new field is ready to be explored.

Plot of reduced light-matter coupling strength over time for several different fields.

New publication by Dr. Forn-Díaz

Dr. Forn-Díaz has participated in a joint work with the group of Prof. Chris Wilson at IQC Waterloo on the area of generation of entangled states of microwave generation, which has just been published to Physical Review Applied. The work, part of which was carried out during Pol’s postdoctoral position at Prof. Wilson’s lab, focuses on a nonlinear multimode resonator that generates entangled states of radiation by the application of external pumping fields at the suitable driving frequencies. The key aspect of the circuit is the presence of a SQUID at the end of the line which mediates the interaction between different modes. This work is very important for the generation of nonclassical states of microwave radiation, which are applicable in the area of quantum communication and quantum sensing.

The reference of the publication is Phys. Rev. Applied 10, 044019 (2018).

Cartoon of the superconducting resonator (top). In the middle, an optical microscope image of the region with the SQUID. At the bottom the lowest 3 modes are shown as function of the magnetic flux applied to the SQUID.

First QUANTIC-only paper!

The QUANTIC group has produced the first work on by QUANTIC-only members, Artur Garcia-Saez and Jose Ignacio Latorre, on one of the main research directions of the team: quantum algorithms! The manuscript reference is arxiv:1806.02287

In this work the focus is on improving the performance of the nowadays popular variational quantum eigensolvers, or VQE. These algorithms are hybrid in that they have a classical part and a quantum part. The classical part consists of optimization methods which then influence the parameters of a quantum circuit that eventually produces an estimation of a certain parameter. This parameter is then important to calculate binding energies of molecules, for instance. This is actually one of the most promising real-life problems which quantum computers, even the noisy, small-scale ones existing these days.

With the new algorithm designed in this work, the Adiabatically-Assisted Variational Quantum Eigensolver (AAVQE) a modification of usual VQE is introduced, in which one starts from a trivial Hamiltonian that produces an exact estimation of a certain parameter. This feeds in a second step in which the Hamiltonian is slightly less trivial. Eventually one arrives at the real-problem Hamiltonian but with a set of parameters evolved in such a way that the result of the problem is obtained directly. In their wok, Artur and José Ignacio have been able to show that the AAVQE algorithm works very well for classical problems, unlike the usual VQE, as has been recently stated.

An instance of the AAVQE algorithm in action.

 

New review article by Dr. Forn-Díaz

A new article has been completed by QUANTIC group member Pol Forn-Díaz. The review titled ‘Ultrastrong coupling regimes of light-matter interaction’ is a compilation of the evolution of this area in quantum optics exploring the boundaries of the field. Dr. Forn-Díaz has been a pioneer with his PhD as well as postdoctoral work with superconducting qubits coupled to resonators and open systems. The review includes an overview of the history of the Rabi model since its inception, and includes progress in other relevant experimental areas such as polaritons in semiconducting microcavities and other hybrid systems such as molecules in cavities and magnons in microwave resonators.

This work is a collaboration with members of the UPV Bilbao group from Enrique Solano, Lucas Lamata and Enrique Rico, as well as Prof. Jun Kono from the Rice University at Houston, in Texas.

The article has been posted on the arxiv repository with reference 1804.09275.

Figure from the review containing a summary of the main results in different areas with maximum coupling strength normalized to the cavity mode.

New publications by QUANTIC team

Last month was busy, we will be updating the news section with more posts soon.

Some of the most important news relate to publications. The QUANTIC team has added a new preprint of a theory paper on Multipartite entanglement on the publications section titled ‘Multipartite entanglement in spin chains and the Hyperdeterminant‘. The work led by Alba Cervera-Lierta from QUANTIC team studied classes of entangled states and a measure for their ‘entanglement-ness’, called hyperdeterminant. It turns out you need four qutrits to yield maximally entangled states.

In addition, one of the papers under review by Dr. Forn-Díaz has been accepted to the prestigious journal Nature Communications. The work entitled ‘Probing the strongly driven spin-boson model in a superconducting quantum circuit‘ is a combined theory and experiment study of the well-known spin-boson model, taken in a new domain of parameters. Using a superconducting flux qubit coupled to a transmission line, combined with a strong drive on the qubit, led to a dressed state picture in the strongly dissipative regime, which had not been studied yet. The result is important in the area of analog simulation of condensed-matter models using superconducting circuits.

Qubit spectral traces as function of applied strong drive amplitude (y-axis). The splitting is a signature of the strong dissipative regime of the spin-boson model and had never before been observed. An extension of the spin-boson model was needed to match the theoretical predictions. Inset shows the superconducting flux qubit used in this work, the arrows represent the superposition of persistent current states of the qubit.

New paper on Two-photon Quantum Rabi model

A new publication has appeared led by Dr. Pol Forn-Díaz on the theoretical implementation of the two-photon quantum Rabi model. A purely genuine, two-photon interaction is a very intriguing effect, and it is very difficult to find a system in nature that exhibits such phenomenon. In this work, which is a collaboration with the group of Prof. Solano at UPV-EHU in Bilbao, the team found a superconducting circuit which is described by a purely two-photon interaction. The work also investigated the very intriguing phenomenon of a spectral collapse, when the two-photon interaction exceeds a certain critical value and the spectrum of the systems becomes unbounded. The work appeared in Physical Review A. The full reference is Phys. Rev. A 97, 013851 (2018).

This circuit, consisting of a dc-SQUID and a flux qubit, implements the two-photon quantum Rabi model.

 

New article on Maximal Entanglement

Alba Cervera-Lierta and José Ignacio Latorre’s work on ‘Maximal Entanglement Generation in High Energy Physics’ has been accepted to SciPost!

This work explores how maximal entanglement is generated at the fundamental level in particle processes such as electron-electron scattering or pair annihilation into photons. It is also shown that the requirement of maximal entanglement constrains the structure of the fundamental interaction. Maximal entanglement is used to obtain a non-trivial prediction for the value of the weak mixing angle, a free parameter of the Standard Model of particles. The results are a first step towards understanding the connections between maximal entanglement and the fundamental symmetries of high-energy physics.

Feynman diagram for electron-positron scattering into a muon-antimuon pair at tree level

The reference of the article is SciPost Phys. 3, 036 (2017).

New article on single photon generation

A new article led by QUANTIC member P. Forn-Díaz has been recently published on Physical Review Applied. The article presents a new method of generating single microwave photons on demand. The new technique consists in controlling the position of nodes of vacuum modes of a transmission line where a superconducting qubit is coupled. This control on the mode profile happens very quickly, in the nanosecond scale, and it results in a real-time tunable qubit-line interaction. This interaction produces photon wavepackets with a shape that can be nearly arbitrary.

Microscope image of the device used to generate single photons. Overlaid with the image are real data traces of single photons generated with a controlled delay

The work was carried out at the laboratory of Prof. Christopher Wilson at the Institute for Quantum Computing in the University of Waterloo (Canada). This result is very relevant for the deployment of microwave quantum networks and to connect sectors of future large-scale quantum computers. The reference of the paper is Physical Review Applied 8, 054015 (2017).