Quantic turns 3 years old

Last week Quantic celebrated its third birthday! Although the COVID-19 situation did not let us to hold the full party, as some of the members could not join the event, it is nice to see how the group grows and improves. This last year has been full of surprises, full of news, full of special moments. This has been, as for most of people, a difficult year. However, at the end of the day year, we are allowed to look back and confess: wow, what a year. Cheers to all who made it possible.

The next months promise to be amazing and exciting, great expectations are yet to be fulfilled. Old friends coming and going, new people getting to know us, new projects to accomplish, discoveries hidden ahead of us… Let us hope the future is as brilliant as in our minds.

Happy Birthday!

New open postdoc position

The QCT group at IFAE has an open postdoctoral position for the SiUCs project funded under the Quantera program. More information is available here and by contacting Pol Forn.

The newly established QCT group in Barcelona offers a postdoctoral position within the project SiUCs ( Superinductor-based Quantum Technologies with Ultrastrong Couplings) funded by the European QuantERA program.

The tasks for this position will involve design, fabrication and measurement of superconducting qubit devices coupled to resonator modes to study the physics of this system in regimes of ultrastrong coupling, and its consequences in superconducting-based quantum technologies.

The candidate will closely work in collaboration with the rest of partners in the SiUCs consortium:

  • Nicolas Roch (Néel Institute CNRS, Grenoble, FR)
  • Ioan Pop (Karlsruhe Institute of Technology, DE)
  • Milena Grifoni (Regensburg University, Institute for Theoretical Physics, DE)
  • Miroslav Grajcar (Institute of Physics, Slovak Academy of Sciences, SK)
  • Elisabetta Paladino (Consiglio Nationale delle Richerche Catania, IT)

Interested candidates are expected to hold a doctoral degree in Physics and must be experienced in experimental superconducting qubit device techniques in at least one of the following areas: quantum optics with superconducting devices, qubit coherent control, quantum simulations with superconducting devices.

Applications should be sent to Pol Forn-Díaz and should include an updated CV with a letter of intent or motivation, and arrange for two or three letters of recommendation. Sending CVs to IFAE implies consent to the legal warning at the bottom of IFAE’s homepage, see the job posting at IFAE’s website.

The appointment will be for a two years term, with the possibility to renew for a third year.


IFAE is an equal opportunity employer committed to diversity in the workplace, and we welcome applications from all qualified candidates. Women are particularly encouraged to apply.

Great News for our PI

Dear all,

we are glad to announce that the Principal Investigator of Quantic, José Ignacio Latorre, has been selected as the new director of the Center for Quantum Technologies in Singapore. We will join the CQT the next 27th July, taking over from founding director Artur Ekert.

Have the best of lucks in this new opportunity!


Bojos per la supercomputació

The Quantic members Carlos Bravo-Prieto and David López-Núñez have shared some time with high-school students that are ”crazy for supercomputing”.

First, they have learnt them the basic concepts of quantum mechanics, a necessary step to understand quantum computing, as well as the potential applications of it.

Presentació powerpoint sobre física quàntica. En aquesta plana, hi ha la introducció a aquesta disciplina.

Then, the foundations of quantum computing have been explained, such as how to represent a qubit in the Bloch sphere, (thanks to Quantum Fracture for the nice drawings)


Finally, our students have learnt to simulate quantum algorithms in classical computers.


Hopefully, some of them will join us in the future in this amazing adventure of researching in Quantum Computing. Thank you very much for your attention, and have luck in your brilliant futures!

New paper by Quantic members!

Diego García-Martín and José Ignacio Latorre recently published “The Prime state and its quantum relatives”, together with E. Ribas, S. Carrazza and G. Sierra.Congratulations! You can see this paper in arXiv (arxiv.org/abs/2005.02422) and scirate (scirate.com/arxiv/2005.024)

The Prime state is the uniform superposition of all the computational-basis states corresponding to prime numbers. This state encodes, quantum mechanically, arithmetic properties of the primes. Moreover, it can be efficiently created on a quantum computer.

In this paper, it is shown that the Quantum Fourier Transform of this state provides direct access to Chebyshev-like biases in the distribution of primes. Also, the entanglement traits of the Prime state are studied. These reveal correlations between prime numbers. In particular, the reduced density matrix for natural bi-partitions is characterized by the Hardy-Littlewood constants.

A relation is found between the scaling of the von Neumann entropy and the Shannon entropy of half the density of square-free numbers. This relation also holds when considering qudit bases, showing this property is intrinsic to the primes. It also holds when considering states defined from prime numbers belonging to arithmetic progressions.



The entanglement of other quantum number-theoretical states is also studied.
An open-source library that diagonalizes matrices using floats of arbitrary precision has been developed for this paper. In summary, a novel approach to Number Theory using the tools of Quantum Information Theory.

Outreach in Quantic

Quantic group has collaborated in a new outreach article for the Journal “Compàs d’Amalgama” in the University of Barcelona. Carlos Bravo-Prieto, Diego García-Martín and Adrián Pérez-Salinas wrote “Computació i supremacia quàntica, per què tothom parla d’això?” (Quantum computing and supremacy, why does everybody talk about that?) You can all read it here (in Catalan)


New paper by the theory team: measuring the tangle

Measuring the tangle of three-qubit states, by A. Pérez-Salinas, C. Bravo-Prieto, D. García-Martín and J. I. Latorre


In this paper, the authors propose a method for transforming an unknown three-qubit state into its canonical form up to phases. This transformation can be achieved variationally and may be used to estimate the tangle. Simulations on this method have been performed.

The tangle is a three-qubit entanglement invariant that quantifies genuine tripartite entanglement. The state with maximum tangle is the GHZ state (and its local transformations). The distribution of tangle for random three-qubit states is depicted below.


The canonical form up to phases can be achieved when the number of measurements of three possible elements in the computational basis are zero. In this form, the tangle is easily measured. The canonical form can be cast by applying local unitaries on each qubit.


The tangle is affected by errors in the circuit. To mitigate these errors, a post-selection scheme can be applied after measuring the output state. This post-selection consists of discarding those results that should not appear in the canonical form.

First, the authors have analyzed the GHZ state. The estimation of the tangle with this method provides lower values as the errors (“t”) in the circuit increase. Post-selection mitigates the errors considerably.


This behavior has found to be a common tendency for many different random states. In the figure depicted below, each dot corresponds to a random three-qubit state.
With errors comparable to those in the state-of-the-art quantum hardware, the tangle presents a mean underestimation of ~30%. Post-selection lowers this result to ~17%.
Although the method herein presented does not provide any speed up respect to quantum tomography, it can be used as a module for other algorithms, such as a classifier.
UPDATE: This paper has been published in Entropy

Postdoctoral position available for the University of Barcelona

There is a new open post-doc position to work at the University of Barcelona with Dr. Luca Tagliacozzo and Dr. Sofyan Iblisdir. The announcement has been recently published

Dear colleagues,
Applications are invited for one or two postdoctoral positions in the field of Quantum Computation.
The successful candidate will work with Dr. Sofyan Iblisdir and Dr. Luca Tagliacozzo, at the
University of Barcelona (Spain) in quantum computation and related topics, from developing and
characterizing new quantum algorithms, to their classical simulation, to the characterization of noise
and imperfections in specific experimental implementations, to quantum machine learning. Beside
quantum computation, the group has strong expertise in the theory of many-body quantum systems
at and out of equilibrium, and in tensor networks techniques.
The positions are for a period of 1 year that will be possibly extended to 2 years depending
on performances and availability of funding. The positions should be filled as soon as possible.
Applicants are expected to have a doctoral degree in Physics, Computer Science, Mathematics, or a
related discipline before the starting date of the position, and have previous expertise in one (or
more) of the following areas: quantum computation, quantum information, tensor networks,
machine learning, quantum many-body systems, condensed matter physics, quantum field theory.
Applicants should have a strong interest in solving challenging problems, as well as a proven record
of research, including publication of original work in at least one of the above areas. Excellent
scientific writing ability and good communication skills are essential.
Applications should be sent to Dr. Sofyan Iblisdir and should include: 1) a motivation letter;
2) a curriculum vitae including a list of publications; 3) a research statement; 4) the name and email
of two references. All qualified applicants will receive equal consideration without regard to
appearance, beliefs, sex, sexual orientation, gender identity, national origin, disability or age. For
full consideration, applications should be submitted by the 10th of March 2020.

Please direct informal enquiries to:
Dr Sofyan Iblisdir: sofyan.iblisdir(at)fqa.ub.edu
Dr Luca Tagliacozzo: luca.tagliacozzo(at)fqa.ub.edu

New arXiv preprint by the theory team

Scaling of variational quantum circuit depth for condensed matter systems, by C. Bravo-Prieto, J. Lumbreras, L. Tagliacozzo and J.I. Latorre

In this paper, the authors have analyzed the performances of a finite depth quantum circuit in order to encode the ground state of local Hamiltonians. They have shown that as expected, the precision improves exponentially with the depth of the circuit in gapped phases.

In conformally invariant gapless phases, the precision improves very slowly up to a number of layers that increase linearly with the system size. They identify that regime with a finite-depth regime, where the depth of the circuit dictates the appearance of an effective correlation length. Beyond that number of layers, the precision improves again exponentially (finite-size regime).


They have provided an explanation of this phenomenon in terms of Lieb-Robinson bounds, and also compared the power of different variational ansatz in representing the ground state of critical systems. Finally, they believe that, in the context of critical systems, the actual entanglement entropy of the wave-function provides a proper measurement of the effective number of free parameters in the ansatz, the number that is ultimately responsible for the accuracy of the results.



New publication in PRX by experimental team

Dr. Forn-Díaz from the Quantic group has participated in a collaboration together with IQC researchers in Waterloo (Canada) where a for the first time spontaneous triple photon downconversion was observed in a superconducting circuit. The article was published in PRX.

Superconducting circuit used to demonstrate triple photon downconversion. The circuit consists of a lambda/4 resonator terminated with a DC-SQUID.

Here is a public summary of the achievement:

For over 30 years, spontaneous parametric down-conversion (SPDC) has been a workhorse for quantum optics. By splitting one “pump photon” into two daughter photons, SPDC has had a crucial role in fundamental tests of quantum theory as well as many applications in quantum information processing. From the early days, researchers have explored splitting the pump photon into three photons (as a possible resource in quantum computation, for example), but it has proven extremely difficult to realize experimentally—until now. Here, we report on an implementation of three-photon SPDC in the microwave domain.

To split one microwave photon into three daughter photons, we use a flux-pumped, superconducting parametric resonator. Our triplet source is bright, producing a propagating photon flux comparable to ordinary two-photon SPDC. We clearly see strong three-photon correlations in the output photons, even in the absence of normal two-photon correlations. The symmetry properties of these correlations allow us to “fingerprint” how the photons were created, clearly demonstrating little contamination from typical SPDC processes.

These results form the basis of an exciting new paradigm of three-photon quantum optics. One can only hope that this new paradigm will be as successful as two-photon quantum optics.

(a) Phase space distribution of the triple photon downconversion. The non-gaussian shape of the distribution is a hallmark of this new effect. (b) same as (a) but at single photon level. (c) Skewness of the distribution, explicitly showing its non-gaussianity.