Lecture d'un couple de qudits nucléaires avec un transistor moléculaire

Abstract : The realization of a quantum computer is one of the most ambitious and promising scientific objectives of the beginning of this century.The strength of quantum computing lies in its ability to use a superposition of states and the interferences between them to overcome the intrinsic limit of classical computers, which is the discrete description of the continuous physical phenomena. This would theoretically allow them to simplify and solve impossible problems for conventional computers.The first step in the realization of a quantum computer, is its basic block: the quantum-bit, or qubit. It is the quantum analogue of the classical bit, which stores information in the form of 0 or 1. In the quantum case, information is formed by the superposition of these two states, leading to an infinity of possibilities. If this step has been done many times by the community, using qubits of different natures, the coupling between several of them remains difficult and limited in number. Indeed, the quantum systems thus formed tend to lose their coherence; or said otherwise, to destroy itself.Among the many possibilities of existing qubit, I have used the nuclear spin. They have the advantage of being relatively well decoupled from their environment, which makes it possible to protect them from external sources of decoherence, and thus to have a longer lifetime than electronic spins.This advantage has a price: it is more difficult to access their reading.To do this, I have made a molecular transistor to connect a single molecule possessing two magnetic centers, the Tb2Pc3, to the source and drain electrodes. The monomolecular magnet used has two magnetic centers (the Tb3 + ions), whose electronic spins J = 6, are coupled to each other via a dipolar interaction. In addition, each of them is coupled to its nuclear spin I = 3/2 via the hyperfine interaction. We thus have a pair of two qudits (d = 4), which brings the size of the Hilbert space to 16, and this inside a single molecule.At first, I have developed the Zeeman diagram of the molecule, which is its energy response to an external magnetic field. Then, I detail the manufacture of the samples, and in particular the use of the electromigration technique. Next, I present the electrical transport measurements, at very low temperatures (milliKelvins) and under a magnetic field, which make it possible to detect the reversal of the electronic spins, which position is dependent on the state of the pair of nuclear spins: it is how the reading of the states of qudits couple is performed.A study of the dynamics of the system is then carried out by correlation measurements among the position of the reversals of the electronic spins between two consecutive scans. This gives a better visualization of the states of the system, but also its relaxation.Finally, I was able to extract its effective temperature, using a Maxwell-Boltzmann distribution. Of the order of 300 mT, it is consistent with the literature, as well as with those extracted on two other molecular transistors obtained at other times of my thesis.In summary, this thesis shows for the first time the use of a single-molecule transistor to access reading of a qudits couple. The large number of existing molecules, and the large number of qubits or qudits that could be coupled inside one of them, makes molecular spintronics a very promising way for possible future molecular quantum computers.The next step will be to operate the coherent manipulation of such a system, in particular via the use of the Stark effect, as it has already been done using a molecule having only a magnetic center.
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Hugo Biard. Lecture d'un couple de qudits nucléaires avec un transistor moléculaire. Autre [cond-mat.other]. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAY006⟩. ⟨tel-02274106⟩

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