A method for trapping neutral atoms, associated system and quantum computer

Abstract

The invention relates to a method for trapping neutral atoms in an array of trapping sites, the neutral atoms having: a fundamental state, a first excited state and a second excited state, the second excited state having a higher principal quantum number than the first excited state, the second excited state having the same hyperfine electronic structure than the first excited state, the method comprising: - a step of cooling down the neutral atoms in at least a magneto-optical trap suitable for imparting a transition between the fundamental state and the second excited state, so as to obtain neutral atoms which are colder than with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state, and - a step of trapping cooled down neutral atoms in an array of trapping sites.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention concerns a method for trapping neutral atoms in an array of trapping sites. The present invention also concerns an associated system for trapping neutral atoms in an array of trapping sites. The present invention also deals with an associated quantum computer.BACKGROUND OF THE INVENTION

[0002] The quest for large-scale quantum computing is nowadays a hot topic, with a fierce race between the different quantum platforms to demonstrate the highest number of qubits. For the last years, the neutral atom platform, in which single atoms are trapped in an array of traps formed by optical tweezers, has witnessed a tremendous growth in qubit numbers, with perspectives up to tens of thousands of qubits in the near future. This growth was triggered by the improved possibilities of control over the generation of large optical tweezers arrays.

[0003] However, a strong limitation might slow down this growth: the required amount of laser power grows linearly with the number of qubits. Eventually, adding more optical tweezers will become difficult due to the lack of laser power.SUMMARY OF THE INVENTION

[0004] Hence, there exists a need for a method enabling to trap more efficiently neutral atoms in trapping sites in order to increase the number of qubits.

[0005] To this end, the invention relates to a method for trapping neutral atoms in an array of trapping sites, the neutral atoms having each a plurality of energy states comprising at least: a fundamental state, a first excited state and a second excited state, the second excited state having a higher principal quantum number than the first excited state, the second excited state having the same hyperfine electronic structure than the first excited state, the method comprising: a step, called reference cooling step, of cooling down the neutral atoms in at least a magneto-optical trap suitable for imparting a transition between the fundamental state and the second excited state, so as to obtain neutral atoms which are colder than with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state, and a step of trapping cooled down neutral atoms in an array of trapping sites, each trapping site being generated with a laser power which is lower than the minimal power required for trapping an atom cooled down with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state.

[0006] The method according to the invention may comprise one or more of the following features considered alone or in any combination that is technically possible: the method also comprises a step of imaging the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms; the method also comprises a step of cooling the trapped neutral atom(s), called trap cooling step, at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms; the method also comprises a preliminary step of cooling down the neutral atoms in a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state, the preliminary step occurring before the reference cooling step such that the neutral atoms cooled down during the reference cooling step have first been cooled down during the preliminary step; the plurality of energy states of the neutral atoms also comprise a third excited state, the third excited state having a higher principal quantum number than the second excited state, the third excited state having the same hyperfine electronic structure than the first excited state and the second excited state, the method also comprising a further step of cooling down the neutral atoms in a magneto-optical trap suitable for imparting a transition between the fundamental state and the third excited state, the further step of cooling occurring after the reference cooling step such that the neutral atoms cooled down during the further step of cooling have first been cooled down during the reference cooling step; the or each magneto-optical trap is generated using three pairs of counter-propagating cooling laser beams and a magnetic quadrupole, the counter-propagating cooling laser beams having a wavelength suitable to address the transition imparted by the magneto-optical trap; each magneto-optical trap is also generated using three pairs of counter-propagating pumping laser beams which are superimposed on the three pairs of counter-propagating cooling laser beams, the three pairs of counter-propagating pumping laser beams being suitable to pump neutral atoms which have been de-excited below the fundamental state; the neutral atoms belong to the alkali column of the periodic table; the neutral atoms are the same chemical element of the periodic table, the first excited state of the neutral atoms being the D2 line; the neutral atoms are Rubidium atoms, the fundamental state being the state 5S 1 / 2 , F=2, the first excited state being the state 5P 3 / 2 , F=3, and the second excited state being the state 6P 3 / 2 , F=3.

[0007] The invention also relates to a system for trapping neutral atoms in an array of trapping sites, the neutral atoms having each a plurality of energy states comprising at least: a fundamental state, a first excited state and a second excited state, the second excited state having a higher principal quantum number than the first excited state, the second excited state having the same hyperfine electronic structure than the first excited state, the system comprising: a magneto-optical trap generator suitable to generate a magneto-optical trap suitable for imparting a transition between the fundamental state and the second excited state, enabling to cool down the neutral atoms so as to obtain neutral atoms which are colder than with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state, and a trapping site generator suitable to generate an array of trapping sites enabling to trap cooled down neutral atoms, each trapping site being generated with a laser power which is lower than the minimal power required for trapping an atom cooled down with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state.

[0008] The system according to the invention may comprise one or more of the following features considered alone or in any combination that is technically possible: the system for trapping neutral atoms also comprises an imaging unit suitable to image the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms; the system for trapping neutral atoms also comprises a cooling unit suitable to cool down the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

[0009] The invention also relates to a quantum computer comprising: a system for generating neutral atoms, the neutral atoms having each a plurality of energy states comprising at least: a fundamental state, a first excited state and a second excited state, the second excited state having a higher principal quantum number than the first excited state, the second excited state having the same hyperfine electronic structure than the first excited state, and a system for trapping the neutral atoms in an array of trapping sites, the system being as previously described. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be easier to understand in view of the following description, provided solely as an example and with reference to the appended drawings in which: Figure 1 is a schematic view of an example of a quantum computer comprising a system for generating neutral atoms and a system for trapping the neutral atoms, Figure 2 is a schematic view of an example of a system for trapping neutral atoms, Figure 3 is a schematic view of an example of two beams generated by the laser assembly of two different magneto-optical trap generators, each laser beam being one of the six superimposed laser beams (cooling + repumping) of a respective magneto-optical trap, Figure 4 is an example of an organigram of a method for trapping neutral atoms in an array of trapping sites, Figure 5 is an example of different energy states for the Rubidium 87 and of different transitions addressed, Figure 6 is an example of a temporal sequence where the following actions are performed on neutral atoms: MOT (magneto-optical trap) cooling, trapping, imaging and cooling, and Figure 7 is another example of a temporal sequence where the following actions are performed on neutral atoms: MOT cooling, trapping, imaging and cooling. DETAILED DESCRIPTION OF SOME EMBODIMENTS

[0011] In the description, the terms "suitable for", "able to" and "configured for" are considered equivalent.

[0012] An example of a quantum computer 10 is illustrated on figure 1.

[0013] A quantum computer, also called quantum processing unit, or QPU, comprises an array of qubits (also called a qubit register), as well as a hardware for manipulating these qubits. A quantum computing device is adapted to perform quantum operations on qubits.

[0014] A quantum computer uses the quantum properties of matter, such as superposition and entanglement, to perform operations on data. Unlike a classical computer based on transistors working on binary data (coded on bits, 0 or 1), the quantum computing device works on qubits whose quantum state can take a continuous rather than discrete number of values.

[0015] As an example, a qubit refers to a two-level quantum mechanical system. For example, a qubit comprises two basic quantum states I0> and I1> representing the possible quantum states of the qubit. According to the superposition principle of quantum mechanics, any superposition of the form al0> + bI1 > (a and b being complex numbers and aa*+bb*=1) is a possible quantum state of the qubit.

[0016] Examples of quantum computing devices based on neutral atoms are described in the article Loic Henriet, Lucas Beguin, Adrien Signoles, Thierry Lahaye, Antoine Browaeys, Georges-Olivier Reymond, and Christophe Jurczak. Quantum computing with neutral atoms. Quantum, 4:327, September 2020. ISSN 2521-327X. doi:10.22331 / q-2020-09-21-327.

[0017] The quantum computer 10 is based on neutral atoms.

[0018] The quantum computer 10 comprises a system 12 for generating neutral atoms and a system 14 for trapping the neutral atoms.

[0019] The system 12 for generating the neutral atoms comprises for example a vacuum chamber in which the neutral atoms are located. In particular, during use, a vacuum is created in the vacuum chamber and a dilute atomic vapor is formed in the vacuum chamber. Other embodiments are nonetheless possible.

[0020] Each neutral atom has a plurality of energy states comprising at least: a fundamental state, a first excited state and a second excited state.

[0021] Each state is defined by an azimuthal quantum number I, a total electron angular momentum J, and a principal quantum number n. Further, each state has a hyperfine electronic structure, which is fully characterized by I, J, and a total atomic angular momentum F. As an example, a state with given n, I, J, is defined as all Zeeman sub-levels as well as the associated electronic fine structure to the given I and J.

[0022] The second excited state has a higher principal quantum number n than the first excited state. The second excited state has the same hyperfine electronic structure than the first excited state.

[0023] More specifically, the first excited state has the same principal quantum number n, but a different azimuthal quantum number I than the fundamental state. The second excited state has a different principal quantum number n, azimuthal quantum number I and total electron angular momentum J as compared to the fundamental state. The first and second excited states have the same azimuthal quantum number I and electron angular momentum J, but different principal quantum number n. Therefore, the first and second excited states have the same hyperfine structure (as they have the same azimuthal quantum number I and electron angular momentum J).

[0024] In some embodiments, the plurality of energy states of each neutral atom also comprises a third excited state. The third excited state has a higher principal quantum number than the second excited state. The third excited state has the same hyperfine electronic structure than the first excited state and the second excited state.

[0025] More generally, the plurality of energy states of each neutral atom may comprise other excited states having a higher principal quantum number than the first, second and third excited state and the same hyperfine electronic structure than the first, second and third excited state.

[0026] Preferably, the neutral atoms belong to the alkali column of the periodic table. Preferably, the neutral atoms are the same chemical element of the periodic table. The first excited state of the neutral atoms is advantageously the D2 line.

[0027] In a particular example of embodiment, the neutral atoms are Rubidium atoms.

[0028] In that case, as visible on Figure 5, the fundamental state is the state 5S 1 / 2 , F=2. The first excited state is the state 5P 3 / 2 , F=3. The second excited state is the state 6P 3 / 2 , F=3. F is the total atomic angular momentum.

[0029] The system 14 for trapping the neutral atoms is suitable for trapping the neutral atoms in an array of trapping sites so as to form an array of qubits (also called qubit register or quantum register). In particular, the system 14 is suitable for trapping at most one neutral atom per trapping site.

[0030] As illustrated on figure 2, the system 14 for trapping the neutral atoms comprises a unit 20 for generating magneto-optical trap(s) and a trapping site generator 22. Optionally, the system 14 also comprises an imaging unit 24 and / or a cooling unit 26.

[0031] The unit 20 for generating magneto-optical trap(s) comprises at least one magneto-optical trap generator 30.

[0032] The magneto-optical trap generator 30 is suitable to generate at least a magneto-optical trap suitable for imparting a transition between the fundamental state and the second excited state, enabling to cool down the neutral atoms so as to obtain neutral atoms which are colder than with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state. Indeed, because it is higher in energy, the second excited state has a narrower linewidth than the first excited state. Due to this narrower linewidth, the neutral atoms can be cooled down at a lower temperature. More generally, an excited state with a higher quantum number, but with the same hyperfine electronic structure than the fundamental state or than another excited state of lower quantum number, has a narrower linewidth. This enables to cool down neutral atoms at a lower temperature, as the minimal temperature reachable via laser cooling depends on the magnitude of the linewidth.

[0033] By the term "colder", it is understood that the atoms have a lower speed. In particular, in a MOT, the atoms are slowed down up to the Doppler temperature limit. By combining the weak impulse of photons tuned to be on resonance with an atomic transition, with a large number of absorption-spontaneous emission cycles, atoms with initial 'thermal' speeds given for example by room temperature, can be slowed down to lower speeds thanks to laser cooling.

[0034] Optionally, the unit 20 for generating magneto-optical trap(s) also comprises at least one of the following elements: a magneto-optical trap generator 32 suitable for imparting a transition between the fundamental state and the first excited state, a magneto-optical trap generator 34 suitable for imparting a transition between the fundamental state and the third excited state, and any other magneto-optical trap generator suitable for imparting a transition between the fundamental state and an excited state with the same hyperfine electronic structure than the first, second and third excited state, but different from the first, second and third excited state.

[0035] For example, the or each magneto-optical trap generator 30, 32, 34 comprises a laser assembly and a magnetic quadrupole.

[0036] Each laser assembly comprises one or several cooling laser sources. The cooling laser sources are suitable for generating three pairs of counter-propagating cooling laser beams suitable for cooling the neutral atoms. The counter-propagating cooling laser beams have a wavelength suitable to address the transition imparted by the magneto-optical trap.

[0037] Preferably, each laser assembly also comprises one or several pumping laser sources. The pumping laser sources are suitable for generating three pairs of counter-propagating pumping laser beams suitable for pumping neutral atoms which have been de-excited below the fundamental state. The pumping laser beams are superimposed on the cooling laser beams.

[0038] In an example, the pumping laser beams have the same wavelength than the cooling laser beams. In a variant, the wavelengths of the pumping laser beams and the cooling laser beams are different.

[0039] The magnetic quadrupole is suitable for generating a magnetic field. The magnetic quadrupole is for example formed of four coils.

[0040] In an example where the unit 20 comprises several magneto-optical trap generators, the magnetic quadrupole and / or the pumping laser source(s) are common to all the magneto-optical trap generators. In this case, the magneto-optical trap generators differ from each other by the laser assembly, and in particular at least by the wavelength of the three pairs of counter-propagating cooling laser beams generated by the laser assembly.

[0041] In other embodiments, each magneto-optical trap generator has its own magnetic quadrupole and / or pumping laser source(s).

[0042] The example of figure 3 illustrates one laser beam B1 generated by the laser assembly of a first magneto-optical trap generator (for example at 780 nm) and one laser beam B1' generated by the laser assembly of a second magneto-optical trap generator 30 (for example at 420 nm). The amplitude and the frequency of each laser beam can be controlled depending on the use of each laser beam. Each laser beam is one of the six superimposed laser beams (cooling + repumping) of a respective magneto-optical trap (one at 780 nm and the other one at 420 nm). A combiner 35, such as a dichroic plate, can be used to combine the two laser beams on the neutral atoms. This enables to use alternatively or simultaneously two types of laser beams (for example for generating two different magneto-optical traps).

[0043] The trapping site generator 22 is suitable to generate an array of trapping sites enabling to trap cooled down neutral atoms. Hence, the trapping site generator 22 is suitable to be activated after the neutral atoms have been cooled down by the magneto-optical trap(s).

[0044] In an example of implementation, as illustrated on figure 2, the trapping site generator 22 comprises a laser source 40, a beam shaper 42 and a focusing unit 44.

[0045] The laser source 40 is able to generate one or several laser beam(s).

[0046] In particular, each trapping site is generated with a laser power which is lower than the minimal power required for trapping an atom, which has been cooled down with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state. Indeed, the neutral atoms being colder, less laser power is needed to trap a single atom.

[0047] The beam shaper 42 is able to shape the laser beam (e.g., to impart a specific phase / intensity pattern to the laser beam) so as to obtain an array of trapping sites for particles when the laser beam is focused.

[0048] The beam shaper 42 is, for example, a spatial light modulator (SLM), such as an optical phase modulator, preferably a liquid crystal optical modulator (LCOS-SLM). In another example, the beam shaper 28 is an acousto-optical deflector (AOD), or a device with metasurfaces.

[0049] The focusing unit 44 is able to focus the shaped laser beam on a target region of the vacuum chamber 12 so as to generate the array of trapping sites.

[0050] The imaging unit 24 is suitable to image the trapped neutral atom(s).

[0051] For example, the imaging unit 24 comprises a laser source and a detector.

[0052] The laser source is suitable to excite the neutral atoms from the fundamental state to an excited state.

[0053] The detector is suitable to detect the fluorescence emitted from the neutral atoms trapped in the trapping sites and de-exciting to their fundamental state.

[0054] The detector is for example a charged-couple device (CCD) sensor or an electron multiplication charged-coupled device (EMCCD) sensor.

[0055] Preferably, the imaging unit 24 is suitable to image the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap enabling to cool down the neutral atoms. This enables to obtain a better image quality because the atoms are colder. In addition, atoms can be imaged with less required trap power, as the atoms are colder.

[0056] In this embodiment, the laser source used to excite the atoms for the imagery is, for example, a laser source of the laser assembly of the magneto-optical generator 30, which is suitable for imparting the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

[0057] For example, when the only or the last magneto-optical trap generator used to cool down the neutral atoms is the magneto-optical trap generator 30, the imaging unit 24 is suitable to image the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the second excited state.

[0058] In a variant, the imaging unit 24 is suitable to image the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the second excited state, even when the second excited state is not the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

[0059] In a variant, the imaging unit 24 is suitable to image the trapped neutral atom(s) at a wavelength, which is different from the wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap enabling to cool down the neutral atoms. For example, the wavelength corresponds to the transition between the fundamental state and the first energy state.

[0060] The cooling unit 26 is suitable to cool down the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

[0061] The cooling unit 26 comprises at least a laser source emitting one or several laser beams at the above described wavelength. For example, at least two counter-propagating laser beams with orthogonal polarization are emitted on the trapped atoms.

[0062] For example, the cooling unit 26 is suitable to cool down the trapped neutral atoms using the polarization gradient cooling technique (also named Sisyphus cooling). The Sisyphus cooling involves the use of specially selected laser light, hitting atoms from various angles to both cool and trap them in a potential well, effectively rolling the atom down a hill of potential energy until it has lost its kinetic energy. This enables to reduce the laser power and the laser frequency shifted away from the transition frequency, in order to minimize the atom temperature.

[0063] For example, the laser source used for the cooling is a laser source of the laser assembly of the magneto-optical generator 30, which is suitable for imparting the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

[0064] For example, when the only or the last magneto-optical trap generator used to cool down the neutral atoms is the magneto-optical trap generator 30, the cooling unit 26 is suitable to image the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the second excited state.

[0065] In a variant, the cooling unit 26 is suitable to image the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the second excited state, even when the second excited state is not the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

[0066] In a variant, the cooling unit 26 is suitable to image the trapped neutral atom(s) at a wavelength, which is different from the wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap enabling to cool down the neutral atoms. For example, the wavelength corresponds to the transition between the fundamental state and the first energy state. An example of a method for trapping neutral atoms in an array of trapping sites, will now be described with reference to the organigram of figure 4 and to the particular examples of figure 5 to 7. The method is carried out using the system 14 for trapping the neutral atoms.

[0067] The method comprises a step, called reference cooling step 100, of cooling down the neutral atoms in a magneto-optical trap suitable for imparting a transition between the fundamental state and the second excited state, so as to obtain neutral atoms which are colder than with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state. The reference cooling step 100 is carried out by the magneto-optical trap generator 30.

[0068] Optionally, the method comprises a preliminary step 90 of cooling down the neutral atoms in a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state. The preliminary step occurs before the reference cooling step 100 such that the neutral atoms cooled down during the reference cooling step 100 have first been cooled down during the preliminary step 90.

[0069] In complement or in a variant, the method comprises a further step 110 of cooling down the neutral atoms in a magneto-optical trap suitable for imparting a transition between the fundamental state and the third excited state. The further step 110 occurs after the reference cooling step such that the neutral atoms cooled down during the further step 110 have first been cooled down during the reference cooling step 100.

[0070] In complement or in a variant, the method comprises another step of cooling down the neutral atoms in a magneto-optical trap suitable for imparting a transition between the fundamental state and an excited state having a higher principal quantum number than the third excited state and the same hyperfine electronic structure than the first, second and third excited state. This step occurs after the reference cooling step (and if applicable the further step 110) such that the neutral atoms cooled down during this step have first been cooled down during the reference cooling step 100 (and if applicable the further step 110).

[0071] Figure 5 illustrates an example of different energy states for the Rubidium 87 and of different transitions addressed. The transition 780 nm MOT is a transition between the fundamental state (5S 1 / 2 , F=2) and the first excited state (5P 3 / 2 , F=3) for cooling down the neutral atoms. The transition marked as "780 nm Repumper" on said figure is a transition suitable to pump neutral atoms that have been excited below the fundamental state. The transition 420 nm MOT is a transition between the fundamental state and the second excited state (6P 3 / 2 , F=3) for cooling down the neutral atoms.

[0072] The method comprises a step 120 of trapping cooled down neutral atoms in an array of trapping sites. The trapping step 120 is carried out by the trapping site generator 22.

[0073] Each trapping site is generated with a laser power which is lower than the minimal power required for trapping an atom cooled down with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state. Hence, either the power of the laser beam generating the trapping site is lower or the power is the same but is used for generating more trapping sites.

[0074] Optionally, the method also comprises a step 130 of imaging the trapped neutral atom(s). The imaging step 130 is carried out by the imagery unit 24.

[0075] Preferably, the trapped neutral atoms are imaged at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

[0076] In an example, the neutral atoms are first excited with a laser beam at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms. Then, the fluorescence emitted by the neutral atoms is imaged on a detector.

[0077] Optionally, the method also comprises a step 140 of cooling the trapped neutral atom(s), called trap cooling step. The trap cooling step 140 is carried out by the cooling unit 26.

[0078] Preferably, the trapped neutral atoms are cooled down at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

[0079] In an example, the cooling is performed with the polarization gradient cooling technique (also named Sisyphus cooling).

[0080] Figures 6 and 7 illustrate two different sequences for cooling, trapping and imaging the neutral atoms.

[0081] In the example of figure 6, only one magneto-optical trap is used to cool down the neutral atoms. The magneto-optical trap has a wavelength of 420 nm (enabling a transition between the fundamental state and the second excited state for Rb 87< ). The magneto-optical trap is first switch on so as to generate a 35 µK (micro kelvins) trap enabling to cool down the neutral atoms. The neutral atoms are then trapped into trapping sites (optical tweezers). During the trapping of the neutral atoms, the laser(s) of the magneto-optical trap at 420 nm is (are) still activated so as to slow down the neutral atoms. The laser(s) at 420 nm is (are) then used to image the neutral atoms by fluorescence. Finally, the laser(s) at 420 nm is (are) used to cool down the neutral atoms in the trapping sites.

[0082] Typically, the power and frequency of the laser at 420 nm vary between the different steps of MOT, trapping, imaging and cooling, depending on the specific needs of the designed quantum evolution. For example during the cooling step, the polarization gradient cooling technique (also named Sisyphus cooling) can be applied, where the laser power will be reduced and the laser frequency shifted away from the transition frequency in order to minimize the atom temperature.

[0083] Depending on the exact application one envisions, laser parameters could be the same or different. Usually it is better in terms of performances to change the parameters depending whether imaging is performed or cooling, for example.

[0084] In the example of figure 7, two magneto-optical traps are used to cool down the neutral atoms. The neutral atoms are first cooled down at 150 µK in a magneto-optical trap at 780 nm (enabling a transition between the fundamental state and the first excited state for Rb 87< ). Then, the neutral atoms are cooled down at 35 µK in a magneto-optical trap at 420 nm (enabling a transition between the fundamental state and the second excited state for Rb 87< ). The neutral atoms are then trapped into trapping sites (optical tweezers). During the trapping of the neutral atoms, the laser(s) of the magneto-optical trap at 780 nm is (are) activated so as to slow down the neutral atoms. The laser(s) at 780 nm is (are) then switched off and the laser(s) at 420 nm is (are) then used to image the neutral atoms by fluorescence. Finally, the laser(s) at 420 nm is (are) used to cool down the neutral atoms in the trapping sites. Typically, the power and frequency of the laser at 420 nm vary between the different steps of MOT, trapping, imaging and cooling. The same applies for the laser at 780 nm and the steps of MOT and trapping.

[0085] These temporal sequences are only given as examples of implementation. Other embodiments are also possible, such as embodiments where two different lasers (for example at 780 nm and 480 nm) are superimposed at the same time during one or several of the steps of trapping, imaging and cooling.

[0086] Hence, the above described solution allows the atoms to reach a lower Doppler temperature for the neutral atoms, by producing a MOT on a narrower transition than the state of the art. Thanks to this, the amount of power required to trap an atom in a tweezers is proportionally reduced. The invention is particularly adapted for alkali species.

[0087] Consequently, the above described method and system 14 for trapping neutral atoms, enable to trap more efficiently single neutral atoms in trapping sites (ie using less power) in order to increase the number of qubits.

[0088] This power-saving of trap laser power applies for (1) trapping single atoms, (2) observing single atoms via fluorescence imaging and (3) cooling already trapped single atoms in optical tweezers.

[0089] The person skilled in the art will understand that the embodiments and variants described above in the description can all be combined provided that they are technically compatible. Many other embodiments are possible without departing from the scope of the invention defined in the appended claims.

Claims

1. A method for trapping neutral atoms in an array of trapping sites, the neutral atoms having each a plurality of energy states comprising at least: a fundamental state, a first excited state and a second excited state, the second excited state having a higher principal quantum number than the first excited state, the second excited state having the same hyperfine electronic structure than the first excited state, the method comprising: - a step, called reference cooling step, of cooling down the neutral atoms in at least a magneto-optical trap suitable for imparting a transition between the fundamental state and the second excited state, so as to obtain neutral atoms which are colder than with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state, and - a step of trapping cooled down neutral atoms in an array of trapping sites, each trapping site being generated with a laser power which is lower than the minimal power required for trapping an atom cooled down with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state.

2. A method according to claim 1, wherein the method also comprises a step of imaging the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

3. A method according to claim 1 or 2, wherein the method also comprises a step of cooling the trapped neutral atom(s), called trap cooling step, at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

4. A method according to any one of claims 1 to 3, wherein the method also comprises a preliminary step of cooling down the neutral atoms in a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state, the preliminary step occurring before the reference cooling step such that the neutral atoms cooled down during the reference cooling step have first been cooled down during the preliminary step.

5. A method according to any one of claims 1 to 4, wherein the plurality of energy states of the neutral atoms also comprise a third excited state, the third excited state having a higher principal quantum number than the second excited state, the third excited state having the same hyperfine electronic structure than the first excited state and the second excited state, the method also comprising a further step of cooling down the neutral atoms in a magneto-optical trap suitable for imparting a transition between the fundamental state and the third excited state, the further step of cooling occurring after the reference cooling step such that the neutral atoms cooled down during the further step of cooling have first been cooled down during the reference cooling step.

6. A method according to any one of claims 1 to 5, wherein the or each magneto-optical trap is generated using three pairs of counter-propagating cooling laser beams and a magnetic quadrupole, the counter-propagating cooling laser beams having a wavelength suitable to address the transition imparted by the magneto-optical trap.

7. A method according to claim 6, wherein each magneto-optical trap is also generated using three pairs of counter-propagating pumping laser beams which are superimposed on the three pairs of counter-propagating cooling laser beams, the three pairs of counter-propagating pumping laser beams being suitable to pump neutral atoms which have been de-excited below the fundamental state.

8. A method according to any one of claims 1 to 7, wherein the neutral atoms belong to the alkali column of the periodic table.

9. A method according to claim 8, wherein the neutral atoms are the same chemical element of the periodic table, the first excited state of the neutral atoms being the D2 line.

10. A method according to any one of claims 1 to 9, wherein the neutral atoms are Rubidium atoms, the fundamental state being the state 5S1 / 2, F=2, the first excited state being the state 5P3 / 2, F=3, and the second excited state being the state 6P3 / 2, F=3.

11. A system (14) for trapping neutral atoms in an array of trapping sites, the neutral atoms having each a plurality of energy states comprising at least: a fundamental state, a first excited state and a second excited state, the second excited state having a higher principal quantum number than the first excited state, the second excited state having the same hyperfine electronic structure than the first excited state, the system (14) comprising: - a magneto-optical trap generator (30) suitable to generate a magneto-optical trap suitable for imparting a transition between the fundamental state and the second excited state, enabling to cool down the neutral atoms so as to obtain neutral atoms which are colder than with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state, and - a trapping site generator (22) suitable to generate an array of trapping sites enabling to trap cooled down neutral atoms, each trapping site being generated with a laser power which is lower than the minimal power required for trapping an atom cooled down with a magneto-optical trap suitable for imparting a transition between the fundamental state and the first excited state.

12. A system according to claim 11, wherein the system (14) for trapping neutral atoms also comprises an imaging unit (24) suitable to image the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

13. A system according to claim 11 or 12, wherein the system (14) for trapping neutral atoms also comprises a cooling unit (26) suitable to cool down the trapped neutral atom(s) at a wavelength enabling the transition between the fundamental state and the excited state of the last magneto-optical trap used to cool down the neutral atoms before trapping the neutral atoms.

14. A quantum computer (10) comprising: - a system (12) for generating neutral atoms, the neutral atoms having each a plurality of energy states comprising at least: a fundamental state, a first excited state and a second excited state, the second excited state having a higher principal quantum number than the first excited state, the second excited state having the same hyperfine electronic structure than the first excited state, and - a system (14) for trapping the neutral atoms in an array of trapping sites, the system (14) being according to any one of claims 11 to 13.

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