A method for generating a radiofrequency control signal for an acousto-optic deflector

Abstract

The invention relates to a method for generating a radiofrequency control signal for each acousto-optic deflector, so called AOD (40), of a device (22) controlling an optical tweezer, the method being implemented by a programmable electronic device (42) and comprising the following steps: - the reception of a computed path for the optical tweezer, - the conversion of the computed path into a frequency modulation, so-called frequency path, for each AOD (40), each frequency path being under the form of a linear segment or multiple linear segments, - the filtering of each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path, and - the generation, for each AOD (40), of a radiofrequency control signal whose frequency is modulated according to the filtered frequency path of the AOD (40).

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention concerns a method for generating a radiofrequency control signal for each acousto-optic deflector of a device controlling an optical tweezer. The invention also relates to an associated method for rearranging particles trapped in trapping sites forming an initial spatial configuration of qubits so as to obtain a target spatial configuration of qubits. The present invention also relates to an associated programmable electronic device. The present invention also relates to an associated device for moving an optical tweezer. The present invention also relates to an associated rearrangement system. The present invention also relates to an associated quantum system.BACKGROUND OF THE INVENTION

[0002] In the field of neutral atom-based quantum computing processors (OPUs), it is common to use a quantum register comprising neutral atoms (acting as qubits) trapped in an array of fixed optical traps, which are for example formed with (fixed) optical tweezers. Often, at least some of these trapped atoms must be moved from one trap to another using a moving optical tweezer.

[0003] For example, after initially populating the traps with atoms, e.g. stochastically from a reservoir of atoms, there is a need for a rearrangement phase in order to build the quantum register, in which the loaded atoms must be selectively placed in desired locations.

[0004] These trapped atoms can be moved with focused lasers called moving tweezers, which are deflected by running said laser beam through AODs (Acousto-Optic Deflector) devices. An AOD deflects the incoming light along a predefined direction based on the frequency modulation of a radiofrequency (RF) control signal. Using this method, 2D moves are made possible by cascading two AODs placed in different axis.

[0005] A basic way to deflect the moving tweezers to move an atom, is to modulate the frequency of the RF signal linearly as a function of time. This approach is simple to implement but suffers from limitations on how fast the atoms can be moved.

[0006] A more elaborated way is to modulate the frequency of the RF control signal with what is referred as S-Curves (e.g., a S-shaped curve). S-Curves are usually generated by using polynomial equations or Sigmoid functions.

[0007] However, a limitation of this approach is that the modulation curves can be computationally very expensive to calculate in real-time especially in the case of optimizing multi-step moves (e.g., when an atom must be moved from trap to trap repeatedly before reaching its destination).

[0008] Because of this, a common approach is to calculate in advance every possible move, but this does not scale well when the register size increases and requires to reduce the possible moves to a set small enough to fit in the memory of the RF signal generator.SUMMARY OF THE INVENTION

[0009] Hence, there exists a need for a method enabling to ease the move of an optical tweezer, so as to rearrange particles, such as atoms, forming an array of qubits.

[0010] To this end, the invention relates to a method for generating a radiofrequency control signal for each acousto-optic deflector, so called AOD, of a device controlling an optical tweezer, so as to move the optical tweezer according to a computed path, each AOD being suitable to deflect a laser beam forming the optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD, the method being implemented by a programmable electronic device and comprising the following steps: the reception of a computed path for the optical tweezer, the conversion of the computed path into a frequency modulation, so-called frequency path, for each AOD, each frequency path being under the form of a linear segment or multiple linear segments, the filtering of each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path, and the generation, for each AOD, of a radiofrequency control signal whose frequency is modulated according to the filtered frequency path of the AOD, so that each AOD deflects a laser beam forming the optical tweezer, enabling to move the optical tweezer according to the computed path.

[0011] 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 or each digital filter has a group delay which is independent of the frequency path to be filtered; the or each digital filter is resettable; the or at least one digital filter is a filter of the Savitzky-Golay family, preferably the or at least one digital filter being a Moving Average filter; the or at least one digital filter is a Moving Average filter, such filter having the following features: when the duration of the computed path is superior to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a constant speed part, and finished with a quadratic deceleration part, and when the duration of the computed path is inferior or equal to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a quadratic deceleration part. the filtering step is performed by applying successively several digital filters on the frequency path to be filtered.

[0012] The invention also deals with a method for rearranging particles trapped in trapping sites forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits, the method comprising, for each particle to be moved, the following phases: determining, by a calculator, a path from an initial position of the particle in the initial spatial configuration of qubits to a target position of the particle in the target spatial configuration of qubits, and generating a radiofrequency control signal for each acousto-optic deflector, so called AOD, of a device for moving an optical tweezer, so as to trap the particle in an optical tweezer and move the optical tweezer according to the determined path, the generation phase being carried out by a programmable electronic device as previously described.

[0013] The invention also relates to a programmable electronic device for generating a radiofrequency control signal for each acousto-optic deflector, so called AOD, of a device so as to move an optical tweezer according to a computed path, each AOD being suitable to deflect a laser beam forming the optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD, the programmable electronic device being configured for: receiving a computed path for the optical tweezer, converting the computed path into a frequency modulation, so-called frequency path, for each AOD, each frequency path being under the form of a linear segment or multiple linear segments, filtering each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path, and generating, for each AOD, a radio-frequency control signal whose frequency is modulated according to the filtered frequency path of the AOD so that each AOD deflects a laser beam forming the optical tweezer, enabling to move the optical tweezer according to the computed path.

[0014] The programmable electronic device according to the invention may comprise the following feature: the programmable electronic device is a Field Programmable Gate Array or an Application-Specific Integrated Circuit.

[0015] The invention also relates to a device for moving an optical tweezer comprising: at least one acousto-optic deflector, so called AOD, each AOD being suitable to deflect a laser beam forming an optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD, and a programmable electronic device configured for generating a radiofrequency control signal for each AOD so as to move the optical tweezer according to a computed path, the programmable electronic device being as previously described.

[0016] The device according to the invention may comprise the following feature: the device comprises two AOD arranged so as to move the optical tweezer in a two-dimensional plane.

[0017] The invention also relates to a rearrangement system for rearranging particles trapped in trapping sites forming an initial spatial configuration of qubits so as to obtain a target spatial configuration of qubits, the rearrangement system comprising at least: a generator of an optical tweezer, the optical tweezer being formed of at least one laser beam, and a device for moving the optical tweezer, the device being as previously described. The invention also relates to a quantum system comprising: a source of particles, a generator of trapping sites for the particles, and a rearrangement system for rearranging particles (P) trapped in the trapping sites, the rearrangement system being as previously described.

[0018] The quantum system according to the invention may comprise the following feature: the particles are chosen among the following particles: neutral atoms, Nitrogen-Vacancy centers, Silicon-Vacancy centers, ions, molecules and quantum dots. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] 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 system comprising a source of particles, a generator of trapping sites for the particles and a rearrangement system for rearranging particles trapped in the trapping sites, Figure 2 is a schematic view of an example of a rearrangement system for rearranging particles trapped in the trapping sites, Figure 3 is an example of a flow chart of some steps of a method for rearranging particles trapped in trapping sites forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits, and Figure 4 is an example of different frequency paths (before filtering and after filtering). DETAILED DESCRIPTION OF SOME EMBODIMENTS

[0020] In the description, the terms "suitable for", "able to" and "configured for" are considered equivalent.

[0021] An example of a quantum system 10 is schematically illustrated on figure 1.

[0022] The quantum system 10 is, for example, a quantum computing device (or quantum processing unit), a quantum sensor, a quantum communication device or any quantum device comprising an array of trapped neutral atoms.

[0023] Typically, the quantum system 10 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.

[0024] In particular, 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> + bl1 > (a and b being complex numbers and aa*+bb*=1) is a possible quantum state of the qubit.

[0025] As illustrated on figure 1, the quantum system 10 comprises a source 12 of particles (noted P on figure 2), a generator 14 of trapping sites for the particles P and a rearrangement system 16 for rearranging particles P trapped in the trapping sites.

[0026] Preferably, the particles P are chosen among the following particles: neutral atoms, Nitrogen-Vacancy centers, Silicon-Vacancy centers, ions, molecules and quantum dots.

[0027] Preferably, the neutral atom particles P are Rubidium atoms, although other elements are possible, such as Strontium atoms, Cesium atoms, or any alkali metal or alkaline earth element.

[0028] In the case of neutral atoms, the source 12 of particles P comprises a vacuum chamber in which the particles P (for example 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. Various cooling processes can be used to cool the dilute atomic vapor, for instance laser cooling, so that the atoms can be trapped. Other embodiments are nonetheless possible.

[0029] The generator 14 of trapping sites is able to generate a laser beam which when focused generates an array of trapping sites for particles P (e.g., optical tweezers).

[0030] In an example of implementation, the generator 14 of trapping sites comprises a laser source, or equivalent, able to generate a laser beam and a beam shaper able to shape the laser beam (e.g., to impart a specific intensity pattern to the laser beam) so as to obtain an array of trapping sites for particles P when the laser beam is focused. The beam shaper is, for example, a spatial light modulator (SLM).

[0031] Typically, the quantum system 10 also comprises a detector configured to detect the presence of particle(s) in the trapping sites (forming a qubit register). The detector is, for example, a camera such as a charge-coupled device (CCD) camera, or an Electron-Multiplying CCD (EMCCD) camera, or any suitable imaging technology. For example, in the case of atoms, the camera is suitable to detect the fluorescence emitted from the atoms.

[0032] Examples of quantum processors based on neutral atoms are described in the article of 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.

[0033] Other embodiments are nonetheless possible.

[0034] The rearrangement system 16 is configured for moving particles P trapped in an array of trapping sites, said particles P forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits.

[0035] Typically, the particles P have first been loaded stochastically in the array of trapping sites. The presence of particles in the trapping sites has been detected using the detector, enabling to detect initial positions of particles in the trapping sites, and to determine which particles P must be moved and where to.

[0036] An example of a rearrangement system 16 is illustrated on figure 2.

[0037] The rearrangement system 16 comprises at least a generator 20 of an optical tweezer and a device 22 for moving the optical tweezer. Hence, the combination of the generator 20 and of the device 22 enables to obtain a so-called moving tweezer.

[0038] The generator 20 of an optical tweezer comprises a laser source 30, a beam shaper 32 and a focusing unit 34.

[0039] The laser source 30 is suitable to generate a laser beam. The wavelength of the laser source 30 is for example suitable to attract the particles P in the optical tweezer.

[0040] The beam shaper 32 is suitable to shape the laser beam (e.g., to impart a specific intensity pattern to the laser beam) so as to obtain an optical tweezer for a particle P when the laser beam is focused. The beam shaper 32 is, for example, a spatial light modulator (SLM).

[0041] The focusing unit 34 is suitable for focusing the laser beam shaped by the beam shaper 32 so as to form an optical tweezer. The focusing unit 34 is for example an assembly of one or several lenses.

[0042] The device 22 is suitable for moving the optical tweezer generated by the generator 20.

[0043] The device 22 comprises at least one acousto-optic deflector, so called AOD 40, and a programmable electronic device 42.

[0044] Each AOD 40 is suitable to deflect a laser beam forming an optical tweezer along a preferred direction as a function of a frequency of a radiofrequency (RF) control signal received by the AOD 40.

[0045] In particular, each AOD is suitable to deflect a laser beam emitted by the laser source 30 of the generator 20.

[0046] For example, the number of AOD 40 depends on the dimensions in which the moving tweezer can be moved. For example, a single AOD 40 enables to obtain a moving tweezer along a first direction X (so in one dimension). Two AOD 40 enable to obtain a moving tweezer along the first direction X and a second direction Y (so in a two-dimension plane).

[0047] As illustrated in the example of figure 2, the device 22 comprises two AOD 40 arranged so as to move the optical tweezer in a two-dimensional plane. More specifically, one AOD 40 is suitable to deflect, along a first direction (e.g., along an X-axis of the array of trapping sites), the laser beam emitted by a laser source 30, and the other AOD 40 is suitable to deflect, along a second direction (e.g., along an Y-axis of the array of trap sites), the deflected laser beam at the output of the first AOD 40.

[0048] Optionally, as illustrated on figure 2, the device also comprises at least one optical element 43 suitable for tuning the focus of the optical tweezer. This enables to move the optical tweezer in a three-dimension space. In an example, the optical element 43 is also controlled by the programmable electronic device 42.

[0049] The programmable electronic device 42 is configured for generating a radiofrequency control signal for each AOD 40 so as to move the optical tweezer according to a computed path. The functioning of the programmable electronic device 42 will be described later in the description.

[0050] Preferably, the programmable electronic device 42 is a Field Programmable Gate Array (FPGA) or an Application-Specific Integrated Circuit (ASIC). However, other programmable electronic devices are nonetheless possible.

[0051] An example of a method for rearranging particles P trapped in trapping sites forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits will now be described, with reference to the flow chart of figure 3.

[0052] The rearranging method is performed for each particle P to be moved in the array of trapping sites. Hence, the different phases of the rearranging method will now be described for a particle to be moved.

[0053] For example, the initial spatial configuration of qubits has been generated previously by loading stochastically particles P generated by the source 12 of particles P in the array of trapping sites generated by the generator 14 of trapping sites.

[0054] In the case of stochastic atom filling as described previously, the initial spatial arrangement of qubits has typically been generated at random without following a specific pattern for the filling of the trapping sites. In a variant, the particles P of the initial spatial configuration of qubits have already been rearranged so as to correspond to an initial pattern.

[0055] The rearranging method comprises a phase 100 of determining, by a calculator which can be a computer or any suitable electronic circuitry, for said trapped particle P, a path (trajectory) from an initial position of the particle P (e.g., a first trap site) in the array of qubits to a target position of the particle P (e.g., a second trap site) in the array of qubits.

[0056] The path for each particle is for example calculated algorithmically or using machine-learning means, such as using a Proximal Policy Optimization (PPO) model, on the basis of a set of data. The set of data comprises for example data relative to the positions of the trapping sites; data relative to the initial positions of the particles P in the initial spatial configuration of qubits; and data relative to the target positions of the particles P of the target spatial configuration of qubits. Other embodiments are nonetheless possible. Determining or calculating the trajectory in real time enables to eliminate the needs to calculate in advance and store all / part of the possible moves in a memory.

[0057] The rearranging method comprises a phase 200 for generating a radiofrequency control signal for each AOD 40, of the device 22 so as to trap the particle P in a mobile optical tweezer and move said optical tweezer according to the determined path. The RF control signal is the signal to be applied over time to each AOD so that the optical tweezer can be moved according to the computed path.

[0058] The generation phase 200 is carried out by the programmable electronic device 42.

[0059] The generation phase 200 comprises a step 210 of reception of a computed path for the optical tweezer. The computed path is for example the path determined at phase 100 for the particle to be moved. Typically, the path of the particle is assimilated to the path of the optical tweezer. In a variant, the path of the particle is different from the path of the optical tweezer as other optical tweezer or environmental conditions also influence the movement of the particle.

[0060] The generation phase 200 comprises a step 220 of converting the computed path into a frequency modulation, so-called frequency path, for each AOD 40.

[0061] Each frequency path is under the form of a linear segment or multiple linear segments. In other words, each frequency path is represented, in the temporal domain, by an evolution of the frequency over time, the evolution having the form of a succession of linear segments (at least one linear segment).

[0062] Figure 4 is an example where different frequency paths have been represented before and after filtering. The frequency paths have the shape of a periodic S-curve. In particular, the frequency path before filtering at the top of figure 4 is composed of a succession of linear segments whose pattern is repeated over a period of time.

[0063] The conversion is for example performed using a correspondence table associating a frequency to a given position in the plane. For example, the correspondence table comprises all the positions or only some of them, and an interpolation algorithm is used to calculate the final frequency.

[0064] The generation phase 200 comprises a step 230 of filtering of each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path. More specifically, the digital filter enables to smooth the frequency path (S-curve modulation).

[0065] Preferably, the or each digital filter has a group delay which is independent of the frequency path to be filtered. For example, "group delay" describes the time shift of the envelope of a wave packet. A wave packet is a "pack" or "group" of oscillations centered around one frequency that travel together. Preferably, the group delay is constant over the digital filter. Consequently, because the group delay through the filter is constant and independent of the slope of the original signal, a 2D linear diagonal move will stay perfectly linear in the particle plane.

[0066] Preferably, the digital filter is resettable. The reset is for example carried out by clearing the memory (state) associated with the filter. In particular, between moves, the moving tweezers have to change location as rapidly as possible. This is done by resetting the filter memory so that the jump to another particle is not affected by the filter group delay.

[0067] For example, the digital filter is a filter of the Savitzky-Golay family. For example, the digital filter is a Moving Average filter, also called Box Car filter.

[0068] We can write the boxcar filter formula as follow: y n = ∑ k = 0 N − 1 x n − k

[0069] Where there are N taps to the filter, x[n] is a sequence of input samples and y[n] is the output of the filter.

[0070] Preferably, when the digital filter is a Box Car filter, the digital filter has the following features: when the duration of the computed path is superior to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a constant speed part, and finished with a quadratic deceleration part, and when the duration of the computed path is inferior or equal to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a quadratic deceleration part.

[0071] In any case, the duration of the processed move is the duration corresponding to the length of the original linear move, plus the group delay of the filter (filter number of samples / sample-rate).

[0072] Preferably, in case of a 2D multi-step move, when the digital filter is a Box Car filter, the digital filter is configured so that, after the original quadratic acceleration, the speed will stay constant and changes of direction will be automatically be smoothed / rounded.

[0073] Figure 4 illustrates three examples of Box Car filtering with different group delays, respectively (from top to bottom): a group delay of 8.2 µs, a group delay of 16.4 µs and a group delay of 32.8 µs. A longer group delay enables to obtain a smoother frequency path.

[0074] In an embodiment, the filtering step is performed by applying successively several digital filters on the frequency path to be filtered. The digital filters can be the same filters or can be different. This enables to smooth even more the RF control signal and the acceleration or jerk seen by the particles.

[0075] The generation phase 200 comprises a step 240 of generating, for each AOD 40, of a RF control signal whose frequency is modulated according to the filtered frequency path of the AOD 40 so that each AOD 40 deflects a laser beam forming the optical tweezer, enabling to move the optical tweezer according to the computed path.

[0076] The rearranging method comprises a phase 300 for effectively moving the considered particle P by the optical tweezer.

[0077] The above phases and steps are carried out for each particle to be moved from an initial position in the initial spatial configuration of qubits to a target position in the target spatial configuration of qubits.

[0078] Hence, the method and elements describe above enable to obtain a modulation (S-curve modulation) that is easily and efficiently implementable in devices like FPGAs and ASICs. In particular, the use of a digital filter enables to easily smooth the frequency modulation of the radiofrequency control signal of each AOD controlling an optical tweezer, enabling to ease the movement of the optical tweezer, so as to rearrange particles forming an array of qubits.

[0079] In some embodiments, the digital filter is a Boxcar / Moving Average filter. This presents the following advantages: Easy to implement on a device like FPGA or ASIC. The group delay through the filter is the same independent of the input signal. Easily resettable by clearing the sample memory. No need to precalculate coefficients as with a polynomial or Sigmoid function. Low latency, processing is done sample by sample and only adds 1 sample delay.

[0080] 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.

Examples

Embodiment Construction

[0020]In the description, the terms "suitable for", "able to" and "configured for" are considered equivalent.

[0021]An example of a quantum system 10 is schematically illustrated on figure 1.

[0022]The quantum system 10 is, for example, a quantum computing device (or quantum processing unit), a quantum sensor, a quantum communication device or any quantum device comprising an array of trapped neutral atoms.

[0023]Typically, the quantum system 10 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.

[0024]In particular, 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 ...

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention concerns a method for generating a radiofrequency control signal for each acousto-optic deflector of a device controlling an optical tweezer. The invention also relates to an associated method for rearranging particles trapped in trapping sites forming an initial spatial configuration of qubits so as to obtain a target spatial configuration of qubits. The present invention also relates to an associated programmable electronic device. The present invention also relates to an associated device for moving an optical tweezer. The present invention also relates to an associated rearrangement system. The present invention also relates to an associated quantum system.BACKGROUND OF THE INVENTION

[0002] In the field of neutral atom-based quantum computing processors (OPUs), it is common to use a quantum register comprising neutral atoms (acting as qubits) trapped in an array of fixed optical traps, which are for example formed with (fixed) optical tweezers. Often, at least some of these trapped atoms must be moved from one trap to another using a moving optical tweezer.

[0003] For example, after initially populating the traps with atoms, e.g. stochastically from a reservoir of atoms, there is a need for a rearrangement phase in order to build the quantum register, in which the loaded atoms must be selectively placed in desired locations.

[0004] These trapped atoms can be moved with focused lasers called moving tweezers, which are deflected by running said laser beam through AODs (Acousto-Optic Deflector) devices. An AOD deflects the incoming light along a predefined direction based on the frequency modulation of a radiofrequency (RF) control signal. Using this method, 2D moves are made possible by cascading two AODs placed in different axis.

[0005] A basic way to deflect the moving tweezers to move an atom, is to modulate the frequency of the RF signal linearly as a function of time. This approach is simple to implement but suffers from limitations on how fast the atoms can be moved.

[0006] A more elaborated way is to modulate the frequency of the RF control signal with what is referred as S-Curves (e.g., a S-shaped curve). S-Curves are usually generated by using polynomial equations or Sigmoid functions.

[0007] However, a limitation of this approach is that the modulation curves can be computationally very expensive to calculate in real-time especially in the case of optimizing multi-step moves (e.g., when an atom must be moved from trap to trap repeatedly before reaching its destination).

[0008] Because of this, a common approach is to calculate in advance every possible move, but this does not scale well when the register size increases and requires to reduce the possible moves to a set small enough to fit in the memory of the RF signal generator.SUMMARY OF THE INVENTION

[0009] Hence, there exists a need for a method enabling to ease the move of an optical tweezer, so as to rearrange particles, such as atoms, forming an array of qubits.

[0010] To this end, the invention relates to a method for generating a radiofrequency control signal for each acousto-optic deflector, so called AOD, of a device controlling an optical tweezer, so as to move the optical tweezer according to a computed path, each AOD being suitable to deflect a laser beam forming the optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD, the method being implemented by a programmable electronic device and comprising the following steps: the reception of a computed path for the optical tweezer, the conversion of the computed path into a frequency modulation, so-called frequency path, for each AOD, each frequency path being under the form of a linear segment or multiple linear segments, the filtering of each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path, and the generation, for each AOD, of a radiofrequency control signal whose frequency is modulated according to the filtered frequency path of the AOD, so that each AOD deflects a laser beam forming the optical tweezer, enabling to move the optical tweezer according to the computed path.

[0011] 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 or each digital filter has a group delay which is independent of the frequency path to be filtered; the or each digital filter is resettable; the or at least one digital filter is a filter of the Savitzky-Golay family, preferably the or at least one digital filter being a Moving Average filter; the or at least one digital filter is a Moving Average filter, such filter having the following features: when the duration of the computed path is superior to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a constant speed part, and finished with a quadratic deceleration part, and when the duration of the computed path is inferior or equal to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a quadratic deceleration part. the filtering step is performed by applying successively several digital filters on the frequency path to be filtered.

[0012] The invention also deals with a method for rearranging particles trapped in trapping sites forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits, the method comprising, for each particle to be moved, the following phases: determining, by a calculator, a path from an initial position of the particle in the initial spatial configuration of qubits to a target position of the particle in the target spatial configuration of qubits, and generating a radiofrequency control signal for each acousto-optic deflector, so called AOD, of a device for moving an optical tweezer, so as to trap the particle in an optical tweezer and move the optical tweezer according to the determined path, the generation phase being carried out by a programmable electronic device as previously described.

[0013] The invention also relates to a programmable electronic device for generating a radiofrequency control signal for each acousto-optic deflector, so called AOD, of a device so as to move an optical tweezer according to a computed path, each AOD being suitable to deflect a laser beam forming the optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD, the programmable electronic device being configured for: receiving a computed path for the optical tweezer, converting the computed path into a frequency modulation, so-called frequency path, for each AOD, each frequency path being under the form of a linear segment or multiple linear segments, filtering each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path, and generating, for each AOD, a radio-frequency control signal whose frequency is modulated according to the filtered frequency path of the AOD so that each AOD deflects a laser beam forming the optical tweezer, enabling to move the optical tweezer according to the computed path.

[0014] The programmable electronic device according to the invention may comprise the following feature: the programmable electronic device is a Field Programmable Gate Array or an Application-Specific Integrated Circuit.

[0015] The invention also relates to a device for moving an optical tweezer comprising: at least one acousto-optic deflector, so called AOD, each AOD being suitable to deflect a laser beam forming an optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD, and a programmable electronic device configured for generating a radiofrequency control signal for each AOD so as to move the optical tweezer according to a computed path, the programmable electronic device being as previously described.

[0016] The device according to the invention may comprise the following feature: the device comprises two AOD arranged so as to move the optical tweezer in a two-dimensional plane.

[0017] The invention also relates to a rearrangement system for rearranging particles trapped in trapping sites forming an initial spatial configuration of qubits so as to obtain a target spatial configuration of qubits, the rearrangement system comprising at least: a generator of an optical tweezer, the optical tweezer being formed of at least one laser beam, and a device for moving the optical tweezer, the device being as previously described. The invention also relates to a quantum system comprising: a source of particles, a generator of trapping sites for the particles, and a rearrangement system for rearranging particles (P) trapped in the trapping sites, the rearrangement system being as previously described.

[0018] The quantum system according to the invention may comprise the following feature: the particles are chosen among the following particles: neutral atoms, Nitrogen-Vacancy centers, Silicon-Vacancy centers, ions, molecules and quantum dots. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] 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 system comprising a source of particles, a generator of trapping sites for the particles and a rearrangement system for rearranging particles trapped in the trapping sites, Figure 2 is a schematic view of an example of a rearrangement system for rearranging particles trapped in the trapping sites, Figure 3 is an example of a flow chart of some steps of a method for rearranging particles trapped in trapping sites forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits, and Figure 4 is an example of different frequency paths (before filtering and after filtering). DETAILED DESCRIPTION OF SOME EMBODIMENTS

[0020] In the description, the terms "suitable for", "able to" and "configured for" are considered equivalent.

[0021] An example of a quantum system 10 is schematically illustrated on figure 1.

[0022] The quantum system 10 is, for example, a quantum computing device (or quantum processing unit), a quantum sensor, a quantum communication device or any quantum device comprising an array of trapped neutral atoms.

[0023] Typically, the quantum system 10 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.

[0024] In particular, 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> + bl1 > (a and b being complex numbers and aa*+bb*=1) is a possible quantum state of the qubit.

[0025] As illustrated on figure 1, the quantum system 10 comprises a source 12 of particles (noted P on figure 2), a generator 14 of trapping sites for the particles P and a rearrangement system 16 for rearranging particles P trapped in the trapping sites.

[0026] Preferably, the particles P are chosen among the following particles: neutral atoms, Nitrogen-Vacancy centers, Silicon-Vacancy centers, ions, molecules and quantum dots.

[0027] Preferably, the neutral atom particles P are Rubidium atoms, although other elements are possible, such as Strontium atoms, Cesium atoms, or any alkali metal or alkaline earth element.

[0028] In the case of neutral atoms, the source 12 of particles P comprises a vacuum chamber in which the particles P (for example 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. Various cooling processes can be used to cool the dilute atomic vapor, for instance laser cooling, so that the atoms can be trapped. Other embodiments are nonetheless possible.

[0029] The generator 14 of trapping sites is able to generate a laser beam which when focused generates an array of trapping sites for particles P (e.g., optical tweezers).

[0030] In an example of implementation, the generator 14 of trapping sites comprises a laser source, or equivalent, able to generate a laser beam and a beam shaper able to shape the laser beam (e.g., to impart a specific intensity pattern to the laser beam) so as to obtain an array of trapping sites for particles P when the laser beam is focused. The beam shaper is, for example, a spatial light modulator (SLM).

[0031] Typically, the quantum system 10 also comprises a detector configured to detect the presence of particle(s) in the trapping sites (forming a qubit register). The detector is, for example, a camera such as a charge-coupled device (CCD) camera, or an Electron-Multiplying CCD (EMCCD) camera, or any suitable imaging technology. For example, in the case of atoms, the camera is suitable to detect the fluorescence emitted from the atoms.

[0032] Examples of quantum processors based on neutral atoms are described in the article of 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.

[0033] Other embodiments are nonetheless possible.

[0034] The rearrangement system 16 is configured for moving particles P trapped in an array of trapping sites, said particles P forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits.

[0035] Typically, the particles P have first been loaded stochastically in the array of trapping sites. The presence of particles in the trapping sites has been detected using the detector, enabling to detect initial positions of particles in the trapping sites, and to determine which particles P must be moved and where to.

[0036] An example of a rearrangement system 16 is illustrated on figure 2.

[0037] The rearrangement system 16 comprises at least a generator 20 of an optical tweezer and a device 22 for moving the optical tweezer. Hence, the combination of the generator 20 and of the device 22 enables to obtain a so-called moving tweezer.

[0038] The generator 20 of an optical tweezer comprises a laser source 30, a beam shaper 32 and a focusing unit 34.

[0039] The laser source 30 is suitable to generate a laser beam. The wavelength of the laser source 30 is for example suitable to attract the particles P in the optical tweezer.

[0040] The beam shaper 32 is suitable to shape the laser beam (e.g., to impart a specific intensity pattern to the laser beam) so as to obtain an optical tweezer for a particle P when the laser beam is focused. The beam shaper 32 is, for example, a spatial light modulator (SLM).

[0041] The focusing unit 34 is suitable for focusing the laser beam shaped by the beam shaper 32 so as to form an optical tweezer. The focusing unit 34 is for example an assembly of one or several lenses.

[0042] The device 22 is suitable for moving the optical tweezer generated by the generator 20.

[0043] The device 22 comprises at least one acousto-optic deflector, so called AOD 40, and a programmable electronic device 42.

[0044] Each AOD 40 is suitable to deflect a laser beam forming an optical tweezer along a preferred direction as a function of a frequency of a radiofrequency (RF) control signal received by the AOD 40.

[0045] In particular, each AOD is suitable to deflect a laser beam emitted by the laser source 30 of the generator 20.

[0046] For example, the number of AOD 40 depends on the dimensions in which the moving tweezer can be moved. For example, a single AOD 40 enables to obtain a moving tweezer along a first direction X (so in one dimension). Two AOD 40 enable to obtain a moving tweezer along the first direction X and a second direction Y (so in a two-dimension plane).

[0047] As illustrated in the example of figure 2, the device 22 comprises two AOD 40 arranged so as to move the optical tweezer in a two-dimensional plane. More specifically, one AOD 40 is suitable to deflect, along a first direction (e.g., along an X-axis of the array of trapping sites), the laser beam emitted by a laser source 30, and the other AOD 40 is suitable to deflect, along a second direction (e.g., along an Y-axis of the array of trap sites), the deflected laser beam at the output of the first AOD 40.

[0048] Optionally, as illustrated on figure 2, the device also comprises at least one optical element 43 suitable for tuning the focus of the optical tweezer. This enables to move the optical tweezer in a three-dimension space. In an example, the optical element 43 is also controlled by the programmable electronic device 42.

[0049] The programmable electronic device 42 is configured for generating a radiofrequency control signal for each AOD 40 so as to move the optical tweezer according to a computed path. The functioning of the programmable electronic device 42 will be described later in the description.

[0050] Preferably, the programmable electronic device 42 is a Field Programmable Gate Array (FPGA) or an Application-Specific Integrated Circuit (ASIC). However, other programmable electronic devices are nonetheless possible.

[0051] An example of a method for rearranging particles P trapped in trapping sites forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits will now be described, with reference to the flow chart of figure 3.

[0052] The rearranging method is performed for each particle P to be moved in the array of trapping sites. Hence, the different phases of the rearranging method will now be described for a particle to be moved.

[0053] For example, the initial spatial configuration of qubits has been generated previously by loading stochastically particles P generated by the source 12 of particles P in the array of trapping sites generated by the generator 14 of trapping sites.

[0054] In the case of stochastic atom filling as described previously, the initial spatial arrangement of qubits has typically been generated at random without following a specific pattern for the filling of the trapping sites. In a variant, the particles P of the initial spatial configuration of qubits have already been rearranged so as to correspond to an initial pattern.

[0055] The rearranging method comprises a phase 100 of determining, by a calculator which can be a computer or any suitable electronic circuitry, for said trapped particle P, a path (trajectory) from an initial position of the particle P (e.g., a first trap site) in the array of qubits to a target position of the particle P (e.g., a second trap site) in the array of qubits.

[0056] The path for each particle is for example calculated algorithmically or using machine-learning means, such as using a Proximal Policy Optimization (PPO) model, on the basis of a set of data. The set of data comprises for example data relative to the positions of the trapping sites; data relative to the initial positions of the particles P in the initial spatial configuration of qubits; and data relative to the target positions of the particles P of the target spatial configuration of qubits. Other embodiments are nonetheless possible. Determining or calculating the trajectory in real time enables to eliminate the needs to calculate in advance and store all / part of the possible moves in a memory.

[0057] The rearranging method comprises a phase 200 for generating a radiofrequency control signal for each AOD 40, of the device 22 so as to trap the particle P in a mobile optical tweezer and move said optical tweezer according to the determined path. The RF control signal is the signal to be applied over time to each AOD so that the optical tweezer can be moved according to the computed path.

[0058] The generation phase 200 is carried out by the programmable electronic device 42.

[0059] The generation phase 200 comprises a step 210 of reception of a computed path for the optical tweezer. The computed path is for example the path determined at phase 100 for the particle to be moved. Typically, the path of the particle is assimilated to the path of the optical tweezer. In a variant, the path of the particle is different from the path of the optical tweezer as other optical tweezer or environmental conditions also influence the movement of the particle.

[0060] The generation phase 200 comprises a step 220 of converting the computed path into a frequency modulation, so-called frequency path, for each AOD 40.

[0061] Each frequency path is under the form of a linear segment or multiple linear segments. In other words, each frequency path is represented, in the temporal domain, by an evolution of the frequency over time, the evolution having the form of a succession of linear segments (at least one linear segment).

[0062] Figure 4 is an example where different frequency paths have been represented before and after filtering. The frequency paths have the shape of a periodic S-curve. In particular, the frequency path before filtering at the top of figure 4 is composed of a succession of linear segments whose pattern is repeated over a period of time.

[0063] The conversion is for example performed using a correspondence table associating a frequency to a given position in the plane. For example, the correspondence table comprises all the positions or only some of them, and an interpolation algorithm is used to calculate the final frequency.

[0064] The generation phase 200 comprises a step 230 of filtering of each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path. More specifically, the digital filter enables to smooth the frequency path (S-curve modulation).

[0065] Preferably, the or each digital filter has a group delay which is independent of the frequency path to be filtered. For example, "group delay" describes the time shift of the envelope of a wave packet. A wave packet is a "pack" or "group" of oscillations centered around one frequency that travel together. Preferably, the group delay is constant over the digital filter. Consequently, because the group delay through the filter is constant and independent of the slope of the original signal, a 2D linear diagonal move will stay perfectly linear in the particle plane.

[0066] Preferably, the digital filter is resettable. The reset is for example carried out by clearing the memory (state) associated with the filter. In particular, between moves, the moving tweezers have to change location as rapidly as possible. This is done by resetting the filter memory so that the jump to another particle is not affected by the filter group delay.

[0067] For example, the digital filter is a filter of the Savitzky-Golay family. For example, the digital filter is a Moving Average filter, also called Box Car filter.

[0068] We can write the boxcar filter formula as follow: y n = ∑ k = 0 N − 1 x n − k

[0069] Where there are N taps to the filter, x[n] is a sequence of input samples and y[n] is the output of the filter.

[0070] Preferably, when the digital filter is a Box Car filter, the digital filter has the following features: when the duration of the computed path is superior to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a constant speed part, and finished with a quadratic deceleration part, and when the duration of the computed path is inferior or equal to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a quadratic deceleration part.

[0071] In any case, the duration of the processed move is the duration corresponding to the length of the original linear move, plus the group delay of the filter (filter number of samples / sample-rate).

[0072] Preferably, in case of a 2D multi-step move, when the digital filter is a Box Car filter, the digital filter is configured so that, after the original quadratic acceleration, the speed will stay constant and changes of direction will be automatically be smoothed / rounded.

[0073] Figure 4 illustrates three examples of Box Car filtering with different group delays, respectively (from top to bottom): a group delay of 8.2 µs, a group delay of 16.4 µs and a group delay of 32.8 µs. A longer group delay enables to obtain a smoother frequency path.

[0074] In an embodiment, the filtering step is performed by applying successively several digital filters on the frequency path to be filtered. The digital filters can be the same filters or can be different. This enables to smooth even more the RF control signal and the acceleration or jerk seen by the particles.

[0075] The generation phase 200 comprises a step 240 of generating, for each AOD 40, of a RF control signal whose frequency is modulated according to the filtered frequency path of the AOD 40 so that each AOD 40 deflects a laser beam forming the optical tweezer, enabling to move the optical tweezer according to the computed path.

[0076] The rearranging method comprises a phase 300 for effectively moving the considered particle P by the optical tweezer.

[0077] The above phases and steps are carried out for each particle to be moved from an initial position in the initial spatial configuration of qubits to a target position in the target spatial configuration of qubits.

[0078] Hence, the method and elements describe above enable to obtain a modulation (S-curve modulation) that is easily and efficiently implementable in devices like FPGAs and ASICs. In particular, the use of a digital filter enables to easily smooth the frequency modulation of the radiofrequency control signal of each AOD controlling an optical tweezer, enabling to ease the movement of the optical tweezer, so as to rearrange particles forming an array of qubits.

[0079] In some embodiments, the digital filter is a Boxcar / Moving Average filter. This presents the following advantages: Easy to implement on a device like FPGA or ASIC. The group delay through the filter is the same independent of the input signal. Easily resettable by clearing the sample memory. No need to precalculate coefficients as with a polynomial or Sigmoid function. Low latency, processing is done sample by sample and only adds 1 sample delay.

[0080] 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.

Examples

Embodiment Construction

[0020]In the description, the terms "suitable for", "able to" and "configured for" are considered equivalent.

[0021]An example of a quantum system 10 is schematically illustrated on figure 1.

[0022]The quantum system 10 is, for example, a quantum computing device (or quantum processing unit), a quantum sensor, a quantum communication device or any quantum device comprising an array of trapped neutral atoms.

[0023]Typically, the quantum system 10 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.

[0024]In particular, 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 ...

Claims

1. A method for generating a radiofrequency control signal for each acousto-optic deflector, so called AOD (40), of a device (22) controlling an optical tweezer, so as to move the optical tweezer according to a computed path, each AOD (40) being suitable to deflect a laser beam forming the optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD (40), the method being implemented by a programmable electronic device (42) and comprising the following steps: - the reception of a computed path for the optical tweezer, - the conversion of the computed path into a frequency modulation, so-called frequency path, for each AOD (40), each frequency path being under the form of a linear segment or multiple linear segments, - the filtering of each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path, and - the generation, for each AOD (40), of a radiofrequency control signal whose frequency is modulated according to the filtered frequency path of the AOD (40), so that each AOD (40) deflects a laser beam forming the optical tweezer, enabling to move the optical tweezer according to the computed path.

2. A method according to claim 1, wherein the or each digital filter has a group delay which is independent of the frequency path to be filtered.

3. A method according to claim 1 or 2, wherein the or each digital filter is resettable.

4. A method according to any one of claims 1 to 3, wherein the or at least one digital filter is a filter of the Savitzky-Golay family, preferably the or at least one digital filter being a Moving Average filter.

5. A method according to any one of claims 1 to 3, wherein the or at least one digital filter is a Moving Average filter, such filter having the following features: - when the duration of the computed path is superior to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a constant speed part, and finished with a quadratic deceleration part, and - when the duration of the computed path is inferior or equal to twice the group delay of the digital filter, the filtered frequency path comprises a quadratic acceleration part, followed by a quadratic deceleration part.

6. A method according to any one of claims 1 to 5, wherein the filtering step is performed by applying successively several digital filters on the frequency path to be filtered.

7. A method for rearranging particles (P) trapped in trapping sites forming an initial spatial configuration of qubits, so as to obtain a target spatial configuration of qubits, the method comprising, for each particle (P) to be moved, the following phases: - determining, by a calculator, a path from an initial position of the particle (P) in the initial spatial configuration of qubits to a target position of the particle (P) in the target spatial configuration of qubits, and - generating a radiofrequency control signal for each acousto-optic deflector, so called AOD (40), of a device (22) for moving an optical tweezer, so as to trap the particle (P) in an optical tweezer and move the optical tweezer according to the determined path, the generation phase being carried out by a programmable electronic device (42) according to the method of any one of claims 1 to 6.

8. A programmable electronic device (42) for generating a radiofrequency control signal for each acousto-optic deflector, so called AOD (40), of a device (22) so as to move an optical tweezer according to a computed path, each AOD (40) being suitable to deflect a laser beam forming the optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD (40), the programmable electronic device (42) being configured for: - receiving a computed path for the optical tweezer, - converting the computed path into a frequency modulation, so-called frequency path, for each AOD (40), each frequency path being under the form of a linear segment or multiple linear segments, - filtering each frequency path by at least one digital filter so as to obtain a corresponding filtered frequency path, and - generating, for each AOD (40), a radio-frequency control signal whose frequency is modulated according to the filtered frequency path of the AOD (40) so that each AOD (40) deflects a laser beam forming the optical tweezer, enabling to move the optical tweezer according to the computed path.

9. A programmable electronic device (42) according to claim 8, wherein the programmable electronic device (42) is a Field Programmable Gate Array or an Application-Specific Integrated Circuit.

10. A device (22) for moving an optical tweezer comprising: - at least one acousto-optic deflector, so called AOD (40), each AOD (40) being suitable to deflect a laser beam forming an optical tweezer as a function of a frequency of a radiofrequency control signal received by the AOD (40), and - a programmable electronic device (42) configured for generating a radiofrequency control signal for each AOD (40) so as to move the optical tweezer according to a computed path, the programmable electronic device (42) being according to claim 8 or 9.

11. A device (22) according to claim 10, wherein the device (22) comprises two AOD (40) arranged so as to move the optical tweezer in a two-dimensional plane.

12. A rearrangement system (16) for rearranging particles (P) trapped in trapping sites forming an initial spatial configuration of qubits so as to obtain a target spatial configuration of qubits, the rearrangement system (16) comprising at least: - a generator (20) of an optical tweezer, the optical tweezer being formed of at least one laser beam, and - a device (22) for moving the optical tweezer, the device (22) being according to claim 10 or 11.

13. A quantum system (10) comprising: - a source (12) of particles (P), - a generator (14) of trapping sites for the particles (P), and - a rearrangement system (16) for rearranging particles (P) trapped in the trapping sites, the rearrangement system (16) being according to claim 12.

14. A quantum system (10) according to claim 13, wherein the particles (P) are chosen among the following particles: neutral atoms, Nitrogen-Vacancy centers, Silicon-Vacancy centers, ions, molecules and quantum dots.

Images