Patent Description:
The goal of the present invention is to design a system and a method for quick, reliable and repeatable construction of a defect free array of cold neutral atoms with a predefined architecture, where preferably all atoms are in the same quantum state.

There are two known ways of atom array construction that are suitable, for example, for quantum computation, namely arrays of cold ions and arrays of cold neutral atoms.

How to construct an array of cold ions is disclosed in Paul1990 [<NUM>], Berkeland2002 [<NUM>], Akerman2011 [<NUM>], Dubost2012 [<NUM>], Pöschl2018 [<NUM>], Thompson2015 [<NUM>] and Heazlewood2019 [<NUM>] where ions within a vacuum chamber are captured by a radio frequency (rf) trap and laser cooled to typically less than few 10mK, to form a Coulomb crystal. A particular cold ion is held in place within the crystal, namely in a particular rf trapping site, by the forces of electric field of the rf trap and electric field from the neighboring cold ions within the crystal, i.e., by the Coulomb repulsion between the ions. Typical spacing between the neighboring cold ions in such arrays are in the order of a few µm to a few <NUM>. The structure of such array can be defined by the architecture of a rf trapping means generating the rf trap with a particular geometry of the rf trapping sites. Particular architectures of linear rf traps (tunnels) where cold ions self-arrange into a simple linear or zig-zag chain structures are disclosed in Akerman2011 [<NUM>], Berkeland2002 [<NUM>], Brownnut2007 [<NUM>], Dubost2012 [<NUM>], Pöschl2018 [<NUM>], Hempel2014 [<NUM>], Pokorny2014 [<NUM>].

Main advantages of thus constructed cold ion arrays and their methods are as follows: (a) it is relatively easy to trap particular ions from the cloud into the rf trap; and (b) due to Coulomb repulsion, the cold ions exhibit self-arranging properties, so that faultless array of cold ions with architecture defined by the rf trapping sites geometry is easily achieved. Due to robustness and reliability of the process usually no additional steps of checking the array for its accuracy is necessary before using the array in practical applications.

The construction of cold neutral atom arrays within an ultra-high vacuum chamber is also known (Kaufman2021 [<NUM>], Schlosser2001 [<NUM>], Schäfer2020 [<NUM>]). In order to capture the neutral atoms into the array, they should be cooled to sufficiently low temperatures, usually by laser cooling. The cold neutral atoms are captured and held within the array with the predefined structure by the optical traps generated by, for example, optical tweezers. When constructing the array of cold neutral atoms, faults in the array, e.g. missing atoms, are highly likely due to statistical nature of capturing cold atoms in the optical traps. Therefore, before using the array of cold neutral atoms in practical applications, usually a process of verifying its accuracy is necessary, which renders the entire process less efficient and slower. The advantage of capturing the cold neutral atoms by optical tweezers is that manipulation of individual cold neutral atoms within the array by moving optical tweezers has less constraints, one of the reasons being that there is virtually no interaction between the cold neutral atoms. Consequently, shorter distances between atoms can be achieved in the array of cold neutral atoms.

The arrays of cold ions have some disadvantages in comparison to arrays of cold neutral atoms, in terms of their limited possibilities of application. For example, shorter distances between individual atoms that can be achieved in the array of cold neutral atoms are advantageous for quantum computation, because with shorter distances it is easier to achieve entanglement between the individual atoms, which is necessary for quantum "programming". Furthermore, in quantum computing or quantum simulations it is desirable to work with more complex arrays, for example 2D or 3D arrays, because each atom within such array has more neighboring atoms than in one dimensional array (e.g. a line), so there are more entanglement possibilities of the neighboring atoms, and consequently possibilities of quantum "programming" are significantly increased. It is much easier to assemble more complex arrays, that are constructed from cold neutral atoms, than from cold ions which are usually trapped in the rf trap in a single line structure, because cold neutral atoms can be held in place by optical traps which in general exhibit greater flexibility than rf traps.

The attribute "cold", used throughout this application in relation to "cold ions" or "cold neutral atoms", is to be understood as a property of ions / atoms having a kinetic energy sufficiently low so that these ions / atoms can be trapped and/or that they can be kept trapped within rf traps and/or optical traps.

The method and the system according to the present invention combine both known methods and devices for assembling cold ion/neutral atom arrays, and their respective advantages, in order to construct quickly an array of cold neutral atoms which is defect free, so no subsequent verification is necessary, and also enable automatic and iterative assembly of more complex arrays of cold neutral atoms.

The invention is further described with reference to the following drawings:.

The system <NUM> for constructing an array of cold neutral atoms according to the present invention comprises the following:.

The vacuum chamber <NUM> is designed to achieve an ultra-high vacuum, in order to enable all the processes according to the present invention of assembling the array of cold neutral atoms. Typical value of the pressure inside the vacuum chamber <NUM> is below <NUM>×<NUM>-<NUM> mbar. The vacuum chamber <NUM> has sufficient optical access for the devices of the system <NUM>, for example the cooling means <NUM> and the optical trapping means <NUM>. For example, the vacuum chamber <NUM> comprises the following ports <NUM>: a low numerical aperture (NA) port 15a for the cooling means <NUM>, which is implemented as laser cooling means, two high NA ports 15c and 15d for the optical trapping means <NUM>, and a port 15f for the neutralization means.

The ion source <NUM> emits the positively charged ions (cations) of a particular atom species into the vacuum chamber <NUM>, preferably in a beam of ions <NUM> towards the rf trap <NUM>, in a controlled way. Preferably, all emitted ions have the same electric charge. In one of possible embodiments of the system <NUM>, a Strontium (Sr) atom source described in Schioppo2012 [<NUM>] in combination with photoionization diode-lasers disclosed in Vant2006 [<NUM>] may be used as the ion source <NUM>.

Species of atoms that are suitable as building blocks of the array assembly according to the present invention should satisfy the following requirements: in their ionized state they should be suitable to be trapped and held in place by the optical trapping means <NUM> and by the rf trapping means <NUM> and suitable to be cooled by laser or other means; and in their neutral state they should be suitable to be trapped and held in place by the optical trapping means <NUM>. For example, alkaline-earth atoms from the second group of periodic system, particularly Strontium (Sr), and alkaline-earth-like atoms, particularly Ytterbium (Yb), are suitable for this purpose, as disclosed in Karpa2019 [<NUM>]. Sr and Yb are particularly suitable because the array constructed according to the present invention can be applied for quantum simulator and/or quantum computers as described in Kaufman2021 [<NUM>], where optically trapped cold neutral atoms are excited to Rydberg states, as disclosed in Morgado2021 [<NUM>] and Henriet2020 [<NUM>].

The cooling means <NUM> is designed to cool the ions, and/or subsequently the neutral atoms, within the vacuum chamber <NUM>. The cooling to sufficiently low temperature is required in order to reduce the kinetic energy of the ions <NUM> and/or neutral atoms <NUM>, so that they can be trapped and held in place in ordered structure by the rf trapping means <NUM> and/or by the optical trapping means <NUM>. In general, the ions <NUM> and/or neutral atoms <NUM> may be cooled by the cooling means <NUM> while they are passing through the vacuum chamber <NUM>, once they are trapped by the rf trapping means <NUM> and/or by the optical trapping means <NUM>, and/or also after they are neutralized and kept within the primary structure <NUM> only by the optical trapping means <NUM>.

Once the ions <NUM> are trapped into the rf trap <NUM>, the cooling is necessary to (further) reduce their kinetic energy in order to form an ordered structure thus defining the rf trapping sites <NUM>, and to further reduce their kinetic energy in order to be held within the optical traps <NUM> before, during and after the neutralization step.

The cooling means <NUM> can be implemented in various known ways, for example by laser cooling as disclosed in Metcalf [<NUM>], sympathetic cooling as disclosed in Tomza2019 [<NUM>], or rf cooling as disclosed in Sriarunothai2017 [<NUM>].

The rf trapping means <NUM> generates the rf trap <NUM> with the rf trapping sites <NUM>, into which cold free ions <NUM> are captured. The geometry of the primary structure <NUM> of the array is defined by the geometry of the rf trapping sites <NUM>. The primary structure <NUM> may contain one single cold ion/cold neutral atom or more. One possible geometry of the primary structure <NUM> known in the state of the art (Paul1990 [<NUM>], Berkeland2002 [<NUM>]) is a line of a few to a few ten cold ions. In order to enable required optical access to the trapped ions <NUM> and provide a trapping potential depth of a few ten eV, the blade shaped design of the rf trapping means <NUM> may be applied as disclosed in Hempel2014 [<NUM>], Pokorny2014 [<NUM>], He2021 [<NUM>]. Preferably, the distance between the neighboring rf trapping sites <NUM> within the primary structure <NUM> is in the order of <NUM>, so that the Coulomb repulsion between the cold ions does not destroy the primary structure <NUM> in embodiments where, before the neutralization, the cold ions <NUM> are held in place within the primary structure <NUM> only by the optical trapping means <NUM>.

The optical trapping means <NUM> generates the optical traps <NUM> within the vacuum chamber <NUM> by optical trapping beams 11f, wherein the optical traps <NUM> have essentially the same geometry as the rf trapping sites <NUM>, namely that of the primary structure <NUM>. Once the ions <NUM> are trapped into the primary structure <NUM> of the rf trap <NUM> and held in place by the rf trapping means <NUM> in the rf trapping sites <NUM>, the trapped cold ions <NUM> are overlaid by the optical traps <NUM> of the optical trapping means, so double-trapping of the cold ions <NUM> is achieved.

The optical trapping means <NUM> can be implemented, for example, as optical tweezers.

The rf trap <NUM> exerts stronger force on the cold ions <NUM> to keep them in the primary structure <NUM>, so the temperature (kinetic energy) of the ions <NUM> can be higher than the temperature of the cold ions <NUM> and the cold neutral atoms <NUM> held in place only by the optical trapping means <NUM> which exerts weaker force on the cold ions <NUM> and the cold neutral atoms <NUM> to keep them in the optical traps <NUM>. That is why the neutralization process is initiated only after the cold ions <NUM> within the primary structure <NUM> are sufficiently cooled by the cooling means <NUM>.

The neutralization means <NUM> comprises a source of electrons 13a to create a controlled flux of electrons, optionally also focusing electrodes to create a neutralization beam 13b of electrons, directed toward the cold ions <NUM>, which are trapped in the primary structure <NUM> by the rf trapping means <NUM> and/or by the optical trapping means <NUM>, in order to neutralize the cold ions through electron-ion recombination. A skilled person in the art can appropriately adjust operating parameters of the neutralization means <NUM>, such as electron energy, electron monochromaticity, electron flux (current), electron density (beam diameter) and the direction of the flux of electrons, depending on particular embodiments of the system and the method according to the present invention, in order to achieve reliable neutralization of all cold ions <NUM> within the primary structure <NUM> as quickly as possible, thus generating neutral atoms <NUM>, while preserving the geometrical arrangement of the neutral atoms <NUM> in the primary structure <NUM>, and to achieve that the neutralized atoms <NUM> are in such quantum state to allow the cold neutral atoms <NUM> to be held within the optical traps <NUM> and to prevent the optical trapping beams 11f to re-ionize them.

The wavelength and intensity of the optical trapping beams 11f should be selected appropriately so that for practical purposes they do not re-ionize the neutralized atoms <NUM> during or after the neutralization.

Once the cold neutral atoms <NUM> are kept in place within the primary structure <NUM> by the optical trapping means <NUM>, further manipulation of the geometry of the structure is possible, because the optical trapping means <NUM> enable such spatial movement of each cold neutral atom <NUM> or a group of individual cold neutral atoms <NUM> by moving the corresponding optical traps <NUM>.

For example, in one possible application of the array of the cold neutral atoms <NUM>, namely for quantum computation, in which it is desired that the distances between neighboring cold neutral atoms <NUM> are in range of µm, the geometry of the primary structure <NUM> can be changed into a secondary structure <NUM>, where the distance between neighboring cold neutral atoms <NUM> is significantly reduced, so, for example, that the cold neutral atoms <NUM> can be entangled via Rydberg interactions. Furthermore, given that the neighboring cold ions <NUM> in the primary structure <NUM> (in the rf trapping sites <NUM>) held in place by the rf trap <NUM>, are not necessarily at equal distances, and consequently neither are the neighboring neutral atoms <NUM> in the primary structure <NUM>, the secondary structure <NUM> can also be used to structure the cold neutral atoms <NUM> equidistantly.

Furthermore, the array of cold neutral atoms <NUM> in the primary structure <NUM> or the secondary structure <NUM>, which typically is a line (i.e. one-dimensional structure), can be combined into more complex final array <NUM>, so for example to form longer one-dimensional structures, arbitrarily shaped two-dimensional structures or even three dimensional structures to be used in different applications. Additional optical traps for the final array <NUM> can either be formed by the optical trapping means <NUM> or by additional optical trapping means (not shown in Figures).

Optionally, the system <NUM> comprises also a high-resolution camera <NUM> in combination with a high numerical aperture (NA) optics 11e, with NA sufficiently high configured for imaging individual cold ions <NUM> in the rf trapping sites <NUM> and/or in the optical traps <NUM> and/or individual cold neutral atoms <NUM> in the optical traps <NUM>. The high-resolution camera <NUM> is directed toward a trapping area, where the rf trapping sites <NUM> and the optical traps <NUM> are located, and possibly also toward a construction area, where, in some embodiments, the final array <NUM> of cold neutral atoms <NUM> is constructed, in order to monitor whether and to what extent the primary structure <NUM>, secondary structure <NUM> and/or the final array <NUM> are filled with the cold ions <NUM> or cold neutral atoms <NUM>. This is especially important in development phase of a particular embodiment of the system <NUM> and in calibration phase, where parameters of the system <NUM> are adjusted to achieve the required performance.

The system <NUM> comprises also an electronic control system (not shown in Figures) with processing, memory and connectivity capabilities, which is connected to the above-described devices of the system and on which the appropriate software modules can be run in order to carry out methods described below, in order to operate, monitor and control the system <NUM>.

The method of constructing an array of cold neutral atoms <NUM> with the geometry of the primary structure <NUM>, held in place by the optical traps <NUM>, according to the present invention comprises the following steps:.

In addition to the above mentioned four steps, a cooling step is necessary for cooling the ions/neutral atoms in order to reduce their kinetic energy (temperature), namely for the ions <NUM> to be trapped, achieve a stable structure and kept in place in the primary structure <NUM> by the rf trap <NUM>, and for the cold ions <NUM> and the cold neutral atoms <NUM> to be kept in place within the optical traps <NUM>. The cooling step can be continuous or comprise intervals of cooling, and can run simultaneously with the above mentioned four steps and/or can run between or after each of these steps. The starting point and duration of the cooling step or its intervals depend on parameters of a particular embodiment of the system <NUM>, such as the kinetic energy of the ions/neutral atoms and the depth of the rf trap <NUM> and the optical traps <NUM>.

For example, in certain embodiments the ions <NUM>, when released from the ion source <NUM>, have sufficiently low kinetic energy (temperature) and the rf trap <NUM> is designed to have sufficient depth, i.e. to exert sufficient electric force, to capture the ions <NUM> floating in the vacuum chamber <NUM> without any cooling. However, the ions <NUM> should always be cooled to achieve their ordered structure within the rf trap <NUM>. Given that in general the optical traps <NUM> have significantly lower depth than the rf trap <NUM> and are not able to hold in place uncooled ions <NUM> / neutral atoms <NUM>, the cooling step should always be initiated before the neutralization step is concluded, namely before the neutral atoms <NUM> are kept in place merely by the optical traps <NUM>, so that once the neutral atoms <NUM> are to be kept in place merely by the optical traps <NUM>, they have sufficiently low kinetic energy.

The above-described method, after completion of step <NUM> and the cooling step, yields an array of cold neutral atoms <NUM> held in place by the optical traps <NUM> in a predefined geometry, namely that of the primary structure <NUM>, an example of which is shown in <FIG>.

Step <NUM> is achieved by the ion source <NUM>, step <NUM> is achieved by the rf trapping means <NUM>, step <NUM> is achieved by the optical trapping means <NUM>, step <NUM> is achieved by the neutralization means <NUM>, and the cooling step is caried out by the cooling means <NUM>.

Step <NUM> can be achieved by switching on the optical trapping means <NUM> only after step <NUM> and the cooling step are completed, or can be achieved with the same result also if the optical trapping means <NUM> are switched on earlier, for example already during step <NUM> or <NUM>. In other words, some of the steps described above can more or less overlap in time depending on a particular embodiment.

Preferably, step <NUM> is ended by switching off the rf trapping means <NUM>, immediately before initiating step <NUM> because the rf trapping means <NUM>, if switched on, may interfere with the neutralization beam 13b of electrons from the neutralization means <NUM>, and therefore interfere with the neutralization of the cold ions <NUM>.

Optionally, additional steps are possible after step <NUM> and the cooling step to achieve various arrays that are more suitable for particular applications. For example, an additional step to reshape the primary structure <NUM> into a more compact secondary structure <NUM> by moving the corresponding optical traps <NUM>, namely to achieve the reduction of the distances between neighboring cold neutral atoms <NUM> within the secondary structure <NUM>, and example of which is shown in <FIG>.

Further additional steps are possible after step <NUM> and the cooling step, or after above mentioned additional steps, to construct more complex final arrays <NUM> of cold neutral atoms <NUM> by combining the arrays in the shape of the primary structure <NUM> or the secondary structure <NUM>. The steps of constructing the final array <NUM> includes moving the cold neutral atoms <NUM> in the primary structure or in the secondary structure from the trapping area into the construction area, both within the vacuum chamber <NUM>. In the construction area the cold neutral atoms <NUM> are held in place by the additional optical traps produced by the optical trapping means <NUM> or by additional optical trapping means (not shown in Figures). In order to reduce time of the construction of the final array <NUM>, the additional steps of moving the primary structure <NUM> or secondary structure <NUM> from the trapping area to the construction area may run simultaneously with the next iteration of the above-mentioned method, namely steps <NUM> to <NUM> and the cooling step.

During or after step <NUM> a verification step can be added, carried out by imaging the cold ions <NUM> in the rf trapping sites <NUM> and/or in the optical traps <NUM> by the high-resolution camera <NUM>, to check whether and to what extent the rf trapping sites <NUM> and/or the optical traps <NUM> are filled with the cold ions <NUM>.

During or after step <NUM> an additional verification step can be added, carried out by imaging the cold neutral atoms <NUM> in the optical traps <NUM> by the high-resolution camera <NUM>, to check whether and to what extent the optical traps <NUM> are filled with the cold neutral atoms <NUM>. The imaging of the cold ions <NUM> and/or cold neutral atoms <NUM> in the rf trapping sites <NUM> and/or in the optical traps <NUM> can be used also in calibration procedures.

The inventive method is not limited to carrying out the individual method steps in a sequential manner or as shown in the preferred embodiment of <FIG>. Rather, the inventive method encompasses carrying out two or more essential and/or optional method steps in parallel or at least partially overlapping in time. For example, the cooling step can be employed at all times, for example in parallel or partially overlapping with any of the essential and/or optional method steps.

The inventive method is not limited by time durations of individual steps, e.g. by those exemplified in <FIG>. Rather, the inventive method encompasses carrying out the individual steps in time intervals of any duration being suited to the actual circumstances. In most applications of the method, it is preferable, that these steps are carried out in as short time intervals as practically achievable, which is particularly advantageous, for example, for use in quantum computing.

An embodiment of the system <NUM> according to the present invention shown in <FIG> and <FIG> produces the array of the cold neutral atoms <NUM> with geometry of the primary structure <NUM> with eight cold neutral atoms <NUM> positioned in a line. The primary structure <NUM> is shown also in <FIG>.

In this embodiment the vacuum chamber <NUM> is implemented as a cylindrical stainless-steel chamber 2a with base pressure less than <NUM>×<NUM>-<NUM> mbar in combination with a vacuum pumping system comprising an ion pump 2b and a titanium sublimation pump 2c, and vacuum gauges 2d.

The ion source <NUM> is implemented as a combination of a Strontium (Sr) oven 3a, with a mechanical shutter 3b, placed inside the vacuum chamber <NUM>, and a photoionization laser system 3c generating two photoionization laser beams of wavelengths of <NUM> and <NUM>, respectively, for two-photon ionization of hot Sr atoms. The photoionization laser beams 3d are directed to intersect the path of a beam <NUM> of hot Sr atoms from the outlet of the Sr oven 3a to the rf trapping sites <NUM>. After ionization the beam <NUM> of Sr+ ions proceeds toward the rf trapping sites, where some of the Sr+ ions are trapped therein. The Sr oven 3a comprises a resistive heated filament and a Sr dendrite of 5N purity in a stainless-steel housing, and has an integrated collimation tube of <NUM> diameter and <NUM> in length which collimates the beam <NUM> of hot Sr atoms with temperature of over <NUM>° C, which is directed toward the rf trapping sites <NUM>. The mechanical shutter 3b with on/off function is used to control the beam <NUM>, <NUM> of the Sr atoms/ions, i.e. to control the loading of the rf trap <NUM>.

The rf trapping means <NUM> is implemented as a linear quadrupole trapping system <NUM> comprising four longitudinal rod electrodes 8a and two on-end electrodes (not shown in <FIG> and <FIG>). The rf trapping means <NUM> is designed to generate a rf trap <NUM> with eight rf trapping sites <NUM> for eight Sr+ ions <NUM> in a linear primary structure <NUM> along its central axis 8c. The rf trap <NUM> has a minimum trap depth well above the mean kinetic energy of ions in the Sr+ ion beam <NUM>, namely of the order of <NUM> eV. The design of the linear quadrupole trapping system <NUM> allows high optical access to the rf trapping sites for the beam <NUM> of Sr+ ions from the ion source <NUM>, for the cooling means <NUM>, for the optical trapping means <NUM>, for the high-resolution camera <NUM>, and for a neutralization beam 13b of electrons from the neutralization means <NUM>.

The cooling means <NUM> is implemented as four laser cooling systems, namely a laser cooling system 7a generating cooling beam of wavelength of <NUM>, a repumper laser system 7b generating <NUM> repumper beam, a sideband cooling laser system 7c generating <NUM> sideband cooling beam, and a quencher laser system 7d generating <NUM> quencher beam. All these beams are directed toward the rf trapping sites <NUM>, namely toward the central axis 8c of the rf trapping means <NUM>. The first two laser cooling systems 7a, 7b are used to cool the trapped ions <NUM> close to Doppler limit of about <NUM> mK. The second two laser cooling systems 7c, 7d are used for cooling the trapped ions <NUM> close to final sub-Doppler temperature limit of about <NUM>µK. This temperature allows the cold ions <NUM> to form ordered linear chain in the rf trapping sites <NUM> and to be held in place by the optical trapping means <NUM>, and also allows, after neutralization, the cold neutral atoms <NUM> to be held in place by the optical trapping means <NUM> for extended periods of time (><NUM>).

The neutralization means <NUM> is implemented as a source of low energy electrons 13a capable of generating various electron beams with electron monochromaticity < <NUM> eV, electron energies between <NUM> eV and <NUM> eV, and beam current (flux) between <NUM> nA and <NUM> mA, which allows the selection of appropriate operating parameters for the neutralization beam 13b of electrons. The operating parameters of the neutralization means <NUM>, which should be adjusted to this particular embodiment of the system, are electron energy, electron flux (current), electron density (beam diameter) and the direction of the neutralization beam 13b of electrons in order to achieve reliable neutralization of all cold ions <NUM> within the primary structure <NUM> as quickly as possible, while preserving the geometrical arrangement of the cold neutral atoms <NUM> in the primary structure <NUM>, and to achieve that the neutralized atoms <NUM> are in the quantum state to be held within the optical traps <NUM> and that the optical trapping beams 11f do not re-ionize them.

The optical trapping means <NUM> in this embodiment comprises a laser tweezer system 11a, generating the optical trapping beams 11f with wavelength of <NUM>-nm, a dichroic mirror 11d and a high numerical aperture (NA) optics 11e with NA > <NUM>. The laser tweezer system 11a comprises two crossed acousto-optic deflectors (AOD) 11b and motorized lens 11c. The AODs 11b split an initial laser beam into eight separate optical trapping beams 11f and are also used to steer the optical trapping beams 11f independently from one another, which consequently enable the optical traps <NUM> to be independently steered within the vacuum chamber <NUM>. Each AOD 11b is used to steer the optical trapping beams 11f independently from one another along one axis (x or y axis), so two crossed AODs 11d enable the steering of the optical trapping beam 11f in an xy target plane. The motorized lens 11c is used to focus the optical trapping beams 11f at a certain distance from the optical trapping means <NUM> (i.e. along z axis), thereby defining the distance of the xy target plane from the optical trapping means <NUM>. The high NA optics 11e with NA > <NUM> is used within the context of the optical trapping means <NUM> to focus the optical trapping beams 11f thereby defining the diameter of the optical traps <NUM>. The optical trapping beams 11f are directed toward the rf trap <NUM> in the vacuum chamber <NUM>. The steering of the optical traps <NUM> is necessary to achieve their spatial overlaying with the rf trapping sites <NUM> and possibly to move the optical traps <NUM> with cold neutral atoms <NUM> to create more compact structure, namely the secondary structure <NUM>, or to move the primary structure <NUM> or the secondary structure <NUM> from the trapping area to the construction area to construct the final array <NUM> as mentioned above. To accomplish that the rf trapping sites <NUM> are spatially overlaid by the optical traps <NUM> in this embodiment of the system <NUM>, a calibration procedure is necessary, and is described below.

The dichroic mirror 11d is used to allow the high NA optics 11e to be used for two purposes: to focus the optical trapping beams 11f, as mentioned above, and for imaging the cold ions <NUM> and/or cold neutral atoms <NUM> in the rf trapping sites <NUM> / optical traps <NUM> with the high-resolution camera <NUM> through the same optical port 15c in the vacuum chamber <NUM> wall as is used to introduce the optical trapping beams 11f into the vacuum chamber <NUM>. The dichroic mirror 11d reflects the optical trapping beams 11f with wavelength of <NUM> from the laser tweezer system 11a towards the high NA optics 11e and accompanying optical port 15c through which the optical trapping beams 11f enter the vacuum chamber <NUM>. On the other hand, the dichroic mirror 11d transmits the light with the wavelengths of <NUM> for imaging the cold ions <NUM>, and/or the light with the wavelengths of <NUM> and/or <NUM> for imaging cold neutral atoms <NUM>, coming from the cold ions <NUM> / cold neutral atoms <NUM> in the rf trapping sites <NUM> and/or the optical traps <NUM> within the vacuum chamber <NUM> to the high-resolution camera <NUM>.

This embodiment of the system <NUM> comprises the high-resolution camera <NUM> which is implemented as an electron-multiplied charge-coupled-device (EMCCD) camera <NUM> with the resolution of more than <NUM> Mpixel and high (single photon) sensitivity in combination with the high NA optics 11e. The EMCCD camera <NUM> is used to monitor whether the rf trapping sites <NUM> are filled with the cold ions <NUM> and whether the optical traps <NUM> are filled with the cold ions <NUM> or the cold neutral atoms <NUM>. To image the cold ions <NUM> in the rf trapping sites <NUM> and/or in the optical traps <NUM>, fluorescence imaging is applied by using the cooling beam at <NUM> and the repumper beam at <NUM>. To image the cold neutral atoms <NUM> in the optical traps <NUM>, fluorescence imaging is applied by using the photoionization laser beam at <NUM> and/or additional beams at <NUM> and/or <NUM> and/or <NUM> from additional beam sources not shown in <FIG>. A particular choice of the laser beams required for imaging the cold neutral atoms depends on the required fidelity and survival probability of the atom imaging.

For example, in a possible calibration process, the high-resolution camera <NUM> can be used to determine a time period in which the rf trapping sites <NUM> are entirely filled with the cold ions <NUM>, namely by monitoring the rf trapping sites <NUM>. The high-resolution camera <NUM> can also be used to determine time periods necessary for cooling the ions <NUM> or the neutral atoms <NUM>. The temperature of the ions <NUM> / neutral atoms <NUM> in the traps can be measured with the high-resolution camera <NUM> indirectly, for example by switching off the rf trap <NUM> or the optical traps <NUM> and measuring the time evolution of ion/atom position. In the calibration process described below, the high-resolution camera <NUM> is used to determine the position of the optical traps <NUM> relative to the rf trapping sites <NUM>, in order to adjust the position of the optical traps <NUM> to spatially overlay them with the rf trapping sites <NUM>, including capturing the image of a calibration mask <NUM> used in the calibration process.

In this embodiment, the system <NUM> according to the present invention comprises a photodetector <NUM> which is used in the calibration process for calibrating the position of the optical traps <NUM>, which is described below. The photodetector <NUM>, can be implemented, for example, as a photodiode <NUM>. The photodetector <NUM> is positioned on its optical port 15d on the same optical axis as the EMCCD camera <NUM> and the optical trapping beams 11f once they enter the vacuum chamber <NUM>, but on the other side of the vacuum chamber <NUM> relative to the optical port 15c for the optical trapping means <NUM> as shown in <FIG>. The optical port 15d is also necessary for functioning of the optical trapping means <NUM>.

In this embodiment, the vacuum chamber <NUM> has one optical port 15a for the laser cooling means, one optical port 15b for photoionization system 3c, one optical port 15c for the high-resolution camera and the optical trapping means, one optical port 15d for the photodetector <NUM>, one port 15e for an electrical high-voltage radio-frequency feedthrough for the rf trapping means <NUM> and for the Sr oven 3a, one port 15f for the neutralization means <NUM>, and a port <NUM> for the vacuum pumping system 2b and the vacuum gauges 2d.

The method according to the present invention as implemented in the embodiment shown in <FIG> and <FIG> is schematically shown in a table in <FIG>.

Columns to the right in the chart represent time intervals A, B, C, D and E, wherein interval A lasts <NUM>, interval B <NUM>, interval C <NUM>, interval D is in the range of <NUM> to <NUM>, and optional interval E lasts <NUM>. The upper rows of the chart show during which time intervals A - E particular steps of the method according to the present invention, namely steps <NUM> - <NUM>, the cooling step and the optional verification step, are carried out, which is shown as hatched cells in the table. The below rows of the chart show during which time intervals A - E particular devices of the system <NUM> are active, which is shown as hatched cells in the table.

For example, step <NUM>, in which the ions <NUM> are released into the vacuum chamber <NUM> toward the rf trapping sites <NUM>, is accomplished in interval A. To achieve that, during interval A the Sr oven 3a, emitting the Sr atoms, is switched on, the mechanical shutter 3b of the Sr oven 3a is open, and the photoionization laser system 3c, generating both photoionization laser beams, namely of wavelengths <NUM> and <NUM>, is active. For practical reasons, the Sr oven 3a is switched on during all intervals, and the flux of the Sr atoms is controlled by the mechanical shutter 3b.

Step <NUM>, namely trapping and holding the ions <NUM> in the rf trapping sites <NUM>, is carried out during intervals A, B and C. There is a short delay between releasing the ions <NUM> into the vacuum chamber <NUM> and trapping first ions <NUM> into the rf trap <NUM>, which is not shown in the chart in <FIG> due to relatively short duration and stochasticity of the delay. To accomplish step <NUM>, the rf trapping means is switched on during intervals A, B and C.

Step <NUM>, double-trapping and holding the cold ions <NUM> by the optical traps <NUM>, and then holding the cold neutral atoms <NUM> by the optical traps <NUM>, is carried out during intervals C, D and E. That is why the optical trapping means <NUM> is switched on during these three intervals. The optical trapping means <NUM> could be switched on during intervals A and B as well, because it would not interfere with other steps of the method.

Step <NUM>, neutralization of the captured cold ions <NUM>, is achieved during interval D, so the neutralization means <NUM> is active during this interval. During step <NUM>, the cold ions <NUM>, until they are neutralized, and then the cold neutral atoms <NUM>, are held in place merely by the optical traps <NUM>, because the rf trap <NUM> is no longer active. The rf trapping means <NUM> is switched off before initiation of step <NUM>, because the electric field of the rf trapping means <NUM> can interfere with the neutralization beam 13b of electrons from the neutralization means <NUM>.

The cooling step is carried out during intervals A and B by the cooling means <NUM>. The ions <NUM> in the rf trap <NUM> are first cooled with the <NUM> cooling beam and <NUM> repumper beam close to Doppler temperature limit of about <NUM> mK during interval A, and then with the <NUM> sub-doppler cooling beam and the <NUM> quencher beam close to sub-Doppler temperature limit of approx. <NUM>µK during interval B.

In the chart in <FIG>, the optional verification step is shown, which is carried out during interval E, to verify whether the primary structure <NUM> comprising the cold neutral atoms <NUM>, held in place by the optical traps <NUM>, is defect free. During this interval, the high-resolution camera <NUM> captures the image of the cold neutral atoms <NUM> in the optical traps <NUM> by applying fluorescence imaging using the photoionization laser beam at <NUM> and/or additional beams at <NUM> and/or <NUM> and/or <NUM>, so in this interval the high-resolution camera <NUM> is active and so is the photoionization laser system 3c, but generating only the photoionization laser beam with the wavelength of <NUM>, and also the additional beam sources (not shown in <FIG>).

The main purpose of the calibration procedure in this embodiment, according to the calibration method described below, is to spatially adjust the position of the optical traps <NUM>, which is done by moving the optical trapping beams 11f and focusing them at various distances from the optical trapping means <NUM>, or more precisely at various distances from the high NA optics 11e, in order to spatially lay the optical traps <NUM> exactly over the rf trapping sites <NUM>.

In calibration step <NUM>, the ions <NUM> are captured in the rf trap <NUM>, sufficiently cooled by the cooling means <NUM>, and held in place by the rf trapping means <NUM> in an ordered structure, namely in the primary structure <NUM>, defined by the rf trapping sites <NUM>; for example by carrying out steps <NUM>, <NUM> and the cooling step of the method according to the present invention. By doing that, the exact positions of the rf trapping sites <NUM> are defined by the location of the cold ions <NUM> in the rf trap <NUM>.

In calibration step <NUM>, the fluorescent imaging by the high-resolution camera <NUM> is applied in order to determine the exact position of the cold ions <NUM>, and thereby the exact position of the rf trapping sites <NUM>, thereby defining an xy plane of the internal coordinate system of the high-resolution camera <NUM>. To image the cold ions <NUM>, the focus of the high-resolution camera <NUM> is set on the cold ions <NUM>. The imaging is done in this embodiment by using the cooling beam at <NUM> and the repumper beam at <NUM>.

In calibration step <NUM>, the calibration mask <NUM>, shown in <FIG>, is inserted into the rf trapping means <NUM> along its central axis 8c, so that the central axis 8c lies on the flat surface of the calibration mask <NUM> and so that the flat surface of the calibration mask <NUM> is perpendicular to the optical axis of the optical trapping beams 11f coming from the high NA optics 11e to the rf trap <NUM>. Optionally, to place the calibration mask <NUM> into the exact position, the calibration mask <NUM> is moved until its image as captured by the high-resolution camera <NUM> is in focus.

The calibration mask <NUM> is essentially a thin plate 20a, for example made of glass and thereby transparent, covered by a thin untransparent metal film, with optical holes 20b with known geometry, for example the diameter of about <NUM>, placed equidistantly in a line with a distance of <NUM> between the neighboring optical holes 20b. The optical holes 20b may be implemented as actual holes in the plate 20a, or in case the plate is transparent and covered by an untransparent film, by absence of the film where the optical holes 20b should be. In this embodiment the calibration mask <NUM> has five optical holes 20b.

In calibration step <NUM>, the high-resolution camera <NUM> captures the image of the calibration mask <NUM>, particularly the position of the optical holes 20b in the calibration mask <NUM>, whereas the surface of the calibration plate 20a is aligned with a correct target plane in which lies the central axis, and whereas the correct target plane is also perpendicular to the optical axis of the optical trapping beams 11f coming from the high NA optics 11e to the rf trap <NUM>. The light for capturing this image can come from a separate source of light or from surrounding light from outside the vacuum chamber <NUM> that is introduced through an optical port (not shown in Figures). From the image of the optical holes 20b in the correct target plane, the coordinates of the optical holes 20b in the xy plane according to an internal coordinate system of the high-resolution camera <NUM> are obtained.

In calibration step <NUM>, the xy target plane of an internal coordinate system of the optical trapping means <NUM> is calibrated with the xy plane of the internal coordinate system of the high-resolution camera <NUM> and the focus of the optical trapping beam 11f, expressed as z axis in the internal coordinate system of the optical trapping means <NUM>, is calibrated at the exact distance, where the calibration mask <NUM> and also the correct target plane are positioned.

The optical trapping beam 11f is systematically steered according to its internal coordinate system across the xy target plane, thereby across the calibration mask <NUM>. The steering of the optical trapping beam 11f in the xy target plane is achieved by the AODs 11b. On the other side of the calibration mask <NUM>, the amount of the light from the optical trapping beam 11f coming through the optical holes 20b of the calibration mask <NUM> is captured by the photodetector <NUM>. Various photodetector signals are shown in <FIG>. The strength of the photodetector signal depends on the position of the optical trapping beam 11f relative to the position of the optical holes 20b of the calibration mask <NUM>, namely when the optical trapping beam 11f shines directly through the optical hole 20b in the calibration mask <NUM>, the photodetector signal is the strongest.

Therefore, the peaks of the photodetector signal indicate the position of the optical holes 20b, which was in calibration step <NUM> expressed in the xy plane of the internal coordinate system of the high resolution camera <NUM>, now expressed in the xy target plane of the internal coordinate system of the optical trapping means <NUM>, by comparing the strength of the photodetector signal with the position of the optical trapping beam 11f, expressed in the xy target plane.

The shape of the photodetector signal indicates also to what extent the optical trapping beam is focused at the exact distance where the calibration mask <NUM> and also the correct target plane are positioned. The first photodetector signal (a) in <FIG> shows that the signal relatively slowly reaches its peak and then descends, because the focus of the optical trapping beam 11f is not yet aligned with the correct target plane. The second photodetector signal (b) in <FIG> reaches its peak and descends from it more steeply, thereby indicating that the focus of the optical trapping beam 11f is closer to the correct target plane. And the third photodetector signal (c) in <FIG> has the steepest shape of rising and falling of the signal, thereby indicating that in this scanning the focus of the optical trapping beam 11f is at the same distance, expressed as z coordinate of the internal coordinate system of the optical trapping means <NUM>, as the correct target plane.

The xy target plane, thereby the calibration mask <NUM>, is systematically scanned by the optical trapping beam 11f; however, several scannings of the entire xy target plane is done, whereas each scanning has a different focus of the optical trapping beam 11f, i.e. with a focus at a different distance from the optical trapping means <NUM>. The motorized lens 11c, which operates according to the internal coordinate system of the optical trapping means <NUM>, is used to achieve different focuses of the optical trapping beam 11f. The photodetector signal is observed while the entire target plane is scanned several times by the optical trapping beam 11f, and compared with the position of the optical trapping beam 11f, expressed in xyz coordinates of the internal coordinate system of the optical trapping means <NUM>.

At the end of calibration step <NUM>, the calibration mask <NUM> is removed from the rf trapping means <NUM>.

In calibration step <NUM>, the exact position of the optical trapping beams 11f, in order to lay the optical traps <NUM> over the rf trapping sites <NUM>, is determined by transforming the position of the cold ions <NUM> obtained in calibration step <NUM>, which is expressed in the xy plane of the internal coordinate system of the high resolution camera <NUM>, into the position of the optical trapping beams 11f, which is expressed in the xy target plane, and focusing the optical trapping beams 11f at the same distance, expressed as z coordinate of the internal coordinate system of the optical trapping means <NUM>, as the correct target plane, which was obtained in calibration step <NUM>.

Claim 1:
System (<NUM>) for constructing an array of cold neutral atoms (<NUM>) characterized in that it comprises:
- a vacuum chamber (<NUM>), within which ultra-high vacuum can be achieved;
- an ion source (<NUM>) configured for populating the vacuum chamber (<NUM>) with positively charged ions (<NUM>);
- a radio frequency (rf) trapping means (<NUM>), generating a rf trap (<NUM>) with a predefined geometry of rf trapping sites (<NUM>), configured for capturing free ions into the rf trap (<NUM>) and for keeping the cold ions (<NUM>) in place within the array with a primary structure (<NUM>), defined by the rf trapping sites (<NUM>);
- an optical trapping means (<NUM>), generating optical traps (<NUM>), configured for double trapping already trapped cold ions (<NUM>) in the primary structure (<NUM>) and for holding the cold ions (<NUM>) and/or the cold neutral atoms (<NUM>) in place within the primary structure (<NUM>);
- a cooling means (<NUM>) configured for cooling the ions (<NUM>) and/or neutral atoms (<NUM>) within the vacuum chamber (<NUM>) to sufficiently low temperature, in order for the cold ions (<NUM>) and/or cold neutral atoms (<NUM>) to be trapped and/or kept in an ordered structure in the rf trapping sites (<NUM>) and/or in the optical traps (<NUM>);
- a neutralization means (<NUM>), comprising a source of electrons (13a) configured for creating a controlled flux of electrons, for neutralizing the cold ions (<NUM>) that are trapped and held in place in the primary structure (<NUM>) by the optical trapping means (<NUM>) and/or by the rf trapping means (<NUM>).