Patent Description:
A quantum processor is a device that is configured to process input data by using quantum states of physical systems, such as atoms or photons, in order to implement an algorithm on said input data and generate output data. Such algorithms can be of various types, as disclosed in [REF <NUM>]. For example, in the so-called digital (or universal) approach, algorithms comprise a sequence of quantum operations that are implemented on predetermined physical systems of the quantum processor. Alternatively, in the analog approach, algorithms comprise the reproduction of a predetermined Hamiltonian evolution by the physical systems of the quantum processor, such reproduction comprising, for example, a plurality of quantum operations.

Neutral atom-based quantum processors generally comprise an ensemble of atoms in a vacuum chamber wherein the quantum state of each atom is used to perform the operations of the algorithm. When an operation of the algorithm is implemented on the quantum state of a single atom, it is referred to as a single-qubit operation and when an operation of the algorithm is implemented on the quantum states of a plurality of atoms, it is referred to as a multi-qubit operation.

Further, said quantum state of the atom can be an excited state (such as a Rydberg state) or a fundamental state, wherein the excitation to such excited state is produced, for example, by the interaction between a laser beam and said atom.

In order to perform an algorithm with a neutral-atom based quantum processor, three steps are generally performed. In a first step, also referred to as preparation step, the atoms are prepared in a predetermined quantum state. In a second step, also referred to as quantum processing step, the quantum states of the atoms are modified to perform operations corresponding to operations of the algorithm on the input data. In a third step, also referred to as readout step, the quantum states of the atoms are read out in order to obtain the output data. Such steps imply the use of a plurality of laser beams to perform a plurality of functions.

In the particular case of a Rubidium-based quantum processor, as disclosed for example in [REF2], Rubidium (Rb) atoms are introduced in a vacuum chamber and maintained in a predetermined volume of said chamber using a magneto-optical trap (MOT) comprising static magnetic fields and a plurality of MOT laser beams at a wavelength of about <NUM>. Such MOT laser beams aim at both reducing the kinetic energy of the atoms (i.e. their temperature) and, in combination with static magnetic fields, at introducing a restoring force that imposes the atoms to remain at a nearly fixed location.

Further, dipolar trapping is implemented in the vacuum chamber in order to trap individual atoms at different predetermined locations so that they can be individually addressed. More specifically, the atoms are arranged in an array of predetermined dimensions by creating a 1D, 2D, or 3D lattice of dipolar optical traps, each trap comprising at most one atom. Such lattice is generated by a dipolar trapping beam at a wavelength of about <NUM> (or, more generally, a wavelength between about <NUM> and about <NUM>) sent to a spatial light modulator that focuses said beam at said predetermined locations.

The atoms are then prepared in a predetermined quantum state, for example a fundamental state, by using an optical pumping beam with a wavelength close to a resonance with a transition between quantum states of Rb atoms. For example, in [REF2], such an optical pumping beam has a wavelength of about <NUM>, but it is also possible to use, inter alia, an optical pumping beam with a wavelength of about <NUM> or <NUM>.

Further, a so-called Rydberg excitation of Rubidium atoms in the vacuum chamber is implemented thanks to a two-photon transition scheme. In practice, a first and a second Rydberg beams at a wavelength of about <NUM> and about <NUM>, respectively (as disclosed, for example in [REF2]), or at a wavelength of about <NUM> and <NUM>, respectively (as disclosed, for example, in [REF3]), are sent to a predetermined number of atoms of the array to induce Rydberg transitions in said atoms so that the combined evolution of said atoms is submitted to their dipolar interaction. In particular, when two atoms are sufficiently close to one another, they cannot be simultaneously excited to a Rydberg state. This phenomenon, referred to as Rydberg blockade, can be used to perform multi-qubit operations of the algorithm on the atoms.

Further, as disclosed for example in [REF3], Raman laser beams (for example at a wavelength of around <NUM>) may also be used to manipulate a quantum state of a predetermined atom of the array of atoms in the vacuum chamber in order to perform a single qubit operation on said atom or as a part of multi-qubit operation involving said atom.

More specifically, the Raman beams and the Rydberg beams can comprise one or more pulses, as explained in [REF1]. For example, in a single-qubit operation, only pulses of Raman laser beams can be used whereas, in a multi-qubit operation comprising a predetermined number of operations, a sequence of Rydberg and Raman laser beams pulses can be used.

<FIG> schematically illustrates a plurality of laser beams configured to perform all the aforementioned functions (MOT, optical pumping, dipolar trapping, Rydberg excitation, Raman excitation) in a quantum processor. Such plurality of laser beams comprise the MOT laser beams <NUM> and the optical pumping beam <NUM> at around <NUM>, the dipolar trapping laser beam <NUM> at around <NUM>, the Raman laser beams <NUM> at around <NUM>, the first Rydberg laser beam <NUM> at around <NUM>, and the second Rydberg laser beam <NUM> at around <NUM>. In particular the MOT laser beams <NUM> comprise three series (<NUM>, <NUM>, <NUM>) of two counterpropagating laser beams.

<FIG> schematically illustrates the laser beams in a plane <NUM> of the array of atoms of the neutral atom quantum processor. The dipolar trapping beam <NUM> is focused into a plurality of spots producing a 1D, 2D or 3D array of optical traps that can generate an array of individual atoms. The first and the second Rydberg beams <NUM>, <NUM> are configured to illuminate a predetermined number of atoms in the array on which a multi-qubit operation is performed. The Raman beams <NUM> are focused on one atom in order to induce a Raman transition in said atom and thus perform a single qubit operation on said atom or as a part of a multi-qubit operation involving such atom.

In order to generate the plurality of laser beams configured to perform all the aforementioned functions (MOT, optical pumping, dipolar trapping, Rydberg excitation, Raman excitation), it is known to use a laser apparatus comprising a plurality of independent laser systems and nonlinear devices. In the example disclosed in [REF2], such laser apparatus comprises: a laser diode at <NUM> for producing non-focused MOT laser beams <NUM> and a non-focused optical pumping beam <NUM>, a laser diode at <NUM> and a free-space tapered amplifier for producing the dipolar trapping beam <NUM>, a laser diode at <NUM> for producing focused Raman laser beams <NUM> and a first Rydberg laser beam <NUM>. Further, a laser diode at <NUM>, a second harmonic generation device (SHG), and a free space tapered amplifier are used for producing the second Rydberg focused laser beam <NUM>. Alternatively, as disclosed in [REF4], such laser apparatus can comprise a plurality of titanium-sapphire lasers for producing the Rydberg laser beams <NUM>,<NUM> and the dipolar trapping beam <NUM>.

[REF5] discloses a laser apparatus for two-photon excitation of trapped Rb atoms wherein Rydberg excitation beams at <NUM> and <NUM> are derived from a frequency-doubled Titanium Sapphire laser and an amplified Fabry-Pérot laser diode.

Such laser apparatuses comprise a plurality of solid state-based and/or semiconductor-based laser systems, which complexifies the structure of the neutral atom-based quantum processor operational system and undermines its robustness and reliability. Further, such laser apparatuses include a large number of free-space coupled elements making them very sensitive to vibration-induced misalignment, hereby deteriorating efficiencies of fiber-coupling and/or of non-linear functions. As a result, such apparatuses may require tedious realignment when transported between different locations. Consequently, such laser apparatuses generally lack reliability, which hampers the commercial use of quantum processors comprising such laser apparatuses.

There is thus a need for a laser apparatus to generate laser beams in a neutral atom-based quantum processor that is more reliable than in the prior art.

In what follows, the term "comprise" is synonym of (means the same as) "include" and "contains", is inclusive and open, and does not exclude other non-recited elements. Moreover, in the present disclosure, when referring to a numerical value, the terms "about" and "substantially" are synonyms of (mean the same as) a range comprised between <NUM>% and <NUM>%, preferably between <NUM>% and <NUM>%, of the numerical value.

According to a first aspect, the present description relates to a laser apparatus for excitation of Rubidium atoms in a quantum processor comprising:.

The laser apparatus of the first aspect provides laser beams to excite Rb atoms of a neutral atom-based quantum processor using an original arrangement of fiber-coupled laser sources and fiber-coupled non-linear devices (SHG and DFG) which drastically reduces the number of free-space coupled elements, therefore making the laser apparatus more robust and reliable than in the prior art.

Further, in such laser apparatus, the laser source emitting light at a wavelength of around <NUM> is used for producing the optical pumping beam, the MOT beam and, in combination with the laser source emitting light at a wavelength of around <NUM>, the dipolar trapping beam and the first Rydberg beam. Therefore, such laser apparatus can provide the MOT beam, the optical pumping beam, the dipolar trapping beam, the first Rydberg beam and the second Rydberg beam with only three fiber-coupled laser sources. As a result, such laser apparatus has a simpler structure and an improved reliability compared to the laser apparatuses of the prior art.

In the present description, an Erbium-based DFB laser (also referred to as Er-doped DFB fiber laser) is a DFB fiber laser comprising an Erbium-doped fiber as a gain medium.

In the present description, an Ytterbium-based DFB laser (also referred to as Yb-doped DFB fiber laser) is a DFB fiber laser comprising an Ytterbium-doped fiber as a gain medium.

In the present description, Thulium-based DFB laser (also referred to as Tm-doped DFB fiber laser) is a DFB fiber laser comprising a Thulium-doped fiber as a gain medium.

In the present description, a DFB fiber laser is a Distributed Feedback fiber laser, i.e. a fiber-coupled laser system comprising a fiber-coupled pump laser diode and a gain medium, wherein the gain medium is a rare earth-doped fiber comprising a diffraction grating. Different types of rare earth can be used depending on the wavelength of the light emitted by the DFB fiber laser.

In the present description, an SHG device is a second harmonic generation device configured to produce, from light at a first frequency, light at a second frequency equal to twice said first frequency.

In the present description, a DFG device is a difference frequency generation device configured to produce, from light at a first frequency and light at a second frequency, light at a third frequency equal to a difference between the largest of the first and second frequencies and the smallest of the first and second frequencies.

According to one or further embodiments, the DFG device can also transmit part of the light at said first frequency and/or part of the light at said second frequency.

According to one or further embodiments, the laser apparatus further comprises:.

wherein said light at a wavelength of around <NUM> is configured to produce Raman laser beams for Raman transition of the Rb atoms.

Such embodiments of the laser apparatus provide laser beams to excite Rb atoms of a neutral atom-based quantum processor to perform single-qubit operations or multi-qubit operations between fundamental states of atoms, using fiber-coupled laser sources and fiber-coupled non-linear devices (SHG and DFG). Therefore, such embodiments are more robust than laser apparatuses of the prior art.

Further, in such embodiments, the laser source emitting light at a wavelength of around <NUM> is further used to produce, in combination with the laser source emitting light at a wavelength of around <NUM>, the Raman beams to induce Raman transition of an atom of the quantum processor. Therefore, such laser apparatus can provide the MOT laser beams, the optical pumping laser beam, the dipolar trapping laser beam, the first Rydberg laser beam, the second Rydberg laser beam and the Raman laser beams with only four fiber-coupled laser sources. As a result, such laser apparatus has a simpler structure and an improved reliability compared to laser apparatuses of the prior art.

According to one or further embodiments, the fiber-coupled laser source for emitting light at a wavelength of around <NUM> comprises a fiber-coupled Tm-doped DFB fiber laser for emitting light at a wavelength of around <NUM> and a fifth fiber-coupled SHG device configured to receive said light at a wavelength of around <NUM> and to produce light at a wavelength of around <NUM>.

In such embodiment, said light at a wavelength of around <NUM> is produced using fiber-coupled elements (a Tm-doped DFB fiber laser and a fiber-coupled SHG device), which makes the laser apparatus more reliable than apparatuses of the prior art comprising free space-coupled elements.

According to one or further embodiments, said fiber-coupled laser source for emitting light at a wavelength of around <NUM> comprises an Yb-doped DFB fiber laser.

In such embodiments, said light at a wavelength of around <NUM> can be produced using a fiber-coupled laser source (the Yb-doped DFB fiber laser) without using an SHG device, hereby simplifying the laser apparatus and making it more reliable than apparatuses of the prior art.

According to one or further embodiments, the laser apparatus further comprises.

In such embodiments, it is possible to produce a dipolar trapping beam using fiber amplifiers. Therefore, such laser apparatus is more reliable than apparatuses of the prior art comprising free space-tapered amplifiers which require very high current and are not reliable over extended period of time. Further, such laser apparatus is also more reliable than apparatuses of the prior art comprising solid state-based lasers such as titanium-sapphire lasers, which are very sensitive to vibrations and are complex to operate outside a laboratory environment.

In such embodiments, the MOT beams, the optical pumping beam, the dipolar trapping beam, the first and the second Rydberg beams and the Raman beams are produced by using amplification of light with fiber amplifiers. Therefore, such a laser apparatus is more reliable than apparatuses of the prior art that use amplification of light relying on free space tapered amplifiers and/or that rely on the high output power of solid state-based lasers such as titanium-sapphire lasers.

According to a second aspect, the present description relates to a quantum processor comprising:.

According to a third aspect, the present description relates to a method for exciting Rubidium atoms in a quantum processor comprising:.

According to one or further embodiments, the method further comprises.

Other advantages and features of the invention will become apparent on reading the description, illustrated by the following figures which represent:.

<FIG> represents a schematic view of a quantum processor <NUM> configured to implement an algorithm on input data <NUM> and generate output data <NUM>.

According to one or further embodiments, the quantum processor <NUM> comprises a laser apparatus <NUM>, a vacuum system assembly <NUM>, a detection system <NUM>, a processing unit <NUM> and a control unit <NUM>.

According to one or further embodiments, the quantum processor <NUM> comprises a processing unit <NUM> in communication with a control unit <NUM>. The processing unit <NUM> is configured to receive input data <NUM> from an external user and generate command data that are sent to the control unit <NUM>. The control unit <NUM> uses the command data in order to control a laser apparatus <NUM> so that the laser apparatus <NUM> produces a plurality of laser beams (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). Such plurality of beams comprises: MOT beams <NUM>, an optical pumping beam <NUM>, a dipolar trapping beam <NUM>, a first Rydberg beam <NUM>, a second Rydberg beam <NUM>, and Raman beams <NUM>.

The vacuum system assembly <NUM> comprises a vacuum chamber and a plurality of Rubidium atoms. The vacuum system assembly <NUM> is configured to receive the plurality of beams (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) in order to implement operations of an algorithm on the input data <NUM>. The operations of the algorithm are performed using quantum states of the Rubidium atoms in the vacuum chamber.

According to one or further embodiments, the vacuum system assembly <NUM> further comprises free-space optical elements configured to direct said beams to said Rubidium atoms, such as optical isolators, lenses, mirrors, waveplates, polarizers. Such vacuum system can also comprise other free-space optical elements, for example optical modulators.

After an algorithm has been implemented on the Rubidium atoms, the quantum states of the Rubidium atoms of the vacuum chamber are detected by a detection system <NUM> and analyzed by a processing unit <NUM> in order to generate output data <NUM>.

According to one or further embodiments, the laser apparatus <NUM> comprises a plurality of fiber-coupled laser sources, for example three different laser sources (<NUM>, <NUM>, <NUM>), and a non-linear fiber-based optical system <NUM>.

The non-linear fiber-based optical system <NUM> is configured to receive light emitted from said plurality of laser sources (<NUM>, <NUM>, <NUM>) and generate said plurality of laser beams (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). In the example shown in <FIG>, only three laser sources are represented, however embodiments of the laser apparatus <NUM> may comprise more than three sources. For example, in order to generated all the beams of the plurality of laser beams (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), four laser sources may be used.

<FIG> represents a laser apparatus <NUM> configured to produce MOT beams <NUM>, an optical pumping beam <NUM>, a dipolar trapping beam <NUM>, a first Rydberg beam <NUM> and a second Rydberg beam <NUM>.

The laser apparatus comprises three laser sources, an Er-doped DFB fiber laser <NUM> emitting light at a wavelength of about <NUM>, an Yb-doped DFB fiber laser <NUM> emitting light at a wavelength of about <NUM>, and a fiber-coupled laser <NUM> emitting light at a wavelength of about <NUM>.

A part of the light emitted by the Er-doped DFB fiber laser <NUM> is sent to a fiber-coupled SHG device <NUM> in order to double the frequency of said part of light and produce light at a wavelength of about <NUM> which is configured to form said MOT beams <NUM> and said optical pumping beam <NUM>.

Light emitted by the Yb-doped DFB fiber laser <NUM> is sent to a fiber-coupled SHG device <NUM> in order to double the frequency of said light and produce light at a wavelength of about <NUM>. Said light at a wavelength of about <NUM> and a part of the light emitted by the Er-doped DFB fiber laser <NUM> are then sent to a fiber-coupled DFG device <NUM> in order to produce light at a wavelength of about <NUM>. Said light at a wavelength of about <NUM> is then sent to an SHG device <NUM> to produce light at a wavelength of about <NUM> which is configured to form said first Rydberg beam <NUM>.

A part of said light produced by said DFG device <NUM> does not pass through the SHG device <NUM> and is configured to form said dipolar trapping beam <NUM>.

According to one or further embodiments, said dipolar trapping beam <NUM> is formed by sending said light at a wavelength of about <NUM>, produced by said DFG device <NUM>, to an optical system, for example a spatial light modulator, in order to produce an array of optical dipole traps that can trap individual atoms. Said array of optical traps can be rearranged using said optical systems.

Light at a wavelength of about <NUM> emitted by the fiber-coupled laser source <NUM> is configured to form the second Rydberg beam <NUM>.

According to one or further embodiments, the fiber-coupled laser source <NUM> comprises a Yb-doped DFB fiber laser optimized to emit light at a wavelength of about <NUM>.

According to one or further embodiments, the fiber-coupled laser source <NUM> comprises a Tm-doped DFB fiber laser emitting light at a wavelength of about <NUM> whose frequency is doubled using a fiber-coupled SHG device in order to produce light at a wavelength of about <NUM>.

According to one or further embodiments, one, two or all of the laser sources <NUM>, <NUM>, <NUM> are frequency-stabilized via frequency stabilization techniques using frequency references <NUM>, <NUM>) and locking electronics (<NUM>, <NUM>).

The frequency (and, therefore, the wavelength) of the Er-doped DFB fiber laser <NUM> is stabilized by sending a part of the light at a wavelength of about <NUM> produced by the fiber-coupled SHG device <NUM> to the frequency reference <NUM>, for example a saturated absorption spectroscopy setup. The frequency of said light at a wavelength of about <NUM> is compared with a frequency of the frequency reference <NUM>, for example a frequency corresponding to a known atomic transition, and an error signal is obtained. Such error signal is processed by the locking electronics <NUM> (comprising, for example, loop filters) and fed to the Er-doped DFB fiber laser <NUM> in order to correct for possible frequency variations and stabilize the frequency of the light emitted by said Er-doped DFB fiber laser <NUM>.

The frequency of the Yb-doped DFB fiber laser <NUM> is stabilized by sending a part of the light at a wavelength of about <NUM> produced by the fiber-coupled DFG device <NUM> to the frequency reference <NUM>, for example a passive ultra-stable optical cavity, in order to obtain an error signal via a Pound-Drever-Hall locking scheme. Such error signal is processed by the locking electronics <NUM> (comprising, for example, loop filters) and fed to the Er-doped DFB fiber laser <NUM> in order to correct for possible frequency variations and stabilize the frequency of the light emitted by said Yb-doped DFB fiber laser <NUM>.

Similarly, the frequency of the fiber-coupled laser source <NUM> is stabilized by sending a part of the light at a wavelength of about <NUM> to the frequency reference <NUM> in order to obtain an error signal. Such error signal is processed by the locking electronics <NUM> and fed to the fiber-coupled laser source <NUM> in order to correct for possible frequency variations and stabilize the frequency of the light emitted by said fiber-coupled laser source <NUM>.

<FIG> represents embodiments of a laser apparatus <NUM> comprising four laser sources: an Er-doped DFB fiber laser <NUM> emitting light at a wavelength of about <NUM>, an Yb-doped DFB fiber laser <NUM> emitting light at a wavelength of about <NUM>, a Tm-doped DFB fiber laser <NUM> emitting light at a wavelength of about <NUM>, and a Yb-doped DFB fiber laser <NUM> emitting light at a wavelength of about <NUM>. Compared to the embodiments represented in <FIG>, the laser apparatus represented in <FIG> additionally produces light at a wavelength of about <NUM> that is configured to form the Raman laser beams <NUM>. Further, such embodiments comprise a Tm-doped DFB fiber laser emitting light at a wavelength of about <NUM> whose frequency is doubled using a fiber-coupled SHG device <NUM> in order to produce light at a wavelength of about <NUM>.

In the embodiments represented in <FIG>, the light emitted by the laser sources (<NUM>, <NUM>, <NUM>, <NUM>) is amplified using fiber-based amplifiers: a first Ytterbium-doped fiber amplifier (Yb-DFA) <NUM>, an Erbium-doped fiber amplifier (Er-DFA) <NUM>, a second Yb-DFA <NUM> and a Thulium-doped fiber amplifier (Tm-DFA) <NUM>, respectively.

In particular, in the laser apparatus represented in <FIG>, the Yb-doped DFB fiber laser <NUM> emits light at a wavelength of about <NUM> that is sent to an Yb-DFA <NUM> to be amplified. Said amplified light produced by the Yb-DFA <NUM> is sent to a SHG device <NUM> that doubles the frequency of said amplified light and produce light at a wavelength of about <NUM>. Said light at a wavelength of about <NUM> and a part of the light at a wavelength of about <NUM> emitted by the Er DFB <NUM> and amplified by the Er-DFA <NUM> are sent to a fiber-coupled DFG device <NUM> in order to produce light at a wavelength of about at a wavelength of about <NUM> that is configured to form Raman laser beams <NUM>.

Although not represented in <FIG>, in the embodiments represented by <FIG>, fiber-based amplifiers may also be used to amplify respectively light emitted by the Er-doped DFB fiber laser <NUM>, light emitted by the Yb-doped DFB fiber laser <NUM> and light emitted by the fiber-coupled laser source <NUM> as described in reference to <FIG>.

According to one or further embodiments, the Raman beams <NUM> comprise two beams with a frequency difference of about <NUM>. Such two beams can be generated with different
setups. In a first exemplary setup, two laser beams at different optical frequencies are emitted by two laser sources and are superimposed. In a second exemplary setup, the two Raman laser beams are generated by modulating a single beam. This is done by modulating the phase or the intensity of the light at a wavelength of about <NUM>, in order to generate modulation sidebands in said light, for example with an electro-optic modulator, wherein said sidebands are separated by said frequency difference of <NUM>.

Claim 1:
A laser apparatus (<NUM>) for excitation of Rubidium atoms in a quantum processor comprising:
- an Er-doped DFB fiber laser (<NUM>) for emitting light at a wavelength of around <NUM>;
- at least a first Yb-doped DFB fiber laser (<NUM>) for emitting light at a wavelength of around <NUM>;
- a fiber-coupled laser source (<NUM>) for emitting light at a wavelength of around <NUM>;
- a first fiber-coupled SHG device (<NUM>) configured to receive said light from said Yb-doped DFB fiber laser (<NUM>) and to produce light at a wavelength of around <NUM>;
- at least a first fiber-coupled DFG device (<NUM>) configured to receive a first part of said light from said Er-doped DFB fiber laser (<NUM>) and said light from said first fiber-coupled SHG device (<NUM>) and produce light at a wavelength of around <NUM>;
- a second fiber-coupled SHG device (<NUM>) configured to receive a first part of said light from said first fiber-coupled DFG device (<NUM>) and produce light at a wavelength of around <NUM>;
- a third fiber-coupled SHG device (<NUM>) configured to receive a second part of said light from said Er-doped DFB fiber laser (<NUM>) and produce light at a wavelength of around <NUM>,
wherein said light at a wavelength of around <NUM> is configured to produce MOT laser beams (<NUM>) for magneto-optical trapping of the Rubidium atoms and an optical pumping laser beam (<NUM>) for optical pumping of the Rubidium atoms;
wherein said light at a wavelength of around <NUM> is configured to produce a dipolar trapping beam (<NUM>) for trapping individual Rubidium atoms at predetermined locations;
wherein said light at a wavelength of around <NUM> and said light at a wavelength of around <NUM> are configured to produce Rydberg laser beams (<NUM>, <NUM>) for transition of Rubidium atoms to Rydberg states.