EP-4742113-A1 - FAST MULTI-PHOTON QUANTUM GATES FOR OPTICAL QUBITS AND QUTRITS

EP4742113A1EP 4742113 A1EP4742113 A1EP 4742113A1EP-4742113-A1

Abstract

The present disclosure relates to a system for performing a quantum manipulation protocol, e.g., a quantum computing algorithm, the system comprising: a trapping laser system for trapping neutral atoms in an optical trap array inside a vacuum chamber operated at a trapping wavelength λ tr , wherein each neutral atom comprises a ground state | 1 S o >, a first metastable excited state | 3 P o > and a second metastable excited state | 3 P 2 >. The system further comprises a state manipulation laser system for generating laser radiation for coupling, via a phase-coherent multi-photon transition, the ground state | 1 S o > to the first metastable excited state | 3 P o > and / or to the second metastable excited state | 3 P 2 > as well as a magnetic field system for generating a magnetic field B ext at a location of the optical trap array inducing a Zeeman splitting for magnetic substates of the second metastable excited state | 3 P 2 >. The system further comprises a control system for controlling the laser radiation generated by the laser system to coherently modify a quantum state of the plurality of neutral atoms based on the quantum manipulation protocol and a a quantum state readout system for determining the quantum state of at least a subset of the plurality of neural atoms based on the quantum manipulation protocol. According to some aspects, a direction of the magnetic field B ext with respect to a polarization direction of the optical trap array is selected such as to render the optical trap array operated at the trapping wavelength λ tr a magic-wavelength optical trap for the second metastable excited state | 3 P 2 > and the first metastable excited state | 3 P o > and / or for the second metastable excited state | 3 P 2 > and the ground state | 1 S o > or both.

Inventors

BLOCH, IMMANUEL
ZEIHER, Johannes
TAO, Renhao
GYGER, Flavien
AMMENWERTH, Maximilian
TIMME, Hendrik

Assignees

Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V.
Ludwig-Maximilians-Universität München, in Vertretung des Freistaates Bayern

Dates

Publication Date: 20260513
Application Date: 20241106

Claims (14)

System (200) for performing a quantum manipulation protocol, comprising: a trapping laser system (210) for trapping neutral atoms in an optical trap array (110) inside a vacuum chamber (115) operated at a trapping wavelength λ tr ; wherein each neutral atom comprises a ground state | 1 S 0 >, a first metastable excited state | 3 P 0 > and a second metastable excited state | 3 P 2 >; a state manipulation laser system (220) for generating laser radiation (130) for coupling, via a phase-coherent multi-photon transition, the ground state | 1 S 0 > to the first metastable excited state | 3 P 0 > and / or to the second metastable excited state | 3 P 2 >; a magnetic field system (230) for generating a magnetic field B ext at a location of the optical trap array inducing a Zeeman splitting for magnetic substates of the second metastable excited state | 3 P 2 >; a control system (240) for controlling the laser radiation generated by the laser system to coherently modify a quantum state of the plurality of neutral atoms based on the quantum manipulation protocol; a quantum state readout system (250) for determining the quantum state of at least a subset of the plurality of neural atoms based on the quantum manipulation protocol; wherein a direction of the magnetic field B ext with respect to a polarization direction of the optical trap array is selected such as to render the optical trap array operated at the trapping wavelength λ tr a magic-wavelength optical trap for the second metastable excited state | 3 P 2 > and the first metastable excited state | 3 P 0 > and / or for the second metastable excited state | 3 P 2 > and the ground state | 1 S 0 >.
System of claim 1, wherein the direction and a strength of the magnetic field B ext , the polarization direction of the optical trap array, and the wavelength λ tr of the optical trap array are selected such that a differential light shift induced by the optical trap array on the second metastable excited state | 3 P 2 > with respect to the first metastable excited state | 3 P 0 > or with respect to the ground state | 1 S 0 >, or with respect to both states affects a fidelity of a single particle quantum operation of the quantum manipulation protocol by less than 10%, preferably by less than 5% and more preferably by less than 1%.
System of claim 2, wherein an angle θ between the polarization direction of the optical trap array and the direction of the magnetic field B ext is selected such that, for a given trapping wavelength λ tr of the optical trap array, the differential light shift induced by the optical trap array on the second metastable state with respect to the first metastable state or with respect to the ground state or with respect to both states affects a fidelity of a single particle quantum operation of the quantum manipulation protocol by less than 10%, preferably by less than 5% and more preferably by less than 1%.
System of claim of any of claims 1 to 3, wherein the neutral atoms are 88 Sr atoms.
System of claim 4, dependent on claim 3, wherein the angle θ is selected between 78° and 79°, and the wavelength λ tr of the optical trap array is selected to be 813 nm.
System of claim 4, dependent on claim 3, wherein the polarization direction of the optical trap array is essentially vertical, the angle θ is selected between 89° and 91°, preferably 90°, and the wavelength λ tr of the optical trap array is selected to be between 1000 nm and 1020 nm.
System of claim 6, wherein the optical trap array is formed by an optical lattice, preferably by a folded optical lattice; and / or the system further comprising: one or more auxiliary optical traps having an auxiliary trap wavelength λ aux and a polarization direction that is essentially horizontal; wherein the auxiliary trap wavelength λ aux and the polarization direction of the one or more auxiliary optical traps with respect to the direction of the magnetic field B ext are selected such that a differential light shift induced by the one or more auxiliary optical traps on the second metastable excited state with respect to the first metastable excited state or with respect to the ground state or with respect to both does not affect a fidelity of the quantum manipulation protocol.
System of any of claims 1 to 7, wherein the laser radiation generated by the atomic state manipulation laser system is applied to the plurality of neutral atoms via a single single-mode, polarization maintaining optical fiber, that, preferably, is length and temperature stabilized; the system optionally further comprising multi-wavelength waveplates which rotate the polarization a first wavelength of the laser radiation but leave a second wavelength essentially unaffected.
Neutral atom quantum computer comprising the system of any of claims 1 to 8, and optionally: an interface, for receiving, via a network (270) from a remote user device (260), instructions of a quantum algorithm, and for sending, via the network, a result of the quantum algorithm to the remote user device.
Method for quantum computing comprising: obtaining (1110) a set of instructions of a quantum algorithm; forming (1120) a quantum register by trapping a plurality of neutral atoms in an optical trap array inside a vacuum chamber; wherein a ground state, a first metastable excited state and a second metastable excited state of the neutral atoms form a qutrit and / or qubits used for quantum computing; performing (1130) , based on the obtained set of instructions of the quantum algorithm, a sequence of quantum gates on a subset of the plurality of neutral atoms, wherein performing the sequence of quantum gates comprises generating a magnetic field B ext at a location of the optical trap array for inducing a Zeeman splitting for magnetic sub-states of the second metastable excited state; and illuminating the subset of the plurality of neutral atoms with laser radiation coupling, via a multi-photon transition, the ground state to the first meta-stable excited state, and / or the ground state to the second metastable excited state.
Method of claim 10, further comprising: applying two-photon coherent depumping for controlled qubit state reset.
Method of claim 11, further comprising laser cooling the plurality of neutral atoms in an optical trap array based at least in part on applying the two-photon coherent depumping for the controlled qubit state reset.
Method of any of claims 10 to 12, further comprising obtaining, via a network form a remote user device, the set of instructions of the quantum algorithm; and sending a result of the quantum algorithm via the network to the remote user device.
Computer program comprising instructions for controlling a quantum computing device to carry out the steps of the method of any of claims 10 to 13.

Description

FIELD OF INVENTION Aspects of the present disclosure relate to quantum technologies using neutral atoms in optical traps as qubits and / or qutrits, and more particularly to techniques, systems, and devices for fast and high-fidelity qubit and / or qutrit state manipulation in engineered trapping potentials without state-dependent light-shifts. INTRODUCTION Quantum technologies, including quantum computers, optical lattice clocks, and / or analog quantum simulators, can outperform classical devices in several applications. In neutral atom-based implementations of such quantum technologies, long-lived internal states of neutral atoms trapped in optical potentials, such as optical tweezer arrays or optical lattices may be used. Neutral atom-based quantum computers, simulators and / or (optical) atom clocks typically trap neutral atoms (i.e. electrically neutral atoms) in optical potentials (e.g. in arrays of optical dipole traps or in optical lattices or combinations thereof), and use two or more long-lived (with respect to operation time) internal (electronic) states for performing quantum manipulation protocols such as a sequence of quantum gates of a quantum comptuting algorithm. The selected states of the atoms may thus form the states of a qubit and / or qutrit. Such quantum manipulation systems typically require precise control over the internal states of the neutral atoms to perform high-fidelity operations such as single qubit gates, Ramsey sequences, etc. For example, quantum computers use physical qubits to store the basic unit of information and perform quantum gates on the qubits to process the stored information, e.g. according to processing instructions of a quantum computing algorithm. Running quantum algorithms requires single- and two-qubit gates, which are the basic computation operations acting on individual qubits and individual pairs of qubits, respectively. However, achieving fast and coherent manipulations of such internal states in large quantum registers of neutral atoms still present significant challenges. SUMMARY One of the main challenges in neutral atom-based quantum technology devices is the requirement for so-called magic trapping conditions (e.g. see: S. Zhang, F. Robicheaux, and M. Saffman: Magic-wavelength optical traps for Rydberg atoms; arXiv:1106.246302 [physics.atom-ph] for general background in this field of technology), where a differential light shift caused by an optical trapping potential between two or more internal states of neutral atoms essentially vanishes (e.g., on time and precision scales relevant for the specific implementation). Such a constraint typically either limits the choice of trapping wavelengths or necessitates specifically engineered trapping potentials. In addition, driving optical clock transitions, e.g., in alkaline earth (like) atoms such as Yb and Sr typically requires significant optical power due to the weak optical coupling on the clock transition, which can lead to spurious light shifts, exacerbate lift-shift induced dephasing and thus may limit quantum operation fidelity. More specifically, ensembles of neutral atoms in optical tweezer arrays or optical lattices are a promising approach for realizing neutral-atom based quantum technologies. The dominant platforms use laser pulses to coherently manipulate the neutral atoms, either for quantum information processing or, for example, sensing, metrology, and / or analog quantum simulation. For example, in 88Sr, established techniques exploit long-lived clock states coherently coupled to the electronic ground state. Driving the corresponding clock transitions typically requires suitable trapping conditions with vanishing differential light shifts (so-called "magic-wavelength" conditions), which constrain the choice of the trapping wavelength of the optical trap array. A known example of such a trapping configuration is the magic wavelength at 813 nm used in modern optical atomic clocks based on Strontium atoms. Alternatively, magic trapping conditions can be engineered with suitable combinations of trap polarization and magnetic fields, as demonstrated recently in several quantum physics experiments (see for example: S. Pucher, et all. Fine-Structure Qubit Encoded in Metastable Strontium Trapped in an Optical Lattice Phys. Rev. Lett. 132, 150605). Further, it is found that driving an optical clock transition even at magic-wavelength trapping conditions typically requires significant laser power, due to the weak (strictly speaking vanishing) optical coupling on the clock transition (e.g., the |1S0> to |3P0> transition shown in Fig. 3). The large, required laser power constitutes a significant bottleneck, as it also is accompanied by spurious light shifts due to nearby levels that limit the achievable operation quality in realistic experimental settings. Further, applying strong magnetic fields at arbitrary angles as required for magic angle tuning is experimentally challenging, which limits t