Method for increasing a coherence time during a quantum computation with at least two qubits and quantum computer

Abstract

A method for increasing a coherence time during a quantum computation with at least two qubits (qb1, qb2) is described herein, comprising the steps of: - applying a quantum gate to a first qubit (qb1) and a second qubit (qb2), - at least during an evolution time (tev) of the quantum gate, applying a first periodic pulse sequence (1) to the first qubit (qb1) and a second periodic pulse sequence (2) to the second qubit (qb2), wherein a ratio between the evolution time (tev) of the quantum gate and a period (T) of the first or second periodic pulse sequence (1, 2) is fixed, - determining if the period (T) of the first or second periodic pulse sequence (1, 2) is resonant with an environmental noise component, - modifying the evolution time (tev) of the quantum gate, such that the period (T) of the first periodic pulse sequence (1) or the period (T) of the second periodic pulse sequence (2) is not resonant with the environmental noise component. Further a quantum computer (10) is disclosed herein.

Description

[0001] P2023,1429 WO N November 14, 2024- 1 -Description Method for increasing a coherence time during a quantum computation with at least two qubits and quantum computer A method for increasing a coherence time during a quantum computation with at least two qubits and a quantum computer are specified herein. At least one object of certain embodiments is to provide a method for increasing a coherence time during a quantum computation involving a conditional quantum gate acting on at least two qubits. This object is achieved by the method with the features according to the independent claim. Advantageous embodiments and further developments of the method and the quantum computer are specified in the dependent claims. According to an embodiment, the method for increasing a coherence time during a quantum computation with at least twoqubits comprises a step of applying a quantum gate to a firstqubit and a second qubit. In particular, each qubit is a quantum mechanical two-state system comprising two linearly independent basis states, denoted by |0> and |1> in the following. For example, the at least two qubits are part of the same quantum register of a quantum computer. In particular, the quantum register comprises multiple qubits and the quantum computer performs quantum computations, such as gate operations, by manipulating states of the qubits in theP2023,1429 WO N November 14, 2024- 2 -quantum register. For example, qubits in the same quantum register interact with each other. In particular, the quantum gate is a conditional quantum gate or an entangling quantum gate that acts simultaneously on the first qubit and the second qubit. For example, the quantum gate is a controlled-not gate (short: CNOT gate) or an interaction gate, such as a Mølmer-Sørensen gate or a generalized Mølmer-Sørensen gate. The quantum gate can also act simultaneously on more than two qubits. For example, the quantum gate entangles the states of the first qubit, the second qubit and further qubits in the quantum register.According to a further embodiment, the method comprises astep of applying a first periodic pulse sequence to the firstqubit and a second periodic pulse sequence to the secondqubit, wherein the first periodic pulse sequence and thesecond periodic pulse sequence are applied at least during an evolution time of the quantum gate, and wherein a ratio between the evolution time of the quantum gate and a period of the first or second periodic pulse sequence is fixed. For example, the first periodic pulse sequence comprises or consists of single qubit gates acting on the first qubit. In other words, each pulse of the first periodic pulse sequence corresponds to one or more single qubit gates. For example, the single qubit gates are rotation operators about the x- axis, the y-axis, and / or the z-axis. In particular, one pulse of the first periodic pulse sequence rotates a state of thefirst qubit by a given angle around a given axis of the Blochsphere. For example, each pulse of the first periodic pulse sequence flips the state of the first qubit such that the basis state |0> is changed to |1> and vice versa.P2023,1429 WO N November 14, 2024- 3 -For example, the second periodic pulse sequence comprises or consists of single qubit gates acting on the second qubit. In other words, each pulse of the second periodic pulse sequence corresponds to one or more single qubit gates. For example, the single qubit gates are rotation operators about the x- axis, the y-axis, and / or the z-axis. In particular, one pulse of the second periodic pulse sequence rotates a state of the second qubit by a given angle around a given axis of the Bloch sphere. For example, each pulse of the second periodic pulse sequence flips the state of the second qubit such that the basis state |0> is changed to |1> and vice versa. In particular, the first periodic pulse sequence and thesecond periodic pulse sequence are periodic in time. Forexample, one period of the first or second periodic pulse sequence comprises one pulse and the pulse is applied to the first or second qubit repeatedly at equidistant time-steps, respectively. The first and second periodic pulse sequences are applied to the first and second qubit, respectively, at least during theevolution time of the quantum gate, or during the entire timeof the quantum computation, i.e. while quantum gates of a quantum circuit operate on the qubits in the quantum register. In particular, the evolution time of the quantum gate corresponds to a time interval during which the quantumgate operation is performed on the respective qubits. Forexample, applying the quantum gate to the first and secondqubits involves time evolving the states of the first andsecond qubits with a specific Hamiltonian corresponding tothe quantum gate for a specific period of time, wherein the specific period of time is the gate operation time.P2023,1429 WO N November 14, 2024- 4 -For example, if the quantum gate can entangles more than two qubits or all qubits in the quantum register, periodic pulse sequences can be applied to all qubits in the quantum register. In particular, the features disclosed above for the first and the second periodic pulse sequence can apply to all periodic pulse sequences that are applied to the respective qubits in the quantum register.For example, during the evolution time of the quantum gate acertain number of pulses of the first periodic pulse sequence and of the second periodic pulse sequence are applied to the first qubit and the second qubit, respectively. For example, the evolution time of the quantum gate determines the period of the first periodic pulse sequence and the period of the second periodic pulse sequence. For example, the first and / or the second periodic pulse sequence comprises or consists of at least five or at least ten pulses that are applied to the first and second qubit, respectively, while the quantum gate acts on the first and second qubits. According to a further embodiment, the method comprises a step of determining if the period of the first or second periodic pulse sequence is resonant with an environmental noise component. For example, the environmental noise component comprises a noise source that couples to the states of the qubits in the quantum register. For example, the noise is a dephasing noise that randomly flips the state of aqubit. For example, the noise source is characterized by anoise spectral density, i.e. a power of the noise per unit of frequency bandwidth. For example, the noise spectral density and / or a coupling between the noise source and the qubits has a frequency dependence with a local maximum. For example, theP2023,1429 WO N November 14, 2024- 5 -period of the first or second periodic pulse sequence is resonant with the environmental noise component if afrequency of the first or second periodic pulse sequence isequal to or approximately equal to the local maximum in the frequency dependence of the noise spectral density and / or the coupling between the noise source and the qubits. For example, the resonance between the period of the first or second periodic pulse sequence and the environmental noise component is determined by measuring a coherence time of the first or second qubit as a function of the period of the first or second periodic pulse sequence, respectively. For example, the resonance corresponds to a local minimum of the coherence time as a function of the period of the first or second periodic pulse sequence, respectively. In particular, the coherence time specifies a characteristic time scale during which a qubit remains quantum coherent and retains its information. In other words, the coherence time can be viewed as a lifetime of a qubit. According to a further embodiment, the method comprises astep of modifying the evolution time of the quantum gate,such that the period of the first periodic pulse sequence or the period of the second periodic pulse sequence is notresonant with the environmental noise component. For example,the evolution time of the quantum gate is increased or decreased and the periods of the first and / or second periodic pulse sequences are increased or decreased accordingly, such that they are no longer resonant with the environmental noise component. For example, the evolution time of the quantum gate is modified by tuning an interaction strength betweenthe qubits taking part in the quantum gate. Accordingly, thecoherence time of the qubits can be advantageously increasedP2023,1429 WO N November 14, 2024- 6 -during the quantum computation. In particular, a resonant enhancement of noise by the first and / or second periodic pulse sequence is advantageously reduced. According to a preferred embodiment, the method for increasing a coherence time during a quantum computation with at least two qubits comprises the steps of:- applying the quantum gate to the first qubit and the secondqubit,- at least during the evolution time of the quantum gate,applying the first periodic pulse sequence to the first qubitand the second periodic pulse sequence to the second qubit,wherein the ratio between the evolution time of the quantumgate and the period of the first or second periodic pulsesequence is fixed,- determining if the period of the first or second periodicpulse sequence is resonant with the environmental noisecomponent,- modifying the evolution time of the quantum gate, such thatthe period of the first periodic pulse sequence or the period of the second periodic pulse sequence is not resonant with the environmental noise component. The method described herein is based on the idea to change an evolution time of a quantum gate such that a period of a dynamical decoupling pulse sequence applied to a qubit can be adjusted accordingly, in order to avoid a resonance between the period of the dynamical decoupling pulse sequence and acomponent of the environmental noise, for example. Inparticular, this method allows to optimize a period of the dynamical decoupling pulse sequences with regard to theenvironmental noise spectrum and thereby advantageouslyincreases a coherence time of the qubits.P2023,1429 WO N November 14, 2024- 7 -According to a further embodiment of the method, the period of the first periodic pulse sequence is equal to the period of the second periodic pulse sequence.According to a further embodiment of the method, the firstperiodic pulse sequence is a dynamical decoupling pulse sequence configured for protecting the first qubit fromdecoherence, and / or the second periodic pulse sequence is adynamical decoupling pulse sequence configured for protectingthe second qubit from decoherence. In particular, decoherencerefers to the loss of quantum coherence of a qubit, such as the first qubit or second qubit, due to an unwanted couplingof the qubit to the environment. For example, the dynamicaldecoupling pulse sequence suppresses decoherence of the qubit by at least approximately averaging the unwanted coupling between the qubit and the environment to zero. In particular,by protecting the qubit from decoherence, the coherence timeof the qubit is increased. For example, the dynamical decoupling pulse sequence is based on a periodically iterated Hahn spin-echo or Carr-Purcell scheme for reducing the dephasing of qubits due to their coupling to environmental noise. For example, the time interval between pulses of the dynamical decoupling pulse sequence is much shorter than a time scale on which the unwanted coupling between the qubit and the environment acts, e.g. the latter is at least by a factor of ten larger than the time interval between pulses.According to a further embodiment of the method, the firstperiodic pulse sequence comprises pi-pulses, wherein each pi-pulse flips the state of the first qubit, and / or the secondP2023,1429 WO N November 14, 2024- 8 -periodic pulse sequence comprises pi-pulses, wherein each pi-pulse flips the state of the second qubit. For example, eachpi-pulse is a single qubit-rotation that rotates the state ofthe respective qubit by an angle of π radians about an axisof the Bloch sphere. In particular, the pi-pulse changes the basis state |0> of the respective qubit to the basis state |1> of the respective qubit and vice versa. According to a further embodiment of the method, the at least two qubits interact via a pairwise Ising or XY interaction. For example, all qubits in the quantum register interact via an all-to-all pairwise Ising or XY interaction. For example, in a quantum register comprising N qubits the interaction takes the form In particular, all pairs of qubits in the quantum register interact via Ising or XY interactions. In equation (1) above Hintrefers to an interaction term in a Hamiltonian, theinteger indices i and j enumerate the N qubits in the quantumregister, and with ^, ^ ∈ {^, ^, ^} denotes one of the three2×2 Pauli matrices acting on the basis states of qubit i with ^ ∈ {1, 2, … , ^}. Here,products of Pauli matrices acting on different qubits are to be understood as tensor products. Further, Jijhas units of energy and parametrizes the interaction strength betweenqubits i and j. For example Jij = J is independent of i andj.For α = the interaction specified in equation (1) isdenoted as “Ising interaction” in the following, whereas forP2023,1429 WO N November 14, 2024- 9 -α β the interaction is denoted as “XY interaction” in thefollowing. For the case of Ising interactions, the basisstates |0> and |1> of the qubits are preferably eigenstates of the corresponding Pauli matrix ^^and are denoted as measurement basis states in the following. For example, pairwise Ising or XY interactions between qubitsarise for various physical qubit realizations, such assuperconducting qubits or trapped ion qubits. For example, the pairwise Ising or XY interaction between the qubits can be used to implement two-qubit gates and / or multi-qubit gates, in particular entangling gates, between the respective qubits. For example, a global entangling gate, such as ageneralized Mølmer-Sørensen gate or a magnetic gradientinduced coupling gate, can be implemented by time evolving the qubits in the quantum register while the qubits interactvia the Hamiltonian specified in Equation (1). In particular,the evolution time of the quantum gate depends on the interaction strength Jijin Equation (1). According to a further embodiment of the method, the evolution time of the quantum gate is modified by tuning an interaction strength between the first qubit and the secondqubit. In particular, the interaction strength Jij of theIsing or XY interaction between the first qubit and the second qubit is tuned. The interaction strength can be tuned by applying electric and / or magnetic fields to the qubits, or by adjusting a spatial separation between qubits, or by applying and / or adjusting a magnetic field gradient to the qubits, for example. According to a further embodiment of the method, theinteraction strength is tuned by applying a time delay toP2023,1429 WO N November 14, 2024- 10 -pulses of the second periodic pulse sequence with respect topulses of the first periodic pulse sequence. For example, thefirst and second periodic pulse sequences flip the states of the first and second qubit, respectively, at a frequency fmuch higher than a corresponding interaction energy scale J,i.e. f >> J / h, where h is Planck’s constant. For example, f ≥10 J / h. Accordingly, the interaction between the qubits iseffectively time averaged over a period of the periodic pulse sequences and the strength of the time averaged interaction can be adjusted by tuning the time-delay of pulses between different periodic pulse sequences. For example, the time averaged interaction strength between the first and second qubits averaged over the period of the first or second periodic pulse sequence is a linear function of the time delay. For example, the first and second periodic pulse sequences flip the measurement basis states of the first and second qubit, respectively. Accordingly, the sign of the interaction strength J12in equation (1) effectivelychanges from + to – and vice versa, if the first and secondqubits are not flipped simultaneously. For example, the time averaged interaction strength between the first qubit and the second qubit takes the value where Δt with 0 ≤ Δt ≤ T is the time delay and T is theperiod of the first and second periodic pulse sequence. Accordingly, the time averaged interaction strength can be continuously tuned between +J12and –J12by adjusting the timedelay Δt accordingly.Advantageously, the first and second periodic pulse sequences have a dual use, i.e. as dynamical decoupling pulse sequences for increasing the respective qubit’s coherence time, as wellP2023,1429 WO N November 14, 2024- 11 -as for tuning the interaction strength between the qubits bytime-shifting pulses of the dynamical decoupling pulse sequences applied to different qubits with respect to each other. According to a further embodiment of the method, basis states of each qubit correspond to different hyperfine states of acorresponding trapped ion. In particular, each qubit isencoded in two different hyperfine states of a corresponding trapped ion. A system of two or more ions trapped in the same ion trap forms the quantum register, for example. According to a further embodiment of the method, the at least two qubits interact via a magnetic gradient induced coupling. For example, a magnetic field gradient along a chain of trapped ions induces the pairwise Ising interaction between all pairs of qubits in the ion trap. For example, this Ising interaction in the presence of the magnetic field gradient is mediated by a common vibrational motion of the ions in the ion trap. Further a quantum computer is specified herein. In particular, the quantum computer implements the method for increasing a coherence time during a quantum computation withat least two qubits described above. All features of themethod are also disclosed for the quantum computer and vice versa. According to an embodiment of the quantum computer, a coherence time during a quantum computation with at least twoqubits is increased using the method described above.P2023,1429 WO N November 14, 2024- 12 -Further advantageous embodiments and further embodiments ofthe method and the quantum computer may become apparent fromthe following exemplary embodiments described in connectionwith the figures. Figure 1 shows a schematic graph of a first periodic pulse sequence, a second periodic pulse sequence, states of a first qubit and a second qubit, as well as an interaction strength between the first qubit and the second qubit as a function of time according to an exemplary embodiment of the method for increasing a coherence time during a quantum computation with at least two qubits.Figure 2 shows a schematic illustration of a quantum computeraccording to an exemplary embodiment. Elements that are identical, similar or have the same effect, are denoted by the same reference signs in the figures. The figures and the proportions of the elements shown in the figures are not to be regarded as true to scale. Rather, individual elements may be shown exaggeratedly large for better representability and / or better understanding. Figure 1 shows a first periodic pulse sequence 1 comprising pulses 11 applied to a first qubit qb1, and a second periodic pulse sequence 2 comprising pulses 21 applied to a second qubit qb2 in a quantum register of a quantum computer according to an exemplary embodiment of the method. The first periodic pulse sequence 1 and the second periodic pulse sequence 2 have the same period T as function of time t. The first qubits qb1 and the second qubit qb2 interact via anIsing interaction ^ ^^^^^^, where ^^^ = ^1 00 −1^ denotes the z-P2023,1429 WO N November 14, 2024- 13 -Pauli matrix acting on the first qubit qb1, ^^^ = denotes the z-Pauli matrix acting on the second qubit qb2,and J is the interaction strength.The pulses 11 of the first periodic pulse sequence 1 are pi- pulses that flip the basis states |0> and |1> of the first qubit qb1 into each other. In other words, after applying one pi-pulse the state |0> is changed to the state |1> and vice versa. Here, |0> and |1> are eigenstates of ^^^with eigenvalues -1 and +1, respectively. Similarly, the pulses 12 of the second periodic pulse sequence 2 are pi-pulses that flip the basis states |0> and |1> of the second qubit qb2 into each other. In other words, after applying one pi-pulse the state |0> is changed to the state |1> and vice versa. Here, |0> and |1> are eigenstates of ^^^with eigenvalues -1 and +1, respectively. The first periodic pulse sequence 1 and the second periodic pulse sequence 2 are dynamical decoupling sequences configured for increasing a coherence time of the first qubitqb1 and the second qubit qb2, respectively. In particular,the dynamical decoupling sequences 1, 2 flip the states of the qubits qb1, qb2 back and forth at a high frequency, such that an unwanted coupling between the qubits qb1, qb2 and the environment is at least approximately averaged to zero. Preferably, the evolution time of the quantum gate is at least five times or at least ten times longer than the period T of the first and second periodic pulse sequences 1, 2. In particular, the dynamical decoupling sequences, i.e. thefirst and second periodic pulse sequences 1, 2, are appliedto the first and second qubits qb1, qb2 during the evolution time tev of a conditional or entangling quantum gate, such asP2023,1429 WO N November 14, 2024- 14 -a CNOT gate, a generalized Mølmer-Sørensen gate, or amagnetic gradient induced coupling gate, that simultaneouslyacts on the first and second qubits qb1, qb2. Here, for better visibility, a fixed number of three pulses 11, 21 of the first and second periodic pulse sequences 1, 2 are applied during the evolution time tev of the quantum gate. The number of pulses 11, 21 applied during the evolution time tev of the quantum gate can also be higher, such as five or ten, for example. If the number of pulses 11, 21 that are applied during the evolution time tev of the quantum gate are fixed, the first and / or the second periodic pulse sequence 1, 2 may be resonant with an environmental noise component. In order to reduce a resonant enhancement of noise, the evolution time tev of the quantum gate is modified while keeping the number of pulses 21, 22 during the evolution time tev fixed. In other words, the period T of the first and second periodic pulse sequence 1, 2 is modified in accordance with the evolution time tev. Accordingly, the frequency of the first and second periodic pulse sequence 1, 2 is shifted such that it is no longer resonant with the environmental noise component, thereby increasing the coherence time of the first and second qubit qb1, qb2. The evolution time tev of the quantum gate is modified by changing the interaction strength J between the first and second qubits qb1, qb2. Specifically, the evolution time tev of the quantum gate is inversely proportional to theinteraction strength J between the qubits. In particular, theevolution time tev of the quantum gate is modified by changing an effective, time averaged interaction strength JP2023,1429 WO N November 14, 2024- 15 -between the qubits qb1, qb2 that is averaged over one period T of the periodic pulse sequences 1, 2. The interaction strength J is tuned by time-shifting pulses 11, 21 between the first periodic pulse sequence 1 and the second periodic pulse sequence 2. In particular, the pulses 21 of the second periodic pulse sequence 2 are time-shifted and thus have a time delay Δt with respect to pulses 11 of the first periodic pulse sequence 1. Accordingly, the second qubit qb2 is flipped at a different time t than the firstqubit qb1 by each respective pi-pulse 11, 21 in each periodT. The second and third panels of Figure 1 show the expectation value 〈^^^ 〉 for the first qubit qb1 and the expectation value the second qubit qb2, respectively, as a function oftime t. In this example, the first qubit qb1 and the second qubit qb2 are both initialized in the basis state |1> at time t=0, accordingly, both expectation values 〈^^^ 〉 and 〈^^^〉 areequal to one at time t=0. After applying the first pi-pulse 11 to the first qubit qb1, the state of the qubit changes to |0> and the expectation value changes accordingly −1. The second pi-pulse 11 flips the state of the first qubit qb1 back to |1> and the expectation value〈^^^ 〉 changesaccordingly. The same is true for the second qubit qb2, albeit at later times delayed by the time delay Δt.The interaction energy ^ = 〈^ ^^^^^^〉 between the first qubit qb1and the second qubit qb2 as a function of time t is depicted in the lowest panel of Figure 1. The interaction energy is +J, if both qubits are in the same state, e.g. both qubits are in the state |0> or both qubits are in the state |1>. The interaction energy is -J, if the first qubit qb1 and theP2023,1429 WO N November 14, 2024- 16 -second qubit qb2 are in different states, e.g. the first qubit qb1 is in state |0> whereas the second qubit qb2 is in state |1>, or vice versa.Accordingly, by tuning the time delay Δt the time average ofthe interaction energy E and thus the time average of the effective interaction strength J can be reduced. Accordingly, the evolution time tev of the quantum gate and thus the period T of the first and second periodic pulse sequences 1, 2 can be increased to avoid a resonance with the environmental noise component. In particular, the frequency 1 / T of the first and second periodic pulse sequences 1, 2 is much larger, e.g. by a factor of at least five or ten, than the frequency scale J / h associated with the interaction strength J (here h denotes Planck’s constant). Accordingly, the first qubit and the second qubit effectively interact via a time averaged interaction with an effective interaction strength J that is a time average of the interaction energy E over one period T of the periodic pulse sequences 1, 2. The quantum computer 10 according to the exemplary embodimentin Figure 2 comprises a quantum register with four qubitsqb1, qb2, qb3, qb4. The two basis states |0> and |1> of each qubit qb1, qb2, qb3, qb4 are two hyperfine states of a corresponding trapped ion. All ions belonging to the quantum register are trapped in the same ion trap, such as a Paul trap or a Penning trap. A magnetic gradient along the chain of trapped ions induces a pairwise Ising interaction between all pairs of qubits in the quantum register. The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments. Rather, the invention encompasses any newP2023,1429 WO N November 14, 2024- 17 -feature and also any combination of features, which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.

[0002] P2023,1429 WO N November 14, 2024- 18 -References1 first periodic pulse sequence11 pulse2 second periodic pulse sequence21 pulse10 quantum computerqb1…4 qubitT periodΔt time delayt timetev gate evolution timeJ interaction strengthE interaction energy

Claims

P2023,1429 WO N November 14, 2024- 19 -Claims 1. A method for increasing a coherence time during a quantum computation with at least two qubits (qb1, qb2), comprising the steps of:- applying a quantum gate to a first qubit (qb1) and a secondqubit (qb2),- at least during an evolution time (tev) of the quantumgate, applying a first periodic pulse sequence (1) to thefirst qubit (qb1) and a second periodic pulse sequence (2) to the second qubit (qb2), wherein a ratio between the evolution time (tev) of the quantum gate and a period (T) of the first or second periodic pulse sequence (1, 2) is fixed,- determining if the period (T) of the first or secondperiodic pulse sequence (1, 2) is resonant with an environmental noise component,- modifying the evolution time (tev) of the quantum gate,such that the period (T) of the first periodic pulse sequence(1) or the period (T) of the second periodic pulse sequence(2) is not resonant with the environmental noise component.

2. The method according to the previous claim, wherein the period (T) of the first periodic pulse sequence (1) is equal to the period (T) of the second periodic pulse sequence (2).

3. The method according to any of the previous claims, wherein- the first periodic pulse sequence (1) is a dynamicaldecoupling pulse sequence configured for protecting the first qubit (qb1) from decoherence, and / or- the second periodic pulse sequence (2) is a dynamicaldecoupling pulse sequence configured for protecting the second qubit (qb2) from decoherence.P2023,1429 WO N November 14, 2024- 20 -4. The method according to any of the previous claims, wherein- the first periodic pulse sequence (1) comprises pi-pulses(11), wherein each pi-pulse (11) flips the state of the first qubit (qb1), and- the second periodic pulse sequence (2) comprises pi-pulses(21), wherein each pi-pulse (21) flips the state of the second qubit (qb2).

5. The method according to any of the previous claims, wherein the at least two qubits (qb1, qb2) interact via a pairwise Ising or XY interaction.

6. The method according to any of the previous claims, wherein the evolution time (tev) of the quantum gate is modified by tuning an interaction strength (J) between the first qubit (qb1) and the second qubit (qb2).

7. The method according to the previous claim, wherein the interaction strength (J) is tuned by applying a time delay (Δt) to pulses (21) of the second periodic pulse sequence (2) with respect to pulses (11) of the first periodic pulse sequence (1).

8. The method according to any of the previous claims, wherein basis states of each qubit (qb1, qb2) correspond to different hyperfine states of a corresponding trapped ion.

9. The method according to the previous claim, wherein the at least two qubits (qb1, qb2) interact via a magnetic gradient induced coupling.P2023,1429 WO N November 14, 2024- 21 -10. A quantum computer (10), wherein a coherence time duringa quantum computation with at least two qubits (qb1) isincreased using the method according to any of claims 1 to 9.

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