Methods and quantum computers for suppressing dephasing of idle qubits in a quantum register comprising multiple qubits
By applying an identity pulse to an idle qubit to perform single-qubit rotation, the problem of suppressing phase loss of idle qubits in a quantum register on a short timescale is solved, thus achieving stability protection of quantum information.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- ELEQTRON GMBH
- Filing Date
- 2024-11-13
- Publication Date
- 2026-06-05
AI Technical Summary
Existing technologies struggle to effectively suppress the dephase of idle qubits in a quantum register with multiple qubits, especially on short timescales, where dephase can destroy the quantum information stored in the idle qubits.
Single-qubit rotation is achieved by applying identity pulses to idle qubits. Short-duration identity pulse sequences are used to suppress phase loss on timescales shorter than the quantum petrified period, including the use of π pulses and iterative Hahn spin echo schemes, to dynamically decouple the undesirable coupling between the idle qubit and its environment.
It effectively suppresses the dephase of idle qubits in a short period of time, maintains the stability of quantum information, and avoids dephase loss caused by environmental noise and the interaction of other qubits.
Smart Images
Figure CN122162145A_ABST
Abstract
Description
[0001] This article details a method and a quantum computer for suppressing dephasing of idle qubits in a quantum register comprising multiple qubits.
[0002] At least one objective of certain embodiments is to describe in detail a method for suppressing dephase of idle qubits in a quantum register comprising multiple qubits on a short timescale. This objective is achieved by the method according to the independent claim.
[0003] Advantageous implementations and further developments of the method and quantum computer are described in detail in the dependent claims.
[0004] According to an embodiment, a method for suppressing dephase of idle qubits in a quantum register comprising multiple qubits includes the following steps: applying an identity pulse to the idle qubit, wherein the identity pulse performs an identity operation on the idle qubit. Specifically, each qubit is a quantum mechanical two-state system comprising two linearly independent ground states (hereinafter denoted by |0> and |1>).
[0005] For example, a quantum computer performs quantum computation, such as gate operations, by manipulating the states of qubits in a quantum register. A quantum computer may include one, two, or more quantum registers physically separated by hardware elements. For example, the ground state of a qubit may be represented by two hyperfine states of a trapped ion, and multiple ions trapped in an ion trap form a quantum register. For example, the ground state of a qubit may be represented by different states of a superconducting qubit, and superconducting qubits connected to a quantum bus form a quantum register. For example, multiple qubits in a quantum register interact with each other. For example, the interaction is an all-to-all interaction, where each qubit in a quantum register interacts with every other qubit in the quantum register via pairwise interactions.
[0006] Specifically, an idle qubit is a qubit in a quantum register that does not participate in quantum gate operations of the quantum circuit during a specific idle time interval. For example, during the idle time interval, quantum gates are applied to qubits in the quantum register other than the idle qubit. For example, an idle qubit is not an idle qubit outside of the idle time interval. Specifically, quantum gate operations of the quantum circuit are applied to the idle qubit before and / or after the idle time interval. For example, different qubits in the quantum register can be idle qubits during different idle time intervals.
[0007] Specifically, here and below, "dephase" refers to the loss of quantum coherence of an idle qubit during the idle time interval. For example, dephase destroys the quantum information stored in the idle qubit. For example, dephase is due to undesired coupling between the idle qubit and its environment. For example, dephase of an idle qubit is caused by external noise coupled to the idle qubit, or it is caused by undesired interaction with other qubits in the quantum register during the idle time interval.
[0008] For example, an identity pulse includes or consists of one or more single-qubit gates acting on the idle qubit during an idle time interval. For example, a single-qubit gate is a rotation operator about the x-axis, y-axis, and / or z-axis. For example, a single-qubit gate causes the state of the idle qubit to rotate by a given angle about a given axis of its Bloch sphere.
[0009] For example, an identity pulse comprises a predetermined number of sub-pulses, particularly at least two. Each of the sub-pulses has properties, for example, one of the single-qubit gates. Exemplarily, the sub-pulses have predetermined phase trajectories for implementing the identity operation on the idle qubit.
[0010] Specifically, the identity pulse performs an identity operation on the idle qubit, such that the state of the idle qubit remains unchanged or approximately unchanged after the identity pulse is applied. For example, the identity pulse maps the ground state |0> to itself and the ground state |1> to itself. For example, the time evolution of the state of the idle qubit when the identity pulse is applied forms a closed loop on the Bloch sphere. In particular, the initial and final states of the idle qubit before and after the application of the identity pulse are the same or approximately the same.
[0011] The method described in this paper is based on the idea of suppressing dephase of idle qubits on a short timescale comparable to or smaller than the Rabi period of a single qubit's rotation. Here and below, for example, when a qubit is resonantly driven by an electromagnetic field coupling two ground states, the Rabi period of the qubit corresponds to the oscillation of the qubit's state between its two ground states |0> and |1>. Specifically, the duration of the Rabi period determines the speed of rotation of the single qubit.
[0012] For example, periodic dynamic decoupling pulse sequences can be used to suppress qubit decoupling by at least approximately averaging the unwanted coupling between the qubit and its environment to zero. For instance, dynamic decoupling pulse sequences are based on an iterative Hahn spin-echo scheme, where the qubit's state is repeatedly flipped back and forth at high frequencies. However, each pulse in a dynamic decoupling pulse sequence that flips the qubit's state is a π pulse, for example, which rotates the qubit's state 180° on a Bloch sphere. Therefore, a π pulse requires a time corresponding to half a Rabi period. Thus, the use of dynamic decoupling pulse sequences is only suitable for suppressing qubit decoupling on timescales much longer than the Rabi period of the qubit.
[0013] In contrast, the method described in this paper can be used to suppress decoupling on timescales smaller than the Rabi period of a qubit. In particular, if the unwanted coupling between the idle qubit and its environment that causes decoupling has an energy scale smaller (e.g., at least five times smaller) than the energy scale associated with the Rabi period of the idle qubit (i.e., the Rabi frequency multiplied by Planck's constant), the identity pulse approximately averages out the unwanted coupling between the idle qubit and its environment.
[0014] According to another embodiment of the method, the identity pulse performs a single-qubit rotation on the idle qubit. For example, the identity pulse causes the state of the idle qubit to evolve over time around a closed loop on a Bloch sphere.
[0015] According to another embodiment of the method, identity pulse suppression is provided for phase loss caused by the interaction between the idle qubit and other qubits in the quantum register, and / or identity pulse suppression is provided for phase loss caused by fluctuations in the ambient electromagnetic field. For example, the fluctuations in the ambient electromagnetic field correspond to electric field noise or magnetic field noise coupled to the qubit.
[0016] According to another embodiment of the method, the identity pulse is applied to the idle qubit on a timescale shorter than or equal to half a Rabi period of the idle qubit. Specifically, for example, the identity pulse has a duration shorter than the duration of the π pulse that flips the state of the idle qubit.
[0017] According to another embodiment of the method, the identity pulse includes a first sub-pulse and a second sub-pulse. The first sub-pulse rotates the state of the idle qubit by an angle θ, and the second sub-pulse rotates the state of the idle qubit by an angle -θ back to its initial state. Specifically, the first sub-pulse rotates the state of the idle qubit about an axis of the Bloch sphere by an angle θ, and the second sub-pulse rotates the state of the idle qubit about the same axis of the Bloch sphere by an angle -θ. For example, angle θ is the polar angle of the Bloch sphere. For example, angle θ is greater than 0°. For example, angle θ is less than or equal to 180°, or less than 90°. Specifically, the second sub-pulse is the inverse of the first sub-pulse. For example, the second sub-pulse is configured to undo the time evolution of the idle qubit's state by the first sub-pulse.
[0018] According to another embodiment of the method, multiple qubits in the quantum register interact via paired Ising interactions or XY interactions. For example, all qubits in the quantum register interact via all-to-all paired Ising interactions or XY interactions.
[0019] For example, in a quantum register comprising N qubits, the interaction takes the following form
[0020] (1)
[0021] Specifically, all qubit pairs in the quantum register interact via the Ising or XY interaction. In equation (1) above, H int This refers to the interaction terms in Hamiltonian, where integer indices i and j enumerate N qubits in the quantum register, and (in ) represents the action on qubit i (where One of the three 2×2 Pauli matrices of the ground state of )
[0022]
[0023] Here, the product of Pauli matrices acting on different qubits will be interpreted as a tensor product. Furthermore, J ij It has energy units and parameterizes the interaction strength between qubits i and j. For example, J ij = J is independent of i and j.
[0024] For α = β, the interaction specified in equation (1) is referred to below as the "Ising interaction", while for α ≠ β, the interaction is referred to below as the "XY interaction". For the case of the Ising interaction where α = β, the ground states |0> and |1> of the qubit are preferably the corresponding Pauli matrices. The eigenstates of , and are referred to as the measurement ground state below.
[0025] For example, pairwise Ising interactions or XY interactions between qubits occur in various physical qubit implementations, such as superconducting qubits or trapped ion qubits. For example, pairwise Ising interactions or XY interactions between qubits can be used to implement two-qubit gates and / or multi-qubit gates between the corresponding qubits, especially entanglement gates. For example, a globally entangled gate (e.g., a generalized Mølmer-Sørensen gate or a magnetic gradient-induced coupling gate) can be implemented by time-evolving the qubits in a quantum register while the qubits are via the Hamiltonian interaction specified in equation (1).
[0026] According to another embodiment of the method, the energy scale of a single qubit rotation is greater than the energy scale of the paired Ising or XY interactions between multiple qubits in a quantum register. Specifically, the Rabi frequency of the qubit multiplied by Planck's constant is greater than the interaction strength J. ij Large, for example, the interaction strength J ij It is at least five times or at least ten times larger. Therefore, the identity pulse can suppress the dephase of the idle qubit due to the interaction between the idle qubit and other qubits in the quantum register.
[0027] According to another embodiment of the method, the idle qubit does not participate in the gate operation of the quantum circuit acting on multiple qubits during a specific idle time interval, during which an identity pulse is applied to the idle qubit.
[0028] According to another embodiment of the method, the ground state of each qubit corresponds to a different hyperfine state of the corresponding trapped ion. Specifically, each qubit is encoded in two different hyperfine states of the corresponding trapped ion. For example, a system of two or more ions trapped in the same ion trap forms a quantum register.
[0029] According to another embodiment of the method, multiple qubits in the quantum register interact via magnetic gradient-induced coupling. For example, a magnetic field gradient along the chain of trapped ions induces paired Ising interactions between all qubit pairs in the ion trap. For example, such Ising interactions in the presence of a magnetic field gradient are mediated by the common vibrational motion of ions in the ion trap.
[0030] This paper further details the quantum computer. Specifically, the quantum computer implements the aforementioned method for suppressing decoupling of idle qubits in a quantum register comprising multiple qubits. All features of this method are also disclosed for the quantum computer, and vice versa.
[0031] According to the implementation, the quantum computer has a quantum register comprising multiple qubits and implements a method for suppressing dephase of idle qubits in the quantum register as specifically described above.
[0032] Other advantageous implementations of this method and quantum computers, as well as other implementations, will become apparent from the exemplary embodiments described below in conjunction with the accompanying drawings.
[0033] Figure 1 A schematic diagram of an identity pulse applied to an idle qubit is shown, according to an exemplary embodiment of a method for suppressing dephase of an idle qubit in a quantum register comprising multiple qubits.
[0034] Figure 2 A schematic diagram of the temporal evolution of the state of an idle qubit on a Bloch sphere during the application of an identity pulse is shown, according to another exemplary embodiment of a method for suppressing dephase of idle qubits in a quantum register comprising multiple qubits.
[0035] Figure 3 A schematic diagram of a quantum computer according to an exemplary embodiment is shown.
[0036] Identical, similar, or equivalent elements are indicated by the same reference numerals in the accompanying drawings. The scale of the drawings and the elements shown therein should not be considered as true scale. Rather, the individual elements may be shown exaggerated for better representativeness and / or better understanding.
[0037] According to an exemplary embodiment of a method for suppressing dephase of idle qubits in a quantum register comprising multiple qubits, Figure 1 The identity pulse 1 shown is applied to the quantum computer 10 during the idle time interval Ti (see [link]). Figure 3 The idle qubit qb1 in the quantum register. During the idle time interval Ti, the idle qubit qb1 does not participate in the quantum gate operations of the quantum circuit that act on the qubits qb1, qb2, qb3, and qb4 in the quantum register. For example, during the idle time interval Ti, at least one gate operation of the quantum circuit is applied only to the other qubits qb2, qb3, and qb4 in the quantum register. Before and / or after the idle time interval Ti, the idle qubit qb1 participates in the quantum gate operations of the quantum circuit; that is, it is not an idle qubit outside the idle time interval Ti.
[0038] To suppress the dephase of idle qubit qb1 during the idle time interval Ti caused by undesired interactions with other qubits qb2, qb3, qb4 and / or undesired coupling with environmental noise, an identity pulse 1 is applied to idle qubit qb1 during the idle time interval Ti. The identity pulse 1 at least approximately averages the undesired coupling or interactions between idle qubit qb1 and its environment to zero. If the idle time interval Ti is longer than the evolution time of the identity pulse 1, two or more identity pulses 1 may subsequently be applied to idle qubit qb1.
[0039] The identity pulse 1 performs a rotation of the state of the idle qubit qb1, such that the final state of the idle qubit qb1 after the identity pulse 1 is applied is the same as or approximately the same as the initial state of the idle qubit qb1 before the identity pulse 1 is applied.
[0040] In this exemplary embodiment, the identity pulse 1 includes a first sub-pulse 11 and a subsequent second sub-pulse 12. The first sub-pulse 11 rotates the initial state of the idle qubit qb1 by an angle θ between 0° and 180° about the axis of the Bloch sphere. The second sub-pulse 12 rotates the state of the idle qubit qb1 back to its initial state by an angle -θ about the same axis of the Bloch sphere, thus performing an identity operation on the idle qubit qb1 in total. For example, the second sub-pulse 12 directly follows the first sub-pulse 11, with no time delay or pause between the two sub-pulses 11 and 12. Alternatively, such as Figure 1 As shown, a pause may exist between the two sub-pulses 11 and 12. The identity pulse 1 may also consist of a single pulse, or it may consist of more than two sub-pulses, with or without a pause between the more than two sub-pulses.
[0041] Figure 2 The state of the idle qubit qb1 at a specific moment during the idle time interval Ti is represented as state vector 2 on Bloch sphere 3. For example, on Bloch sphere 3, state vector 2 pointing to the south pole corresponds to the ground state |0>, state vector 2 pointing to the north pole corresponds to the ground state |1>, and state vector 2 pointing in other directions corresponds to the complex superposition of the ground states |0> and |1>.
[0042] The identity pulse 1 consists of a single-qubit rotation that rotates the state of the idle qubit qb1 such that, during its time evolution, the tip of the state vector 2 traces a closed loop 4 on the surface of the Bloch sphere 3. Therefore, the initial state of the idle qubit qb1 before the application of identity pulse 1 and the final state of the idle qubit qb1 after the application of identity pulse 1 are the same or approximately the same.
[0043] according to Figure 3 The quantum computer 10 in the exemplary embodiment includes a quantum register with four qubits qb1, qb2, qb3, and qb4. The two ground states |0> and |1> of each qubit qb1, qb2, qb3, and qb4 are the two hyperfine states of the corresponding trapped ions. All ions belonging to the quantum register are trapped in the same ion trap, such as a Paul trap or a Penning trap. The magnetic gradient along the chain of trapped ions induces paired Ising interactions between all paired qubits qb1, qb2, qb3, and qb4 in the quantum register.
[0044] This patent application claims priority to German patent application 10 2023 131 673.7, the disclosure of which is incorporated herein by reference.
[0045] This invention is not limited to the exemplary embodiments described herein. Rather, the invention covers any new features and any combination of features, even if the feature or combination itself is not expressly specified in the patent claims or exemplary embodiments. The invention particularly includes any combination of features in the patent claims and any combination of features in the exemplary embodiments.
[0046] Figure Labels
[0047] 1. Identical Pulse
[0048] 11 First Sub-Pulse
[0049] 12 Second Sub-Pulse
[0050] 10 Quantum Computers
[0051] 2. State Vector
[0052] 3 Bloch Ball
[0053] 4 Closed loop
[0054] qb1 idle qubit
[0055] qb2…4 qubits
[0056] t time
[0057] Ti (Idle Time Interval)
[0058] θ angle
Claims
1. A method for suppressing decoupling of an idle qubit (qb1) in a quantum register comprising multiple qubits (qb1, qb2, qb3, qb4), comprising the following steps: - An identity pulse (1) is applied to the idle qubit (qb1), wherein the identity pulse (1) performs an identity operation on the idle qubit (qb1), wherein, - The identity pulse (1) is applied to the idle qubit (qb1) on a timescale shorter than or equal to half a Rabi period of the idle qubit (qb1).
2. The method according to the preceding claim, wherein, The identity pulse (1) performs a single-qubit rotation on the idle qubit (qb1).
3. The method according to any one of the preceding claims, wherein, - The identity pulse (1) suppresses phase loss caused by the interaction between the idle qubit (qb1) and the other qubits (qb2, qb3, qb4) in the quantum register, and / or - The constant pulse (1) suppresses phase loss caused by fluctuations in the ambient electromagnetic field.
4. The method according to any one of the preceding claims, wherein, The identity pulse (1) includes a first sub-pulse (11) and a second sub-pulse (12). The first sub-pulse rotates the state of the idle qubit (qb1) by an angle θ, and the second sub-pulse rotates the state of the idle qubit (qb1) back to its initial state by an angle -θ.
5. The method according to any one of the preceding claims, wherein, The plurality of qubits (qb1, qb2, qb3, qb4) in the quantum register interact with each other via paired Ising interactions or XY interactions.
6. The method according to the preceding claim, wherein, The energy scale of a single qubit rotation is greater than the energy scale of the paired Ising interaction or XY interaction between the plurality of qubits (qb1, qb2, qb3, qb4) in the quantum register.
7. The method according to any one of the preceding claims, wherein, The idle qubit (qb1) does not participate in the gate operation of the quantum circuit of the plurality of qubits (qb1, qb2, qb3, qb4) during a specific idle time interval (Ti), during which the identity pulse (1) is applied to the idle qubit (qb1).
8. The method according to any one of the preceding claims, in, The ground state of each qubit (qb1, qb2, qb3, qb4) corresponds to a different hyperfine state of the trapped ion.
9. The method according to the preceding claim, wherein, The plurality of qubits (qb1, qb2, qb3, qb4) in the quantum register interact via magnetic gradient-induced coupling.
10. A quantum computer (10), wherein, The method according to any one of claims 1 to 9 is used to suppress the dephase of the idle qubit (qb1) in a quantum register comprising a plurality of qubits (qb1, qb2, qb3, qb4).