Quantum bit manipulation method and related computing device

Abstract

The invention relates to a quantum bit control method and computing equipment thereof. The method comprises the following steps: acquiring a control bit atom and a target bit atom; and performing a qubit gate or qubit entanglement operation based on a Rydberg blocking effect between both the control bit atom and the target bit atom using a single photon process or a multi-photon process, the qubit gate or qubit entanglement operation comprising: a step a) wherein the qubit gate or qubit entanglement operation is performed on the basis of the Rydberg blocking effect between both the control bit atom and the target bit atom; applying a Pi pulse consisting of a first group of Pi / 2 pulses and a second group of Pi / 2 pulses in succession to the control bit atom, the first group of Pi / 2 pulses and the second group of Pi / 2 pulses having the same wavelength and being applied in opposite directions to the control bit atom, wherein a Pi pulse is defined as a pulse that resonates in frequency from a first ground state of the control bit atom to a Rydberg state, and an energy allows the control bit atom to fully transition from the first ground state to the Rydberg state for a pulse duration.

Description

Technical Field

[0001] The present application relates to the field of quantum computing, and more specifically to a method for manipulating quantum bits and related computing equipment. Background Art

[0002] Neutral atoms have been shown to be useful for building large-scale quantum computing and quantum simulation systems, thanks to their unique characteristics compared to other quantum systems. For example, neutral atoms can be well isolated from the external environment and have quantum coherence for a long time. The technology developed in the past four decades makes it easier to initialize, manipulate, and read the internal and motional states of neutral atoms. Atomic arrays with large numbers of atoms can be quickly constructed while maintaining independent control of individual atoms.

[0003] Based on the above characteristics, neutral atoms can now construct quantum processors of about 300 atoms, and can manipulate individual atoms to achieve single-bit gates and two-bit gates. The fidelity of single-bit gates achieved using microwaves is greater than 99.99%, meeting the requirements for quantum computing error correction, but the fidelity of two-bit gates still lags behind superconducting and trapped ion systems. Therefore, at this stage, the most urgent thing for neutral atoms is to increase the fidelity of two-bit gates above the error correction threshold. Summary of the invention

[0004] The purpose of the present disclosure is to provide a method for manipulating quantum bits and a related computing device, which can have improved fidelity of quantum bit gates or entangled states.

[0005] According to the first aspect of the present disclosure, a method for manipulating a quantum bit is provided. The method comprises: obtaining a control bit atom and a target bit atom; and using a single photon process or a multi-photon process, performing a quantum bit gate or a quantum bit entanglement operation based on the Rydberg blockade effect between the control bit atom and the target bit atom, wherein the quantum bit gate or the quantum bit entanglement operation comprises: step a), applying a π pulse consisting of a first group of π / 2 pulses and a second group of π / 2 pulses to the control bit atom, wherein the first group of π / 2 pulses and the second group of π / 2 pulses have the same wavelength and are applied to the control bit atom in opposite directions, wherein the π pulse is defined as a pulse whose frequency resonates with the transition from the first ground state to the Rydberg state of the control bit atom, and whose energy allows the control bit atom to completely transition from the first ground state to the Rydberg state within the pulse duration.

[0006] It will be understood that by utilizing the quantum bit manipulation method disclosed in the present invention, the phase accumulated due to the Doppler effect can be completely offset or eliminated, thereby completely suppressing or reducing the error caused by the Doppler effect.

[0007] In some embodiments, the quantum bit gate or quantum bit entanglement operation may also include: step b), after applying the one π pulse to the control bit atom, applying a 2π pulse to the target bit atom, wherein the 2π pulse is defined as a pulse whose energy allows the target bit atom to transition from the first ground state to the Rydberg state and then back to the first ground state.

[0008] In some embodiments, the qubit gate or qubit entanglement operation may further include: step c), after applying the 2π pulse to the target bit atom, applying another π pulse consisting of another first group of π / 2 pulses and another second group of π / 2 pulses to the control bit atom; wherein the another first group of π / 2 pulses and the another second group of π / 2 pulses have the same wavelength and are applied to the control bit atom in opposite directions. It will be understood that the above steps a), b) and c) construct a two-bit CZ gate.

[0009] In some embodiments, when utilizing the single photon process, the first group of π / 2 pulses, the second group of π / 2 pulses, the another first group of π / 2 pulses, and the another second group of π / 2 pulses are all realized by a single laser pulse of the same wavelength.

[0010] In some embodiments, when the multi-photon process is a two-photon process, the first group of π / 2 pulses, the second group of π / 2 pulses, the another first group of π / 2 pulses and the another second group of π / 2 pulses are respectively composed of a combination of a π / 2 pulse of a first wavelength and a π / 2 pulse of a second wavelength applied simultaneously to the control bit atom, the first wavelength is different from the second wavelength, and the π / 2 pulse of the first wavelength and the π / 2 pulse of the second wavelength are applied to the control bit atom in opposite directions.

[0011] In some embodiments, the qubit gate or qubit entanglement operation may further include: step d), before step a), applying a π / 2 Raman pulse to the target bit atom, and after step c), applying another π / 2 Raman pulse to the target bit atom to respectively implement an H gate operation. It will be understood that the above steps a), b), c) and d) construct a two-bit CNOT gate.

[0012] In some embodiments, the method may further include: before executing the quantum bit gate or quantum bit entanglement operation, performing a pre-quantum bit operation on the control bit atom and the target bit atom, the pre-quantum bit operation including: making the initial states of both the control bit atom and the target bit atom in the first ground state.

[0013] In some embodiments, the pre-qubit operation may further include: after initial states of both the control bit atom and the target bit atom are in the first ground state, preparing the control bit atom to a superposition state.

[0014] In some embodiments, the method may further include: after executing the quantum bit gate or quantum bit entanglement operation, performing a post-quantum bit operation on the control bit atom and the target bit atom, the post-quantum bit operation including: determining the fidelity of the entangled state by measuring the parity signal of the entangled state of the control bit atom and the target bit atom.

[0015] In some embodiments, the pre-qubit operation or the post-qubit operation may also include: switching the control bit atom and / or the target bit atom between a corresponding first ground state and a second ground state by applying a Raman laser pulse to the control bit atom and / or the target bit atom, the second ground state having a lower energy level than the corresponding first ground state.

[0016] In some embodiments, the method may further include calculating a duration of the π / 2 pulse or the π pulse based on a Rabi frequency of the single-photon process or the multi-photon process.

[0017] In some embodiments, the control bit atoms and the target bit atoms are selected from neutral atoms.

[0018] According to a second aspect of the present disclosure, a computing device is provided, comprising: a quantum bit gate implemented according to the aforementioned method.

[0019] In some embodiments, the computing device is a quantum computer, a quantum processor, or a quantum circuit.

[0020] In some embodiments, the qubit gates include a CZ gate and a CNOT gate.

[0021] According to a third aspect of the present disclosure, a quantum system is provided, which is configured to perform the aforementioned method. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and other features, advantages and aspects of various embodiments of the present disclosure will become more apparent with reference to the following detailed description in conjunction with the accompanying drawings.

[0023] In the accompanying drawings, the same or similar reference numerals denote the same or similar elements, wherein:

[0024] Figure 1 A schematic diagram of the principle of a controlled Z-gate scheme based on the Rydberg blocking effect is shown;

[0025] Figure 2 The schematic diagram of energy levels of a rubidium 87 atom as an example is shown in which a single photon is excited to a Rydberg state and a two-photon is excited to a Rydberg state;

[0026] Figure 3 A schematic diagram of a controlled Z-gate pulse sequence based on the Rydberg blocking effect is shown;

[0027] Figure 4 A schematic diagram of a conventional two-photon opposed Rydberg excitation scheme is shown;

[0028] Figure 5 A schematic diagram showing the principle of controlling bit atoms by laser pulse irradiation according to an exemplary embodiment of the present disclosure is shown;

[0029] Figure 6 A schematic diagram showing a pulse sequence of a two-bit controlled Z-gate according to an example embodiment of the present disclosure;

[0030] Figure 7 A flowchart of a method for manipulating a quantum bit according to an exemplary embodiment of the present disclosure is shown;

[0031] Figure 8 shows an energy level diagram of a rubidium 87 atom being excited to a Rydberg state by two-photon excitation according to a first exemplary embodiment of the present disclosure;

[0032] Fig. 9 shows an optical structure diagram and a pulse timing diagram according to a first exemplary embodiment of the present disclosure;

[0033] Fig.10 shows an energy level diagram of two-photon excitation of rubidium 87 atoms to Rydberg states according to a second exemplary embodiment of the present disclosure;

[0034] Fig.11 shows an optical structure diagram and a pulse timing diagram according to a second exemplary embodiment of the present disclosure;

[0035] Fig.12 A schematic diagram showing the energy levels of a cesium atom excited to a Rydberg state by a single photon according to a third exemplary embodiment of the present disclosure; and

[0036] Fig.13 An optical structure diagram and a pulse timing diagram according to a third exemplary embodiment of the present disclosure are shown. DETAILED DESCRIPTION

[0037] Embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although certain embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be construed as being limited to the embodiments described herein, which are instead provided for a more thorough and complete understanding of the present disclosure. It should be understood that the drawings and embodiments of the present disclosure are only for exemplary purposes and are not intended to limit the scope of protection of the present disclosure.

[0038] As mentioned above, there is an urgent need to improve the fidelity of multi-qubit gates (especially two-qubit gates) on neutral atoms to above the error correction threshold.

[0039] For multi-qubit gates including two-qubit gates, they typically need to be constructed based on the Rydberg blockade effect between the control bit atom and the target bit atom. This is because the atomic interaction in the ground state is too weak to realize a fast two-qubit gate. When the atom is excited from the ground state to the Rydberg state, the atoms in the Rydberg state have a strong dipole-dipole interaction, which causes the Rydberg energy level of the atom to move, resulting in the so-called Rydberg blockade effect, which is the basis for realizing multi-qubit gates (especially two-qubit gates). Usually, in neutral atom quantum computing, qubits are encoded on two magnetically insensitive energy levels of the atomic ground state hyperfine energy level, which are recorded here as the first ground state |1> and the second ground state |0>. Then, the basis of the quantum state of the two qubits can be written as {|00>,|01>,|10>,|11>}, so the standard controlled-Z gate (Controlled-Z or CZ gate) can be written as follows:

[0040]

[0041] Figure 1 A schematic diagram showing the principle of a controlled Z-gate (or CZ-gate) scheme based on the Rydberg blockade effect between a control bit atom and a target bit atom.

[0042] like Figure 1 As shown in the figure, in addition to the first ground state |1> and the second ground state |0>, the Rydberg state |r> is also introduced. It should be noted here that the π pulse in the figure refers to a pulse whose frequency resonates with the transition from |1> to |r>, and the pulse duration allows the control bit atom to completely transition from the |1> state to |r>, while the 2π pulse refers to a pulse that transitions the target bit atom from |1> to |r> and then back to the |1> state. It should be understood that the 2π pulse does not cause a change in the number of atoms but a change in phase.

[0043] Here, when the control bit atom and the target bit atom are in different initial states, different evolution processes will occur. Specifically, when the initial state of the two atoms is |00>, the system does not evolve, and the overall phase change is 0; when the atoms are in |01>, |10>, |11>, the control bit atom accumulates phase π, that is, when the system is in |01>, |10>, |11>, the system accumulates phase π. Therefore, with the two-bit quantum state {|00>, |01>, |10>, |11>} as the basis, the controlled Z gate represented by formula (2) can be finally formed.

[0044]

[0045] It should be understood that the above formula (2) can be obtained by a simple basis vector transformation to obtain the above formula (1). Therefore, the above CZ′ can be equivalent to the above CZ. In addition, theoretical research related to quantum computing has also proved that all quantum circuits can be realized by single-bit gates and two-bit CZ gates, among which, in particular, CZ gates combined with Hadamard gates (Hadmard gates or H gates) can realize two-bit controlled NOT gates (or CNOT gates). Therefore, it is very important to realize high-fidelity two-bit controlled Z gates in experiments.

[0046] From above Figure 1 It can be seen that an important step in realizing a two-qubit gate is to excite the atom to the Rydberg state. This process is usually achieved through a single-photon process or a multi-photon process (typically a two-photon process), that is, the atom simultaneously absorbs one or more (for example, two) photons of a specific frequency and then transitions from the ground state to the Rydberg state.

[0047] As an example, Figure 2 The figure shows the energy level diagram of the single photon excitation to the Rydberg state and the two-photon excitation to the Rydberg state of the rubidium 87 atom. Figure 2 As shown in the left figure, a rubidium 87 atom can absorb a single photon at 297 nm from the ground state 5S 1 / 2 Transition to the Rydberg state nP; or Figure 2 As shown in the right figure, it can also be achieved by using the intermediate energy level 5P 3 / 2 Absorbing a 780nm and a 480nm photon from the ground state 5S 1 / 2 Transition to the Rydberg state nS or nD. However, since the atom absorbs a photon during the process of being excited to the Rydberg state, it is bound to be subject to the recoil effect of the photon momentum. The effective momentum of the photon is At the same time, since the atoms have a finite temperature T, the velocity distribution of the atoms is Therefore, when the atom undergoes Rydberg excitation, there will be a random detuning caused by the Doppler effect.

[0048] To further illustrate the problem, Figure 3 A schematic diagram of a controlled Z-gate pulse sequence based on the Rydberg blocking effect is shown.

[0049] like Figure 3 As shown in the figure, after the control bit atom is excited to the Rydberg state by the first π pulse, due to the random detuning δ or Doppler effect mentioned above, the control bit atom will accumulate an additional phase φ=δ(2t π,t +2t gap ), where t π,t represents the time for applying a 2π pulse to the target bit atom, and t gap It represents the time interval between the first π pulse or the second π pulse and the 2π pulse, resulting in a reduction in the fidelity of the two-bit gate.

[0050] In order to reduce the impact of the above-mentioned Doppler effect, there are several conventional solutions:

[0051] The first method is to lower the temperature of atoms so that the temperature distribution of atoms Try to reduce it as much as possible, but this puts higher requirements on the complexity of the system. At present, the temperature of atoms can usually be cooled to several μK.

[0052] The second is to reduce the time for the free evolution of the control bit atom. Figure 3 It can be seen that the control of the free evolution time of the bit atom is determined by the length of the 2π pulse applied to the target bit atom, which requires increasing the power of the laser to achieve, but high-power lasers also place higher requirements on the complexity of the system and related technologies.

[0053] The third method uses a two-photon excitation light scheme, in which a laser pulse of a first wavelength and a laser pulse of a second wavelength are irradiated towards each other onto a control bit atom, such as a rubidium 87 atom. Figure 4 Schematic diagram of conventional two-photon counter-irradiation Rydberg excitation scheme is shown. Figure 4 As shown, when an atom absorbs a photon, the momentum of the two photons can cancel each other out, making the effective wave vector felt by the atom However, this two-photon counter-radiation scheme still cannot completely eliminate the influence of the Doppler effect on the fidelity of the two-bit gate. This is because the momentum of a photon is inversely proportional to the wavelength of light, that is, k∝1 / λ. Due to the limitation of the interatomic energy level, the wavelengths of the two photons undergoing Rydberg excitation must be different. Therefore, even if an atom absorbs two opposite photons, the effective momentum felt by the atom is It is still not zero, so in the process of realizing the two-bit gate, controlling the bit atom will still produce additional phase accumulation, which will lead to a decrease in the fidelity of the two-qubit gate.

[0054] The purpose of the present disclosure is to suppress the Doppler effect of two-qubit atoms in a Rydberg excitation scheme, eliminate or reduce the extra phase accumulation of atoms in a two-qubit gate due to the Doppler effect, thereby improving the fidelity of a two-qubit gate or two-qubit entanglement constructed based on the Rydberg blockade effect. The concept of the present disclosure is to divide the conventional π pulse applied to the control bit atom into two π / 2 pulses, and make the wavelengths of the two π / 2 pulses before and after the same but in opposite directions, thereby offsetting the random detuning caused by the Doppler effect, so that the final random detuning δ=0, thereby greatly improving the fidelity of the two-qubit gate or entangled state of the scheme.

[0055] In order to more clearly understand the concept of the present disclosure, the following will use the two-photon process and refer to Figure 5 and Figure 6 To describe the principles of the concept of the present disclosure, Figure 5 A schematic diagram showing the principle of controlling a bit atom by using two-photon laser pulse irradiation according to an example embodiment of the present disclosure; and Figure 6 A schematic diagram showing a pulse sequence of a two-bit controlled Z-gate according to an example embodiment of the present disclosure is shown.

[0056] like Figure 5 As shown, reference numeral 50 indicates an optical dipole trap, in which the control bit atom 10 is trapped. Figure 5 and Figure 6 It can be seen that in the two-photon process scheme, Figure 3 The two previous and next π pulses used to manipulate and control the bit atom are decomposed into two consecutive groups of π / 2 pulses, where the two consecutive groups of π / 2 pulses are designed to have the same wavelength but opposite directions of action, as will be described in further detail later.

[0057] More specifically, for the two-photon process, for the formation of the first π pulse of the two π pulses, firstly, two pulses of different wavelengths Ω r1 ,Ω b1 The first group of pulses Ω 1 At the same time, it is applied to the control bit atom, and its action time is π / 2 (therefore, the first group of pulses Ω 1 It can also be called the first group of π / 2 pulses Ω 1 ); Then, the other two pulses of different wavelengths Ω r2 ,Ω b2 The second group of pulses Ω 2At the same time, it is applied to the control bit atom, and its action time is also π / 2 (therefore, the second group of pulses Ω 2 It can also be called the second group of π / 2 pulses Ω 2 ), wherein the first group of π / 2 pulses and the second group of π / 2 pulses have the same wavelength but act in opposite directions (more specifically, the first group of π / 2 pulses Ω 1 Ω r1 and the second group of π / 2 pulses Ω 2 Ω r2 The wavelengths are the same but in opposite directions, while the first group of π / 2 pulses Ω 1 Ω b1 and the second group of π / 2 pulses Ω 2 Ω b2 The wavelengths of the first group of π / 2 pulses Ω are the same but in opposite directions. 1 and the second group of π / 2 pulses Ω 2 The combination of will form the first π pulse mentioned above.

[0058] Similarly, for the formation of the second π pulse of the two π pulses, the second π pulse can also be formed by a combination of another first group of π / 2 pulses and another second group of π / 2 pulses, wherein the another first group of π / 2 pulses and the another second group of π / 2 pulses have the same wavelength and are applied to the control bit atom in opposite directions. The combination of the another first group of π / 2 pulses and the another second group of π / 2 pulses will form the second π pulse.

[0059] In particular, in some embodiments, the composition and action direction of the another first group of π / 2 pulses are exactly the same as the first group of π / 2 pulses, and the composition and action direction of the another second group of π / 2 pulses are exactly the same as the second group of π / 2 pulses. It should be understood that in some other embodiments, the composition of the another first group of π / 2 pulses is different from the first group of π / 2 pulses, and the composition of the another second group of π / 2 pulses is different from the second group of π / 2 pulses.

[0060] It should also be understood that since the wavelengths between the first group of π / 2 pulses and the second group of π / 2 pulses and / or between another first group of π / 2 pulses and another second group of π / 2 pulses are the same but the directions are completely opposite, the accumulated phase of the aforementioned Doppler effect can be completely offset or eliminated, thereby completely suppressing or reducing the error caused by the Doppler effect.

[0061] Here, it should be noted that, although the expressions of the first group of π / 2 pulses and the second group of π / 2 pulses and another first group of π / 2 pulses and another second group of π / 2 pulses are used above for the composition of the first π pulse and the second π pulse, respectively, the above expressions "first group", "second group", "another first group" and "another second group" are not intended to: indicate an order or to constitute any limitation on the scope of the present disclosure. They are used only for the convenience of description. Therefore, in various embodiments or contexts of the present disclosure, unless the context is obviously contradictory, the above "first group" and "second group" may also be used to refer to or cover the π / 2 pulses in the two groups of π / 2 pulses decomposed in the above second π pulse. At the same time, the above "another first group" and "another second group" may also be used to refer to or cover the π / 2 pulses in the two groups of π / 2 pulses decomposed in the above first π pulse.

[0062] In addition, it should be understood that although the above description refers to a two-photon process and how to decompose the two preceding and following π pulses into two groups of π / 2 pulses, the above description can also be similarly applied to a single-photon process, a three-photon process, or even a process with more photons. For example, the first π pulse and the second π pulse of a single-photon process and a three-photon process can also be decomposed into two groups of π / 2 pulses, wherein in a single-photon process, each group of π / 2 pulses will be formed by only a single pulse of a single wavelength, and in a three-photon process, each group of π / 2 pulses will be formed by a combination of pulses of three different wavelengths; and for processes with more photons, the same can be applied.

[0063] The following will refer to Figure 7 A flowchart of a method for manipulating a quantum bit according to an example embodiment of the present disclosure is generally described.

[0064] like Figure 7 As shown, the method 700 includes: in box 710, obtaining control bit atoms and target bit atoms.

[0065] It will be understood that the present disclosure aims to realize qubit gate or qubit entanglement operation based on the Rydberg blockade effect between the control bit atom and the target bit atom.

[0066] As an example of obtaining a control bit atom and a target bit atom, the control bit atom and the target bit atom can be trapped in two independent light traps, for example. Typically, in some embodiments, both the control bit atom and the target bit atom can be selected from neutral atoms. As an example, the neutral atom can include an alkali metal atom, such as a cesium atom, a rubidium atom, etc.

[0067] Next, in box 720, a qubit gate or qubit entanglement operation based on the Rydberg blockade effect between the control bit atom and the target bit atom is performed using a single photon process or a multi-photon process. More specifically, the qubit gate or qubit entanglement operation may include: step a), applying a π pulse consisting of a first group of π / 2 pulses and a second group of π / 2 pulses to the control bit atom, wherein the first group of π / 2 pulses and the second group of π / 2 pulses have the same wavelength and are applied to the control bit atom in opposite directions, wherein the π pulse is defined as a pulse whose frequency resonates with the transition from the first ground state to the Rydberg state of the control bit atom and whose energy allows the control bit atom to completely transition from the first ground state to the Rydberg state within the pulse duration.

[0068] It should be understood that the role of the above step a) is to apply a π pulse having an energy capable of allowing the control bit atom to transition from the first ground state to the Rydberg state to the control bit atom. It should be particularly noted that in the scheme of realizing a quantum bit gate or a quantum bit entangled state such as a controlled Z gate (or CZ gate) based on the Rydberg effect, this π pulse can be any one of the two π pulses mentioned above. It will also be understood that, unlike conventional schemes, this π pulse is composed of two groups of successive π / 2 pulses (i.e., a first group of successive π / 2 pulses and a second group of successive π / 2 pulses), and these two groups of successive π / 2 pulses have the same wavelength and act in opposite directions.

[0069] It will be understood that in the above step a), the phase accumulated by the Doppler effect can be completely offset or reduced, thereby completely suppressing or reducing the error caused by the Doppler effect.

[0070] In some embodiments, the above-mentioned qubit gate or qubit entanglement operation may further include other steps, such as: step b), after applying the above-mentioned one π pulse to the control bit atom, applying a 2π pulse to the target bit atom, wherein the 2π pulse is defined as a pulse whose energy allows the target bit atom to transition from the first ground state to the Rydberg state and then return to the first ground state; and / or step c), after applying the 2π pulse to the target bit atom, applying another π pulse consisting of another first group of π / 2 pulses and another second group of π / 2 pulses to the control bit atom; wherein the another first group of π / 2 pulses and the another second group of π / 2 pulses have the same wavelength and are applied to the control bit atom in opposite directions. It will be understood that the above steps a), b) and c) construct a two-bit CZ gate.

[0071] In addition, in some embodiments, the above-mentioned qubit gate or qubit entanglement operation may further include step d), that is, before step a), applying a π / 2 Raman pulse to the target bit atom, and after step c), applying another π / 2 Raman pulse to the target bit atom to respectively implement an H gate operation. It will be understood that the above-mentioned steps a), b), c) and d) construct a two-bit CNOT gate.

[0072] It should be understood that the laser pulses used in the quantum bit manipulation method of the present disclosure are different in the single-photon process or the multi-photon process. For example, in an embodiment utilizing a single-photon process, the above-mentioned first group of π / 2 pulses, the second group of π / 2 pulses, another first group of π / 2 pulses, and another second group of π / 2 pulses can be realized by a single laser pulse of the same wavelength. In an embodiment of a two-photon process, the above-mentioned first group of π / 2 pulses, the second group of π / 2 pulses, another first group of π / 2 pulses, and another second group of π / 2 pulses can be, for example, respectively composed of a combination of a π / 2 pulse of a first wavelength and a π / 2 pulse of a second wavelength applied simultaneously to the control bit atom, wherein the first wavelength is different from the second wavelength, and the π / 2 pulse of the first wavelength and the π / 2 pulse of the second wavelength are both applied to the control bit atom in opposite directions.

[0073] It should be understood that in order to conveniently implement the above-mentioned quantum bit gate or quantum bit entanglement operation, in some embodiments, the quantum bit manipulation method may also include: a pre-quantum bit operation before performing the quantum bit gate or quantum bit entanglement operation.

[0074] As an example, the pre-qubit operation may, for example, include: making the initial states of both the control bit atom and the target bit atom in the first ground state. It should be understood that this operation may be performed, for example, by an optical pumping process, which helps to confirm the initial states of the control bit atom and the target bit atom. As another example, the pre-qubit operation may also, for example, include: by applying a Raman laser pulse to the control bit atom and / or the target bit atom, the control bit atom and / or the target bit atom are switched between the corresponding first ground state and the second ground state, and the second ground state has an energy level lower than the corresponding first ground state. As another example, the pre-qubit operation may also include: after making the initial states of both the control bit atom and the target bit atom in the first ground state, preparing the control bit atom to a superposition state. It will be understood that after subsequently passing the superposition state of the control bit atom through the above-mentioned two-qubit gate, the entangled state of both the control bit atom and the target bit atom can be obtained.

[0075] In some other embodiments, in order to conveniently implement detection such as fidelity of the above-mentioned qubit gate or qubit entanglement operation, the above-mentioned qubit manipulation method may also include: post-qubit operation after performing the qubit gate or qubit entanglement operation. As an example, the post-qubit operation may, for example, include: determining the fidelity of the entangled state or qubit gate by measuring the parity signal of the entangled state of the control bit atom and the target bit atom. As another example, the post-qubit operation may also include: applying Raman laser pulses to the above-mentioned control bit atom and / or the above-mentioned target bit atom to achieve switching of the control bit atom and / or the target bit atom between the corresponding first ground state and the second ground state.

[0076] It will also be understood that in some embodiments, the duration of the π / 2 pulse, π pulse or 2π pulse used in the above-mentioned single photon process or multi-photon process can be calculated by the Rabi frequency of the single photon process or multi-photon process.

[0077] To better understand the above principles, Figure 8 A schematic diagram showing the energy levels of a rubidium 87 atom being excited to a Rydberg state by two-photon excitation according to a first exemplary embodiment of the present disclosure is shown.

[0078] It will be understood that this first exemplary embodiment will utilize a two-photon scheme to achieve the manipulation of quantum bits. More specifically, quantum entanglement of two rubidium 87 atoms (i.e., between the control bit atom and the target bit atom) is achieved through two-photon transitions of 780nm laser and 480nm laser.

[0079] like Figure 8 As shown, the quantum bit of the first exemplary embodiment can be defined as, for example, the ground state hyperfine levels of two rubidium 87 atoms, where the second ground state |0>=|5S 1 / 2 ,F=1,m F =0>, first ground state |1>=|5S 1 / 2 ,F=2,m F =0>. In some embodiments, the flip between the second ground state |0> and the first ground state |1> can be achieved by stimulated Raman transition. For example, the transition between the second ground state |0> and the first ground state |1> can be achieved by two laser beams with a wavelength of about 795 nm and a frequency difference of 6834.683 MHz, wherein the two laser beams can be designed to be in contact with the excited state |5P 1 / 2 > Detuning 50-100 GHz. Usually, in experiments, for simplicity, the above two laser beams can be combined into one laser beam, which is recorded as a Raman laser pulse.

[0080] It should be understood that the interaction between the control bit atom and the target bit atom is achieved through Rydberg state interaction. In this example embodiment, when the control bit atom is excited to the Rydberg state, the control bit atom can be excited from the first ground state |1> through the intermediate energy level |5P by a two-photon transition with a first wavelength of 780nm and a second wavelength of 480nm. 3 / 2 >Excited to Rydberg state|79D 5 / 2 >.

[0081] It should be noted here that usually the above-mentioned first wavelength of 780nm laser pulse is designed to be in contact with the intermediate energy level |5P 3 / 2 >Detune by several hundred MHz to several GHz to avoid leakage excitation of control bit atoms to intermediate energy levels.

[0082] Fig. 9 An optical structure diagram and a pulse timing diagram according to a first exemplary embodiment of the present disclosure are shown.

[0083] First, see Fig. 9 (a) in FIG. 1 shows the optical structure diagram of the pulse irradiation control bit atom and the target bit atom. Fig. 9 As shown in (a), the control bit atom 10 and the target bit atom 11 are respectively trapped in two independent light traps (not shown). As an example of quantum bit manipulation of the control bit atom 10, the laser irradiated on the control bit atom 10 may include five laser pulses, including: a first wavelength laser pulse 12 with a wavelength of 780nm, a second wavelength laser pulse 13 with a wavelength of 480nm, another first wavelength laser pulse 14 with a wavelength of 780nm, another second wavelength laser pulse 15 with a wavelength of 480nm, and a Raman laser pulse 16 with a wavelength of 795nm.

[0084] Therefore, the wavelength, frequency and intensity of the first wavelength laser pulse 12 and the other first wavelength laser pulse 14 are the same, and the wavelength, frequency and intensity of the second wavelength laser pulse 13 and the other second wavelength laser pulse 15 are also the same. In addition, the first wavelength laser pulse 12 and the other second wavelength laser pulse 15 have the same action direction, and the second wavelength laser pulse 13 and the other first wavelength laser pulse 14 have the same action direction; but the first wavelength laser pulse 12 and the other first wavelength laser pulse 14 have opposite action directions; the second wavelength laser pulse 13 and the other second wavelength laser pulse 15 have opposite action directions. In addition, the above-mentioned first wavelength laser pulse 12 and second wavelength laser pulse 13 act on the control bit atom 10 at the same time, and the other first wavelength laser pulse 14 and the other second wavelength laser pulse 15 act on the control bit atom at the same time, and their action time is designed to be π / 2.

[0085] Therefore, the first wavelength laser pulse 12 and the second wavelength laser pulse 13 may correspond to Figure 5 Ω r1 and Ω b1 , to form the first group of π / 2 pulses in any one of the two π pulses mentioned above; and the other first wavelength laser pulse 14 and the other second wavelength laser pulse 15 can correspond to Figure 5 Ω r2 and Ω b2 , to form the second group of π / 2 pulses in any of the π pulses. It should be understood that the first group of π / 2 pulses and the second group of π / 2 pulses will be combined to form a π pulse, the energy of which is suitable for exciting the control bit atom 10 from the first ground state to the Rydberg state.

[0086] The role of the Raman laser pulse 16 with a wavelength of 795nm is to achieve the flipping of the control bit atom 10 between the second ground state |0> and the first ground state |1>, thereby realizing a single-bit gate. It will be understood that the single-bit gate combined with the quantum bit operation based on the Rydberg effect can realize other quantum bit gates or realize quantum entanglement between the control bit atom and the target bit atom.

[0087] As an example of quantum bit operation on the target bit atom 11, the laser irradiated on the target bit atom 11 may include three laser pulses, including: another first wavelength laser pulse 18 with a wavelength of 780nm, another second wavelength laser pulse 17 with a wavelength of 480nm, and another Raman laser pulse 19 with a wavelength of 795nm. As an example, the wavelength, frequency and intensity of the second wavelength laser pulse 13 and the other second wavelength laser pulse 15 acting on the control bit atom 10 are exactly the same. As another example, the wavelength, frequency and intensity of the first wavelength laser pulse 12 and the other first wavelength laser pulse 14 acting on the control bit atom 10 are exactly the same. As another example, the wavelength, frequency and intensity of the other Raman laser pulse 19 acting on the target bit atom 11 can be exactly the same as the first Raman laser pulse 16 acting on the control bit atom 10. According to the design, the above-mentioned second wavelength laser pulse 17 and the first wavelength laser pulse 18 will act on the target bit atom simultaneously, and the action time is 2π.

[0088] The combination of the above-mentioned second wavelength laser pulse 17 and the first wavelength laser pulse 18 is used to realize the target bit atom 11 to be excited from the first ground state to the Rydberg state and then return to the first ground state. The role of another Raman laser pulse 19 is to realize the flipping of the target bit atom 11 between the second ground state |0> and the first ground state |1>, thereby realizing a single-bit gate. It will be understood that the single-bit gate combined with the quantum bit operation based on the Rydberg effect can realize other quantum bit gates or realize the quantum entanglement between the control bit atom and the target bit atom.

[0089] Fig. 9 (b) in the figure shows the relevant timing pulse diagram. For the convenience of illustration and description, the subscript r in the figure represents a laser pulse with a wavelength of 780nm, the subscript b represents a laser pulse with a wavelength of 480nm, the subscript Ram represents a Raman laser pulse with a wavelength of 795nm used for ground state manipulation, and at the same time, the subscript c represents the control bit atom, the subscript t represents the target bit atom, the subscript c1 represents the first group of π / 2 pulses in the first π pulse of the two previous and subsequent π pulses or the other first group of π / 2 pulses in the second π pulse, and the subscript c2 represents the second group of π / 2 pulses in the first π pulse of the two previous and subsequent π pulses or the other second group of π / 2 pulses in the second π pulse. According to the above marking method, the above-mentioned first wavelength laser pulse 12 can be recorded as Ω r_c1 , the second wavelength laser pulse 13 can be recorded as Ω b_c1 , the above-mentioned other first wavelength laser pulse 14 can be recorded as Ω r_c2 , the above-mentioned other second wavelength laser pulse 15 can be recorded as Ω b_c2 , the above Raman laser pulse 16 is recorded as Ω Ram_c The above-mentioned second wavelength laser pulse 17 can be recorded as Ω b_t The above-mentioned second wavelength laser pulse 18 can be recorded as Ω r_t , the other Raman laser pulse 19 can be recorded as Ω Ram_t .

[0090] In some embodiments, the duration of the above-mentioned π / 2 pulse, π pulse or 2π pulse can be calculated according to the Rabi frequency of the two-photon process.

[0091] As an example, the single-photon Rabi frequency of all first wavelength laser pulses with 780 nm is Ω 780 =Ω r_c1 =Ω r_c2 =Ω r_t = 60 MHz, the single-photon Rabi frequency of all second wavelength laser pulses with 480 nm is Ω 480 =Ω b_c1 =Ω b_c2 =Ω b_t=40MHz, all the first wavelength laser pulses with 780nm and the intermediate energy level |5P 3 / 2 > Detuning Δ = 600MHz, then the two-photon Rabi frequency Ω 0 =Ω 480 Ω 780 / 2Δ=2MHz,π pulse time t π =1 / Ω 0 =0.5μs, π / 2 pulse time is t π / 2 =0.25μs. At the same time, consider selecting the intensity of the Raman laser pulse with 795nm and |5P 1 / 2 >The detuning amount δ of the state can make the Rabi frequency of the ground state transition Ω Ram = 2MHz, then there is a π pulse time t for the ground state π =1 / Ω Ram =0.5μs, π / 2 pulse time is t π / 2 =0.25μs.

[0092] The following will refer to Fig. 9 The timing pulse diagram of (b) in FIG. 1 is used to describe the quantum bit gate constructed according to the first exemplary embodiment of the present disclosure, and the quantum entangled state is realized based on the exemplary constructed quantum bit gate. The manipulation process of quantum bits.

[0093] It should be noted here that the pulses included in mark 60 (or the gray box) construct a two-bit controlled NOT gate (or CNOT gate), the pulses included in the dashed box indicated by mark 70 construct a two-bit controlled Z gate (or CZ gate), the pulses included in the dashed box indicated by mark 80 represent the pre-qubit operation before performing the quantum bit gate or quantum bit entanglement operation, and the pulses included in the dashed box indicated by mark 90 represent the post-qubit operation after performing the quantum bit gate or quantum bit entanglement operation.

[0094] According to this example embodiment of the present disclosure, the qubit operation process may first start with the pre-qubit operation indicated by label 80. In some embodiments, the pre-qubit operation may be performed so that the initial states of the control bit atom 10 and the target bit atom 11 are both in the first ground state, i.e., the |1> state, which may be achieved, for example, by an optical pumping process. In this way, those skilled in the art may confirm the initial state of the two qubit atoms. Note that the above-mentioned qubit operation for achieving the initial state is not shown in the above-mentioned label 80.

[0095] The first exemplary embodiment aims to realize an exemplary quantum entangled state through the constructed quantum bit gate As a preparatory step to realize the above quantum entangled state, Figure 8 As shown in (b) of FIG. 1 , the above-mentioned pre-qubit operation may also include: first, applying a Raman laser pulse 16 (i.e., Raman light) of π / 2 (i.e., 0.25 μs) to the control bit atom 10, thereby preparing the control bit atom 10 to a superposition state At the same time, another Raman laser pulse 19 of π (i.e., 0.5 μs) is applied to the target bit atom 11 to prepare the target bit atom 11 to the second ground state |0 t >, then the quantum states of the two atoms can be expressed as Then, we can make the above quantum state The above quantum entangled state is realized by the following two-bit controlled NOT gate indicated by the mark 60:

[0096] Specifically, the operation of the two-bit controlled NOT gate indicated by the mark 60 may include:

[0097] First, another Raman laser pulse 19 of π / 2 (i.e., 0.25 μs) is applied to the target bit atom 11 to implement a Hadamard gate (Hadamard gate or H gate) operation;

[0098] Next, a first π pulse consisting of a first group of π / 2 pulses and a second group of π / 2 pulses is applied to the control bit atom 10, that is, a first wavelength laser pulse 12 and a second wavelength laser pulse 13 of π / 2 (i.e., 0.25 μs) are simultaneously applied to the control bit atom 10, and then another first wavelength laser pulse 14 and another second wavelength laser pulse 15 of another π / 2 (i.e., 0.25 μs) are simultaneously applied to the control bit atom 10;

[0099] Next, a second wavelength laser pulse 17 of 2π (i.e., 1 μs) and a first wavelength laser pulse 18 are simultaneously applied to the target bit atom 11, i.e., a 2π pulse is applied to the target bit atom 11;

[0100] Next, a second π pulse consisting of another first group of π / 2 pulses and another second group of π / 2 pulses is applied to the control bit atom 10, that is, a first wavelength laser pulse 12 and a second wavelength laser pulse 13 of π / 2 (i.e., 0.25 μs) are applied to the control bit atom 10 at the same time, and then another first wavelength laser pulse 14 and another second wavelength laser pulse 15 of another π / 2 (i.e., 0.25 μs) are applied to the control bit atom 10 at the same time;

[0101] Finally, another Raman laser pulse 19 of π / 2 (ie, 0.25 μs) is applied to the target bit atom 11 again, that is, a Hadamard gate (Hadamard gate or H gate) is applied again.

[0102] It should be understood that the function of the above-mentioned controlled NOT gate (or CNOT gate) is to control the bit atom to be in the second ground state |0 c >, the state of the target bit atom does not change, and when the control bit atom is in the first ground state |1 c >, the state of the target bit atom is flipped, that is, |0 t >→|1 t > or |1 t >→|0 t >. Therefore, the quantum state of the two atoms After passing through the CNOT gate, it becomes a two-atom entangled state.

[0103] In some embodiments, the qubit operation process of the present disclosure may also include: as indicated by mark 90, a post-qubit operation after performing the qubit gate or qubit entanglement operation. For example, in order to achieve the measurement of the above-mentioned two-qubit entangled state, the above-mentioned post-bit processing operation may include: determining the fidelity of the entangled state by measuring the parity signal of the entangled state of the control bit atom and the target bit atom. Specifically, for example, by simultaneously and separately applying a π / 2 (i.e., 0.25μs) Raman laser pulse 16 and another Raman laser pulse (i.e., Raman light) 19 to the control bit atom 10 and the target bit atom 11, the phases of the two pulses can be changed, thereby measuring the parity signal of the entangled state and determining the fidelity of the above-mentioned entangled state.

[0104] The following is an analysis and verification of the fidelity of the CNOT gate or quantum entangled state obtained according to the first exemplary embodiment described above.

[0105] In order to illustrate the advantages of the first exemplary embodiment of the present disclosure, Figure 5 and Figure 6 The conventional scheme and the scheme of the first exemplary embodiment are shown for numerical comparison.

[0106] Assume that the atomic temperature T = 5μK. When other decoherence factors are not considered, the decoherence time from the atomic ground state to the Rydberg state is considered with the two-photon emission scheme of 780nm and 480nm. Where m is the atomic mass of rubidium 87, k B is the Boltzmann constant, T is the atomic temperature, k eff =2π(1 / 780nm-1 / 480nm) is the effective wave vector during two-photon excitation.

[0107] For the conventional two-photon irradiation scheme, when the control bit atom is excited to the Rydberg state, the control bit atom will accumulate an additional phase δ=δ(2t π,t +2t gap ), there are already public documents proving that the dephasing time of the control bit caused by this is exactly the decoherence time from the atomic ground state to the Rydberg state. Therefore, the fidelity of this scheme depends on the interval between the two π pulses applied to the control bit, that is, the fidelity F = exp(-t gap / T 2 ). Under the parameters set here, t gap It mainly depends on the length of the 2π pulse and the rise and fall time of the optical switch. Here, t 2π =1μs, the optical switch rise and fall time Δt = 50ns, then t gap =t 2π +2Δt=1.01μs, so F=0.917.

[0108] Different from the conventional technical solutions mentioned above, the solution of the first exemplary embodiment of the present disclosure uses two sets of excitation pulses with completely opposite directions to control the bit atom, so the effective detuning amount in the process is Therefore, in this process, the decoherence of the control bit atom caused by the Doppler effect completely disappears. At the same time, since in this scheme, the two π pulses applied to the control bit atom are decomposed into two π / 2 pulses, these two processes are actually a Ramsey process, that is, in this process, the control bit atom is subjected to the decoherence time T from the ground state to the Rydberg state. 2 Since the minimum interval between two pulses is limited by the optical switching time Δt = 50ns, the fidelity F in this scheme is F = exp(-Δt / T 2 ) 2 =0.992, that is, the fidelity of this scheme is improved by about 8% compared with the conventional scheme.

[0109] The first exemplary embodiment is described above mainly with reference to the two-photon beaming scheme of 780nm and 480nm. It should be understood that two-photon beaming schemes with other wavelengths are also possible. The second exemplary embodiment of the present disclosure will be described below with reference to the two-photon beaming scheme of 420nm and 1013nm.

[0110] Fig.10 A schematic diagram of energy levels of two-photon excitation of rubidium 87 atoms to Rydberg states according to a second exemplary embodiment of the present disclosure is shown.

[0111] like Fig.10As shown, the quantum bit of the second exemplary embodiment is still defined on the ground state hyperfine level of the rubidium 87 atom, wherein the second ground state |0>=|5S 1 / 2 ,F=1,m F =0>, first ground state |1>=|5S 1 / 2 ,F=2,m F =0>. The flip between the second ground state |0> and the first ground state |1> is still achieved through stimulated Raman transition, that is, for example, the transition between the second ground state |0> and the first ground state |1> is achieved by two laser beams with a wavelength of about 795nm and a frequency difference of 6834.683MHz, wherein the above two laser beams can be designed to be in contact with the excited state |5P 1 / 2 > Detuning 50-100 GHz. In experiments, for simplicity, the above two laser beams can be combined into one laser beam, which is called Raman laser pulse.

[0112] It should be understood that the interaction between the control bit atom and the target bit atom is achieved through Rydberg state interaction. In this example embodiment, when the control bit atom is excited to the Rydberg state, the control bit atom can be experimentally realized by a two-photon transition with a first wavelength of 420nm and a second wavelength of 1013nm from the first ground state |1> state to the intermediate energy level |6P 3 / 2 >Excited to Rydberg state|79D 5 / 2 >.

[0113] It should be noted here that usually the above-mentioned first wavelength of 420nm laser pulse will be with the intermediate energy level |6P 3 / 2 > Detune from a few hundred MHz to a few GHz to avoid spontaneous emission from intermediate energy levels.

[0114] Fig.11 An optical structure diagram and a pulse timing diagram according to a second exemplary embodiment of the present disclosure are shown, wherein (a) shows an optical structure diagram, and (b) shows a related pulse timing diagram. It can be understood that the optical structure and pulse sequence of the second exemplary embodiment are almost the same as those of the first exemplary embodiment, and the only difference is that the laser pulse with a first wavelength of 780nm in the first exemplary embodiment is replaced by a laser pulse with a first wavelength of 420nm here, and the laser pulse with a second wavelength of 480nm in the first exemplary embodiment is replaced by a laser pulse with a second wavelength of 1013nm here. Therefore, with respect to Fig.11 The optical structure diagram and pulse timing are shown in detail and will not be described in detail. Fig.11 The reference numerals in FIG. 1 are used except for the control target atom 20 and the target bit atom 21. Fig. 9 The reference numerals in .

[0115] In addition, the relevant parameters in the second exemplary embodiment are also set to be the same as those in the first exemplary embodiment, namely: Ω 420nm =Ω b_c1 =Ω b_c2 =Ω b_t =60MHz,Ω 1013nm =Ω r_c1 =Ω r_c2 =Ω r_t =40MHz, the first laser pulse with a wavelength of 420nm is designed to be detuned from the intermediate energy level by 600MHz, then the Rabi frequency of the two photons is 0 =2MHz, π pulse time t π =0.5μs, π / 2 pulse time is t π / 2 = 0.25μs. In addition, the Rabi frequency of the ground state transition is set to Ω 0 =2MHz, then the ground state π pulse time t π =0.5μs, π / 2 pulse time is t π / 2 = 0.25μs. At the same time, the temperature of the atom is set to T = 5μK, then the decoherence time from the atomic ground state to the Rydberg state is Where m is the atomic mass of rubidium 87, k is B is the Boltzmann constant, T is the atomic temperature, k eff =2π(1 / 1013nm-1 / 420nm) is the effective wave vector during two-photon excitation.

[0116] Based on the fidelity analysis similar to that of the first exemplary embodiment described above, the fidelity of the conventional solution can be calculated as F = exp(-t gap / T 2 )=0.861, and the fidelity of the solution of the second exemplary embodiment of the present disclosure is F=exp(-Δt / T 2 ) 2 =0.997. Therefore, compared with the conventional solution, the fidelity of the solution of the second exemplary embodiment of the present disclosure is improved by 13%.

[0117] The exemplary embodiments of the present disclosure are described above mainly with reference to the two-photon process. As mentioned above, the concepts of the present disclosure can also be implemented using the single-photon process. Fig.12 and Fig.13 A third exemplary embodiment of the present disclosure implemented using a single photon process is described.

[0118] Fig.12 A schematic diagram showing the energy levels of a cesium atom excited to a Rydberg state by a single photon according to a third exemplary embodiment of the present disclosure is shown.

[0119] like Fig.12As shown, similar to the first and second exemplary embodiments, the quantum bit of the third exemplary embodiment can be defined on the ground state hyperfine level of the cesium 137 atom, wherein the second ground state |0>=|6S 1 / 2 ,F=3,m F =0>, first ground state |1>=|6S 1 / 2 ,F=4,m F =0>. The flip between the second ground state |0> and the first ground state |1> is achieved by stimulated Raman transition. For example, the transition between the second ground state |0> and the first ground state |1> is achieved by two laser beams with a frequency difference of 9192.632MHz and a wavelength of about 459nm, where the two laser beams are excited to the excited state |6P 1 / 2 > The detuning is about tens of GHz. Usually, in experiments, for simplicity, two laser beams can be combined into one laser beam, which is recorded as a Raman laser pulse.

[0120] It should be understood that the interaction between the control bit atom and the target bit atom is achieved through Rydberg state interaction. In this example embodiment, when the control bit atom is excited to the Rydberg state, the atom can be excited from the first ground state |1> to the Rydberg state |70P by a single photon transition with a wavelength of 319nm. 3 / 2 >.

[0121] Fig.13 An optical structure diagram and a pulse timing diagram according to a third exemplary embodiment of the present disclosure are shown.

[0122] First, see Fig.13 (a) in FIG. 1 shows the optical structure diagram of the pulse irradiation control bit atom and the target bit atom. Fig.13 As shown in (a) in FIG. 1 , the control bit atom 30 and the target bit atom 31 are respectively trapped in two independent optical traps (not shown). As an example of quantum bit manipulation of the control bit atom 10, the laser irradiated on the control bit atom 30 may include three laser beams, including: a first wavelength laser pulse 32 with a wavelength of 319nm, another first wavelength laser pulse 33 with a wavelength of 319nm, and a Raman laser pulse 34 with a wavelength of 459nm, wherein the first wavelength laser pulse 32 and the other first wavelength laser pulse 33 have the same wavelength, frequency and intensity, but the two act on the control bit atom 30 in opposite directions. .

[0123] According to the design, the first wavelength laser pulse 32 and another first wavelength laser pulse 33 can be successively irradiated onto the control bit atom 30, and their action time is π / 2. In this case, the first wavelength laser pulse 32 can be regarded as the first group of π / 2 pulses, and the other first wavelength laser pulse 33 can be regarded as the second group of π / 2 pulses. It should be understood that the combination of the above-mentioned first wavelength laser pulse 32 and the other first wavelength laser pulse 33 can thus constitute any one of the two π pulses mentioned above, that is, the first π pulse or the second π pulse. The π pulse formed by the combination of the first wavelength laser pulse 32 and the other first wavelength laser pulse 33 can realize the excitation of the control bit atom 30 from the first ground state |1> state to the Rydberg state |70P 3 / 2 >. The Raman laser pulse 34 can be used to control the flipping of the bit atom 30 between the first ground state |1> and the second ground state |0> to realize a single-bit gate. It will be understood that the above-mentioned single-bit gate combined with the quantum bit operation based on the Rydberg effect can realize other quantum bit gates or realize quantum entanglement between the control bit atom and the target bit atom.

[0124] As an example of quantum bit manipulation of the target bit atom 11, the laser irradiated onto the target bit atom 31 may include three laser pulses, including another first wavelength laser pulse 35 with a wavelength of 319 nm and another Raman laser pulse 36 with a wavelength of 459 nm. Another first wavelength laser pulse 35 may have the same wavelength, frequency and intensity as the first wavelength laser pulse 32 and another first wavelength laser pulse 33 acting on the control bit atom 30. Another Raman laser pulse 36 may have the same wavelength, frequency and intensity as the Raman laser pulse 34 acting on the control bit atom 30. According to the design, the action time of another first wavelength laser pulse 35 on the target bit atom 30 is 2π.

[0125] The function of the first wavelength laser pulse 35 is to realize the excitation of the target bit atom 31 from the first ground state to the Rydberg state and then return to the first ground state, and the function of the other Raman laser pulse 36 is to realize the flipping of the target bit atom 31 between the first ground state |1> and the second ground state |0>, thereby realizing a single-bit gate. The single-bit gate can realize other quantum bit gates or realize quantum entanglement between the control bit atom and the target bit atom in combination with the quantum bit operation based on the Rydberg effect.

[0126] Fig.13(b) in FIG. 1 shows the related timing pulse diagram. For the convenience of illustration and description, similar to the first and second exemplary embodiments, here the subscript r represents a laser pulse with a wavelength of 319 nm, the subscript Ram represents a laser pulse of 419 nm for ground state manipulation, and at the same time, the subscript c represents a control bit atom, the subscript t represents a target bit atom, the subscript c1 represents a first group of π / 2 pulses in the first π pulse or another first group of π / 2 pulses in the second π pulse, and the subscript c2 represents a second group of π / 2 pulses in the first π pulse or another second group of π / 2 pulses in the second π pulse. According to the above marking method, the first wavelength laser pulse 32 is denoted as Ω r_c1 , another first wavelength laser pulse 33 is denoted as Ω r_c2 , Raman laser pulse 34 is recorded as Ω Ram_c , and the first wavelength laser pulse 35 is denoted as Ω r_t , and the other Raman laser pulse 36 is denoted as Ω Ram_t .

[0127] In some embodiments, the duration of the above-mentioned π / 2 pulse, π pulse or 2π pulse can be calculated according to the Rabi frequency of the single-photon process.

[0128] As an example, the single-photon Rabi frequency Ω of a first wavelength laser pulse of 319 nm is 319 =Ω 0 =2MHz, then the π pulse time t π =0.5μs, π / 2 pulse time is t π / 2 = 0.25μs. In addition, the Rabi frequency of the ground state transition is set to Ω Ram = 2MHz, then the π pulse time for the ground state is t π =0.5μs, π / 2 pulse time is t π / 2 =0.25μs.

[0129] The following will refer to Fig.13 The timing pulse diagram of (b) in FIG. 1 is used to describe the quantum bit gate constructed according to the first exemplary embodiment of the present disclosure, and the quantum entangled state is realized based on the exemplary constructed quantum bit gate. The quantum bit operation process.

[0130] Similar to the previous first and second example embodiments, the pulses included in the mark 60 (or the gray box) construct a two-bit controlled NOT gate (or CNOT gate), the pulses included in the dashed box indicated by the mark 70 construct a two-bit controlled Z gate (or CZ gate), the pulses included in the dashed box indicated by the mark 80 represent the pre-qubit operation before performing the qubit gate or qubit entanglement operation, and the pulses included in the dashed box indicated by the mark 90 represent the post-qubit operation after performing the qubit gate or qubit entanglement operation.

[0131] According to this example embodiment of the present disclosure, the qubit operation process may first start with the pre-qubit operation indicated by the mark 80. In some embodiments, the pre-qubit operation may be performed so that the initial states of the control bit atom 30 and the target bit atom 31 are both in the first ground state, i.e., the |1> state, which may be achieved, for example, by an optical pumping process. In this way, those skilled in the art may confirm the initial state of the two qubit atoms. Note that the above-mentioned qubit operation for achieving the initial state is not shown in the above-mentioned mark 80.

[0132] The third exemplary embodiment aims to realize the relevant quantum entangled state by constructing a quantum bit gate As a preparatory step to realize the above quantum entangled state, Fig.13 As shown in (b) of FIG. 1 , the above-mentioned pre-qubit operation may also include: first, applying a Raman laser pulse 34 of π / 2 (i.e., 0.25 μs) to the control bit atom 30, which can prepare the control bit atom 30 to a superposition state. At the same time, another Raman laser pulse 36 of π (ie, 0.5 μs) is applied to the target bit atom 31 to prepare the target bit atom 11 to the second ground state |0 t >, then the quantum states of the two atoms can be expressed as Then, we can make the above quantum state The above quantum entangled state is realized by the following two-bit controlled NOT gate indicated by the mark 60:

[0133] Specifically, the operation of the two-bit controlled NOT gate indicated by the mark 60 may include:

[0134] First, a Raman laser pulse 36 of π / 2 (i.e., 0.25 us) is applied to the target bit atom 31 to implement a Hadamard gate (Hadamard gate or H gate) operation;

[0135] Next, the first π pulse composed of the first group of π / 2 pulses and the second group of π / 2 pulses is applied to the control bit atom 30, that is, a first wavelength laser pulse 32 of π / 2 (i.e., 0.25 μs) and another first wavelength laser pulse 33 of another π / 2 (i.e., 0.25 μs) are applied to the control bit atom 30, and the first wavelength laser pulse 32 and the other first wavelength laser pulse 33 act in opposite directions;

[0136] Next, a 2π (i.e., 1 μs) first wavelength laser pulse 35 is applied to the target bit atom 31, i.e., a 2π pulse is applied to the target bit atom 11;

[0137] Next, a second π pulse consisting of another first group of π / 2 pulses and another second group of π / 2 pulses is applied to the control bit atom 30, that is, a first wavelength laser pulse 32 of π / 2 (i.e., 0.25 μs) and another first wavelength laser pulse 33 of another π / 2 (i.e., 0.25 μs) are applied to the control bit atom 30, and the first wavelength laser pulse 32 and the other first wavelength laser pulse 33 act in opposite directions;

[0138] Finally, another Raman laser pulse 36 of π / 2 is applied to the target bit atom 31 again, that is, a Hadamard gate (Hadamard gate or H gate) is applied again.

[0139] It should be understood that the function of the controlled NOT gate or CNOT gate is to control the bit atom to be in the second ground state |0 c >, the state of the target bit atom does not change, and when the control bit atom is in the first ground state |1 c >, the state of the target bit atom is flipped, that is, |0 t >→|1 t > or |1 t >→|0 t >. Therefore, the quantum state of the two atoms After passing through the CNOT gate, it becomes a two-atom entangled state.

[0140] In some embodiments, the qubit operation process of the present disclosure may also include: as indicated by mark 90, a post-qubit operation after performing the qubit gate or qubit entanglement operation. For example, in order to achieve the measurement of the above-mentioned two-qubit entangled state, the above-mentioned post-bit processing operation may include: determining the fidelity of the entangled state by measuring the parity signal of the entangled state of the control bit atom and the target bit atom. For example, a π / 2 (i.e., 0.25μs) Raman laser pulse 34 and another Raman laser pulse 36 can be applied to the control bit atom 30 and the target bit atom 31 simultaneously and respectively to change the phases of the two laser pulses, thereby measuring the parity signal of the entangled state, thereby determining the fidelity of the above-mentioned entangled state.

[0141] The following is an analysis and verification of the fidelity of the CNOT gate or quantum entangled state obtained according to the third exemplary embodiment described above.

[0142] In order to illustrate the advantages of the third exemplary embodiment of the present disclosure, Figure 5 and Figure 6 Numerical comparison is made between the conventional scheme and the scheme of the third exemplary embodiment shown.

[0143] Assume that the atomic temperature T = 5μK. When other decoherence factors are not considered, the decoherence time from the atomic ground state to the Rydberg state is considered at 319nm. Where m is the atomic mass of cesium 137, k is B is the Boltzmann constant, T is the atomic temperature, k eff =2π / 319nm is the effective wave vector during single-photon excitation.

[0144] For the conventional single-photon excitation scheme, after the control bit atom is excited to the Rydberg state, the control bit atom will accumulate an additional phase φ=δ(2t π,t +2t gap ), which causes the decoherence time of the control bit atom to be exactly the decoherence time from the atomic ground state to the Rydberg state. Therefore, the fidelity of this scheme depends on the interval between the two π pulses applied to the control bit atom, that is, the fidelity F = exp(-t gap / T 2 ). Under the parameters set here, t gap It mainly depends on the length of the 2π pulse and the rise and fall time of the optical switch. Here, t 2π =1μs, the optical switch rise and fall time Δt = 50ns, then t gap =t 2π +2Δt=1.01μs, so F=0.78.

[0145] Different from the conventional technical solutions mentioned above, the solution of the third exemplary embodiment of the present disclosure uses two sets of excitation pulses with completely opposite directions to control the bit atom, so the effective detuning amount in the process is Therefore, in this process, the decoherence of the control bit atom caused by the Doppler effect completely disappears. At the same time, since in this scheme, the two π pulses applied to the control bit atom are decomposed into two π / 2 pulses, these two processes are actually a Ramsey process, that is, in this process, the control bit atom is subjected to the decoherence time T from the ground state to the Rydberg state. 2 In addition, since the minimum interval between two pulses is limited by the optical switching time Δt = 50ns, the fidelity F in this scheme is F = exp(-Δt / T 2 ) 2 =0.995, that is, the fidelity of this scheme is improved by about 22% compared with the conventional scheme.

[0146] The above has described in detail the method for manipulating quantum bits according to the present disclosure and its specific embodiments. It should be understood that the scheme of the present disclosure is not limited to the above method, but may also relate to a computing device, which may include a quantum bit gate implemented according to the aforementioned method. In particular, the quantum bit gate includes but is not limited to a CZ gate and a CNOT gate. The computing device may be a quantum processor or a quantum computer or a quantum circuit. In addition, the scheme of the present disclosure may also relate to a quantum system, which may be configured to execute the aforementioned method.

[0147] Although the present invention has been described and illustrated in detail in the drawings and the foregoing description, these descriptions and illustrations should be considered illustrative or exemplary rather than restrictive; the present invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments can be understood and practiced by those skilled in the art in practicing the claimed invention by studying the drawings, the disclosure, and the appended claims.

[0148] In addition, it should be understood that the methods, steps or processes described above are merely examples. Although the steps of the method are described in a specific order in the specification, this does not require or imply that these operations must be performed in this specific order, or that all the operations shown must be performed to achieve the desired result. On the contrary, the steps described above may be changed in the order of execution if the context is not contradictory. Additionally or alternatively, some steps may be omitted, multiple steps may be combined into one step, and / or one step may be decomposed into multiple steps.

[0149] In the claims, the word "comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfil the functions of a plurality of items set out in the claims. The mere fact that certain features are recited in mutually different embodiments or dependent claims does not mean that a combination of these features cannot be used to advantage. The scope of protection of the present application covers any possible combination of the various features recited in the various embodiments or dependent claims without departing from the spirit and scope of the present application.

[0150] Any reference signs in the claims should not be construed as limiting the scope of the disclosure.

Claims

1. A method for manipulating a quantum bit, characterized in that: include: Obtaining control bit atoms and target bit atoms; as well as Using a single photon process or a multi-photon process, a quantum bit gate or a quantum bit entanglement operation based on the Rydberg blockade effect between the control bit atom and the target bit atom is performed, wherein the quantum bit gate or the quantum bit entanglement operation includes: Step a), applying a π pulse consisting of a first group of π / 2 pulses and a second group of π / 2 pulses successively to the control bit atom, wherein the first group of π / 2 pulses and the second group of π / 2 pulses have the same wavelength and are applied to the control bit atom in opposite directions, wherein the π pulse is defined as a pulse whose frequency is resonant with the transition of the control bit atom from the first ground state to the Rydberg state and whose energy allows the control bit atom to completely transition from the first ground state to the Rydberg state within the pulse duration.

2. The control method according to claim 1, characterized in that: The qubit gate or qubit entanglement operation also includes: Step b), after applying the one π pulse to the control bit atom, applying a 2π pulse to the target bit atom, The 2π pulse is defined as a pulse whose energy allows the target bit atom to transition from the first ground state to the Rydberg state and then back to the first ground state.

3. The control method according to claim 2, characterized in that: The qubit gate or qubit entanglement operation also includes: Step c), after applying the 2π pulse to the target bit atom, applying another π pulse consisting of another first group of π / 2 pulses and another second group of π / 2 pulses to the control bit atom; wherein the another first group of π / 2 pulses and the another second group of π / 2 pulses have the same wavelength and are applied to the control bit atom in opposite directions; The above steps a), b) and c) construct a two-bit CZ gate.

4. The control method according to claim 3, characterized in that: When utilizing the single photon process, the first group of π / 2 pulses, the second group of π / 2 pulses, the another first group of π / 2 pulses and the another second group of π / 2 pulses are all realized by a single laser pulse of the same wavelength.

5. The control method according to claim 3, characterized in that: When the multi-photon process is a two-photon process, the first group of π / 2 pulses, the second group of π / 2 pulses, the another first group of π / 2 pulses and the another second group of π / 2 pulses are respectively composed of a combination of a π / 2 pulse of a first wavelength and a π / 2 pulse of a second wavelength applied simultaneously to the control bit atom, the first wavelength is different from the second wavelength, and the π / 2 pulse of the first wavelength and the π / 2 pulse of the second wavelength are applied to the control bit atom in opposite directions.

6. The control method according to claim 3, characterized in that: The qubit gate or qubit entanglement operation also includes: Step d), before step a), applying a Raman pulse of π / 2 to the target bit atom, and after step c), applying another Raman pulse of π / 2 to the target bit atom, to respectively implement an H-gate operation; The above steps a), b), c) and d) construct a two-bit CNOT gate.

7. The control method according to any one of claims 1 to 6, characterized in that: Also includes: Before executing the qubit gate or qubit entanglement operation, a pre-qubit operation is performed on the control bit atom and the target bit atom, the pre-qubit operation comprising: The initial states of both the control bit atom and the target bit atom are set to the first ground state.

8. The control method according to claim 7, characterized in that: The pre-qubit operation also includes: After making the initial states of both the control bit atom and the target bit atom in the first ground state, the control bit atom is prepared to a superposition state.

9. The control method according to any one of claims 1 to 6, characterized in that: Also includes: After performing the qubit gate or qubit entanglement operation, performing a post-qubit operation on the control bit atom and the target bit atom, the post-qubit operation comprising: The fidelity of the entangled state is determined by measuring the parity signal of the entangled state of the control bit atom and the target bit atom.

10. The control method according to claim 9, characterized in that: The pre-qubit operation or the post-qubit operation further includes: By applying Raman laser pulses to the control bit atom and / or the target bit atom, the control bit atom and / or the target bit atom are switched between a corresponding first ground state and a second ground state, wherein the second ground state has an energy level lower than the corresponding first ground state.

11. The control method according to any one of claims 1 to 6, characterized in that: Also includes: The duration of the π / 2 pulse or the π pulse is calculated based on the Rabi frequency of the single-photon process or the multi-photon process.

12. The control method according to any one of claims 1 to 6, characterized in that: The control bit atoms and the target bit atoms are selected from neutral atoms.

13. A computing device comprising: A quantum bit gate implemented by the method according to any one of claims 1 to 12.

14. The computing device of claim 13, which is a quantum computer, a quantum processor, or a quantum circuit.

15. The computing device of claim 13, wherein the qubit gates include a CZ gate and a CNOT gate.

16. A quantum system configured to perform the method according to any one of claims 1-12.

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