A scalable device for fast reading and resetting superconducting qubits

Abstract

The invention discloses an expandable fast reading and resetting device for superconducting quantum bits, which relates to the field of quantum information technology. The device comprises a bandpass filter and a transmission line, a DC voltage-superconducting quantum interference device is added in the middle of the transmission line, the transmission line itself constitutes a resonant cavity, the reading cavity is coupled to the transmission line through inductance or capacitance, the quantum bit is coupled to the reading cavity through capacitance or inductance, the bandpass filter and the transmission line are connected through capacitance or inductance coupling, the vertex frequency of the transmission line is set to the reading cavity frequency, and the quantum bit frequency is lower than the reading cavity frequency; when resetting the quantum state, the frequency of the transmission line is modulated to the quantum bit frequency, so that the transmission line resonates with the quantum bit, thereby resetting the quantum bit; the fast reading and resetting device for superconducting quantum bits can be quickly integrated into various superconducting quantum chips, the leakage rate of the reading cavity is improved, and the quantum bit is quickly read.

Description

Technical Field

[0001] The present invention relates to the field of quantum information technology, and in particular to a scalable fast reading and resetting device for superconducting quantum bits. Background Art

[0002] The rapid reading of quantum bits is of great significance in experiments such as quantum error correction and quantum feedback. Superconducting quantum bits use a dispersive reading method. The quantum bits are detuned and coupled to a resonant cavity as a reading cavity. The state of the quantum bits can be known by detecting the frequency or phase of the reading cavity. In order to achieve rapid reading, it is necessary to increase the leakage rate of the reading cavity. However, due to the existence of the Purcell effect, the quantum bits are decoherent due to unnecessary coupling with the outside world through the reading cavity. Therefore, it is necessary to add a Purcell filter between the reading cavity and the external environment to protect the coherence of the quantum bits while achieving a rapid reading cavity leakage rate.

[0003] The initialization of quantum states is an important condition for the construction of quantum computers, which involves restoring the state of quantum bits to the initial state or a specific reference state for subsequent quantum operations or measurements. At present, there are many methods for resetting superconducting quantum bits, including cavity dissipation method, measurement-based feedback method, etc. The cavity-based dissipation method is to transfer the quantum state to the photon state of the coupled reading cavity, and dissipate it through the fast leakage channel of the reading cavity.

[0004] There are currently two main modes. One is that the reading cavity frequency is lower than the bit frequency. Under this condition, the bit frequency needs to be modulated to the reading cavity frequency, wait for a while, and wait for the quantum state to be transferred to the photon state of the reading cavity, and then modulate the bit frequency back to a position away from the reading cavity frequency, and then wait for the photons in the reading cavity to dissipate. This method requires precise control of the bit frequency, and the reading cavity frequency is lower than the bit frequency, so the bit is more likely to be excited during reading; the other is that the reading cavity frequency is higher than the bit frequency. Under this condition, the Raman process needs to be used to excite the bit with microwaves of a certain frequency to exchange the quantum state to the high-frequency reading cavity photon state, and then wait for the photons in the reading cavity to dissipate. This method requires strong excitation on the bit, and there may be problems such as heating and crosstalk, which is not conducive to the manipulation of multiple quantum bits.

[0005] The measurement-based feedback mechanism is that the electronic system decides whether the quantum bit needs to be reset based on the reading results. This method has extremely high requirements on reading accuracy, and the reset time depends on the reading time and line delay. Summary of the invention

[0006] Based on the technical problems existing in the background technology, the present invention proposes a scalable fast reading and resetting device for superconducting quantum bits, which can be quickly integrated into various superconducting quantum processor chips, improve the leakage rate of the reading cavity, and realize fast reading of quantum bits.

[0007] The present invention proposes an expandable fast reading and resetting device for superconducting quantum bits, comprising a bandpass filter and a transmission line, wherein a DC voltage-superconducting quantum interference device is added in the middle of the transmission line to achieve frequency adjustment of the transmission line, the transmission line itself constitutes a resonant cavity, the reading cavity is coupled to the transmission line through inductance or capacitance, and the quantum bit is coupled to the reading cavity through capacitance or inductance;

[0008] The bandpass filter and the transmission line are connected through capacitance or inductance coupling, the vertex frequency of the transmission line is set to be at the reading cavity frequency, and the quantum bit frequency is lower than the reading cavity frequency;

[0009] When resetting the quantum state, the frequency of the transmission line is modulated by applying an external magnetic flux to the DC voltage-superconducting quantum interference device. The frequency of the transmission line is modulated to the quantum bit frequency, so that the transmission line and the quantum bit resonate. That is, after the quantum bit and the resonant cavity are fully exchanged, the external magnetic flux of the DC voltage-superconducting quantum interference device is canceled, thereby resetting the quantum bit.

[0010] Furthermore, two bandpass filters are respectively arranged on both sides of the transmission direction of the transmission line, and are connected through capacitor or inductor coupling.

[0011] Furthermore, the response curve of the bandpass filter is set to be the flattest response or the Chebyshev response, and the response curve is implemented using a step impedance transformation.

[0012] Furthermore, the passband of the bandpass filter is set to the reading cavity frequency band, and the rest is the stopband frequency band.

[0013] Furthermore, the reading cavity frequency band is 6-7 GHz, and the quantum bit is located in the stop band frequency band at 4-5 GHz.

[0014] A scalable superconducting quantum bit fast reading and resetting device, characterized in that it includes a bandpass filter and a transmission line, the transmission line itself constitutes a resonant cavity, the reading cavity is coupled to the transmission line through inductance or capacitance, and the quantum bit is coupled to the reading cavity through capacitance and inductance;

[0015] The bandpass filter and the transmission line are connected via capacitive coupling, the vertex frequency of the transmission line is set at the reading cavity frequency, and the quantum bit frequency is higher than the reading cavity frequency;

[0016] When resetting the quantum state, the frequency of the quantum bit is modulated to the frequency of the transmission line, causing the transmission line and the quantum bit to resonate. That is, after the quantum bit and the resonant cavity have fully exchanged, the modulated quantum bit frequency is pulled back to the original quantum bit frequency, thereby resetting the quantum bit.

[0017] Furthermore, the reading cavity frequency band is 4-5 GHz, and the quantum bit frequency band is 6-7 GHz.

[0018] The advantages of the scalable superconducting quantum bit fast reading and resetting device provided by the invention are: two bandpass filters are set to couple the transmission line in the middle through capacitance or inductance to form a resonant mode, and by adding an adjustable DC voltage-superconducting quantum interference device to the transmission line, the frequency of the transmission line can be modulated. By designing the vertex frequency of the transmission line at the reading cavity frequency, not only fast and high-fidelity reading can be achieved, but also the bit can be reset through the resonant mode of the transmission line. Regardless of whether the quantum bit frequency is higher or lower than the transmission line frequency, the quantum bit can be reset. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a structural schematic diagram of the present invention;

[0020] Among them, 1-bandpass filter, 2-transmission line, 3-reading cavity, 4-qubit, 5-DC voltage-superconducting quantum interference device. DETAILED DESCRIPTION

[0021] Below, the technical solution of the present invention is described in detail through specific embodiments. Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific implementation disclosed below.

[0022] like Figure 1 As shown, the present invention proposes an expandable superconducting quantum bit fast reading and resetting device, including a bandpass filter 1 and a transmission line 2, a DC voltage-superconducting quantum interference device 5 is added in the middle of the transmission line 2 to achieve frequency adjustment of the transmission line 2, the transmission line 2 itself constitutes a resonant cavity, the reading cavity 3 is coupled to the transmission line 2 through inductance or capacitance, the quantum bit 4 is coupled to the reading cavity 3 through capacitance and inductance, and two bandpass filters 1 are respectively arranged on both sides of the transmission direction of the transmission line 2, and are both connected through capacitance or inductance coupling;

[0023] In practical applications, the vertex frequency of transmission line 2 is set to be near the frequency of reading cavity 3. Regardless of whether the frequency of quantum bit 4 is higher or lower than the frequency of reading cavity 3, there will be a large detuning with the frequency of the transmission line. At this time, the decoherence time of the bit is not affected by the transmission line. Under this condition, the bit can be read. Since the reading cavity 3 is coupled with a bandpass filter, the leakage rate of the reading cavity 3 is faster, and fast high-fidelity reading can be achieved.

[0024] When it is necessary to reset the quantum bit 4, two situations are considered respectively, so as to adopt different modulation methods. (1) If the frequency of the quantum bit 4 is lower than the frequency of the reading cavity 3, because the vertex frequency of the transmission line 2 is designed to be near the frequency of the reading cavity 3, the frequency of the quantum bit 4 is lower than the vertex frequency of the transmission line. When resetting the quantum state, the frequency of the transmission line 2 is modulated by applying a magnetic flux to the DC voltage-superconducting quantum interference device 5 (dc SQUID), and the frequency of the transmission line 2 is modulated to be near the frequency of the quantum bit 4, so that the transmission line and the quantum bit 4 resonate, thereby realizing the resetting of the bit. After the quantum bit 4 and the resonant cavity are fully exchanged, the external magnetic flux of the DC voltage-superconducting quantum interference device 5 is cancelled. That is, in this embodiment, when the frequency of the quantum bit 4 is lower than the frequency of the reading cavity 3, when fast reading is performed, by applying a microwave pulse of a certain intensity to the reading cavity 3 for a period of time, the photons in the reading cavity 3 can quickly reach a steady state and then quickly dissipate, and the state of the quantum bit 4 is obtained by detecting the frequency or phase of the reading cavity 3. (2) If the frequency of quantum bit 4 is higher than the frequency of reading cavity 3, since the vertex frequency of the transmission line is designed to be near the frequency of reading cavity 3, at this time the vertex frequency of the transmission line is lower than the frequency of quantum bit 4. Therefore, when resetting the quantum state, the frequency of quantum bit 4 is modulated to be near the frequency of the transmission line, so that the transmission line and quantum bit 4 resonate, thereby achieving the resetting of the bit; after the quantum bit 4 and the resonant cavity have fully exchanged, the modulated frequency of quantum bit 4 is pulled back to the original frequency of quantum bit 4; that is, under the condition that the frequency of quantum bit 4 is higher than the frequency of reading cavity 3, when performing rapid reading, by applying a microwave pulse of a certain intensity to the reading cavity 3 and continuing it for a period of time, the photons in the reading cavity 3 can quickly reach a steady state and then quickly dissipate, and the state of quantum bit 4 can be obtained by detecting the frequency or phase of the reading cavity 3.

[0025] The bandpass filter 1 is a bandpass Purcell filter, and the response curve of the bandpass Purcell filter is set to the flattest response or Chebyshev response, and the implementation method is to use step impedance transformation. The bandpass Purcell filter obtained by this setting has a simple structure, is easy to design, has a compact size, is compatible with the processing technology of the quantum bit 4, and is very easy to design on the chip.

[0026] In addition, a DC voltage-superconducting quantum interference device 5 (dc SQUID) is added in the middle of the transmission line 2. This dc SQUID is composed of two Josephson junctions in parallel, which is equivalent to a Josephson junction with adjustable critical current. The critical current is regulated by an external magnetic flux. Therefore, by adding the dc SQUID, the frequency of the transmission line 2 can be adjusted.

[0027] For quantum bit 4 whose frequency is lower than reading cavity 3 frequency, the passband of bandpass filter 1 is designed to be reading cavity 3 frequency band, which is generally 6-7 GHz, and the rest is stopband frequency band, among which quantum bit 4 is in the stopband frequency band of 4-5 GHz. By increasing the order of bandpass filter 1, the stopband attenuation can be easily increased to achieve better protection for quantum bit 4. Therefore, the leakage rate of reading cavity 3 can be designed to be in the range of 10~20 MHz, so as to achieve fast reading without affecting the coherence time of quantum bit 4.

[0028] For quantum bit 4 whose frequency is higher than reading cavity 3 frequency, the passband of bandpass filter 1 is designed to be the reading cavity frequency band, which is generally 4-5 GHz, and the rest is the stopband frequency band, where reading cavity 3 is in the passband frequency band of 4-5 GHz, and quantum bit is in the stopband frequency band of 6-7 GHz. By increasing the order of bandpass filter 1, the stopband attenuation can be easily increased to achieve better protection for quantum bit 4. Therefore, the leakage rate of reading cavity 3 can be designed to be in the range of 10~20 MHz, thereby achieving fast reading without affecting the coherence time of quantum bit 4.

[0029] Bandpass filter 1 and transmission line 2 are connected through capacitive coupling. Since both sides of the capacitive coupling are open to the ground, the transmission line 2 naturally forms a wavelength resonant cavity, is the number of cycles, is the product.

[0030] This embodiment realizes fast reading of quantum bit 4 by integrating bandpass filter 1. Bandpass filter 1 adopts a step impedance transformation structure, has the advantages of simple structure, easy design, compact size, etc., is compatible with superconducting quantum chip preparation process, consumes less simulation resources, and has accurate results, etc. It can be quickly integrated into various superconducting quantum processor chips, improves the leakage rate of reading cavity 3, and realizes fast reading of quantum bit 4.

[0031] In this embodiment, two bandpass filters 1 are arranged to couple the transmission line 2 in the middle through capacitance or inductance to form a resonant mode. By adding an adjustable dc SQUID loop to the transmission line 2, the frequency of the transmission line 2 can be modulated. By designing the vertex frequency of the transmission line 2 to be at the frequency of the reading cavity 3, not only fast and high-fidelity reading can be achieved, but also the bit can be reset through the resonant mode of the transmission line 2. Regardless of whether the frequency of the quantum bit 4 is higher or lower than the transmission line frequency, the quantum bit 4 can be reset.

[0032] By designing the length of the transmission line 2 segment between the two bandpass filters 1, a frequency below the frequency band of the quantum bit 4 can be achieved. By using the reading cavity 3 in the transmission line 2 as the dissipative cavity of the quantum bit 4, the quantum state can be reset without adding other devices or other resources.

[0033] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A scalable superconducting quantum bit fast reading and resetting device, characterized in that: It comprises a bandpass filter (1) and a transmission line (2), wherein a DC voltage-superconducting quantum interference device (5) is added in the middle of the transmission line (2) to achieve frequency adjustment of the transmission line (2), the transmission line (2) itself constitutes a resonant cavity, the reading cavity (3) is coupled to the transmission line (2) through inductance or capacitance, and the quantum bit (4) is coupled to the reading cavity (3) through capacitance or inductance; The bandpass filter (1) and the transmission line (2) are connected via capacitive or inductive coupling, the vertex frequency of the transmission line (2) is set to be at the frequency of the reading cavity (3), and the frequency of the quantum bit (4) is lower than the frequency of the reading cavity (3); When resetting the quantum state, a magnetic flux is applied to the DC voltage-superconducting quantum interference device (5) to modulate the frequency of the transmission line (2), and the frequency of the transmission line (2) is modulated to the frequency of the quantum bit (4), so that the transmission line (2) and the quantum bit (4) resonate, thereby resetting the quantum bit (4).

2. The scalable superconducting quantum bit fast reading and resetting device according to claim 1, characterized in that: The two bandpass filters (1) are respectively arranged on both sides of the transmission line (2) in the transmission direction, and are both connected via capacitor or inductor coupling.

3. The scalable superconducting quantum bit fast reading and resetting device according to claim 1, characterized in that: The response curve of the bandpass filter (1) is set to be the flattest response or Chebyshev response, and the response curve is implemented using step impedance transformation.

4. The scalable superconducting quantum bit fast reading and resetting device according to claim 1, characterized in that: The passband of the bandpass filter (1) is set to the reading cavity (3) frequency band, and the rest is the stopband frequency band.

5. The scalable superconducting quantum bit fast reading and resetting device according to claim 4, characterized in that: The frequency band of the reading cavity (3) is 6-7 GHz, and the frequency band of the quantum bit (4) is 4-5 GHz.

6. A scalable superconducting quantum bit fast reading and resetting device, characterized in that: It comprises a bandpass filter (1) and a transmission line (2), wherein the transmission line (2) itself forms a resonant cavity, a reading cavity (3) is coupled to the transmission line (2) via inductance or capacitance, and a quantum bit (4) is coupled to the reading cavity (3) via capacitance or inductance; The bandpass filter (1) and the transmission line (2) are connected via capacitive coupling, the vertex frequency of the transmission line (2) is set to be at the frequency of the reading cavity (3), and the frequency of the quantum bit (4) is higher than the frequency of the reading cavity (3); When resetting the quantum state, the frequency of the quantum bit (4) is modulated to the frequency of the transmission line, so that the transmission line (2) and the quantum bit (4) resonate, thereby achieving the resetting of the quantum bit (4).

7. The scalable superconducting quantum bit fast reading and resetting device according to claim 6, characterized in that: The frequency band of the reading cavity (3) is 4-5 GHz, and the frequency band of the quantum bit (4) is 6-7 GHz.

Images