Superconducting quantum chip with double coupling bit frequency lower than bit frequency

Abstract

The invention discloses a superconductive quantum chip with double coupling bit frequency lower than bit frequency. In order to improve the expandability of a two-dimensional quantum bit coupling technology and improve the fidelity of a CZ gate, the invention provides a quantum bit coupling scheme of double coupling bits, and discloses a superconductive quantum chip design scheme of which the double coupling bit frequency is lower than the bit frequency (BBQ). Compared with a single-coupling bit BBQ scheme, the double-coupling BBQ scheme is easier to physically realize on a large-scale superconducting quantum chip. According to the technical scheme disclosed by the invention, the physical spacing of a large number of sub-bits can be pulled, so that the routing is not crowded any more, the signal crosstalk is reduced, the measurement accuracy is improved, the difficulty in realizing a minimum bypass capacitance process is overcome, and the method is easy to physically realize on a large-scale superconducting quantum chip.

Description

Technical Field

[0001] The present invention relates to superconducting quantum bit design, and in particular to a design scheme for tunable coupled superconducting quantum bits. Background Art

[0002] The coupling of qubits is the basis for realizing two-qubit gate operations. The design goal of qubit coupling is to improve gate fidelity and facilitate large-scale expansion.

[0003] The BAQ scheme was initially proposed. The BAQ scheme is a three-body system that includes a coupling bit and two quantum bits. The two quantum bits are indirectly coupled through the coupling bit, that is, virtual exchange interaction. Adjusting the frequency of the coupling bit can adjust the coupling strength between the two quantum bits. The BAQ scheme can turn off and on the coupling and solve the problem of next-nearest-neighbor coupling. However, when the frequency detuning of the two quantum bits is less than the anharmonicity and the coupling bit frequency is slightly greater than the bit frequency, the ZZ interaction has a very obvious depression. This depression is caused by the interaction between the |11|> state of the quantum bit and the |2|> state of the coupling bit. Since the CZ gate is required to pass through this depression, it causes the |2|> state to leak to the coupling bit.

[0004] Later, IBM proposed a BBQ improvement plan, so that the CZ gate does not need to pass through the depression. This plan takes a negative sign for the direct coupling strength of the two quantum bits. In order to achieve a negative direct coupling strength of the two quantum bits, the quantum bits are designed to be ungrounded, and the electrodes of the same polarity of the two quantum bits are coupled with the coupling bit respectively to obtain a positive coupling strength between the quantum bit and the coupling bit. The electrodes of the opposite polarity of the two quantum bits are coupled with a bypass capacitor to obtain a negative coupling strength of the direct coupling of the two quantum bits. This solution solves the problem of the CZ gate leaking to the |2|> state of the coupling bit, but the bypass capacitor is physically difficult to implement. The solution needs to meet the requirement that the capacitance of the bypass capacitor to the ground is extremely small, and the ground around the bypass capacitor needs to be hollowed out, which is difficult to implement in terms of technology. Summary of the invention

[0005] The purpose of the present invention is to propose a design scheme for a double-coupled BBQ superconducting quantum chip.

[0006] In order to achieve the above-mentioned purpose, the technical solution of the present invention is to disclose a superconducting quantum chip with a dual-coupling coupling bit frequency lower than the bit frequency, characterized in that it is composed of a quantum bit module, a fixed frequency coupling bit module and an adjustable frequency coupling bit module, the quantum bit module is not grounded, and the two quantum bit modules are connected through the fixed frequency coupling bit module and the adjustable frequency coupling bit module. The electrodes of the same polarity of the two quantum bit modules are respectively coupled with one of the coupling bits through a capacitor to obtain a positive coupling strength between the quantum bit and the coupling bit, and the electrodes of the opposite polarity of the two quantum bit modules are respectively coupled with the other coupling bit through a capacitor to obtain a negative coupling strength between the quantum bit and the coupling bit.

[0007] Preferably, the quantum bit module, the fixed frequency coupling bit module and the adjustable frequency coupling bit module can be combined in a two-dimensional array.

[0008] Preferably, the fixed frequency coupling bit module is composed of a ground capacitor and a Josephson junction.

[0009] Preferably, the adjustable frequency coupling bit module is composed of a ground capacitor and a superconducting quantum interference device.

[0010] Preferably, the capacitance to ground of the fixed frequency coupling bit module and the capacitance to ground of the adjustable frequency coupling bit module are of the same design.

[0011] In order to improve the scalability of two-dimensional quantum bit coupling technology and improve the fidelity of CZ gates, the present invention proposes a quantum bit coupling scheme of dual-coupled bits, and discloses a superconducting quantum chip design scheme in which the dual-coupled coupling bit frequency is lower than the bit frequency (BBQ). The quantum bit is Transmon. The quantum chip adopts BBQ design, which can avoid leakage to the high-energy state of the coupled bit compared to the coupling bit frequency higher than the bit frequency (BAQ). Dual coupling means that there are two coupled bits, replacing the traditional single coupled bit, one of which has an adjustable coupling bit frequency and the other has a fixed coupling bit frequency, turning the three-body system into a four-body system, and turning the three-body system into a four-body system. The dual-coupled BBQ scheme is easier to physically implement on a large-scale superconducting quantum chip than the single-coupled bit BBQ scheme. The technical solution disclosed in the present invention can increase the physical spacing of quantum bits, so that the routing is no longer crowded, reduce signal crosstalk, and improve measurement accuracy. The present invention overcomes the difficulties in realizing the extremely small bypass capacitor process and is easy to physically implement on a large-scale superconducting quantum chip. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 The design of a 2-qubit superconducting qubit is shown;

[0013] Figure 2 The 2D array design of 6 superconducting qubits is shown;

[0014] Figure 3 This is the unit circuit diagram. DETAILED DESCRIPTION

[0015] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0016] like Figure 1 As shown, the invention discloses a superconducting quantum chip with a dual-coupled coupling bit frequency lower than the bit frequency, including a quantum bit module, a fixed frequency coupling bit module and an adjustable frequency coupling bit module. Extended to multi-bit quantum bit control, it can be composed of a combination of a quantum bit module, a fixed frequency coupling bit module and an adjustable frequency coupling bit module, which can be arranged in a two-dimensional array.

[0017] The quantum bit module is composed of electrodes 4, 6, 9, 10, Josephson junctions 5, 11, quantum signal XY controls 1, 3, and quantum signal reading resonant cavities 15, 16. Quantum signal XY controls 1, 3 respectively excite the quantum bit pulses and input signals. The frequencies of quantum signal reading resonant cavities 15, 16 are different, corresponding to the corresponding quantum bit data readings.

[0018] The fixed frequency coupling bit module is composed of a ground capacitor 13 and a Josephson junction 12 .

[0019] The adjustable frequency coupling bit module is composed of a ground capacitor 8 and a superconducting quantum interference device (SQUID) 7 (ie, a Josephson junction).

[0020] The ground capacitors 8 and 13 of the fixed frequency coupling bit module and the adjustable frequency coupling bit module adopt the same design, and only differ in the design of the Josephson junction. The fixed frequency coupling bit module adopts a single junction, and the adjustable frequency coupling bit module adopts a double junction.

[0021] Electrode 4 is connected to ground capacitor 8 via a capacitor. Electrode 8 is connected to ground capacitor 9 via a capacitor. Electrode 6 is connected to ground capacitor 13 via a capacitor. Electrode 13 is connected to ground capacitor 9 via a capacitor.

[0022] Figure 1In the figure, the components corresponding to serial numbers 1 and 3 are respectively the XY control lines of the bit, and their function is to provide XY control pulse signals for the bit. The component corresponding to serial number 2 is the Z control line of the adjustable frequency coupling bit, and its function is to provide the adjustable frequency coupling bit with a Z control pulse signal of adjustable frequency. The component corresponding to serial number 18 is the quantum bit module, the component corresponding to serial number 19 is the adjustable frequency coupling bit module, and the component corresponding to serial number 20 is the fixed frequency coupling bit module. The components corresponding to serial numbers 14 and 17 are respectively the input and output ends of the read feed line, and their function is to read the quantum bit.

[0023] like Figure 2 As shown, the 6-bit superconducting quantum bit two-dimensional array design disclosed in the embodiment of the present invention includes 6 quantum bits, namely electrodes 13, 14, 20, 21, 27, 28 and 44, 45, 51, 53, 58, 59, and Josephson junctions 12, 19, 26 and 46, 52, 60. It also includes quantum signal XY control lines 2, 5, 7 and 41, 43, 66, and quantum signal reading resonant cavities 9, 10, 11 and 61, 62, 63.

[0024] The 6-bit superconducting quantum bit two-dimensional array design includes a read feed line, which includes a feed line input end 4 and a feed line output end 8. The read feed line surrounds the quantum signal reading resonant cavities 9, 10, 11, 61, 62, and 63 of the 6 quantum bits through a signal transmission line.

[0025] The 6-bit superconducting quantum bit two-dimensional array design includes 7 fixed-frequency coupling bits, namely ground capacitors 18, 25, 31, 35, 38, 47, 54, and Josephson junctions 17, 24, 32, 36, 37, 48, 55.

[0026] The 6-bit superconducting quantum bit two-dimensional array design includes 7 adjustable frequency coupling bits, namely ground capacitors 15, 23, 29, 34, 39, 49, 57, SQUIDs 16, 22, 30, 33, 40, 50, 56, and Z control lines 1, 3, 6, 65, 42, 64, 67.

[0027] The six quantum bits are connected via 10 fixed-frequency coupling bits and 10 adjustable-frequency coupling bits.

[0028] The 6-bit superconducting quantum bit two-dimensional array design uses air bridges to cross the lines at the intersections.

[0029] The 6-bit superconducting qubit two-dimensional array design can realize 6-qubit signal coupling, separate the qubit spacing by dual-coupled bits, and realize qubit control of various bit numbers through array arrangement.

[0030] like Figure 3 As shown, a circuit schematic diagram of a superconducting quantum chip with a dual-coupling coupling bit frequency lower than the bit frequency disclosed in an embodiment of the present invention includes 2 quantum bits, 1 fixed frequency coupling bit and 1 adjustable frequency coupling bit.

[0031] Figure 3 In the example, quantum bit 1 includes the capacitance C of electrode 1 and electrode 2 to ground. 11 and C 22 , the capacitance C between electrode 1 and electrode 2 12 Josephson knot Jq1 The quantum bit 2 includes the capacitance C of the electrodes 3 and 4 to the ground. 33 and C 44 , the capacitance C between electrode 3 and electrode 4 34 Josephson knot Jq2 The fixed frequency coupling bit includes the capacitance C of electrode 6 to ground. 66 Josephson knot Jc2 The adjustable frequency coupling bit includes the capacitance C of electrode 5 to ground. 55 and SQUID L Jc1 .

[0032] Figure 3 The adjustable frequency coupling bits are respectively connected through capacitors C 15 and C 35 Capacitive coupling is performed with qubit 1 and qubit 2 to form two positive coupling strengths. The fixed frequency coupling bits are respectively connected through capacitors C 16 and C 46 Capacitive coupling is performed with qubit 1 and qubit 2, forming a positive and a negative coupling strength.

Claims

1. A superconducting quantum chip with a double coupling coupling bit frequency lower than the bit frequency, characterized in that: It is composed of a quantum bit module, a fixed frequency coupling bit module and an adjustable frequency coupling bit module. The quantum bit module is not grounded. The two quantum bit modules are connected through the fixed frequency coupling bit module and the adjustable frequency coupling bit module. The electrodes of the same polarity of the two quantum bit modules are respectively coupled with one of the coupling bits through a capacitor to obtain a positive coupling strength between the quantum bit and the coupling bit. The electrodes of the opposite polarity of the two quantum bit modules are respectively coupled with the other coupling bit through a capacitor to obtain a negative coupling strength between the quantum bit and the coupling bit.

2. A superconducting quantum chip with a double coupling coupling bit frequency lower than the bit frequency as claimed in claim 1, characterized in that: The quantum bit module, the fixed frequency coupling bit module and the adjustable frequency coupling bit module can be combined in a two-dimensional array.

3. A superconducting quantum chip with a double coupling coupling bit frequency lower than the bit frequency as claimed in claim 1, characterized in that: The fixed frequency coupling bit module is composed of a ground capacitor and a Josephson structure.

4. A superconducting quantum chip with a double coupling coupling bit frequency lower than the bit frequency as claimed in claim 3, characterized in that: The adjustable frequency coupling bit module is composed of a ground capacitor and a superconducting quantum interference device.

5. A superconducting quantum chip with a double coupling coupling bit frequency lower than the bit frequency as claimed in claim 4, characterized in that: The capacitance to ground of the fixed frequency coupling bit module and the capacitance to ground of the adjustable frequency coupling bit module are of the same design.

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