CN115238901A - Capacitive Adjustable Coupling Unit - Google Patents

Abstract

The invention provides a capacitance adjustable coupling unit, comprising: the circuit comprises a first qubit, a second qubit, an adjustable coupler and a first bypass capacitor; wherein the first qubit and the adjustable coupler are each capacitively coupled to a first pole of the first bypass capacitance, a second pole of the first bypass capacitance is grounded, and the second qubit is capacitively coupled to the adjustable coupler.

Description

Capacitive Adjustable Coupling Unit

technical field

The present invention generally relates to the field of multi-qubit chips, and in particular, to a capacitively adjustable coupling unit.

Background technique

The configuration design of the capacitively tunable coupling unit plays a crucial role in the large-scale expansion of multi-bit quantum chips, which can realize the tunable and turn-off coupling between qubits, and at the same time can reduce the operation of bit entanglement gates. parasitic coupling, thereby improving the fidelity of the entanglement gate. The capacitively tunable coupling unit usually consists of three parts, a qubit Q1, a qubit Q2 and a tunable coupler Qc sandwiched between the two qubits. The coupling coefficient between the qubit Q1 and the qubit Q2 is adjusted by biasing the adjustable coupler Qc between the qubit Q1 and the qubit Q2 by applying an external magnetic flux. This coupling coefficient can be adjusted continuously from positive to negative to completely turn off or arbitrarily adjust the coupling size between qubits.

Existing capacitively adjustable coupling units include the following two designs:

(1) Qubit Q1, tunable coupler Qc, and qubit Q2 are all grounded transmon forms (see Y. Fei, et. al. 2018 PHYS. REV. APPLIED 10, 054062). The grounded qubit is composed of a non-linear inductance formed by a ground capacitance and a Josephson junction in parallel. It has a smaller volume at a specified capacitance value, and can save a lot of area on a two-dimensional chip for extensive expansion. However, the capacitance-tunable coupling unit is not easy to be applied to the fabrication of three-dimensional superconducting quantum chips based on flip-chip technology, because it is difficult to leave space for wiring, arranging resonant cavities, and the like.

(2) Introduce floating transmons as qubits or tunable couplers. This configuration not only realizes the above functions, but also realizes large-area wiring through the large size of floating qubits (see E.A.Sete, et.al. 2021 PHYSICAL REVIEW APPLIED 16, 024050). However, the increase in the size of floating qubits brings about a reduction in the decoherence time, which will severely restrict the realization of quantum computers.

SUMMARY OF THE INVENTION

Based on the above problems of the prior art, the present invention provides a capacitively adjustable coupling unit, which includes: a first quantum bit, a second quantum bit, an adjustable coupler and a first bypass capacitor;

The first qubit and the adjustable coupler are both capacitively coupled to the first pole of the first bypass capacitor, the second pole of the first bypass capacitor is grounded, and the second quantum A bit capacitively coupled to the tunable coupler.

In one embodiment, the capacitance-adjustable coupling unit further includes a second bypass capacitor, which includes:

a first pole capacitively coupled to the second qubit, the tunable coupler and the first pole of the first bypass capacitor; and

The second pole, which is connected to ground.

In one embodiment, the capacitance-adjustable coupling unit further includes a second bypass capacitor, which includes:

a first pole capacitively coupled to the second qubit and the first pole of the first bypass capacitor; and

The second pole, which is connected to ground.

In one embodiment, the second qubit is capacitively coupled to the tunable coupler via the second bypass capacitor and the first bypass capacitor.

In one embodiment, the intermediate dielectric layer of the second bypass capacitor is an air layer.

In one embodiment, the second qubit is capacitively coupled to the first pole of the first bypass capacitor.

In one embodiment, the first qubit, the second qubit and the tunable coupler are not on the same substrate.

In one embodiment, capacitive coupling with ground isolation or direct capacitive coupling without ground is performed between the first qubit, the second qubit, the tunable coupler and the first bypass capacitor.

In one embodiment, the intermediate dielectric layer of the first bypass capacitor is an air layer.

In one embodiment, the first qubit, the second qubit, and the tunable coupler are in the form of a grounded transmon or a floating transmon.

In the capacitive tunable coupling unit of the present invention, a bypass capacitor is introduced between the qubit and the tunable coupler. Bypass capacitors can be made very long, which facilitates wiring, vias, etc. in large-scale qubit scaling. In the capacitive tunable coupling unit of the present invention, the qubit Q1, the qubit Q2 and the tunable coupler Qc can be either grounded transmon or floating transmon. For the grounded transmon form, the overall area of the qubit can be designed to be smaller, which can increase the decoherence time of the qubit, and in the design of flip-chip bonding, it can also weaken the qubit capacitor when the two chips are inverted. The resulting increase in additional capacitance greatly facilitates the extended design of large-scale superconducting qubits based on flip-chip bonding.

Description of drawings

FIG. 1A is a schematic diagram of a circuit structure of a grounded transmon qubit in the prior art.

FIG. 1B is a top view of the process structure of the qubit of FIG. 1A .

Fig. 1C is a cross-sectional view along the dashed line MM' in Fig. 1B.

FIG. 2A is a schematic diagram of a circuit structure of a floating-type transmon qubit in the prior art.

FIG. 2B is a top view of the process structure of the qubit of FIG. 2A.

Fig. 2C is a cross-sectional view along the dashed line NN' in Fig. 2B.

3A is a schematic diagram of a circuit structure of a capacitively adjustable coupling unit according to the first embodiment of the present invention.

FIG. 3B is a top view of the process structure of the capacitively adjustable coupling unit of FIG. 3A .

4A is a schematic diagram of a circuit structure of a capacitively adjustable coupling unit according to a second embodiment of the present invention.

FIG. 4B is a top view of the process structure of the capacitively adjustable coupling unit of FIG. 4A .

5A is a schematic diagram of a circuit structure of a capacitively adjustable coupling unit according to a third embodiment of the present invention.

FIG. 5B is a top view of the process structure of the capacitively adjustable coupling unit of FIG. 5A .

6A is a schematic diagram of a circuit structure of a capacitively adjustable coupling unit according to a fourth embodiment of the present invention.

FIG. 6B is a top view of the process structure of the capacitively adjustable coupling unit of FIG. 6A .

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below through specific embodiments in conjunction with the accompanying drawings. It should be noted that the embodiments of the present invention are given for illustration only and do not limit the scope of the present invention.

FIG. 1A is a schematic diagram of a circuit structure of a grounded transmon qubit in the prior art. The grounded transmon qubit includes a capacitor 101 and a Josephson junction 102 connected in parallel. The capacitor 101 includes a first pole 101a, which is connected to the first pole 102a of the Josephson junction 102, and a second pole 101b, which is connected together with the second pole 102b of the Josephson junction 102 and grounded (GND). FIG. 1B is a top view of the process structure of the qubit of FIG. 1A , and FIG. 1C is a cross-sectional view along the dotted line MM' in FIG. 1B . As shown in FIGS. 1B and 1C, the first pole 101a of the capacitor 101 is connected to the first pole 102a of the Josephson junction 102, the second pole 101b of the capacitor 101 is GND, and the dielectric layer of the capacitor 101 is an air layer. The second pole 102b of the Josephson junction 102 is grounded, and the Josephson junction 102 further includes an insulating layer 102c.

It should be noted that the circuit structure of the grounded transmon qubits in FIGS. 1A-1C is only an exemplary simplified structure, and other forms of circuit structures may be adopted in practical applications as required. For example, the Josephson junction in FIG. 1A may be a Squid double junction, or the capacitor 101 may adopt other capacitor forms (eg, two capacitors in parallel). The grounded transmon qubit should satisfy the ratio of the tunneling energy E _J of its Josephson junction to the capacitive energy E _C in the range of 10-10 ³ , the inductive energy E _L of its linear inductance and the tunneling energy E of the Josephson junction The ratio of _J is 0 and the Josephson junction is grounded. In practical applications, an appropriate circuit structure of grounded transmon qubits can be selected according to the above constraints.

FIG. 2A is a schematic diagram of a circuit structure of a floating-type transmon qubit in the prior art. The floating type transmon qubit includes capacitors 201 , 202 , 204 and a Josephson junction 203 . The capacitor 201 includes a first pole 201a, which is connected to the first pole 203a of the Josephson junction 203 and the first pole 202a of the capacitor 202, and a second pole 201b, which is connected to ground. The second pole 203b of the Josephson junction 203 and the second pole 202b of the capacitor 202 are connected and jointly connected to the first pole 204a of the capacitor 204, and the second pole 204b of the capacitor 204 is connected to the ground. In floating-type transmon qubits, the Josephson junction 203 is not grounded.

FIG. 2B is a top view of the process structure of the qubit of FIG. 2A , and FIG. 2C is a cross-sectional view along the dotted line NN' in FIG. 2B . 2B and 2C, the first pole 201a of the capacitor 201 is connected to the first pole 203a of the Josephson junction 203, the second pole 201b of the capacitor 201 is GND, and the dielectric layer of the capacitor 201 is an air layer. The second pole 203b of the Josephson junction 203 is connected to the first pole 204a of the capacitor 204, and the Josephson junction 203 further includes an insulating layer 203c. The second pole 204b of the capacitor 204 is GND, the first pole 201a of the capacitor 201 and the first pole 204a of the capacitor 204 form the capacitor 202, that is, the first pole 201a of the capacitor 201 forms the first pole 202a of the capacitor 202, and the first pole 202a of the capacitor 204 One pole 204a constitutes the second pole 202b of the capacitor 202 .

It should be noted that the circuit structures of the floating-type transmon qubits in FIGS. 2A-2C are merely exemplary, and other forms of circuit structures may be adopted in practical applications as required. For example, the Josephson junction in FIG. 2A may be a Squid double junction, or the capacitors 201/204 may adopt other capacitor forms (eg, two capacitors in parallel). The floating-type transmon qubit should satisfy the ratio of the tunneling energy E _J of its Josephson junction to the capacitive energy E _C in the range of 10-10 ³ , and the inductive energy E _L of its linear inductance and the tunneling energy of the Josephson junction. The ratio of E to _J is 0 and the Josephson junction is not grounded. In practical applications, an appropriate circuit structure of the floating transmon qubit can be selected according to the above constraints.

The qubit Q1, the tunable coupler Qc and the qubit Q2 in the capacitive tunable coupling unit of the present invention can be either grounded transmon or floating transmon. The following description will be given by taking the quantum bit Q1, the tunable coupler Qc and the quantum bit Q2 all in the form of grounded transmon as an example.

3A is a schematic diagram of a circuit structure of a capacitively adjustable coupling unit according to the first embodiment of the present invention. The capacitive tunable coupling unit includes a grounded transmon qubit Q1 , an adjustable coupler Qc in the form of a grounded transmon, a grounded transmon qubit Q2 , a bypass capacitor 304 and a bypass capacitor 310 . The specific circuit structure and connection relationship of the grounded transmon qubit have been introduced in detail in FIGS. 1A-1C , and will not be repeated here. The tunable coupler Qc is shown in dashed lines in Figure 3A, the Josephson junctions 302, 309 and 312 are Squid double junctions.

The bypass capacitor 304 includes a first pole 304a, which is coupled to the qubit Q1 via a capacitor 303 and coupled to the tunable coupler Qc via a capacitor 305, and a second pole 304b, which is connected to ground. Bypass capacitor 310 includes a first pole 310a, which is coupled to qubit Q2 via capacitance 311 and to tunable coupler Qc via capacitance 307, and a second pole 310b, which is connected to ground. Bypass capacitor 304 is coupled to bypass capacitor 310 via capacitor 306 .

FIG. 3B is a top view of the process structure of the capacitively adjustable coupling unit of FIG. 3A . As shown in FIG. 3B , the first pole 304a of the bypass capacitor 304 is capacitively coupled to the qubit Q1 and the adjustable coupler Qc, the second pole 304b of the bypass capacitor 304 is GND, and the dielectric layer of the bypass capacitor 304 is an air layer . The first pole 310a of the bypass capacitor 310 is capacitively coupled to the qubit Q2 and the adjustable coupler Qc, the second pole 310b of the bypass capacitor 310 is GND, and the dielectric layer of the bypass capacitor 310 is an air layer.

The first pole 304a of the bypass capacitor 304 and the first pole 301a of the capacitor 301 constitute the capacitor 303, that is, the first pole 301a of the capacitor 301 constitutes the first pole 303a of the capacitor 303, and the first pole 304a of the bypass capacitor 304 constitutes the capacitor 303 the second pole 303b. The first pole 304a of the bypass capacitor 304 and the first pole 308a of the capacitor 308 constitute the capacitor 305 , that is, the first pole 304a of the bypass capacitor 304 constitutes the first pole 305a of the capacitor 305 , and the first pole 308a of the capacitor 308 constitutes the capacitor 305 the second pole 305b. The first pole 310a of the bypass capacitor 310 and the first pole 313a of the capacitor 313 constitute the capacitor 311, that is, the first pole 310a of the bypass capacitor 310 constitutes the first pole 311a of the capacitor 311, and the first pole 313a of the capacitor 313 constitutes the capacitor 311 the second pole 311b. The first pole 310a of the bypass capacitor 310 and the first pole 308a of the capacitor 308 constitute the capacitor 307, that is, the first pole 308a of the capacitor 308 constitutes the first pole 307a of the capacitor 307, and the first pole 310a of the bypass capacitor 310 constitutes the capacitor 307 the second pole 307b. The first pole 304a of the bypass capacitor 304 and the first pole 310a of the bypass capacitor 310 constitute the capacitor 306 , that is, the first pole 304a of the bypass capacitor 304 constitutes the first pole 306a of the capacitor 306 , and the first pole of the bypass capacitor 310 310a constitutes the second pole 306b of the capacitor 306 . The capacitors 303, 305, 306, 307, and 311 are coupling capacitors, and the intermediate dielectric layers are all air layers.

In the embodiment of FIGS. 3A-3B , the conditions for tunable coupling and turn-off between qubits Q1 and Q2 need to be determined by the following parameters: qubits Q1 , Q2 and the respective capacitances of the tunable coupler Qc (ie the capacitance 301, 308 and 313) and Josephson junctions (ie Josephson junctions 302, 309 and 312) parameters; capacitances of bypass capacitors 304 and 310; and qubits Q1, Q2, adjustable coupler Qc and bypass capacitors 304, 310 Coupling capacitance between them (ie capacitances 303, 305, 306, 307, 311). By changing the bias current of the tunable coupler Qc, the coupling strength between the first qubit Q1 and the second qubit Q2 can be changed, so the coupling strength between the qubits can be completely turned off or arbitrarily adjusted.

4A is a schematic diagram of a circuit structure of a capacitively adjustable coupling unit according to a second embodiment of the present invention. The capacitive tunable coupling unit includes a grounded transmon qubit Q1 , an adjustable coupler Qc in the form of a grounded transmon, a grounded transmon qubit Q2 , a bypass capacitor 404 and a bypass capacitor 410 . The specific circuit structure and connection relationship of the grounded transmon qubit have been introduced in detail in FIGS. 1A-1C , and will not be repeated here. The tunable coupler Qc is shown in dashed lines in Figure 4A, and the Josephson junctions 402, 409 and 412 are Squid double junctions.

The bypass capacitor 404 includes a first pole 404a, which is coupled to the qubit Q1 via a capacitor 403 and coupled to the tunable coupler Qc via a capacitor 405, and a second pole 404b, which is connected to ground. Bypass capacitor 410 includes a first pole 410a, which is coupled to qubit Q2 via capacitance 411, and a second pole 410b, which is connected to ground. Bypass capacitor 404 is coupled to bypass capacitor 410 via capacitor 406 . Adjustable coupler Qc is not coupled to bypass capacitor 410 .

FIG. 4B is a top view of the process structure of the capacitively adjustable coupling unit of FIG. 4A . As shown in FIG. 4B , the first pole 404a of the bypass capacitor 404 is capacitively coupled to the qubit Q1 and the adjustable coupler Qc, the second pole 404b is GND, and the dielectric layer of the bypass capacitor 404 is an air layer. The first pole 410a of the bypass capacitor 410 is capacitively coupled to the qubit Q2, the second pole 410b is GND, and the dielectric layer of the bypass capacitor 410 is an air layer.

The first pole 404 a of the bypass capacitor 404 and the first pole 401 a of the capacitor 401 form the capacitor 403 . The first pole 404 a of the bypass capacitor 404 and the first pole 408 a of the capacitor 408 form the capacitor 405 . The first pole 410a of the bypass capacitor 410 and the first pole 413a of the capacitor 413 constitute the capacitor 411 . The first pole 404 a of the bypass capacitor 404 and the first pole 410 a of the bypass capacitor 410 form the capacitor 406 . The capacitors 403, 405, 406, and 411 are coupling capacitors, and the intermediate dielectric layers are all air layers.

In the embodiment of FIGS. 4A-4B , the conditions for tunable coupling and turn-off between qubits Q1 and Q2 need to be determined by the following parameters: qubits Q1 , Q2 and the respective capacitances of the tunable coupler Qc (ie capacitance 401, 408 and 413) and Josephson junctions (i.e. Josephson junctions 402, 409 and 412) parameters; capacitances of bypass capacitors 404 and 410; and qubits Q1, Q2, adjustable coupler Qc and bypass capacitors 404, 410 Coupling capacitance between them (ie capacitances 403, 405, 406, 411). By changing the bias current of the tunable coupler Qc, the coupling strength between the first qubit Q1 and the second qubit Q2 can be changed, so the coupling strength between the qubits can be completely turned off or arbitrarily adjusted.

5A is a schematic diagram of a circuit structure of a capacitively adjustable coupling unit according to a third embodiment of the present invention. The capacitive tunable coupling unit includes a grounded transmon qubit Q1 , an adjustable coupler Qc in the form of a grounded transmon, a grounded transmon qubit Q2 and a bypass capacitor 504 . The specific circuit structure and connection relationship of the grounded transmon qubit have been introduced in detail in FIGS. 1A-1C , and will not be repeated here. The tunable coupler Qc is shown in dashed lines in Figure 5A, and the Josephson junctions 502, 509 and 512 are Squid double junctions.

The bypass capacitor 504 includes a first pole 504a, which is coupled to the qubit Q1 via a capacitor 503, is coupled to the tunable coupler Qc via a capacitor 505, and is coupled to the qubit Q2 via a capacitor 511; and a second pole 504b, which is connected to to the ground.

FIG. 5B is a top view of the process structure of the capacitively adjustable coupling unit of FIG. 5A . As shown in FIG. 5B , the first pole 504a of the bypass capacitor 504 is capacitively coupled to the qubit Q1, the qubit Q2 and the adjustable coupler Qc, the second pole 504b of the bypass capacitor 504 is GND, and the medium of the bypass capacitor 504 The layer is the air layer.

The first pole 504 a of the bypass capacitor 504 and the first pole 501 a of the capacitor 501 form the capacitor 503 . The first pole 504 a of the bypass capacitor 504 and the first pole 508 a of the capacitor 508 form the capacitor 505 . The first pole 504a of the bypass capacitor 504 and the first pole 513a of the capacitor 513 constitute the capacitor 511 . The capacitors 503, 505, and 511 are coupling capacitors, and the intermediate dielectric layers are all air layers.

In the embodiment of FIGS. 5A-5B , the conditions for tunable coupling and turn-off between qubits Q1 and Q2 need to be determined by the following parameters: qubits Q1 , Q2 and the respective capacitances of the tunable coupler Qc (that is, the capacitance 501, 508 and 513) and Josephson junctions (ie Josephson junctions 502, 509 and 512) parameters; the capacitance of the bypass capacitor 504; (ie capacitances 503, 505, 511). By changing the bias current of the tunable coupler Qc, the coupling strength between the first qubit Q1 and the second qubit Q2 can be changed, so the coupling strength between the qubits can be completely turned off or arbitrarily adjusted.

6A is a schematic diagram of a circuit structure of a capacitively adjustable coupling unit according to a fourth embodiment of the present invention. The capacitive tunable coupling unit includes a grounded transmon qubit Q1 , an adjustable coupler Qc in the form of a grounded transmon, a grounded transmon qubit Q2 and a bypass capacitor 604 . The specific circuit structure and connection relationship of the grounded transmon qubit have been introduced in detail in FIGS. 1A-1C , and will not be repeated here. The tunable coupler Qc is shown in dashed lines in Figure 6A, and the Josephson junctions 602, 609 and 612 are Squid double junctions.

The bypass capacitor 604 includes a first pole 604a, which is coupled to the qubit Q1 via a capacitor 603 and coupled to the tunable coupler Qc via a capacitor 605; and a second pole 304b, which is connected to ground. Tunable coupler Qc is coupled to qubit Q2 via capacitor 614 .

FIG. 6B is a top view of the process structure of the capacitively adjustable coupling unit of FIG. 6A . As shown in FIG. 6B , the first pole 604a of the bypass capacitor 604 is capacitively coupled to the qubit Q1 and the adjustable coupler Qc, the second pole 604b of the bypass capacitor 604 is GND, and the dielectric layer of the bypass capacitor 604 is an air layer .

The first pole 604 a of the bypass capacitor 604 and the first pole 601 a of the capacitor 601 form the capacitor 603 . The first pole 604 a of the bypass capacitor 604 and the first pole 608 a of the capacitor 608 form the capacitor 605 . The first pole 608a of the capacitor 608 and the first pole 613a of the capacitor 613 constitute the capacitor 614 . The capacitors 603, 605, and 614 are coupling capacitors, and the intermediate dielectric layers are all air layers.

In the embodiment of FIGS. 6A-6B , the conditions for tunable coupling and turn-off between qubits Q1 and Q2 need to be determined by the following parameters: qubits Q1 , Q2 and the respective capacitances of the tunable coupler Qc (that is, the capacitance 601, 608 and 613) and Josephson junctions (i.e. Josephson junctions 602, 609 and 612) parameters; capacitance of bypass capacitor 604; and coupling capacitance between qubits Q1, Q2, adjustable coupler Qc and bypass capacitor 604 (ie capacitances 603, 605, 614). By changing the bias current of the tunable coupler Qc, the coupling strength between the first qubit Q1 and the second qubit Q2 can be changed, so the coupling strength between the qubits can be completely turned off or arbitrarily adjusted.

In the above four embodiments, a bypass capacitor is introduced into the adjustable capacitance coupling unit. The capacitively adjustable coupling unit may include a quantum bit Q1, a quantum bit Q2, an adjustable coupler Qc, and a first bypass capacitor. The quantum bit Q1 and the adjustable coupler Qc are both capacitively coupled to the first bypass capacitor, and the quantum bit Q2 is capacitively coupled to the adjustable coupler Qc. In the first embodiment, the capacitive adjustable coupling unit further includes a second bypass capacitor, the qubit Q2 and the adjustable coupler Qc are capacitively coupled to the second bypass capacitor, and the qubit Q2 is coupled to the adjustable coupler via the second bypass capacitor. Adjust the coupler Qc. In the second embodiment, the adjustable capacitance coupling unit further includes a second bypass capacitor, the qubit Q2 is capacitively coupled to the second bypass capacitor, the first bypass capacitor is capacitively coupled to the second bypass capacitor, and the qubit Q2 is capacitively coupled to the second bypass capacitor via The second bypass capacitor and the first bypass capacitor are capacitively coupled to the adjustable coupler Qc. In a third embodiment, the qubit Q2 is capacitively coupled to a first bypass capacitor, which is coupled to the tunable coupler Qc via the first bypass capacitor. In the fourth embodiment, the qubit Q2 is capacitively coupled directly to the tunable coupler Qc.

The capacitive coupling between the quantum bit Q1, the quantum bit Q2, the adjustable coupler Qc, the first bypass capacitor, and the second bypass capacitor may be capacitive coupling with ground isolation or direct capacitive coupling without ground.

In the above four embodiments, after introducing the bypass capacitor, the effective coupling coefficient g between the qubit Q1 and the qubit Q2 can be expressed as

g=g ₁₂ -g _eff (1)

where _geff is the virtual exchange interaction term introduced by the tunable coupler:

Among them, j=1, 2; represent qubits Q1, Q2 respectively; energy level difference w _c and w _j are the energy level difference between the ground state and the first excited state of tunable coupler Qc and qubit Q _j , respectively; coupling coefficient g _1c and g _2c are the equivalent coupling coefficients of the qubit Q1 and the tunable coupler Qc and the qubit Q2 and the tunable coupler Qc after the bypass capacitors, respectively; g ₁₂ is the direct coupling coefficient of the qubit Q1 and the qubit Q2 . From the expression of the effective coupling coefficient g, it can be seen that the effective coupling between qubits consists of two parts, namely the virtual exchange interaction term _geff introduced by the tunable coupler and the direct coupling between qubit Q1 and qubit Q2 term g ₁₂ . The competition between g _eff and g ₁₂ is realized by adjusting the frequency detuning between the tunable coupler and the qubit, so that the effective coupling g of the qubits Q1 and Q2 can be adjusted and turned off.

In an embodiment of the present invention, taking the capacitance-tunable coupling unit in FIG. 3A and FIG. 3B as an example, the capacitance 301 of the superconducting qubit Q1 is 75fF, and the Josephson junction 302 is in the form of a Squid double junction. The junction resistance is 20000Ω. The parameters of the superconducting qubit Q2 are the same as those of the qubit Q1. The capacitance 308 of the adjustable coupler Qc is 44fF, and the Josephson junction 309 is in the form of a Squid double junction, and the junction resistance of each junction is 20000Ω. Bypass capacitors 304 and 310 are both 36fF. The coupling capacitor 303 is 15fF, the coupling capacitor 305 is 8fF, the coupling capacitor 307 is 8fF, the coupling capacitor 306 is 8fF, and the coupling capacitor 311 is 15fF. By changing the bias current of the tunable coupler Qc, the coupling coefficient between the superconducting qubit Q1 and the superconducting qubit Q2 can be adjusted in the range of 5MHz to -40Hz.

According to an embodiment of the present invention, the shapes of the qubits Q1, Q2, the tunable coupler Qc and the bypass capacitor can be any desired shapes such as circles and polygons. It only needs to satisfy that the coupling coefficient between qubits Q1 and Q2 can be adjusted and turned off.

In the capacitance adjustable coupling unit of the present invention, the arrangement of the qubits Q1, Q2, the adjustable coupler Qc and the bypass capacitor includes but is not limited to the four forms shown in Figs. 3A-6B. As long as the bypass capacitor can be introduced simultaneously, the quantum The bits Q1, Q2, and the adjustable coupler Qc can achieve adjustable coupling between bits.

In the capacitance-adjustable coupling unit of the present invention, the substrate may be sapphire, silicon, or the like. A superconducting metal layer may be plated on the substrate to serve as a grounding layer, and the superconducting metal may be, for example, Al, Nb, Ta, or the like. Materials for making capacitors and Josephson junctions include Al, Nb, Ta, and the like.

In the capacitive tunable coupling unit of the present invention, the first qubit Q1, the second qubit Q2 and the tunable coupler Qc may be coplanar, that is, coexist on one substrate; or may not be coplanar, That is not on one substrate, for example, the first qubit Q1 is on substrate A, the second qubit Q2 and tunable coupler Qc are on another substrate B, and then substrates A and B are face-to-face close but not in contact, The first quantum bit Q1, the second quantum bit Q2 and the adjustable coupler Qc are still capacitively coupled. The substrates A and B can be metal-connected through indium pillars. For example, one plate of a bypass capacitor can be distributed on two opposite surfaces of substrates A and B at the same time, and connected through indium pillars. In this way, the first qubit Q1, the second qubit Q2 and the tunable coupler Qc can be separately fabricated more conveniently, thereby increasing the success rate.

In the capacitance-adjustable coupling unit of the present invention, a bypass capacitor is introduced between the qubit and the adjustable coupler, and the adjustable coupling and turn-off between the qubit Q1 and the qubit Q2 can still be realized after calculation. The length of the bypass capacitor between the qubit and the tunable coupler can be very long (as long as it conforms to the characteristics of the lumped capacitor), which facilitates wiring, perforation, etc. in large-scale qubit expansion. In the capacitive tunable coupling unit of the present invention, the qubit Q1, the qubit Q2 and the tunable coupler Qc can be either grounded transmon or floating transmon. For the grounded transmon form, the overall area can be designed to be smaller, which can increase the decoherence time of the qubit, and in the design of flip-chip bonding, it can also reduce the effect on the qubit capacitor when the two chips are inverted to each other. The addition of additional capacitance greatly facilitates the extended design of large-scale superconducting qubits based on flip-chip bonding.

A single or multiple bypass capacitors are introduced into the capacitance adjustable coupling unit of the present invention, and the adjustable coupling between two qubits can be realized. Due to the introduction of bypass capacitors, the distance between qubits is greatly increased, thereby providing sufficient wiring space for the expansion of large-scale superconducting qubits. In addition, the capacitive adjustable coupling unit of the present invention can provide a connection in the form of adjustable coupling between chips for the modularization of the quantum chip, and can play an important role in the large-scale expansion of a single quantum chip and the expansion of the modular form.

Although the present invention has been described in terms of the preferred embodiments, the present invention is not limited to the embodiments described herein, and various changes and changes can be made without departing from the scope of the present invention.

Claims (10)

1. A capacitively tunable coupling unit, comprising: the circuit comprises a first qubit, a second qubit, an adjustable coupler and a first bypass capacitor;

wherein the first qubit and the adjustable coupler are each capacitively coupled to a first pole of the first bypass capacitance, a second pole of the first bypass capacitance is grounded, and the second qubit is capacitively coupled to the adjustable coupler.

2. The capacitively adjustable coupling unit of claim 1, wherein the capacitively adjustable coupling unit further comprises a second bypass capacitor comprising:

a first pole capacitively coupled to the second qubit, the adjustable coupler, and a first pole of the first bypass capacitance; and

a second pole connected to ground.

3. The capacitively adjustable coupling unit of claim 1, wherein the capacitively adjustable coupling unit further comprises a second bypass capacitor comprising:

a first pole capacitively coupled to the second qubit and to a first pole of the first bypass capacitance; and

a second pole connected to ground.

4. The capacitively adjustable coupling unit of claim 3, wherein the second qubit is capacitively coupled to the adjustable coupler via the second bypass capacitance and the first bypass capacitance.

5. The capacitively tunable coupling unit of any of claims 2 to 4, wherein the intermediate dielectric layer of the second bypass capacitor is an air layer.

6. The capacitively adjustable coupling unit of claim 1, wherein the second qubit is capacitively coupled to a first pole of the first bypass capacitance.

7. The capacitively adjustable coupling unit of claim 1, wherein the first qubit, the second qubit, and the adjustable coupler are not on a same substrate.

8. The capacitively adjustable coupling unit of claim 1, wherein the first qubit, the second qubit, the adjustable coupler, and the first bypass capacitor are capacitively coupled with ground isolation or directly capacitively coupled without ground.

9. The capacitively tunable coupling unit of claim 1, wherein the middle dielectric layer of the first bypass capacitor is an air layer.

10. The capacitively adjustable coupling unit of claim 1, wherein the first qubit, the second qubit, and the adjustable coupler are in the form of grounded or floating transmon.

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