CN115238901B - Capacitance-adjustable coupling unit - Google Patents

Capacitance-adjustable coupling unit Download PDF

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CN115238901B
CN115238901B CN202210717997.7A CN202210717997A CN115238901B CN 115238901 B CN115238901 B CN 115238901B CN 202210717997 A CN202210717997 A CN 202210717997A CN 115238901 B CN115238901 B CN 115238901B
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capacitance
qubit
pole
tunable
capacitor
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CN115238901A (en
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相忠诚
宋小会
梁珪涵
赵思路
梅铮扬
李想
李力
许凯
范桁
郑东宁
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Institute of Physics of CAS
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Abstract

The invention provides a capacitance-adjustable coupling unit, which comprises: a first qubit, a second qubit, a tunable coupler, and a first bypass capacitor; wherein the first qubit and the tunable coupler are both capacitively coupled to a first pole of the first bypass capacitor, a second pole of the first bypass capacitor is grounded, and the second qubit is capacitively coupled to the tunable coupler.

Description

Capacitance-adjustable coupling unit
Technical Field
The present invention relates generally to the field of multiple-qubit chips, and more particularly to a capacitively tunable coupling unit.
Background
The configuration design of the capacitance adjustable coupling unit plays a vital role in the large-scale expansion of the multi-bit quantum chip, can realize the adjustable coupling and the turn-off of the quantum bits, and can reduce the parasitic coupling in the operation of the bit entanglement gate, thereby improving the fidelity of the entanglement gate. The capacitively tunable coupling unit typically includes 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 an externally applied magnetic flux. The coupling coefficient can be continuously adjusted from positive to negative to turn off completely or to adjust the coupling size between qubits arbitrarily.
The existing capacitance-tunable coupling unit comprises the following two designs:
(1) Qubit Q1, tunable coupler Qc, and qubit Q2 are all in the form of ground transmon (see Y.Fei, et.al.2018PHYS.REV.APPLIED 10,054062). The grounding type qubit is formed by connecting a nonlinear inductor formed by a capacitance to ground and a Josephson junction in parallel. The volume is smaller under the appointed capacitance value, and the area can be saved on a two-dimensional chip for large expansion. However, the capacitance-adjustable coupling unit is not easy to be applied to the preparation of three-dimensional superconducting quantum chips based on flip-chip technology, because space is difficult to be reserved for wiring, resonant cavities are arranged, and the like.
(2) Floating transmon is introduced as a qubit or tunable coupler. This configuration can realize the above functions while realizing a large-area wiring by the large size of the floating-type qubit (see E.A.Sete, et.al.2021PHYSICAL REVIEW APPLIED 16,024050). However, the increase in size of the floating-type qubit brings about a decrease in decoherence time, which severely restricts the implementation of a quantum computer.
Disclosure of Invention
Based on the above-mentioned problems of the prior art, the present invention provides a capacitance-tunable coupling unit, comprising: a first qubit, a second qubit, a tunable coupler, and a first bypass capacitor;
wherein the first qubit and the tunable coupler are both capacitively coupled to a first pole of the first bypass capacitor, a second pole of the first bypass capacitor is grounded, and the second qubit is capacitively coupled to the tunable coupler.
In one embodiment, the capacitance-tunable coupling unit further includes a second bypass capacitance comprising:
A first pole capacitively coupled to the second qubit, the tunable coupler, and a first pole of the first bypass capacitance; and
And a second pole connected to ground.
In one embodiment, the capacitance-tunable coupling unit further includes a second bypass capacitance comprising:
a first pole capacitively coupled to the second qubit and a first pole of the first bypass capacitance; and
And a second pole connected to ground.
In one embodiment, the second qubit is capacitively coupled to the tunable coupler via the second bypass capacitance and the first bypass capacitance.
In one embodiment, the intermediate dielectric layer of the second bypass capacitor is an air layer.
In one embodiment, the second qubit capacitance is coupled to a first pole of the first bypass capacitance.
In one embodiment, the first qubit, the second qubit, and the tunable coupler are not on the same substrate.
In one embodiment, the first qubit, the second qubit, the tunable coupler, and the first bypass capacitor are capacitively coupled with or without a direct capacitive coupling.
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 take the form of ground transmon or floating transmon.
In the capacitively tunable coupling unit of the present invention, a bypass capacitance is introduced between the qubit and the tunable coupler. The bypass capacitance can be made very long, which facilitates wiring, puncturing, etc. in large scale qubit expansion. In the capacitance tunable coupling unit of the present invention, the qubit Q1, the qubit Q2, and the tunable coupler Qc can be either in the form of a ground transmon or a floating transmon. For the grounding transmon mode, the whole area of the quantum bit can be designed to be smaller, so that the decoherence time of the quantum bit can be increased, and the extra capacitance increase on the quantum bit capacitance caused by the mutual back-off of two chips can be weakened in the flip-chip design, thereby greatly facilitating the expansion design of the large-scale superconducting quantum bit based on the flip-chip.
Drawings
Fig. 1A is a schematic diagram of a circuit structure of a grounded transmon qubit according to the prior art.
Fig. 1B is a top view of the process structure of the qubit of fig. 1A.
Fig. 1C is a sectional view along the broken line MM' in fig. 1B.
Fig. 2A is a schematic diagram of a circuit structure of a floating-type transmon qubit according to the prior art.
Fig. 2B is a top view of the process structure of the qubit of fig. 2A.
Fig. 2C is a sectional view along a broken line NN' in fig. 2B.
Fig. 3A is a schematic circuit diagram of a capacitive tunable coupling unit according to a first embodiment of the present invention.
Fig. 3B is a top view of the process structure of the capacitance tunable coupling unit of fig. 3A.
Fig. 4A is a schematic circuit diagram of a capacitive tunable coupling unit according to a second embodiment of the present invention.
Fig. 4B is a top view of the process structure of the capacitance tunable coupling unit of fig. 4A.
Fig. 5A is a schematic circuit diagram of a capacitive tunable coupling unit according to a third embodiment of the present invention.
Fig. 5B is a top view of the process structure of the capacitance tunable coupling unit of fig. 5A.
Fig. 6A is a schematic circuit diagram of a capacitive tunable coupling unit according to a fourth embodiment of the present invention.
Fig. 6B is a top view of the process structure of the capacitance tunable coupling unit of fig. 6A.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail by means of specific embodiments with reference to the accompanying drawings. It should be noted that the examples given herein are for illustration only and are not intended to limit the scope of the present invention.
Fig. 1A is a schematic diagram of a circuit structure of a grounded transmon qubit according to the prior art. The grounded transmon qubit comprises a parallel connected capacitor 101 and a josephson junction 102. The capacitance 101 comprises a first pole 101a connected to a first pole 102a of the josephson junction 102; and a second diode 101b connected together with the second diode 102b of the josephson junction 102 and connected to Ground (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 dashed line MM' in fig. 1B. As shown in fig. 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 comprises an insulating layer 102c.
It should be noted that the circuit configuration of the grounded transmon qubit in fig. 1A-1C is merely an exemplary simplified configuration, and that other forms of circuit configurations may be employed as desired in practical applications. For example, the josephson junction in fig. 1A may be a Squid double junction, or the capacitor 101 may take other forms of capacitance (e.g., a form in which two capacitors are connected in parallel). The grounded transmon qubit should meet that the ratio of the tunneling energy E J to the capacitance energy E C of the Josephson junction is in the range of 10-10 3, the ratio of the inductance energy E L of the linear inductance to the tunneling energy E J of the Josephson junction is 0, and the Josephson junction is grounded. In practical application, a suitable circuit structure of the grounding transmon qubits can be selected according to the constraint condition.
Fig. 2A is a schematic diagram of a circuit structure of a floating-type transmon qubit according to the prior art. Floating-ground transmon qubits include capacitors 201, 202, 204 and josephson junctions 203. The capacitance 201 comprises a first pole 201a connected to a first pole 203a of the josephson junction 203 and to a first pole 202a of the capacitance 202; a second pole 201b connected to ground. The second pole 203b of the josephson junction 203 and the second pole 202b of the capacitor 202 are connected and commonly connected to the first pole 204a of the capacitor 204, the second pole 204b of the capacitor 204 being connected to ground. In floating-ground 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 dashed line NN' in fig. 2B. As shown in fig. 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, the josephson junction 203 further comprising 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, i.e. the first pole 201a of the capacitor 201 forms the first pole 202a of the capacitor 202, and the first pole 204a of the capacitor 204 forms the second pole 202b of the capacitor 202.
It should be noted that the floating transmon qubit circuit configuration in fig. 2A-2C is merely exemplary, and that other forms of circuit configurations may be employed as desired in practical applications. For example, the josephson junction in fig. 2A may be a Squid double junction, or the capacitors 201/204 may take other forms of capacitance (e.g., two capacitors in parallel). The floating-ground transmon qubit should meet that the ratio of the tunneling energy E J to the capacitance energy E C of the Josephson junction is in the range of 10-10 3, the ratio of the inductance energy E L of the linear inductance to the tunneling energy E J of the Josephson junction is 0, and the Josephson junction is not grounded. In practical application, a suitable circuit structure of the floating-type transmon qubits can be selected according to the constraint conditions.
The quantum bit Q1, the tunable coupler Qc and the quantum bit Q2 in the capacitance tunable coupling unit of the present invention may be in the form of either a grounded transmon or a floating transmon form. The following description will take, as an example, the form of the qubit Q1, the tunable coupler Qc, and the qubit Q2 being all grounded transmon.
Fig. 3A is a schematic circuit diagram of a capacitive tunable coupling unit according to a first embodiment of the present invention. The capacitance adjustable coupling unit comprises 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 of the grounded transmon qubits are described in detail in fig. 1A-1C, and are not described in detail herein. The adjustable coupler Qc is shown in dashed lines in fig. 3A, the josephson junctions 302, 309 and 312 being Squid double junctions.
Wherein bypass capacitor 304 comprises a first pole 304a coupled to qubit Q1 via capacitor 303 and to tunable coupler Qc via capacitor 305; and a second pole 304b connected to ground. Bypass capacitor 310 includes a first pole 310a coupled to qubit Q2 via capacitor 311 and to tunable coupler Qc via capacitor 307; and a second pole 310b 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 capacitance tunable 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 tunable 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 tunable 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, i.e. 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 second pole 303b of the capacitor 303. The first pole 304a of the shunt capacitance 304 and the first pole 308a of the capacitance 308 constitute the capacitance 305, i.e. the first pole 304a of the shunt capacitance 304 constitutes the first pole 305a of the capacitance 305 and the first pole 308a of the capacitance 308 constitutes the second pole 305b of the capacitance 305. The first pole 310a of the shunt capacitance 310 and the first pole 313a of the capacitance 313 constitute the capacitance 311, i.e. the first pole 310a of the shunt capacitance 310 constitutes the first pole 311a of the capacitance 311 and the first pole 313a of the capacitance 313 constitutes the second pole 311b of the capacitance 311. The first pole 310a of the shunt capacitance 310 and the first pole 308a of the capacitance 308 constitute the capacitance 307, i.e. the first pole 308a of the capacitance 308 constitutes the first pole 307a of the capacitance 307 and the first pole 310a of the shunt capacitance 310 constitutes the second pole 307b of the capacitance 307. The first pole 304a of the bypass capacitor 304 and the first pole 310a of the bypass capacitor 310 constitute the capacitor 306, i.e. the first pole 304a of the bypass capacitor 304 constitutes the first pole 306a of the capacitor 306 and the first pole 310a of the bypass capacitor 310 constitutes the second pole 306b of the capacitor 306. The capacitor 303,305,306,307,311 is a coupling capacitor, and the middle dielectric layers are all air layers.
In the embodiment of fig. 3A-3B, the conditions for adjustable coupling and switching off between qubits Q1 and Q2 need to be determined by the following parameters: the respective capacitance (i.e., capacitances 301,308 and 313) and josephson junction (i.e., josephson junctions 302, 309 and 312) parameters of qubits Q1, Q2 and tunable coupler Qc; the capacitance of bypass capacitors 304 and 310; and qubits Q1, Q2, the coupling capacitance (i.e., capacitance 303,305,306,307,311) between the tunable coupler Qc and the bypass capacitances 304, 310. 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, and thus the coupling size between the qubits can be completely turned off or arbitrarily adjusted.
Fig. 4A is a schematic circuit diagram of a capacitive tunable coupling unit according to a second embodiment of the present invention. The capacitance adjustable 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 of the grounded transmon qubits are described in detail in fig. 1A-1C, and are not described in detail herein. The adjustable coupler Qc is shown in dashed lines in fig. 4A, the josephson junctions 402, 409 and 412 being Squid double junctions.
Wherein bypass capacitor 404 comprises a first pole 404a coupled to qubit Q1 via capacitor 403 and to tunable coupler Qc via capacitor 405; and a second pole 404b connected to ground. Bypass capacitor 410 includes a first pole 410a coupled to qubit Q2 via capacitor 411; and a second pole 410b connected to ground. Bypass capacitor 404 is coupled to bypass capacitor 410 via capacitor 406. The adjustable coupler Qc is not coupled to the bypass capacitor 410.
Fig. 4B is a top view of the process structure of the capacitance tunable coupling unit of fig. 4A. As shown in fig. 4B, a first pole 404a of the bypass capacitor 404 is capacitively coupled to the qubit Q1 and the tunable coupler Qc, a second pole 404B is GND, and a 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 404a of the shunt capacitance 404 and the first pole 401a of the capacitance 401 constitute a capacitance 403. The first pole 404a of the shunt capacitance 404 and the first pole 408a of the capacitance 408 constitute a capacitance 405. The first pole 410a of the shunt capacitance 410 and the first pole 413a of the capacitance 413 constitute the capacitance 411. The first pole 404a of the bypass capacitor 404 and the first pole 410a of the bypass capacitor 410 constitute the capacitor 406. The capacitor 403,405,406,411 is a coupling capacitor, and the middle dielectric layers are all air layers.
In the embodiment of fig. 4A-4B, the conditions for adjustable coupling and switching off between qubits Q1 and Q2 need to be determined by the following parameters: the respective capacitance (i.e., capacitances 401,408, and 413) and josephson junction (i.e., josephson junctions 402, 409, and 412) parameters of qubits Q1, Q2 and tunable coupler Qc; the capacitance of bypass capacitors 404 and 410; and qubits Q1, Q2, the coupling capacitance (i.e., capacitance 403,405,406,411) between the tunable coupler Qc and the shunt capacitance 404, 410. 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, and thus the coupling size between the qubits can be completely turned off or arbitrarily adjusted.
Fig. 5A is a schematic circuit diagram of a capacitive tunable coupling unit according to a third embodiment of the present invention. The capacitance tunable coupling unit includes a grounded transmon qubit Q1, a tunable coupler Qc in the form of a grounded transmon, a grounded transmon qubit Q2, and a bypass capacitance 504. The specific circuit structure and connection of the grounded transmon qubits are described in detail in fig. 1A-1C, and are not described in detail herein. The adjustable coupler Qc is shown in dashed lines in fig. 5A, and josephson junctions 502, 509 and 512 are Squid double junctions.
Wherein bypass capacitor 504 includes a first pole 504a coupled to qubit Q1 via capacitor 503, to tunable coupler Qc via capacitor 505 and to qubit Q2 via capacitor 511; and a second pole 504b connected to ground.
Fig. 5B is a top view of the process structure of the capacitance tunable coupling unit of fig. 5A. As shown in fig. 5B, a first pole 504a of the bypass capacitor 504 is capacitively coupled to the qubit Q1, the qubit Q2, and the tunable coupler Qc, a second pole 504B of the bypass capacitor 504 is GND, and a dielectric layer of the bypass capacitor 504 is an air layer.
The first pole 504a of the shunt capacitance 504 and the first pole 501a of the capacitance 501 constitute a capacitance 503. The first pole 504a of the shunt capacitance 504 and the first pole 508a of the capacitance 508 constitute a capacitance 505. The first pole 504a of the shunt capacitance 504 and the first pole 513a of the capacitance 513 constitute a capacitance 511. The capacitor 503,505,511 is a coupling capacitor, and the middle dielectric layers are all air layers.
In the embodiment of fig. 5A-5B, the conditions for adjustable coupling and switching off between qubits Q1 and Q2 need to be determined by the following parameters: the respective capacitance (i.e., capacitances 501,508 and 513) and josephson junction (i.e., josephson junctions 502, 509 and 512) parameters of qubits Q1, Q2 and tunable coupler Qc; the capacitance of the bypass capacitor 504; and qubits Q1, Q2, the coupling capacitance (i.e., capacitance 503,505,511) between tunable coupler Qc and bypass capacitance 504. 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, and thus the coupling size between the qubits can be completely turned off or arbitrarily adjusted.
Fig. 6A is a schematic circuit diagram of a capacitive tunable coupling unit according to a fourth embodiment of the present invention. The capacitance adjustable 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 capacitance 604. The specific circuit structure and connection of the grounded transmon qubits are described in detail in fig. 1A-1C, and are not described in detail herein. The adjustable coupler Qc is shown in dashed lines in fig. 6A, the josephson junctions 602, 609 and 612 being Squid double junctions.
Wherein bypass capacitor 604 comprises a first pole 604a coupled to qubit Q1 via capacitor 603 and to tunable coupler Qc via capacitor 605; and a second pole 304b connected to ground. The tunable coupler Qc is coupled to the qubit Q2 via a capacitance 614.
Fig. 6B is a top view of the process structure of the capacitance tunable coupling unit of fig. 6A. As shown in fig. 6B, a first pole 604a of the bypass capacitor 604 is capacitively coupled to the qubit Q1 and the tunable coupler Qc, a second pole 604B of the bypass capacitor 604 is GND, and a dielectric layer of the bypass capacitor 604 is an air layer.
The first pole 604a of the bypass capacitor 604 and the first pole 601a of the capacitor 601 constitute the capacitor 603. The first pole 604a of the bypass capacitor 604 and the first pole 608a of the capacitor 608 constitute a capacitor 605. The first pole 608a of the capacitor 608 and the first pole 613a of the capacitor 613 constitute the capacitor 614. The capacitor 603,605,614 is a coupling capacitor, and the middle dielectric layers are all air layers.
In the embodiment of fig. 6A-6B, the conditions for adjustable coupling and switching off between qubits Q1 and Q2 need to be determined by the following parameters: the respective capacitance (i.e., capacitances 601,608 and 613) and josephson junction (i.e., josephson junctions 602, 609 and 612) parameters of qubits Q1, Q2 and tunable coupler Qc; the capacitance of the bypass capacitor 604; and qubits Q1, Q2, the coupling capacitance (i.e., capacitance 603,605,614) between tunable coupler Qc and bypass capacitance 604. 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, and thus the coupling size between the qubits can be completely turned off or arbitrarily adjusted.
In the four embodiments described above, a bypass capacitor is introduced in the capacitance tunable coupling unit. The capacitively tunable coupling unit may include a qubit Q1, a qubit Q2, a tunable coupler Qc, and a first bypass capacitor. Wherein the qubit Q1 and the tunable coupler Qc are both capacitively coupled to the first bypass capacitor, and the qubit Q2 is capacitively coupled to the tunable coupler Qc. In the first embodiment, the capacitively tunable coupling unit further includes a second bypass capacitor, to which the qubit Q2 and the tunable coupler Qc are capacitively coupled, and via which the qubit Q2 is coupled to the tunable coupler Qc. In a second embodiment, the capacitance tunable coupling unit further includes a second bypass capacitance, the qubit Q2 is capacitively coupled to the second bypass capacitance, the first bypass capacitance is capacitively coupled to the second bypass capacitance, and the qubit Q2 is capacitively coupled to the tunable coupler Qc via the second bypass capacitance and the first bypass capacitance. In a third embodiment, the qubit Q2 is capacitively coupled to a first bypass capacitance, which is coupled to the tunable coupler Qc via the first bypass capacitance. In a fourth embodiment, qubit Q2 is directly capacitively coupled to tunable coupler Qc.
The capacitive coupling between the qubit Q1, the qubit Q2, the tunable coupler Qc, the first bypass capacitor and the second bypass capacitor may be capacitive coupling with or without isolation.
In the four embodiments described above, the effective coupling coefficient g between the qubit Q1 and the qubit Q2 after the bypass capacitance is introduced can be expressed as
g＝g12-geff (1)
Wherein g eff is the virtual switching interaction term introduced by the tunable coupler:
Wherein j=1, 2; respectively representing qubits Q1, Q2; the energy level differences w c and w j are the energy level differences of the ground state and the first excited state of the tunable coupler Qc and the qubit Q j, respectively; coupling coefficients g 1c and g 2c are equivalent coupling coefficients of the quantum bit Q1 and the adjustable coupler Qc and the quantum bit Q2 and the adjustable coupler Qc after bypass capacitance folding; g 12 is the direct coupling coefficient of qubit Q1 and qubit Q2. From the expression of the effective coupling coefficient g, it can be seen that the effective coupling between the qubits consists of two parts, namely a virtual exchange interaction term g eff introduced by the tunable coupler and a direct coupling term g 12 of the qubits Q1 and Q2. The competition of g eff and g 12 is realized by adjusting the frequency detuning of the adjustable coupler and the quantum bit, so that the adjustable turn-off of the effective coupling g of the quantum bits Q1 and Q2 is achieved.
In one embodiment of the invention, taking the capacitance tunable coupling unit in fig. 3A and 3B as an example, the capacitance 301 of the superconducting qubit Q1 is 75fF, the josephson junction 302 takes the form of a Squid double junction, and the junction resistance of each junction is 20000 Ω. The parameters of superconducting qubit Q2 are the same as qubit Q1. The capacitance 308 of the adjustable coupler Qc is 44fF, the josephson junctions 309 take the form of Squid double junctions, each junction having a junction resistance of 20000 Ω. Bypass capacitors 304 and 310 are each 36fF. Coupling capacitance 303 is 15fF, coupling capacitance 305 is 8fF, coupling capacitance 307 is 8fF, coupling capacitance 306 is 8fF, and coupling capacitance 311 is 15fF. The coupling coefficient between superconducting qubit Q1 and superconducting qubit Q2 can be adjusted in the range of 5MHz to-40 Hz by changing the bias current of adjustable coupler Qc.
According to one embodiment of the invention, the shape of the qubits Q1, Q2, the tunable coupler Qc and the bypass capacitor may be any desired shape, such as circular, polygonal, etc. The coupling coefficient between the quantum bits Q1 and Q2 can be regulated and switched off only.
In the capacitance tunable coupling unit of the present invention, the arrangement of the quantum bits Q1, Q2, the tunable coupler Qc and the bypass capacitance includes, but is not limited to, four forms of fig. 3A to 6B, as long as the bypass capacitance, the quantum bits Q1, Q2, and the tunable coupler Qc can be simultaneously introduced and the inter-bit tunable coupling can be realized.
In the capacitance-tunable coupling unit of the present invention, the substrate may be sapphire, silicon, or the like. A layer of superconducting metal, such as Al, nb, ta, etc., may be plated on the substrate to serve as a ground layer. Materials for preparing the capacitor and the Josephson junction include Al, nb, ta and the like.
In the capacitance tunable coupling unit of the present invention, the first qubit Q1, the second qubit Q2, and the tunable coupler Qc may be coplanar, i.e., co-exist on one substrate; or may be non-coplanar, i.e., not on one substrate, e.g., the first qubit Q1 on substrate a, the second qubit Q2 and the tunable coupler Qc on the other substrate B, and then the substrates a and B are brought into face-to-face proximity but not contact, such that capacitive coupling remains between the first qubit Q1, the second qubit Q2 and the tunable coupler Qc. The substrates a and B may be connected by indium posts, for example, one plate of the bypass capacitor may be distributed on opposite sides of the substrates a and B at the same time, and connected by indium posts. In this way, the first qubit Q1, the second qubit Q2, and the tunable coupler Qc can be manufactured more conveniently and separately, increasing the success rate.
In the capacitance adjustable coupling unit, bypass capacitance is introduced between the quantum bit and the adjustable coupler, and the adjustable coupling and the turn-off between the quantum bit Q1 and the quantum bit Q2 can still be realized through calculation. The bypass capacitance length between the qubit and the tunable coupler can be made long (only by conforming to the characteristics of lumped capacitance), which is helpful for wiring, perforation, etc. in large-scale qubit expansion. In the capacitance tunable coupling unit of the present invention, the qubit Q1, the qubit Q2, and the tunable coupler Qc can be either in the form of a ground transmon or a floating transmon. For the grounding transmon mode, the whole area can be designed smaller, so that the decoherence time of the quantum bit can be increased, and the extra capacitance increase on the quantum bit capacitance caused by the mutual back-off of two chips can be weakened in the flip-chip design, thereby greatly facilitating the expansion design of the large-scale superconducting quantum bit based on the flip-chip.
The capacitance adjustable coupling unit of the invention introduces a single or a plurality of bypass capacitances, and can realize adjustable coupling between two quantum bits. Due to the introduction of the bypass capacitor, the distance between the qubits is greatly increased, so that sufficient wiring space is provided for the expansion of the large-scale superconducting qubits. In addition, the capacitance adjustable coupling unit can provide connection in an inter-chip adjustable coupling mode for modularization of the quantum chips, and plays an important role in large-scale expansion and modularization expansion of single quantum chips.
While the invention has been described in terms of preferred embodiments, the invention is not limited to the embodiments described herein, but encompasses various changes and modifications that may be made without departing from the scope of the invention.

Claims (9)

1. A capacitance-tunable coupling unit, comprising: a first qubit, a second qubit, a tunable coupler, and a first bypass capacitor;
Wherein the first qubit and the tunable coupler are both capacitively coupled to a first pole of the first bypass capacitor, a second pole of the first bypass capacitor is grounded, and the second qubit is capacitively coupled to the tunable coupler;
wherein the first qubit, the second qubit and the adjustable coupler are in a grounded transmon mode or a floating transmon mode.
2. The capacitance-tunable coupling unit of claim 1, wherein the capacitance-tunable coupling unit further comprises a second bypass capacitance comprising:
A first pole capacitively coupled to the second qubit, the tunable coupler, and a first pole of the first bypass capacitance; and
And a second pole connected to ground.
3. The capacitance-tunable coupling unit of claim 1, wherein the capacitance-tunable coupling unit further comprises a second bypass capacitance comprising:
a first pole capacitively coupled to the second qubit and a first pole of the first bypass capacitance; and
And a second pole connected to ground.
4. The capacitance-tunable coupling unit of claim 3, wherein the second qubit is capacitively coupled to the tunable coupler via the second bypass capacitance and the first bypass capacitance.
5. The capacitance-tunable coupling unit of any one of claims 2 to 4, wherein an intermediate dielectric layer of the second bypass capacitance is an air layer.
6. The capacitance-tunable coupling unit of claim 1, wherein the second qubit capacitance is coupled to a first pole of the first bypass capacitance.
7. The capacitively tunable coupling unit of claim 1, wherein the first qubit, the second qubit, and the tunable coupler are not on the same substrate.
8. The capacitively tunable coupling unit of claim 1, wherein the first qubit, the second qubit, the tunable coupler, and the first bypass capacitor are capacitively coupled with or without a direct capacitive coupling.
9. The capacitance-tunable coupling unit of claim 1, wherein the intermediate dielectric layer of the first bypass capacitance is an air layer.
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