CN115238901A - Capacitance-adjustable coupling unit - Google Patents
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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
Technical Field
The invention relates to the field of multi-quantum bit chips in general, and in particular relates to a capacitance adjustable coupling unit.
Background
The configuration design of the capacitance adjustable coupling unit plays a crucial role in large-scale expansion of the multi-bit quantum chip, coupling adjustment and disconnection among quantum bits can be achieved, meanwhile, parasitic coupling in operation of the bit entanglement gate can be reduced, and therefore the fidelity of the entanglement gate is improved. The capacitively tunable coupling unit typically comprises three parts, a qubit Q1, a qubit Q2 and a tunable coupler Qc sandwiched between the two qubits. The coupling coefficient between qubit Q1 and qubit Q2 is adjusted by biasing the adjustable coupler Qc between qubit Q1 and qubit Q2 with an applied magnetic flux. The coupling coefficient can be continuously adjusted from positive to negative to completely turn off or arbitrarily adjust the magnitude of coupling between qubits.
The existing capacitance adjustable coupling unit comprises the following two designs:
(1) Qubit Q1, tunable coupler Qc, and qubit Q2 are all ground-connected transmon versions (see y.fei, et.al.2018phys.rev.applied 10,054062). The grounding type qubit is formed by connecting a ground capacitor and a nonlinear inductor formed by a Josephson junction in parallel. The capacitance value of the capacitor is smaller, and the area of the two-dimensional chip can be saved and a large amount of expansion can be performed. However, the capacitance-adjustable coupling unit is not easily applied to the preparation of a three-dimensional superconducting quantum chip based on a flip chip technology, because a space is difficult to reserve for wiring and arranging resonant cavities and the like.
(2) Floating type transmon is introduced as a qubit or tunable coupler. This configuration can realize the above-described functions and, at the same time, realize large-area wiring by the large size of the floating-type qubit (see e.a. sete, et.al.2021 photonic REVIEW APPLIED 16,024050). However, the increase in the size of the floating qubit brings about a decrease in the decoherence time, which severely restricts the implementation of quantum computers.
Disclosure of Invention
Based on the above problems of the prior art, the present invention provides a capacitance adjustable coupling unit, which includes: 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.
In one embodiment, 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.
In one embodiment, 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.
In one embodiment, the second qubit is capacitively coupled to the adjustable coupler via the second bypass capacitance and the first bypass capacitance.
In one embodiment, the middle dielectric layer of the second bypass capacitor is an air layer.
In one embodiment, the second qubit is capacitively coupled to a first pole of the first bypass capacitor.
In one embodiment, the first qubit, the second qubit, and the adjustable coupler are not on the same substrate.
In one embodiment, the first qubit, the second qubit, the adjustable coupler, and the first bypass capacitance are capacitively coupled with ground isolation or directly capacitively coupled without ground.
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 adjustable coupler take the form of a grounded or floating transmon.
In the capacitance-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 extensions. In the capacitance-adjustable coupling unit of the present invention, qubit Q1, qubit Q2, and adjustable coupler Qc may be in the form of either a grounded or floating transmon. For the grounded transmon form, the whole area of the qubit can be designed to be smaller, so that the decoherence time of the qubit can be increased, the additional capacitance increase caused by mutual reverse buckling of two chips on the qubit capacitor can be weakened in the flip-chip bonding design, and great convenience is brought to the large-scale superconducting qubit extension design based on the flip-chip bonding.
Drawings
Fig. 1A is a schematic circuit diagram 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 sectional view along a broken line MM' in fig. 1B.
Fig. 2A is a schematic circuit diagram of a prior art floating type transmon qubit.
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 structure diagram of a capacitance adjustable coupling unit according to a first embodiment of the invention.
Fig. 3B is a top view of the process structure of the capacitively tunable coupling unit of fig. 3A.
Fig. 4A is a schematic circuit diagram of a capacitance-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 tunable coupling unit of fig. 4A.
Fig. 5A is a schematic circuit structure diagram of a capacitance adjustable coupling unit according to a third embodiment of the invention.
Fig. 5B is a top view of the process structure of the capacitively tunable coupling unit of fig. 5A.
Fig. 6A is a schematic circuit structure diagram of a capacitance-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 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 way of specific embodiments with reference to the accompanying drawings. It should be noted that the examples given herein are for illustration only and do not limit the scope of the invention.
Fig. 1A is a schematic circuit diagram of a grounded type 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 comprises a first pole 101a connected to a first pole 102a of the josephson junction 102; and a second pole 101b 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 dashed line MM' in fig. 1B. As shown in fig. 1B and 1C, a first pole 101a of the capacitor 101 is connected to a first pole 102a of the josephson junction 102, a second pole 101B of the capacitor 101 is GND, and a 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 structure of the grounded type transmon qubit in fig. 1A-1C is merely an exemplary simplified structure, and other circuit structures may be adopted in practical applications as needed. 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 parallel form of two capacitors).The grounded transmon qubit should satisfy the tunneling energy E of its Josephson junction J And the capacitance energy E C In the range of 10-10 3 Within the range of (1), the inductance energy E of the linear inductor L Tunneling energy E with Josephson junction J Is 0 and the josephson junction is grounded. In practical applications, a suitable circuit structure of the grounded type transmon quantum bit can be selected according to the above constraints.
Fig. 2A is a schematic circuit diagram of a prior art floating type transmon qubit. The floating type transmon qubit includes capacitors 201, 202, 204 and josephson junction 203. The capacitor 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 capacitor 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, and the second pole 204b of the capacitor 204 is connected to ground. In floating type transmon qubits, 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, a first pole 201a of the capacitor 201 is connected to a first pole 203a of the josephson junction 203, a second pole 201B of the capacitor 201 is GND, and a 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 comprises 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 circuit structure of the floating-type transmon qubit in fig. 2A-2C is merely exemplary, and other circuit structures may be adopted in practical applications as needed. For example, the josephson junction in fig. 2A may be a Squid double junction, or the capacitors 201/204 may take other capacitive forms (e.g., two capacitors in parallel). The floating type transmon qubit should satisfy the tunneling energy E of its Josephson junction J And a capacitance energy E C In the range of 10-10 3 Within the range of (1), the inductance energy E of the linear inductor L Tunneling energy E with Josephson junction J Is 0 and the josephson junction is not grounded. In practical application, a suitable circuit structure of the floating type transmon qubit can be selected according to the above constraint conditions.
The qubit Q1, the adjustable coupler Qc and the qubit Q2 in the capacitance-adjustable coupling unit of the present invention may be in the form of either a grounded transmon or a floating transmon. The following description will be given taking as an example that qubit Q1, tunable coupler Qc, and qubit Q2 are all of the grounded type transmon type.
Fig. 3A is a schematic circuit structure diagram of a capacitance-adjustable coupling unit according to a first embodiment of the present invention. The capacitance-adjustable coupling unit comprises a grounded transmon qubit Q1, a grounded transmon form adjustable coupler Qc, a grounded transmon qubit Q2, a bypass capacitance 304 and a bypass capacitance 310. The specific circuit structure and connection relationship of the grounded type transmon qubit are described in detail in fig. 1A-1C, and are not described herein again. In fig. 3A the adjustable couplers Qc are shown in dashed lines and the josephson junctions 302, 309 and 312 are Squid double junctions.
Wherein the bypass capacitor 304 includes a first pole 304a coupled to the qubit Q1 via a capacitor 303 and to the adjustable coupler Qc via a 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 adjustable coupler Qc via capacitor 307; and a second pole 310b connected to ground. The bypass capacitance 304 is coupled to a bypass capacitance 310 via a capacitance 306.
Fig. 3B is a top view of the process structure of the capacitively tunable coupling unit of fig. 3A. As shown in fig. 3B, a first pole 304a of the bypass capacitor 304 is capacitively coupled to the qubit Q1 and the tunable coupler Qc, a second pole 304B of the bypass capacitor 304 is GND, and a dielectric layer of the bypass capacitor 304 is an air layer. A first pole 310a of the bypass capacitor 310 is capacitively coupled to the qubit Q2 and the tunable coupler Qc, a second pole 310b of the bypass capacitor 310 is GND, and a 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 form the capacitor 303, i.e. the first pole 301a of the capacitor 301 forms the first pole 303a of the capacitor 303 and the first pole 304a of the bypass capacitor 304 forms the second pole 303b of the capacitor 303. The first pole 304a of the bypass capacitor 304 and the first pole 308a of the capacitor 308 form the capacitor 305, i.e. the first pole 304a of the bypass capacitor 304 forms the first pole 305a of the capacitor 305 and the first pole 308a of the capacitor 308 forms the second pole 305b of the capacitor 305. The first pole 310a of the bypass capacitor 310 and the first pole 313a of the capacitor 313 form the capacitor 311, i.e., the first pole 310a of the bypass capacitor 310 forms the first pole 311a of the capacitor 311 and the first pole 313a of the capacitor 313 forms the second pole 311b of the capacitor 311. The first pole 310a of the bypass capacitor 310 and the first pole 308a of the capacitor 308 form the capacitor 307, i.e. the first pole 308a of the capacitor 308 forms the first pole 307a of the capacitor 307 and the first pole 310a of the bypass capacitor 310 forms the second pole 307b of the capacitor 307. The first pole 304a of the bypass capacitor 304 and the first pole 310a of the bypass capacitor 310 form the capacitor 306, i.e., the first pole 304a of the bypass capacitor 304 forms the first pole 306a of the capacitor 306 and the first pole 310a of the bypass capacitor 310 forms 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 turn-off between qubits Q1 and Q2 need to be determined by the following parameters: qubits Q1, Q2 and the respective capacitances (i.e., capacitances 301,308 and 313) and josephson junction (i.e., josephson junctions 302, 309 and 312) parameters of the adjustable coupler Qc; the capacitances of bypass capacitances 304 and 310; and qubits Q1, Q2, the coupling capacitance between the adjustable coupler Qc and the bypass capacitors 304, 310 (i.e., capacitor 303,305,306,307,311). By changing the bias current of the adjustable coupler Qc, the coupling strength between the first qubit Q1 and the second qubit Q2 can be changed, so that the coupling between the qubits can be completely switched off or arbitrarily adjusted.
Fig. 4A is a schematic circuit diagram of a capacitance-adjustable coupling unit according to a second embodiment of the present invention. The capacitance-adjustable coupling unit includes a grounded transmon qubit Q1, a grounded transmon form adjustable coupler Qc, a grounded transmon qubit Q2, a bypass capacitance 404, and a bypass capacitance 410. The specific circuit structure and connection relationship of the grounded type transmon qubit are described in detail in fig. 1A-1C, and are not described herein again. In fig. 4A the adjustable couplers Qc are shown in dashed lines and the josephson junctions 402, 409 and 412 are Squid double junctions.
Wherein the bypass capacitor 404 includes a first pole 404a coupled to the qubit Q1 via a capacitor 403 and coupled to the adjustable coupler Qc via a 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 capacitively 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 the dielectric layer of the bypass capacitor 404 is a layer of air. 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 bypass capacitor 404 and the first pole 401a of the capacitor 401 constitute a capacitor 403. The first pole 404a of the bypass capacitor 404 and the first pole 408a of the capacitor 408 constitute a capacitor 405. The first pole 410a of the bypass capacitor 410 and the first pole 413a of the capacitor 413 form a capacitor 411. The first pole 404a of the bypass capacitor 404 and the first pole 410a of the bypass capacitor 410 form 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 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 (i.e., capacitances 401,408 and 413) and josephson junction (i.e., josephson junctions 402, 409 and 412) parameters; the capacitance of bypass capacitors 404 and 410; and qubits Q1, Q2, the coupling capacitance between the adjustable coupler Qc and the bypass capacitors 404, 410 (i.e., capacitor 403,405,406,411). By changing the bias current of the adjustable coupler Qc, the coupling strength between the first qubit Q1 and the second qubit Q2 can be changed, so that the coupling between the qubits can be completely switched off or arbitrarily adjusted.
Fig. 5A is a schematic circuit diagram of a capacitance-adjustable coupling unit according to a third embodiment of the present invention. The capacitance-adjustable coupling unit includes a grounded transmon qubit Q1, a grounded transmon form adjustable coupler Qc, a grounded transmon qubit Q2, and a bypass capacitance 504. The specific circuit structure and connection relationship of the grounded type transmon qubit are described in detail in fig. 1A-1C, and are not described herein again. In fig. 5A the adjustable couplers Qc are shown in dashed lines and the 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 adjustable 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 capacitively tunable coupling unit of fig. 5A. As shown in FIG. 5B, a first pole 504a of the bypass capacitor 504 is capacitively coupled to qubit Q1, qubit Q2 and the tunable coupler Qc, a second pole 504B of the bypass capacitor 504 is GND, and the dielectric layer of the bypass capacitor 504 is an air layer.
The first pole 504a of the bypass capacitor 504 and the first pole 501a of the capacitor 501 form a capacitor 503. The first pole 504a of the bypass capacitor 504 and the first pole 508a of the capacitor 508 form a capacitor 505. The first pole 504a of the bypass capacitor 504 and the first pole 513a of the capacitor 513 form a capacitor 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 turn-off between qubits Q1 and Q2 need to be determined by the following parameters: qubits Q1, Q2 and the respective capacitances (i.e. capacitances 501,508 and 513) and josephson junction (i.e. josephson junctions 502, 509 and 512) parameters of the adjustable coupler Qc; the capacitance of the bypass capacitance 504; and qubits Q1, Q2, a coupling capacitance between the adjustable coupler Qc and the bypass capacitance 504 (i.e., capacitance 503,505,511). By changing the bias current of the adjustable coupler Qc, the coupling strength between the first qubit Q1 and the second qubit Q2 can be changed, so that the coupling between the qubits can be completely switched off or arbitrarily adjusted.
Fig. 6A is a schematic circuit structure diagram of a capacitance-adjustable coupling unit according to a fourth embodiment of the present invention. The capacitance-adjustable coupling unit includes a grounded transmon qubit Q1, a grounded transmon form adjustable coupler Qc, a grounded transmon qubit Q2, and a bypass capacitance 604. The specific circuit structure and connection relationship of the grounded type transmon qubit are described in detail in fig. 1A-1C, and are not described herein again. In fig. 6A the adjustable couplers Qc are shown in dashed lines and the josephson junctions 602, 609 and 612 are Squid double junctions.
Wherein the bypass capacitor 604 comprises a first pole 604a coupled to the qubit Q1 via a capacitor 603 and to the adjustable coupler Qc via a capacitor 605; and a second pole 304b connected to ground. The adjustable coupler Qc is coupled to the qubit Q2 via a capacitor 614.
Fig. 6B is a top view of the process structure of the capacitively 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 a capacitor 603. The first pole 604a of the bypass capacitor 604 and the first pole 608a of the capacitor 608 form a capacitor 605. The first pole 608a of the capacitor 608 and the first pole 613a of the capacitor 613 form a 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 turn-off between qubits Q1 and Q2 need to be determined by the following parameters: qubits Q1, Q2 and the respective capacitances (i.e. capacitances 601,608 and 613) and josephson junction (i.e. josephson junctions 602, 609 and 612) parameters of the adjustable coupler Qc; the capacitance of the bypass capacitance 604; and qubits Q1, Q2, a coupling capacitance between the adjustable coupler Qc and the bypass capacitance 604 (i.e., capacitance 603,605,614). By changing the bias current of the adjustable coupler Qc, the coupling strength between the first qubit Q1 and the second qubit Q2 can be changed, so that the coupling between the qubits can be completely switched off or arbitrarily adjusted.
In the above four embodiments, a bypass capacitor is introduced into the capacitance-adjustable coupling unit. The capacitance adjustable coupling unit may include a qubit Q1, a qubit Q2, an adjustable coupler Qc, and a first bypass capacitance. And the qubit Q1 and the adjustable coupler Qc are both capacitively coupled to the first bypass capacitor, and the qubit Q2 is capacitively coupled to the adjustable coupler Qc. In the first embodiment, the capacitance-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 Qc via the second bypass capacitor. In a second embodiment, the capacitance adjustable coupling unit further comprises 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 adjustable coupler Qc via the second bypass capacitance and the first bypass capacitance. In a third embodiment, qubit Q2 is capacitively coupled to a first bypass capacitor, which is coupled to the adjustable coupler Qc via the first bypass capacitor. In a fourth embodiment, qubit Q2 is capacitively coupled directly to the adjustable coupler Qc.
The capacitive coupling among the qubit Q1, the qubit Q2, the adjustable coupler Qc, the first bypass capacitor, and the second bypass capacitor may be capacitive coupling with ground isolation or may be direct capacitive coupling without ground.
In the above four embodiments, after introducing the bypass capacitance, the effective coupling coefficient g between qubit Q1 and qubit Q2 can be expressed as
g＝g 12 -g eff (1)
Wherein g is eff Is derived from the virtual exchange interaction term introduced by the tunable coupler:
wherein j =1,2; represent qubits Q1, Q2, respectively; energy level difference w c And w j Respectively a tunable coupler Qc and a qubit Q j The difference in energy levels of the ground state and the first excited state of (a); coefficient of coupling g 1c And g 2c Equivalent coupling coefficients of the qubit Q1 and the adjustable coupler Qc and the qubit Q2 and the adjustable coupler Qc after being converted by bypass capacitors are respectively obtained; 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 qubits is composed of two parts, namely the virtual exchange interaction term g introduced by the adjustable coupler eff And a direct coupling term g for qubit Q1 and qubit Q2 12 . G is realized by adjusting the frequency detuning of the tunable coupler and the qubit eff And g 12 So as to achieve the adjustability and the turn-off of the effective coupling g of the qubits Q1, Q2.
In one embodiment of the present invention, taking the tunable capacitance 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, and the junction resistance of each junction is 20000 Ω. Both bypass capacitors 304 and 310 are 36fF. Coupling capacitor 303 is 15fF, coupling capacitor 305 is 8fF, coupling capacitor 307 is 8fF, coupling capacitor 306 is 8fF, and coupling capacitor 311 is 15fF. The coupling coefficient between the superconducting qubit Q1 and the superconducting qubit Q2 is tunable in the range of 5MHz to-40 Hz by varying the bias current of the tunable coupler Qc.
According to an embodiment of the present invention, the qubits Q1, Q2, the adjustable coupler Qc and the bypass capacitor may have any desired shape, such as circular, polygonal, etc. The coupling coefficient between the qubits Q1 and Q2 can be adjusted and turned off only by meeting the requirement.
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 of fig. 3A-6B, as long as the bypass capacitor, the qubits Q1, Q2 and the adjustable coupler Qc can be introduced at the same time and the adjustable coupling between the bits can be realized.
In the capacitance-adjustable 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 plane. Materials for fabricating capacitors and josephson junctions include Al, nb, ta, and the like.
In the capacitance adjustable coupling unit, the first qubit Q1, the second qubit Q2 and the adjustable coupler Qc can be coplanar, namely, the first qubit Q1, the second qubit Q2 and the adjustable coupler Qc coexist on one substrate; it may also be non-coplanar, i.e., not on one substrate, e.g., first qubit Q1 on substrate a, second qubit Q2 and tunable coupler Qc on another substrate B, and then substrates a and B in close face-to-face proximity but not in contact, such that capacitive coupling remains between first qubit Q1, second qubit Q2 and tunable coupler Qc. The substrates a and B may be metal connected by indium posts, for example, one plate of a shunt capacitor may be simultaneously disposed on both opposing surfaces of the substrates a and B, and connected by indium posts. In this way, the first qubit Q1, the second qubit Q2 and the adjustable coupler Qc can be manufactured more easily and separately, increasing the success rate.
In the capacitance adjustable coupling unit, a bypass capacitor is introduced between the qubit and the adjustable coupler, and the adjustable coupling and the turn-off between the qubit Q1 and the qubit Q2 can still be realized through calculation. The length of the bypass capacitor between the qubit and the tunable coupler can be made very long (only the lumped capacitor characteristic needs to be met), which is helpful for wiring, puncturing and the like in large-scale qubit expansion. In the capacitance-adjustable coupling unit of the present invention, qubit Q1, qubit Q2, and adjustable coupler Qc may be in the form of either a grounded or floating transmon. For adopting the grounded transmon form, the whole area can be designed to be smaller, so that the coherent fading time of the qubit can be increased, the additional capacitance increase brought on the qubit capacitance when two chips are mutually inverted can be weakened in the flip-chip bonding design, and the extensive design of the large-scale superconducting qubit based on the flip-chip bonding is greatly facilitated.
The capacitance adjustable coupling unit of the invention introduces a single branch or a plurality of branch bypass capacitors and can realize adjustable coupling between two qubits. Due to the introduction of the bypass capacitor, the distance between the qubits is greatly increased, thereby providing sufficient wiring space for the expansion of large-scale superconducting qubits. In addition, the capacitance adjustable coupling unit can provide adjustable coupling type connection among chips for modularization of the quantum chip, and can play an important role in large-scale expansion of a single quantum chip and expansion of a modularization form.
Although the present invention has been described by way of preferred embodiments, the present invention is not limited to the embodiments described herein, and various changes and modifications may 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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