CN117332865A - A superconducting qubit energy level regulator - Google Patents

Abstract

The utility model discloses a superconductive quantum bit energy level regulator can regulate and control the SQUID magnetic flux, changes its jump current, and then regulates and control the quantum bit energy level, that is to say, the superconductive quantum bit energy level regulator that this application embodiment provided can be with the energy level of the first excited state of magnetic flux adjustable transmission sub-quantum bit repeatedly adjust to a series of discrete heights to realize Z control and some two door operation, and can adapt to the trend that superconductive quantum bit quantity expanded after combining with RSFQ digital circuit.

Description

A superconducting qubit energy level regulator

Technical field

This application relates to but is not limited to the field of quantum computing technology, and in particular, to a superconducting qubit energy level regulator.

Background technique

Split transmon is a qubit commonly used in the design of superconducting quantum processors. Its Z control and certain double-gate operations (such as Gate) needs to be realized by regulating the level of its energy level. Usually, a DC bias line, the Z control line, is designed next to the superconducting quantum interference device (SQUID, Superconducting Quantum Interference Device) of the qubit, and part of the magnetic field generated by the current from the room temperature control device on the Z control line passes through the SQUID , by regulating the current size to regulate the SQUID magnetic flux to change its jump current, thereby regulating the qubit energy level. Although this method is convenient for continuously changing the current of the Z control line at the room temperature end, it requires each qubit to be equipped with a Z control line and a corresponding room temperature control port. As the number of qubits increases, the number of Z control lines will also increase, which will inevitably occupy a large amount of internal space in the dilution refrigerator, thereby hindering the expansion of the number of qubits in the future.

Contents of the invention

This application provides a superconducting qubit energy level regulator that can regulate the SQUID magnetic flux, change its jump current, and thereby regulate the qubit energy level, and can adapt to the expansion of the number of superconducting qubits after being used in combination with an RSFQ digital circuit. trend.

Embodiments of the present invention provide a superconducting qubit energy level regulator, including: an inductor loop, at least one magnetic flux forward bias circuit, and at least one magnetic flux reverse bias circuit; wherein,

The inductance loop is respectively coupled to the superconducting quantum interferor SQUID, the magnetic flux forward bias circuit, and the magnetic flux reverse bias circuit in the qubit to be controlled. The inductance loop is used to maintain the SQUID. External magnetic flux required for jump current;

The magnetic flux forward bias circuit is used to receive the first single magnetic flux quantum SFQ pulse signal and increase the external magnetic flux of the SQUID in the positive direction;

The magnetic flux reverse bias circuit is used to receive the second SFQ pulse signal and reversely increase the external magnetic flux of the SQUID.

In an illustrative example, the qubit to be controlled is a flux-tunable transport sub-qubit.

In an illustrative example, the inductor loop includes a plurality of inductors connected in series for coupling with the SQUID in the qubit to be controlled, the magnetic flux forward bias circuit, and the magnetic flux reverse bias circuit.

In an illustrative example, the flux forward bias circuit includes: a first Josephson junction connected in parallel and an inductor for coupling with the inductor loop.

In an illustrative example, the flux forward bias circuit further includes: a first resistor connected in parallel with the first Josephson junction.

In an illustrative example, the magnetic flux reverse bias circuit includes: a second Josephson junction connected in parallel and another inductor for coupling with the inductor loop.

In an exemplary example, the magnetic flux reverse bias circuit further includes: a second resistor connected in parallel with the second Josephson junction.

In an illustrative example, the inductance loop includes a first inductor, a second inductor and a third inductor connected in series; the third inductor is coupled with the seventh inductor in the SQUID to form a mutual inductance, and the second inductor is The inductor is coupled with the sixth inductor in the magnetic flux reverse bias circuit to form a mutual inductance, and the first inductor is coupled with the fifth inductor in the magnetic flux forward bias circuit to form a mutual inductance;

The magnetic flux forward bias circuit includes a first Josephson junction and the fifth inductor connected in parallel; wherein the connection node between the first Josephson junction and the fifth inductor is the input of the first SFQ pulse signal. Node, the fifth inductor and the first inductor in the inductor loop are coupled to form a mutual inductance and have the same terminal direction;

The magnetic flux reverse bias circuit includes a parallel second Josephson junction and the sixth inductor; wherein the connection node between the second Josephson junction and the sixth inductor is the input of the second SFQ pulse signal. Node, the sixth inductor is coupled with the second inductor in the inductor loop to form a mutual inductance and the direction of the same terminal is opposite;

The flux-adjustable transmission sub-qubit includes: a capacitor connected in parallel for coupling the SQUID with the third inductor and a grounding capacitor.

In an exemplary example, the inductor loop further includes a fourth inductor connected in series with the first inductor, the second inductor and the third inductor.

In an illustrative example, the SQUID includes two Josephson junctions and a seventh inductor for coupling with the third inductor.

In an illustrative example, the fifth inductor and the sixth inductor are coupled to generate mutual inductance.

The superconducting qubit energy level regulator provided by the embodiments of the present application can regulate the SQUID magnetic flux, change its jump current, and thereby regulate the qubit energy level. In other words, the superconducting qubit energy level regulator provided by the embodiments of the present application can The device can reproducibly adjust the energy level of the first excited state of the flux-tunable transport sub-qubit to a series of discrete heights, thereby achieving Z control and certain double-gate operations (such as gate), and can adapt to the trend of expanding the number of superconducting qubits when combined with RSFQ digital circuits.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention may be realized and obtained by the structure particularly pointed out in the written description, claims and appended drawings.

Description of drawings

The drawings are used to provide a further understanding of the technical solution of the present application and constitute a part of the specification. They are used to explain the technical solution of the present application together with the embodiments of the present application and do not constitute a limitation of the technical solution of the present application.

Figure 1 is a schematic structural diagram of a superconducting qubit energy level regulator in an embodiment of the present application;

Figure 2 is a schematic structural diagram of the first embodiment of the superconducting qubit energy level regulator in the embodiment of the present application;

Figure 3 is a schematic structural diagram of a second embodiment of the superconducting qubit energy level regulator in the embodiment of the present application.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present application more clear, the embodiments of the present application will be described in detail below with reference to the accompanying drawings. It should be noted that, as long as there is no conflict, the embodiments and features in the embodiments of this application can be arbitrarily combined with each other.

Figure 1 is a schematic structural diagram of a superconducting qubit energy level regulator in an embodiment of the present application. As shown in Figure 1, it at least includes: an inductor loop 10 and at least one magnetic flux forward bias circuit 20 (as shown in Figure 1 As shown in Figure 1, it may include a first magnetic flux forward bias circuit 20... an Nth magnetic flux forward bias circuit 20) and at least one magnetic flux reverse bias circuit 30 (as shown in Figure 1, it may include a first magnetic flux reverse bias circuit 30...Mth magnetic flux reverse bias circuit 30); where,

The inductor loop 10 is coupled to the SQUID 40, the flux forward bias circuit 20, and the flux reverse bias circuit 30 in the qubit 50 to be controlled respectively. The inductor loop 10 is used to maintain the external circuit required for the jump current of the SQUID 40. magnetic flux;

The magnetic flux forward bias circuit 20 is used to receive the first single flux quantum (SFQ, Single Flux Quantum) pulse signal and increase the external magnetic flux of the SQUID 40 in the positive direction;

The magnetic flux reverse bias circuit 30 is used to receive the second SFQ pulse signal and increase the external magnetic flux of the SQUID 40 in a reverse direction.

In an illustrative example, a Rapid Single Flux Quantum (RSFQ) superconducting digital circuit has the advantages of high speed, low power consumption, and compatibility with traditional integrated circuit processes, and can be integrated with superconducting qubits in the same processor. , thereby eliminating the need for microwave coaxial lines used to directly control superconducting qubits. Therefore, using RSFQ superconducting digital circuits to control qubits will be a feasible solution to expand the number of qubits in the future.

The superconducting qubit energy level regulator provided by the embodiments of the present application can regulate the SQUID magnetic flux, change its jump current, and thereby regulate the qubit energy level. In other words, the superconducting qubit energy level regulator provided by the embodiments of the present application can The device can reproducibly adjust the energy level of the first excited state of the flux-tunable transport sub-qubit to a series of discrete heights, thereby achieving Z control and certain double-gate operations (such as gate), and can adapt to the trend of expanding the number of superconducting qubits when combined with RSFQ digital circuits.

In an exemplary example, the values of N and M may be the same or different. The values of N and M can be determined according to actual needs. That is to say, the number of magnetic flux forward bias circuits 20 and magnetic flux reverse bias circuits 30 can be designed according to actual needs.

In an illustrative example, the qubit 50 to be controlled is a flux-adjustable transport sub-qubit (Splittransmon), and the inductor loop 10 may include multiple series-connected inductors, which are used to interact with the SQUID 40 and the magnetic flux forward bias. The circuit 20 and the magnetic flux reverse bias circuit 30 are coupled. In one embodiment, as shown in Figure 2, it includes at least a first inductor 101, a second inductor 102 and a third inductor 103; the third inductor 103 and the SQUID 40 in the magnetic flux adjustable transport sub-qubit 50 The seventh inductor 402 is coupled to form a mutual inductance, the second inductor 102 is coupled to the sixth inductor 302 in the magnetic flux reverse bias circuit 30 to form a mutual inductance, and the first inductor 101 is coupled to the fifth inductor 202 in the magnetic flux forward bias circuit 20 to form a mutual inductance.

In an illustrative example, as shown in FIG. 2 , the magnetic flux forward bias circuit 20 includes: a first Josephson junction 201 and a fifth inductor 202 for coupling with the inductor loop 10 . The first Josephson junction 201 and the fifth inductor 202 are connected in parallel to form the magnetic flux forward bias circuit 20. Among them, the connection node between the first Josephson junction 201 and the fifth inductor 202 (node A in Figure 2) is the input node of the first SFQ pulse signal, and the fifth inductor 202 and the first inductor 101 in the inductor loop 10 The coupling forms mutual inductance and the same ends are in the same direction. When the frequency corresponding to the energy level needs to be increased, a corresponding SFQ pulse is input at node B to positively increase the external magnetic flux required for the jump current of SQUID 40.

In an illustrative example, as shown in FIG. 2 , the magnetic flux reverse bias circuit 30 includes: a second Josephson junction 301 and another sixth inductor 302 for coupling with the inductor loop 10 . The junction 301 and the sixth inductor 302 are connected in parallel to form the magnetic flux reverse bias circuit 30 . Among them, the connection node between the second Josephson junction 301 and the sixth inductor 302 (node B in Figure 2) is the input node of the second SFQ pulse signal, and the sixth inductor 302 and the second inductor 102 in the inductor loop 10 Coupling creates mutual inductance with opposite ends. When the frequency corresponding to the energy level needs to be lowered, a corresponding SFQ pulse is input from node B to reversely increase the external magnetic flux required for the jump current of SQUID 40.

In an illustrative example, as shown in FIG. 2 , the magnetic flux forward bias circuit 20 and the magnetic flux reverse bias circuit 30 may be coupled to generate mutual inductance. In one embodiment, the second inductor 202 in the flux forward bias circuit 20 and the sixth inductor 302 in the flux reverse bias circuit 30 may be coupled to generate mutual inductance.

In an illustrative example, the flux forward bias circuit 20 may further include: a first resistor (not shown in the drawing), the first resistor is connected in parallel with the first Josephson junction 201 in the flux forward bias circuit 20, To form an overdamped junction or a critically damped junction.

In an illustrative example, the flux reverse bias circuit 30 may further include: a second resistor (not shown in the drawing), the second resistor is connected in parallel with the second Josephson junction 301 in the flux reverse bias circuit 30, To form an overdamped junction or a critically damped junction.

In an illustrative example, as shown in FIG. 2 , the flux-adjustable transmission sub-qubit 50 includes: a SQUID 40 and a ground capacitor 501 , the SQUID 40 and the ground capacitor 501 are connected in parallel, and are used to connect with the inductor loop 10 The third inductor 103 is coupled to the seventh inductor 402 . In one embodiment, the SQUID 40 in the flux-adjustable transport sub-qubit 50 includes two Josephson junctions and a self-inductor, that is, a seventh inductor 402, wherein the two Josephson junctions are shown in the embodiment of Figure 2 The Josephson Knot 4011 and the Josephson Knot 4012. In one embodiment, Josephson junction 4011 and Josephson junction 4012 are identical and SQUID 40 is symmetrical.

The following is a detailed description taking the superconducting qubit energy level regulator including: an inductor loop 10, a magnetic flux forward bias circuit 20 and a magnetic flux reverse bias circuit 30 as an example.

As shown in Figure 3, in this embodiment, the qubit 50 to be controlled is a magnetic flux adjustable transport sub-qubit; the superconducting qubit energy level regulator in this embodiment includes an inductor loop 10, a magnetic A forward bias circuit 20 and a magnetic flux reverse bias circuit 30; among which,

The inductor loop 10 is used to maintain the external magnetic flux required for the jump current of the SQUID 40 in the magnetic flux-adjustable transmission sub-qubit 50. In this embodiment, the inductor loop 10 includes four series-connected inductors: An inductor 101 is the inductor L _n1 , the second inductor 102 is the inductor L _n2 , the third inductor 103 is the inductor L ₀ , and the fourth inductor 105 is the inductor _Lu , wherein the first inductor 101 is the inductor L _n1 and the second inductor 102 That is, the inductor L _n2 and the third inductor 103 or the inductor L _n are respectively connected with the fifth inductor 202 in the flux forward bias circuit 20 or the inductor L ₁ , the sixth inductor 302 in the flux reverse bias circuit 30 or the inductor L ₂ and SQUID. The seventh inductor 104 in 40 is the inductor L _s coupling, the corresponding mutual inductances are M ₁ , M ₂ and M respectively, and the corresponding coupling coefficients are k ₁ , k ₂ and k respectively. In one embodiment, by adjusting the value of the fourth inductance _Lu in the inductance loop 10, the induced current corresponding to a single SFQ pulse input to the magnetic flux forward bias circuit 20 or the magnetic flux reverse bias circuit 30 can be adjusted, Then adjust the external magnetic flux corresponding to SQUID 40.

The magnetic flux forward bias circuit 20 is used to receive the first SFQ pulse signal and increase the external magnetic flux of the SQUID 40 in the positive direction. In this embodiment, the magnetic flux forward bias circuit 20 consists of a Josephson junction 101, that is, a Josephson junction J ₁ and a fifth inductor 202, that is, the inductor L _1, are formed in parallel. The same end of the inductor L ₁ is consistent with the same end of the inductor L _n1 in the coupled inductor loop 10. The inductor L ₁ is also connected to the magnetic flux reverse bias circuit 30 The inductor L ₂ in is coupled, the mutual inductance is M ₀ , the coupling coefficient is k ₀ , and the same ends are consistent;

The magnetic flux reverse bias circuit 30 is used to receive the second SFQ pulse signal and reduce or reversely increase the external magnetic flux of the SQUID 40. In this embodiment, the magnetic flux reverse bias circuit 30 consists of a Josephson junction 201, that is, a Josephson junction. J ₂ is formed in parallel with a sixth inductor 302 , that is, the inductor L ₂ . The same end of the inductor L ₂ is opposite to the same end of the inductor L _n2 in the coupled inductor loop 10 .

In the embodiment shown in Figure 3, the qubit controlled by the superconducting qubit energy level regulator in this embodiment, namely the flux-adjustable transmission sub-qubit 50, is composed of a SQUID 40 and a grounding capacitor 501 connected in parallel. Among them, SQUID 40 is symmetrical, and its two Josephson junctions are Josephson junctions 4011 (Josephson junctions J _q1 and J _q2 in Figure 3 are the same, and the seventh inductor 104 in SQUID 40 is the inductor L _s .

The parameter design in the embodiment of the superconducting qubit energy level regulator shown in Figure 3 is shown in Table 1:

Note: In Table 1, Φ ₀ ≈2.0678×10 ^-15 Wb is the magnetic flux quantum.

Table 1

As shown in Figure 3, combined with Table 1, in one embodiment, the idle frequency of the qubit is 6.0GHz. If you want to adjust its frequency to 5.5GHz, that is, change the frequency corresponding to the energy level of the first excited state If the frequency is adjusted lower than the base state frequency by 0.5GHz, then one SFQ pulse can be input at node B. In one embodiment, if you want to restore the frequency to 6.0 GHz, then you can input 1 SFQ pulse at node A. Here, the change in the energy level height of the qubit's first excited state or the frequency of the qubit is related to the number of input SFQ pulses. If the control accuracy needs to be improved, in one embodiment, this can be achieved by increasing the number of magnetic flux forward bias circuits 20 and/or magnetic flux reverse bias circuits 30 that have different inductive coupling coefficients from the inductive loop 10 .

The working principle of the superconducting qubit energy level regulator in this embodiment is as follows:

According to Kirchhoff's voltage law, the potentials of node A, node B, and node C can be expressed as formula (1), formula (2), and formula (3) respectively:

Among them, i ₁ , i ₂ , i and i _s are the currents of the inductors in the magnetic flux forward bias circuit 20 , the magnetic flux reverse bias circuit 20 , the inductance loop 10 and the SQUID 40 respectively. Integrate formula (1), formula (2) and formula (3) over time (from 0 to t) respectively to obtain formula (4), formula (5) and formula (6):

-M ₂ i ₂ (t)+L _o i(t)+Mi _s (t)+L _u i(t)

Since node C is grounded, then, Assume,

Formula (7) and formula (8) are the integral of the inductor voltage with respect to time of the magnetic flux forward bias circuit 20 and the magnetic flux reverse bias circuit 30 respectively. Suppose further:

Inductor L ₃ =L _n1 +L _n2 +L _o +L _u (9)

Then, formula (6) can be expressed as formula (10):

0＝L ₃ i(t)+M ₁ i ₁ (t)-M ₂ i ₂ (t)+Mi _s (t) (10)

In this embodiment, assuming that when designing the circuit layout, it is ensured that there is weak coupling between the inductor loop 10 and the qubit SQUID 40, then the impact of the current of the SQUID 40 on the inductor loop 10 will be very small, that is, the mutual inductance M is much less than Inductor L ₃ , and because the size of the current i _s (t) does not exceed the critical current of the Josephson junction (such as tens of nanoamps (nA)), the circuit design can be made such that i (t) (tens of microamps (uA) )) is much larger than it, so Mi _s (t) is much smaller than L ₃ i(t). Mi _s (t) in formula (10) can be omitted, and formula (10) is simplified to formula (11):

0＝L ₃ i(t)+M ₁ i ₁ (t)-M ₂ i ₂ (t) (11)

Simultaneously combine formula (4), formula (5), formula (7), formula (8), and formula (11) to obtain formula (12):

In formula (12),

can be solved That is shown in formula (16):

Therefore, formula (17) can be obtained:

The qubit frequency is f _q , and the energy level difference between its ground state and the first excited state is shown in formula (18):

Among them, the capacitor charging energy Reduced external magnetic flux/> Equivalent Josephson junction energy of SQUID/>

In this embodiment, it is assumed that at the beginning, there is no voltage signal at node A and node B. When the external magnetic flux is 0, the qubit is idle. f _q =6.0GHz. In one embodiment, if 1 SFQ pulse signal is input at node B, that is Then, according to formula (17), i(t)=66.17uA is obtained. Therefore, the external magnetic flux formed by the current of the inductor loop 10 on the SQUID 40 of the qubit is Φ _sq =Mi(t)=3.705×10 ^-16 Wb, then, Decreased by its regulation, calculated from formula (18), the first excited state energy level of the qubit decreases relative to the ground state energy level, the frequency becomes f _q =5.5GHz, and the qubit is busy. In one embodiment, if you want to restore the idle state, you need to input an SFQ pulse signal at node A, so Φ _A =Φ ₀ and Φ _B =Φ ₀ . According to formula (17), this makes i(t)=0, that is, the external magnetic flux returns to 0, and the qubit returns to the idle state.

In one embodiment, controlling the time a qubit is in a busy state enables a single gate operation that changes the phase of the qubit. If a qubit is coupled to other qubits when it is busy and the coupling time is controlled well, then it can be achieved door operation.

Although the embodiments disclosed in the present application are as above, the described contents are only used to facilitate the understanding of the present application and are not intended to limit the present application. Anyone skilled in the field to which this application belongs can make any modifications and changes in the form and details of the implementation without departing from the spirit and scope disclosed in this application. However, the scope of patent protection of this application still must The scope is defined by the appended claims.

Claims (11)

1. A superconducting qubit energy level regulator, including: an inductor loop, at least one magnetic flux forward bias circuit, and at least one magnetic flux reverse bias circuit; wherein, The inductance loop is respectively coupled to the superconducting quantum interferor SQUID, the magnetic flux forward bias circuit, and the magnetic flux reverse bias circuit in the qubit to be controlled. The inductance loop is used to maintain the SQUID. External magnetic flux required for jump current; The magnetic flux forward bias circuit is used to receive the first single magnetic flux quantum SFQ pulse signal and increase the external magnetic flux of the SQUID in the positive direction; The magnetic flux reverse bias circuit is used to receive the second SFQ pulse signal and reversely increase the external magnetic flux of the SQUID. 2. The superconducting qubit energy level regulator according to claim 1, wherein the qubit to be controlled is a flux-adjustable transport sub-qubit. 3. The superconducting qubit energy level regulator according to claim 1, wherein the inductance loop includes a plurality of SQUIDs and the magnetic flux forward bias circuit connected in series with the qubit to be controlled. , the inductor coupled to the magnetic flux reverse bias circuit. 4. The superconducting qubit energy level regulator according to claim 1, wherein the magnetic flux forward bias circuit includes: a first Josephson junction connected in parallel and an inductor for coupling with the inductor loop. . 5. The superconducting qubit energy level regulator according to claim 4, wherein the magnetic flux forward bias circuit further comprises: a first resistor connected in parallel with the first Josephson junction. 6. The superconducting qubit energy level regulator according to claim 1, wherein the magnetic flux reverse bias circuit includes: a second Josephson junction connected in parallel and another for coupling with the inductance loop. inductance. 7. The superconducting qubit energy level regulator according to claim 6, wherein the magnetic flux reverse bias circuit further comprises: a second resistor connected in parallel with the second Josephson junction. 8. The superconducting qubit energy level regulator according to claim 2, wherein, The inductance loop includes a first inductor, a second inductor and a third inductor connected in series; the third inductor is coupled with the seventh inductor in the SQUID to form a mutual inductance, and the second inductor is counter-biased to the magnetic flux. The sixth inductance coupling in the circuit forms a mutual inductance, and the first inductance couples with the fifth inductance in the magnetic flux forward bias circuit to form a mutual inductance; The magnetic flux forward bias circuit includes a first Josephson junction and the fifth inductor connected in parallel; wherein the connection node between the first Josephson junction and the fifth inductor is the input of the first SFQ pulse signal. Node, the fifth inductor and the first inductor in the inductor loop are coupled to form a mutual inductance and have the same terminal direction; The magnetic flux reverse bias circuit includes a parallel second Josephson junction and the sixth inductor; wherein the connection node between the second Josephson junction and the sixth inductor is the input of the second SFQ pulse signal. Node, the sixth inductor is coupled with the second inductor in the inductor loop to form a mutual inductance and the direction of the same terminal is opposite; The flux-adjustable transmission sub-qubit includes: a capacitor connected in parallel for coupling the SQUID with the third inductor and a grounding capacitor. 9. The superconducting qubit energy level regulator according to claim 8, wherein the inductor loop further includes a fourth inductor connected in series with the first inductor, the second inductor and the third inductor. . 10. The superconducting qubit energy level regulator according to claim 8 or 9, wherein the SQUID includes two Josephson junctions and a seventh inductor for coupling with the third inductor. 11. The superconducting qubit energy level regulator according to claim 8 or 9, wherein the fifth inductor and the sixth inductor are coupled to generate mutual inductance.