Coupled-line bus to suppress classical crosstalk for superconducting qubits

A system includes a first quantum circuit plane that includes a first qubit, a second qubit and a third qubit. A coupled-line bus is coupled between the first qubit and the second qubit. A second circuit plane is connected to the first quantum circuit plane, comprising a control line coupled to the third qubit. The control line and the coupled-line bus are on different planes and crossing over each other, and configured to mitigate cross-talk caused by the crossing during signal transmission.

BACKGROUND

Technical Field

The present disclosure generally relates to superconducting devices, and more particularly, qubit control.

Description of the Related Art

Superconducting quantum computing is an implementation of a quantum computer in superconducting electronic circuits. Quantum computation studies the application of quantum phenomena for information processing and communication. Various models of quantum computation exist, and the most popular models include the concepts of qubits and quantum gates. A qubit is a generalization of a bit that has two possible states, but can be in a quantum superposition of both states. A quantum gate is a generalization of a logic gate, however the quantum gate describes the transformation that one or more qubits will experience after the gate is applied on them, given their initial state.

SUMMARY

According to various embodiments, a system and quantum circuit structure are provided for reducing cross-talk between circuit elements that are on different planes. A first quantum circuit plane includes a first qubit, a second qubit, and a coupled-line bus coupled between a first qubit and a second qubit. There is a third qubit. A second circuit plane, sometimes referred to herein as an interposer chip, is connected to the first quantum circuit plane and includes a control line that is coupled to the third qubit. The control line and the coupled-line bus are on different planes and crossing over each other and configured to mitigate cross talk caused by the crossing during signal transmission.

In one embodiment, the coupled-line bus is configured to transmit differential mode signals between the first and second qubits of the first quantum circuit plane.

In one embodiment, the first quantum circuit plane and the second circuit plane are located on separate chips of a flip chip and connected together via bump bonds.

In one embodiment, the control line is a feed line that is configured to transmit a signal to drive the third qubit.

In one embodiment, the control line is a resonator that is configured to read a signal from the third qubit.

In one embodiment, the control line is orthogonal to the coupled-line bus at the crossing to suppress an inductive coupling between the control line and the coupled-line bus.

In one embodiment, the coupled-line bus is a dual strip coplanar waveguide (CPW) transmission-line resonator.

In one embodiment, the first qubit and the second qubit are coupled to the coupled-line bus differentially and configured to excite only an odd mode of the coupled-line bus.

In one embodiment, a wavelength of a qubit excitation in the control line is longer than a width and a gap of the coupled-line bus.

In one embodiment, a suppression ratio of a crosstalk between the coupled-line bus and the control line depends on a gap between lines of the coupled-line bus.

According to one embodiment, a method of reducing crosstalk between different circuit planes of a quantum circuit, such as a qubit chip and an interposer chip, is provided. A coupled-line bus is provided between a first qubit and a second qubit of a first quantum circuit plane. A control line is provided on a second circuit plane. The control line connects to a third qubit. The first circuit plane can be on a first quantum circuit plane, the second circuit plane can be on a second circuit plane, and the first quantum circuit plane is bonded to the second circuit plane such that the control line and the coupled-line bus are on different planes and crossing over each other and mitigate cross talk caused by the crossing during signal transmission.

In one embodiment, differential mode signals are transmitted between the first and second qubits of the first quantum circuit plane.

In one embodiment, the first quantum circuit plane and the second circuit plane are on different chips, and the chips are coupled (e.g., bonded or connected) together via bump bonds.

In one embodiment, the control line is a feed line and the third qubit is driven through the feed line.

In one embodiment, a common mode signal is generated on the coupled-line bus to electrically mitigate a cross talk from the control line to the coupled-line bus.

In one embodiment, an inductive coupling between the control line and the coupled-line bus is suppressed by arranging the control line to be orthogonal to the coupled-line bus.

In one embodiment, the first qubit and the second qubit are coupled to the coupled-line bus differentially to excite only an odd mode of the coupled-line bus.

By virtue of the features discussed herein, classical crosstalk between a first quantum circuit plane and a second circuit plane, such as a qubit chip and an interposer chip in a flip chip geometry, is substantially reduced. These and other features will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.

DETAILED DESCRIPTION

Overview

The present disclosure generally relates to superconducting devices, and more particularly, to efficient qubit control that suppresses classical crosstalk. The electromagnetic energy associated with a qubit can be stored in so-called Josephson junctions and in the capacitive and inductive elements that are used to form the qubit. In one example, to read out the qubit state, a microwave signal is applied to the microwave readout cavity that couples to the qubit at the cavity frequency. The transmitted (or reflected) microwave signal goes through multiple thermal isolation stages and low-noise amplifiers that are used to block or reduce the noise and improve the signal-to-noise ratio. The amplitude and/or phase of the returned/output microwave signal carries information about the qubit state, such as whether the qubit has decohered to the ground or excited state. The microwave signal carrying the quantum information about the qubit state is usually weak (e.g., on the order of a few microwave photons). To measure this weak signal, low-noise quantum-limited amplifiers (QLAs), such as Josephson amplifiers and travelling-wave parametric amplifiers (TWPAs), may be used as preamplifiers (i.e., first amplification stage) at the output of the quantum system to boost the quantum signal, while adding the minimum amount of noise as dictated by quantum mechanics, in order to improve the signal to noise ratio of the output chain. In addition to Josephson amplifiers, certain Josephson microwave components that use Josephson amplifiers or Josephson mixers such as Josephson circulators, Josephson isolators, and Josephson mixers can be used in scalable quantum processors.

The ability to include more qubits is salient to being able to realize the potential of quantum computers. Applicants have recognized that to increase the computational power and reliability of a quantum computer, improvements can be made along two main dimensions. First, is the qubit count itself. The more qubits in a quantum processor, the more states can in principle be manipulated and stored. Second is low error rates, which is relevant to manipulate qubit states accurately and perform sequential operations that provide consistent results and not merely unreliable data. Thus, to improve fault tolerance of a quantum computer, a large number of physical qubits should be used to store a logical quantum bit. In this way, the local information is delocalized such that the quantum computer is less susceptible to local errors and the performance of measurements in the qubits' eigenbasis, similar to parity checks of classical computers, thereby advancing to a more fault tolerant quantum bit.

As the number of qubits increases, the cross-talk between its wires becomes more prominent. Classical crosstalk is a phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit or channel to another. In the context of qubit architectures, crosstalk may occur when one a qubit is driven through its control line and unwanted signal is leaked to other qubits via spurious microwave coupling. Reference now is made in detail to the examples illustrated in the accompanying drawings and discussed below.

Example Architectures

FIG.1illustrates a typical superconducting flip-chip architecture100, where qubits are coupled to each other via buses. Architecture100includes first, second, and third qubits140,142, and150that are on a first qubit circuit plane. There is a single-line bus120between the first and second qubits140,142. For example, the first and second qubits140,142are coupled via a single-line coplanar waveguide (CPW) transmission-line resonator bus120on the first circuit plane. Other than bump bonded flip chips, the circuit planes can be on the same substrate, either on the (e.g., front/back) surfaces or buried, and connected by vias (e.g., either through silicon vias (TSVs) or regular vias). The coupling of the circuit planes can occur through a conductive connection (e.g., bumps, TSVs, or vias) or through electromagnetic coupling (e.g., capacitive or inductive).

The third qubit150is driven through its control line110, which is on a second circuit plane (e.g., interposer chip). In one embodiment, the interposer chip includes the control and/or readout resonators/lines for one or more qubits. The single-line bus120between the first and second qubits140,142of the first circuit plane and the control line110of the third qubit150intersect at the crossing point130, represented by a relatively large circle. It is noted that the smaller circles (e.g.,150) represent, by way of example only and not by way of limitation, bump bonds that may be used in flip-chip architectures, sometimes referred to as controlled collapse chip connection (C4). More generally, the small circles represent possible connection points between the first circuit plane and the second circuit plane. In the case of circuit planes on the same substrate, for example, the small circles represent TSVs or vias. It is at the intersection point130where crosstalk is manifest.

Thus, control lines (e.g.,110) of the second circuit plane (e.g., interposer chip) may cross the single-line buses (e.g.,120) between qubits in the first plane. Classical crosstalk is generated due to unwanted microwave coupling at the crossing points between the control lines and buses. In various scenarios, crossings can be arranged to be orthogonal to suppress the inductive coupling between the control lines and the bus lines. However, capacitive coupling will remain finite and continues to be a source of classical crosstalk.

Accordingly, in one aspect, what is provided herein is a coupled-line bus architecture that reduces the classical crosstalk between wires of different circuit planes, such as those in flip-chip architectures, by several orders of magnitude.

FIG.2is a superconducting qubit architecture,200consistent with an illustrative embodiment. Architecture200includes, by way of example only and not by way of limitation, first and second qubits240and242that are on a first plane, and a third qubit250. There is a coupled line bus220between the first and second qubits240,242. Accordingly, in contrast to known architectures that use a single-line bus between qubits, in the architecture ofFIG.2the first and second qubits240,242are coupled by a coupled-line bus220. In one embodiment, the coupled line bus220is implemented as a dual strip coplanar waveguide (CPW) transmission-line resonator coupled line bus220on a first circuit plane (e.g., chip).

In various embodiments, the coupled-line bus220provides odd mode, even mode, or both types of signal propagation. Even and odd modes are the two main modes of propagation of the signal through a coupled transmission line pair. In odd mode impedance is defined as impedance of a single transmission line when the two lines in a pair are driven differentially (with signals of the same amplitude and opposite polarity). In even mode impedance is defined as impedance of a single transmission line when the two lines in a pair are driven with a common mode signal (i.e., having the same amplitude and the same polarity).

These two modes of propagation can affect the signal integrity properties of these two lines. The impact of even-mode and odd-mode signal propagation on the signal integrity properties of two coupled lines may lead to increased or decreased values of the intended isolated characteristic impedance. For example, when two transmission lines are coupled to each other, the intended characteristic impedances of the two lines are affected by their relative switching characteristics.

For example, when the signal on a transmission line is synchronously switching with the signal from a nearby transmission line, and its signal polarity is opposite of the signal polarity from the nearby transmission line, this situation is referred to herein as odd-mode signal propagation.

The third qubit250inFIG.2is driven through its control line210. The third qubit has a corresponding control line210that is on a second plane (e.g., interposer chip). It will be understood that the control line210and its corresponding qubit250need not literally be on a same circuit level. For example, the control line210may be at a different metallization level that is one or more layers higher (or lower) than that of the qubit. Accordingly, the term “plane” is used herein to describe one or more groups of layers, such as those of quantum chips. The two circuit planes can be on separate chips in a flip chip geometry, where the second circuit plane on a second chip is bonded to the first quantum circuit plane on a first chip via bump bonds. In another example, the two circuit planes can be located on the same chip, for example, on opposite surfaces of the same chip, be buried, or on a surface circuit plane, where the first and the second circuit planes are connected by through-silicon vias or regular vias, respectively.

The coupled-line bus220between the first and second qubits240,242of the first circuit plane, and the control line210of the third qubit250intersect at the crossing point230, represented by a relatively large circle. As inFIG.1, the smaller circles (e.g.,250) represent bump bonds that may be used in flip-chip architectures.

InFIG.2, qubits240and242are coupled to the coupled-line bus220in a differential way such that they will excite only the odd mode of the coupled-line bus, whereas the control line210will excite only the even mode of the coupled bus line220. Thus, there will be no signal leaked from the control line210to the first and second qubits240,242of the first plane and the classical crosstalk significantly suppressed.

Reference now is made toFIG.3, which provides a close-up view300of the first qubit ofFIG.2, consistent with an illustrative embodiment. The first qubit240and the second qubit (not shown inFIG.3) are coupled differentially by the coupled-line bus220. Each qubit may be in each corresponding silicon substrate (e.g.,342for first qubit240). By way of example only and not by way of limitation, the structure ofFIG.3is illustrated to be on top of a ground plane330. As explained above, by virtue of using a coupled-line bus220the first and second qubits240,242excite only the odd-mode of the coupled bus line220. The +/− symbols inFIG.3are used to illustrate the differential excitation of the odd-mode of the coupled-line bus220.

FIG.4provides a close-up view400of the crossing point region ofFIG.2, consistent with an illustrative embodiment. In the example ofFIG.4, the control line210crosses the coupled-line bus220orthogonally, thereby suppressing the inductive coupling. A finite capacitive coupling may remain. However, applicants have determined that qubits are insensitive to this capacitive crosstalk since the control line210only excites the even-mode of the coupled-line bus220. In one embodiment, the voltage on the control line210is assumed to stay constant across the crossing region, which provides a good approximation, given the wavelength of the excitation of a qubit (e.g., first qubit240, second qubit242, and/or third qubit250) being much longer than the width of the coupled-line bus220slot (i.e., the gap between the lines of the coupled-line bus).

Using a dual strip CPW coupled-line bus (on the first quantum circuit plane) to couple the qubits, suppresses the capacitive crosstalk from the control line (on the interposer chip related to the control line210) by several orders of magnitude compared to using a single-line CPW transmission-line resonator ofFIG.1. In one aspect, the suppression ratio may mainly depend on a gap between the lines of the coupled-line bus220. Accordingly, the smaller the gap, the better the suppression ratio. However, smaller gap sizes reduce the internal quality factor. The internal quality factor at the single photon limit is determined by the coupling to TLS's (two-level systems) at the interfaces. The coupling happens by electric dipole coupling. Smaller gaps produce larger electric fields, hence, lower quality factors. Accordingly, there is a sweet spot where the internal quality factor is favorable and the crosstalk suppression is sufficient. In this regard, the coupled-line bus220has a gap that is based on the minimum pitch possible for that particular circuit design. With dimensions used in current practice the classical crosstalk can be reduced, for example, by up to two orders of magnitude compared to using a single-line bus having similar dimensions.

Conclusion