Patent Description:
As the number of qubits on a given quantum processor increases, it becomes necessary to move quantum information between qubits fabricated on separate chips, especially for applications such as quantum error correction. The current state of the art uses planar structures, such as bus resonators, to transmit quantum information between qubits.

<CIT> describes novel qubit device packages, as well as related computing devices and methods. In one embodiment, an exemplary qubit device package includes a qubit die and a package substrate, where the qubit die is coupled to the package substrate using one or more preforms. In particular, a single preform may advantageously be used to replace a plurality of individual contacts, e.g., a plurality of individual solder bumps, electrically coupling the qubit die to the package substrate. Such packages may reduce design complexity and undesired coupling and enable inclusion of larger numbers of qubits in a single qubit die.

<CIT> describes quantum circuit assemblies that implement adaptive programming of quantum dot qubit devices. An example quantum circuit assembly includes a quantum circuit component including a quantum dot qubit device, and a control logic coupled to the quantum circuit component. The control logic is configured to adaptively program the quantum dot qubit device by iterating a sequence of applying one or more signals to the quantum dot qubit device, determining a state of at least one qubit of the quantum dot qubit device, and using the determined state to modify the signals to be applied to the quantum dot qubit device in the next iteration. In this manner, the signals may be fine-tuned to achieve a higher probability of the qubit(s) in the quantum dot qubit device being set to the desired state.

<CIT> describes quantum computing performed by operating a quantum processor cell that includes a three-dimensional device. The quantum processor cell can be operated in a fault-tolerant regime.

<FIG> is a schematic illustration of a system <NUM> for transmission of quantum information for quantum error correction according to an embodiment of the invention. The system <NUM> includes an ancilla qubit chip <NUM> comprising a plurality of ancilla qubits <NUM>, <NUM>, <NUM>. The system <NUM> includes a data qubit chip <NUM> spaced apart from the ancilla qubit chip <NUM>. The data qubit chip <NUM> comprising a plurality of data qubits <NUM>, <NUM>, <NUM>. The system <NUM> includes an interposer <NUM> coupled to the ancilla qubit chip <NUM> and the data qubit chip <NUM>. The interposer <NUM> includes a dielectric material <NUM> and a plurality of superconducting structures <NUM>, <NUM>, <NUM> formed in the dielectric material <NUM>. The superconducting structures <NUM>, <NUM>, <NUM> enable transmission of quantum information between the plurality of data qubits <NUM>, <NUM>, <NUM> on the data qubit chip <NUM> and the plurality of ancilla qubits <NUM>, <NUM>, <NUM> on the ancilla qubit chip <NUM> via virtual photons for quantum error correction.

As shown in <FIG>, the interposer <NUM> according to an embodiment of the invention includes a first surface <NUM> and a second surface <NUM> opposite the first surface <NUM>. The ancilla qubit chip <NUM> is coupled to the first surface <NUM> of the interposer <NUM>, and the data qubit chip <NUM> is coupled to the second surface <NUM> of the interposer <NUM>.

According to an embodiment of the current invention, each superconducting structure of the plurality of superconducting structures extends from a data qubit of the plurality of data qubits to an ancilla qubit of the plurality of ancilla qubits. For example, in <FIG>, the superconducting structure <NUM> extends from the data qubit <NUM> to the ancilla qubit <NUM>. According to an embodiment of the current invention, the data qubit <NUM> has a first frequency, the ancilla qubit <NUM> has a second frequency, and a superconducting resonator comprising the superconducting structure <NUM>, the solder bumps <NUM> and <NUM>, and the right-angle capacitor couplers on chips <NUM> and <NUM> has a third frequency. The superconducting structure <NUM> formed in the interposer allows the transmission of quantum information because it forms a part of a superconducting resonator. The superconducting resonator may also include the solder bumps <NUM> and <NUM> and as well as structures on the ancilla and data qubit chips that are galvanically coupled to the solder bumps <NUM> and <NUM>. The structures on the ancilla and data qubit chips may be coplanar waveguide transmission lines, as is the case in the figures, although embodiments of the invention are not limited to coplanar waveguide transmission lines. In some embodiments, the superconducting structure <NUM> itself may form the superconducting resonator.

The frequency of the superconducting resonator, referred to herein as the third frequency, is sufficiently detuned from the first frequency and the second frequency to prevent real photon transfer between the data qubit and the ancilla qubit. Instead, quantum information is transferred from the data qubit <NUM> and the ancilla qubit <NUM> by virtual photon transfer. Virtual photon transfer ensures that the quantum information stored in the data qubit <NUM> is immune from the electromagnetic Purcell effect. Virtual photon transfer also protects the quantum information from the effects of dielectric loss of the insulating material forming the interposer <NUM>.

According to an embodiment of the invention, the ancilla qubit chip is bonded to the interposer. In <FIG>, the ancilla qubit chip <NUM> is bonded to the interposer <NUM> using a plurality of solder bumps <NUM>, <NUM>, <NUM>. The solder bumps couple the ancilla qubits <NUM>, <NUM>, <NUM> to the superconducting structures <NUM>, <NUM>, <NUM>. As shown in <FIG>, the solder bumps may be galvanically coupled to the superconducting structures <NUM>, <NUM>, <NUM>, and capacitively coupled the ancilla qubits <NUM>, <NUM>, <NUM>. Embodiments of the current invention are not limited to the particular number of ancilla qubits, data qubits, and superconducting structures, and solder bumps shown in <FIG>.

According to an embodiment of the invention, the data qubit chip is bonded to the interposer. In <FIG>, the data qubit chip <NUM> is bonded to the interposer <NUM> using a plurality of solder bumps <NUM>, <NUM>, <NUM>. As shown in <FIG>, the solder bumps may be galvanically coupled to the superconducting structures <NUM>, <NUM>, <NUM>, and capacitively coupled to the data qubits <NUM>, <NUM>, <NUM>. The solder bumps may be formed from a superconducting material, although the embodiments of the invention are not limited to solder bumps formed from superconducting materials. One example material for the solder bumps is indium. The system <NUM> according to an embodiment of the present invention may include multiple ancilla qubit chips and data qubit chips. The ancilla qubit chips and data qubit chips may be bonded to a single interposer, or to multiple interposers.

<FIG> is a schematic illustration of a top-down view of an ancilla qubit chip <NUM> according to an embodiment of the current invention. The ancilla qubit <NUM> includes three ancilla qubits <NUM>, <NUM>, <NUM>. However, ancilla qubit chips according to other embodiments of the current invention are not limited to any particular number of ancilla qubits. There can be more than three, or less than three ancilla qubits in other embodiments.

<FIG> is a schematic illustration of a top-down view of an interposer <NUM>. The interposer includes a plurality of superconducting structures <NUM>, <NUM>, <NUM>, <NUM>. The superconducting structures may be superconducting vias, for example. The superconducting vias could be part of superconducting transmission line resonators that are partially or wholly formed within the interposer. The superconducting structures may be formed from one or more of niobium, aluminum, tin, electroplated rhenium, or indium, for example. Although the embodiment of <FIG> shows an example of four superconducting structures <NUM>, <NUM>, <NUM>, <NUM>, other embodiments could have less than four or more than four.

<FIG> is a schematic illustration of a top-down view of a data qubit chip <NUM>. The data qubit chip <NUM> includes two data qubits <NUM>, <NUM>. Other embodiments of data qubit chips could have more than or less than two data qubits.

<FIG> is a schematic illustration of a top-down view of the interposer coupled to the ancilla chip and the data chip. The ancilla qubits <NUM>, <NUM>, <NUM> and the data qubits <NUM>, <NUM> are connected to each other by the superconducting structures <NUM>, <NUM>, <NUM>, <NUM>.

Embodiments of the current invention enable transmission of quantum information for quantum error correction. Quantum error correction often requires a large number of data qubits and ancilla qubits to be coupled to each other. The data qubit are qubits that have relatively long relaxation and coherence times, while the ancilla qubits may be qubits that have relatively short relaxation and coherence times. Quantum information is spread over a collection of data qubits. The data qubits are coupled to ancilla qubits such that errors in the quantum information are mapped form the data qubits to the ancilla qubits. The ancilla qubits can be measured to detect and/or correct the errors.

Quantum error correction algorithms, such as, but not limited to, the Surface Code, the Shor Code, and the Steane Code require frequent measurements of the ancilla qubits. These measurements provide information about the data qubits to which the ancilla qubits are coupled, and also stabilize the data qubits. The frequency of the measurements necessitates fast measurements, which require strong coupling between the measurement resonators coupled to the ancilla qubits and the environment. Although the strong coupling enables fast measurement of the ancilla qubits, it also makes the ancilla qubits more susceptible to environmental noise and increases the spontaneous decay rate of the ancilla qubits through the Purcell effect. This strong coupling, if made to the data qubits, would shorten the lifetime of the quantum states in the data qubits.

Embodiments of the current invention enable strong coupling between the ancilla qubits and the environment, while reducing the coupling between the data qubits and the environment. The ancilla qubits are physically separated from the data qubits, and are coupled to the data qubits by superconducting structures formed in the interposer.

The physical separation also allows different materials and processes to be used for the formation of the data qubit chip and the ancilla qubit chip. Although both chips may include a plurality of qubits, the quality requirements for the data qubits and ancilla qubits may be very different. The requirements for the ancilla qubits may be based on how frequently they are measured. According to some embodiments, the ancilla qubit measurement cycle may be about <NUM>, so the ancilla qubits may have coherence times greater than <NUM>, for example, on the order of a few microseconds. The material requirements for such qubits are not as stringent as those used to fabricate higher quality qubits, such as the data qubits. Further, the ancilla qubit chip can be formed and modified using fabrication methods such as lithography that can change the frequency of the ancilla qubits. The ancilla qubit chip can also be formed such that the ancilla qubits are tunable qubits. While having tunable qubits can aid in system control, the process of forming the tunable qubits can require breaking the ground plane of the microwave resonators coupled to the qubits. This could be undesirable for the data qubits because of the introduction of flux noise susceptibility and spurious microwave modes, but may be acceptable for the ancilla qubits, which are allowed a shorter coherence time.

According to an embodiment of the current invention, the interposer includes a dielectric material that is, for example, a printed circuit board, an organic laminate, a silicon chip, a ceramic, a glass-reinforced epoxy laminate material such as FR-<NUM>, duroid, or polyether ether ketone (PEEK).

According to an embodiment of the current invention, the ancilla qubit chip includes ancilla measurement resonators coupled to the plurality of ancilla qubits. The ancilla measurement resonators are configured for measurement of the plurality of ancilla qubits. The ancilla measurement resonators may be, for example, superconducting microwave coplanar waveguide resonators. <FIG> is a schematic illustration of an ancilla qubit chip <NUM> that includes an ancilla qubit <NUM> and an ancilla measurement resonator <NUM> configured for measurement of the ancilla qubit <NUM>. The ancilla measurement resonator <NUM> may capacitively couple to the ancilla qubit <NUM> to measurement and control instruments. <FIG> shows a capacitor <NUM> that capacitively couples the ancilla measurement resonator <NUM> and the ancilla qubit <NUM> to a port <NUM> with measurement and control instruments.

According to an embodiment of the current invention, the data qubit chip includes data measurement resonators coupled to the plurality of data qubits. The data measurement resonators may be, for example, superconducting microwave resonators. <FIG> is a schematic illustration of a data qubit chip <NUM> that includes a data qubit <NUM> and a data measurement resonator <NUM> configured for measurement of the data qubit <NUM>. <FIG> shows a capacitor <NUM> that capacitively couples the data measurement resonator <NUM> and the data qubit <NUM> to a port <NUM> for measurement and control instruments.

<FIG> is a schematic illustration of the ancilla qubit chip <NUM> of <FIG> coupled to the data qubit chip <NUM> of <FIG> by an interposer. As shown in <FIG>, the capacitor <NUM> coupling the ancilla qubit <NUM> and the ancilla measurement resonator <NUM> to measurement and control instruments is much larger than the capacitor <NUM> coupling the data qubit <NUM> and the data measurement resonator <NUM> to measurement and control instruments. The strong coupling between the ancilla measurement resonator <NUM> and the readout electronics enables fast measurement of the ancilla qubit <NUM>. This is useful for quantum error correction, which may require measurement cycles on the order of one per <NUM>. In contrast, the data measurement resonator <NUM> is weakly coupled to the measurement electronics because the data qubit <NUM> may only be read when the quantum algorithm is complete, instead of every <NUM>. A longer measurement time may be used to make up for the weak coupling. The weak coupling between the data qubit <NUM> and the measurement electronics helps preserve the coherence of the data qubit <NUM>. The data qubit <NUM> may have relaxation and coherence times that are greater than <NUM>, for example. The data qubit <NUM> may have relaxation and coherence times that are on the order of <NUM>, for example.

As an alternative to the configuration shown in <FIG>, the ancilla qubit chip and the data qubit chip may be coupled to the same surface of the interposer. <FIG> is a schematic illustration of an ancilla qubit chip <NUM> and a data cubit chip <NUM> coupled to the same surface <NUM> of an interposer <NUM>.

According to an embodiment of the current invention, for each data qubit of the plurality of data qubits, the superconducting structures enable transmission of quantum information between the data qubit and at least two ancilla qubits of the plurality of ancilla qubits. Similarly, for each ancilla qubit of the plurality of ancilla qubits, the superconducting structures may enable transmission of quantum information between the ancilla qubit and at least two data qubits of the plurality of data qubits. Quantum information can be mapped from at least two data qubits to ancilla qubits to allow measurement of an eigenstate of the data qubits, so that performing the measurement does not destroy the quantum information.

<FIG> is a schematic illustration of a system <NUM> for transmission of quantum information for quantum error correction according to an embodiment of the current invention. An ancilla qubit <NUM> on the ancilla qubit chip <NUM> is coupled to two superconducting structures <NUM>, <NUM>. The superconducting structures enable transmission of quantum information between the ancilla qubit <NUM> and two data qubits <NUM>, <NUM>. Another superconducting structure <NUM> enables transmission of quantum information between the data qubit <NUM> and a second ancilla qubit <NUM>. Although only two of the qubits in <FIG> are illustrated as being coupled to two other qubits, each of the qubits on the ancilla chip and the data chip may be coupled to two or more qubits on the other chip by superconducting structures. Quantum error correction codes, such as the Surface Code, for example, may require that each data qubit be coupled to multiple ancilla qubits, and each ancilla qubit be coupled to multiple data qubits. In the example of the Surface Code, errors can be mapped from the data qubits to the ancilla qubits using CNOT gates, via virtual photons exchanged through the superconducting structures. Measurement of the ancilla qubits may give the parity of the data qubits, for example in the Surface Code. Since parity is an eigenvalue of the Bell state, measurement of the ancilla qubits stabilizes the quantum information in the data qubits. Although the Surface Code is discussed herein, the embodiments of the current invention are not limited to the Surface Code. Other quantum error correction algorithms may be used.

<FIG> is a flowchart that illustrates a method <NUM> of transmitting quantum information for quantum error correction according to an embodiment of the current invention. The method <NUM> includes providing a plurality of ancilla qubits <NUM>, and providing a plurality of data qubits <NUM> spaced apart from the plurality of ancilla qubits. The method <NUM> includes mapping errors from the plurality of data qubits to the plurality of ancilla qubits via virtual photons in a superconducting microwave transmission line <NUM>. The method <NUM> further includes measuring the plurality of ancilla qubits to detect the errors <NUM>, and performing quantum error correction <NUM> based on the detected errors.

According to an embodiment of the invention, measuring the plurality of ancilla qubits <NUM> gives a parity of the plurality of data qubits.

<FIG> is a schematic illustration of a quantum computer <NUM> according to an embodiment of the present invention. The quantum computer <NUM> includes a refrigeration system under vacuum including a containment vessel <NUM>. The quantum computer <NUM> includes an ancilla qubit chip <NUM> contained within a refrigerated vacuum environment defined by the containment vessel <NUM>. The ancilla qubit chip <NUM> includes a plurality of ancilla qubits <NUM>, <NUM>, <NUM>. The quantum computer <NUM> includes a data qubit chip <NUM> contained within the refrigerated vacuum environment defined by the containment vessel <NUM>. The data qubit chip <NUM> is spaced apart from the ancilla qubit chip <NUM> and includes a plurality of data qubits <NUM>, <NUM>, <NUM>. The quantum computer <NUM> includes an interposer <NUM> contained within the refrigerated vacuum environment defined by the containment vessel <NUM>. The interposer <NUM> is coupled to the ancilla qubit chip <NUM> and the data qubit chip <NUM> and includes a dielectric material <NUM> and a plurality of superconducting structures <NUM>, <NUM>, <NUM> formed in the dielectric material <NUM>. The superconducting structures <NUM>, <NUM>, <NUM> enable transmission of quantum information between the plurality of data qubits <NUM>, <NUM>, <NUM> on the data qubit chip <NUM> and the plurality of ancilla qubits <NUM>, <NUM>, <NUM> on the ancilla qubit chip <NUM> via virtual photons for quantum error correction.

The quantum computer according to an embodiment of the current invention may include a plurality of ancilla qubit chips, data qubit chips, and interposers. Further, the embodiments of the invention are not limited to the particular number of ancilla qubits, data qubits, and superconducting structures, and solder bumps shown in <FIG>.

Embodiments of the current invention enable transfer of quantum information using a dielectric interposer with partially embedded microwave transmission line bus resonators. The quantum information is communicated via virtual photons in the resonators. The use of virtual photons ensures that quantum information is not lost due to the electromagnetic Purcell effect or the dielectric loss of the material. By separating qubit chips into those that include data qubits (long-lived, high quality qubits) and those that include ancilla qubits (need fast measurement and control, and are therefore more susceptible to loss channels), errors may be mapped onto the ancilla qubits using the superconducting interposer.

Claim 1:
A system (<NUM>) for transmission of quantum information for quantum error correction, comprising:
an ancilla qubit chip (<NUM>) comprising a plurality of ancilla qubits (<NUM>, <NUM>, <NUM>);
a data qubit chip (<NUM>) spaced apart from said ancilla qubit chip (<NUM>), said data qubit chip (<NUM>) comprising a plurality of data qubits (<NUM>, <NUM>, <NUM>);
an interposer (<NUM>) coupled to said ancilla qubit chip (<NUM>) and said data qubit chip, said interposer (<NUM>) comprising a dielectric material (<NUM>); and
a plurality of superconducting resonators each comprising one of the plurality of superconducting structures (<NUM>, <NUM>, <NUM>, <NUM>);
wherein each respective superconducting structure of the plurality of superconducting structures (<NUM>, <NUM>, <NUM>, <NUM>) forms a part of a respective superconducting resonator having a third frequency;
wherein the respective superconducting structure (<NUM>, <NUM>, <NUM>, <NUM>) extends from a respective data qubit, having a first frequency, of the plurality of data qubits (<NUM>, <NUM>, <NUM>) to a respective ancilla qubit, having a second frequency, of the plurality of ancilla qubits (<NUM>, <NUM>, <NUM>);
wherein the third frequency is detuned from the first and second frequency to prevent real photon transfer between the respective data qubit (<NUM>, <NUM>, <NUM>) and the respective ancilla qubit (<NUM>, <NUM>, <NUM>);
wherein the respective superconducting structure enables transmission of quantum information between the respective data qubit on said data qubit chip (<NUM>) and the respective ancilla qubit on said ancilla qubit chip (<NUM>) via virtual photons for quantum error correction.