Patent Description:
Superconducting qubits are one of the main candidates for building a quantum processor. There are several different styles of superconducting qubits. Some have a frequency that is tunable with magnetic flux, enabling the qubit frequency to be tweaked to avoid cross-talk between qubits. Magnetic flux is applied to the qubit using an external flux bias line, wherein a current through the flux bias line creates a magnetic field that changes the qubit frequency. However, flux tunable qubits are very sensitive to noise coming from the external flux bias line. Noise in the external flux bias line causes dephasing of the qubit, shortening the lifetime of any quantum state, and limiting the effectiveness of the qubit in the quantum processor. <NPL>, pertains to a tunable qubit device comprising: a tunable qubit, the tunable qubit comprising a superconducting quantum interference device (SQUID) loop; and a flux bias line inductively coupled to the SQUID loop to provide a persistent bias to the tunable qubit.

According to an embodiment of the present invention, a tunable qubit device includes a tunable qubit, the tunable qubit including a superconducting quantum interference device (SQUID) loop. The tunable qubit device further includes a superconducting loop inductively coupled to the SQUID loop, and a flux bias line inductively coupled to the superconducting loop. The superconducting loop is formed from a superconducting material having a critical temperature that is a lower temperature than a critical temperature of any superconducting material of the tunable qubit. In operation, the superconducting loop provides a persistent bias to the tunable qubit.

According to an embodiment of the present invention, a method of producing a tunable qubit device includes forming, on a first surface of a substrate, a tunable qubit comprising a SQUID loop. The method further includes forming, on a second surface of the substrate, the second surface opposing the first surface, a superconducting loop formed from a superconducting material having a critical temperature that is a lower temperature than a critical temperature of each superconducting material of the tunable qubit. The method further includes forming, on the second surface of the substrate, a flux bias line inductively coupled to the superconducting loop. The superconducting loop is inductively coupled to the SQUID loop, and, in operation, the superconducting loop provides a persistent bias to the tunable qubit.

According to an embodiment of the present invention, a method of tuning a tunable qubit device including a SQUID loop and a bias superconducting loop includes raising the temperature of the tunable qubit device from a temperature suitable for operation of the tunable qubit to a temperature above a critical temperature of the bias superconducting loop but below a critical temperature of each superconducting material of the SQUID loop. The method further includes applying a magnetic field to the bias superconducting loop using a flux bias line, and reducing the temperature of the tunable qubit device to a temperature below the critical temperature of the bias superconducting loop, thereby trapping a flux in the bias superconducting loop. The method further includes removing the magnetic field applied by the flux bias line while maintaining the persistent bias current in the bias superconducting loop. The magnetic field created by the persistent bias current in the superconducting loop penetrates the SQUID loop, tuning a frequency of the tunable qubit.

According to an embodiment of the present invention, a quantum computer includes a refrigeration system under vacuum comprising a containment vessel, and a qubit chip contained within a refrigerated vacuum environment defined by the containment vessel, wherein the qubit chip comprises a plurality of tunable qubit devices. The quantum computer further includes a plurality of electromagnetic waveguides arranged within the refrigerated vacuum environment so as to direct electromagnetic energy to and receive electromagnetic energy from at least a selected one of the plurality of tunable qubit devices. Each of the plurality of tunable qubit devices includes a tunable qubit, the tunable qubit comprising a SQUID loop; a superconducting loop inductively coupled to the SQUID loop; and a flux bias line inductively coupled to the superconducting loop. The superconducting loop is formed from a superconducting material having a critical temperature that is a lower temperature than a critical temperature of any superconducting material of the tunable qubit. In operation, the superconducting loop provides a persistent bias to the tunable qubit.

<FIG> is a schematic illustration of a tunable qubit device <NUM> according to an embodiment of the invention. The tunable qubit device <NUM> includes a tunable qubit <NUM>. The tunable qubit <NUM> includes a superconducting quantum interference device (SQUID) loop <NUM>. The tunable qubit device <NUM> also includes a superconducting loop <NUM> inductively coupled to the SQUID loop <NUM>, and a flux bias line <NUM> inductively coupled to the superconducting loop <NUM>. The superconducting loop <NUM> is formed from a superconducting material having a critical temperature that is a lower temperature than a critical temperature of any superconducting material of the tunable qubit <NUM>. In operation, the superconducting loop <NUM> provides a persistent bias to the tunable qubit <NUM>.

Herein, the word "persistent" is defined as constant and continuous. The persistent bias may persist for a period of time that is much longer than other characteristic times of the system. For example, the period of time may be much longer than the relaxation and dephasing times of the tunable qubit. The period of time may be a period of minutes, hours, or days. The period of time may continue for the length of time that the temperature of the system is maintained below the critical temperature of the superconducting material of the superconducting loop.

According to an embodiment of the invention, the tunable qubit device <NUM> further includes a substrate <NUM>. The SQUID loop <NUM> may be formed on a first surface of the substrate <NUM>, and the superconducting loop <NUM> may be formed on a second surface of the substrate opposing the first surface, as shown in <FIG>.

<FIG> is a schematic illustration of a tunable qubit device <NUM> according to an embodiment of the invention. <FIG> shows a side view of a substrate <NUM>. The substrate <NUM> has a first surface <NUM> and a second surface <NUM> opposite the first surface <NUM>. The tunable qubit device <NUM> has a tunable qubit <NUM> including a SQUID loop <NUM> formed on the first surface <NUM>. The tunable qubit device <NUM> has a superconducting loop <NUM> formed on the second surface <NUM>. The superconducting loop <NUM> is inductively coupled to the SQUID loop <NUM>. The tunable qubit device <NUM> also has a flux bias line <NUM> inductively coupled to the superconducting loop <NUM>.

According to an embodiment of the invention, the SQUID loop <NUM> is formed to be substantially in a first plane, and the superconducting loop <NUM> is formed to be substantially in a second plane that is spaced apart from and substantially parallel to the first plane. According to an embodiment of the invention, the SQUID loop <NUM> and the superconducting loop <NUM> are aligned so as to maximize inductive coupling between the SQUID loop <NUM> and the superconducting loop <NUM>. For example, in a side view of the substrate <NUM>, the superconducting loop <NUM> may be vertically aligned with the SQUID loop <NUM>, as shown in <FIG>. The center of the superconducting loop <NUM> may be vertically aligned with the center of the SQUID loop <NUM>. Alternatively, the center of the superconducting loop <NUM> may be laterally offset from the center of the SQUID loop <NUM> in the side view. The superconducting loop <NUM> may completely or partially overlay the SQUID loop <NUM>.

The inductive coupling between the superconducting loop <NUM> and the SQUID loop <NUM> may depend on the extent to which the superconducting loop <NUM> and the SQUID loop <NUM> are aligned. For example, the inductive coupling may be maximized when the superconducting loop <NUM> and the SQUID loop are aligned such that the superconducting loop <NUM> completely overlays the SQUID loop <NUM>.

According to an embodiment of the invention, the superconducting loop creates a magnetic field that tunes a frequency of the tunable qubit. Often, only a very small tuning of the qubit frequency is needed to avoid frequency collisions. The tuning is conventionally achieved by applying a magnetic flux to the tunable qubit using a flux bias line. However, the flux bias line introduces noise into the system, which results in dephasing of the tunable qubit. Even small fluctuations in the bias current that is used to control the frequency of the tunable qubit can have a devastating effect on the qubit's coherence. Accordingly, embodiments of the invention employ a superconducting loop to create a persistent magnetic field that tunes the frequency of a tunable qubit.

According to an embodiment of the invention, the superconducting loop continues to create the magnetic field when no magnetic field is created by the flux bias line. As described in more detail below, a user can introduce a magnetic flux into the superconducting loop using a flux bias line. By controlling the temperature of the system, the user can trap the magnetic flux in the superconducting loop such that the magnetic flux remains in the loop even after the flux bias line ceases to create the magnetic field. In this way, the tunable qubit is isolated from the noisy flux bias line, while still being tuned by the persistent flux trapped in the superconducting loop.

According to an embodiment of the invention, the superconducting loop includes one or more of titanium, zirconium, or hafnium, for example. The tunable qubit may include one or more of niobium, aluminum, and titanium nitride, for example.

<FIG> is a flowchart that illustrates a method <NUM> of producing a quantum computer chip according to an embodiment of the current invention. The method <NUM> includes forming, on a first surface of a substrate, a tunable qubit <NUM> comprising a superconducting quantum interference device (SQUID) loop. The method <NUM> further includes forming, on a second surface of the substrate, the second surface opposing the first surface, a superconducting loop <NUM> formed from a superconducting material having a critical temperature that is a lower temperature than a critical temperature of each superconducting material of the tunable qubit. The method further includes forming, on the second surface of the substrate, a flux bias line <NUM> inductively coupled to the superconducting loop. The superconducting loop is inductively coupled to the SQUID loop, and in operation, the superconducting loop provides a persistent bias to the tunable qubit.

According to an embodiment of the invention, the method further includes tuning the tunable qubit <NUM> using a persistent bias current in the superconducting loop, wherein the persistent bias current persists when flux from the flux bias line is removed. <FIG> is a flowchart that illustrates a method <NUM> of tuning the tunable qubit using the persistent bias current in the superconducting loop. The method <NUM> includes raising the temperature of the tunable qubit device from a temperature suitable for operation of the tunable qubit to a temperature above the critical temperature of the superconducting loop <NUM> but below the critical temperature of each superconducting material of the tunable qubit. At this temperature, the material of the superconducting loop ceases to be superconducting, but the superconducting material or materials forming the tunable qubit remain superconducting. The temperature suitable for operation of the tunable qubit may be less than <NUM> mK, for example. The temperature of the tunable qubit device may be raised from less than <NUM> mK to a temperature above the critical temperature of the superconducting loop <NUM> but below the critical temperature of each superconducting material of the tunable qubit. For example, if the superconducting loop is formed from titanium and the tunable qubit is formed from niobium and aluminum, then the temperature of the tunable qubit device may be raised from less than <NUM> mK to a temperature above <NUM>, the critical temperature of titanium, but below <NUM>, which is the lower of the critical temperatures of niobium (TC = <NUM>) and aluminum (TC = <NUM>).

The method <NUM> further includes applying a magnetic field to the superconducting loop using the flux bias line <NUM>. Because the flux bias line is inductively coupled to the superconducting loop, a current in the flux bias line will have a corresponding magnetic field that acts on the superconducting loop, causing a flux to penetrate the superconducting loop. The method <NUM> further includes reducing the temperature of the tunable qubit device to a temperature below the critical temperature of the superconducting loop <NUM>, thereby trapping a flux in the superconducting loop. As the material forming the superconducting loop returns to a superconducting state, the flux that is penetrating the superconducting loop area will be trapped in the superconducting loop.

The method <NUM> further includes removing the magnetic field applied by the flux bias line <NUM> while maintaining the persistent bias current in the superconducting loop. Because the flux penetrating the superconducting loop is trapped in the superconducting loop, the current in the flux bias line is no longer needed. Accordingly, the magnetic field applied by the flux bias line can be removed. When the current in the flux bias line is reduced to zero, the coherence of the tunable qubit is no longer affected by noise in the flux bias line.

The magnetic field created by the persistent bias current in the superconducting loop penetrates the SQUID loop, tuning a frequency of the tunable qubit. Because the superconducting loop is inductively coupled to the SQUID loop, the trapped magnetic flux inside the superconducting loop partially penetrates the SQUID loop. The extent to which the magnetic flux trapped in the superconducting loop penetrates the SQUID loop is determined by the mutual inductance M between the superconducting loop and the SQUID loop. The penetrating flux causes a displacement of the critical current of the SQUID. The change in the critical current results in a change in the frequency of the tunable qubit. Further, because the magnetic flux is trapped in the superconducting loop, the superconducting loop will provide a persistent, i.e., constant and continuing, flux that penetrates the SQUID loop. Thus, the frequency of the qubit will remain tuned as long at the temperature of the tunable qubit device is maintained below the critical temperature of the superconducting loop.

The method according to an embodiment of the invention includes further tuning the frequency of the tunable qubit. For example, after the initial tuning of the tunable qubit, a user may decide to adjust frequency of the qubit by changing the magnetic field created by the superconducting loop. <FIG> is a flowchart that illustrates a method <NUM> for further tuning the frequency of the tunable qubit. The method <NUM> includes raising the temperature of the tunable qubit device to a temperature above the critical temperature of the superconducting loop <NUM> but below the critical temperature of each superconducting material of the tunable qubit. The method <NUM> further includes applying a magnetic field to the superconducting loop that is weaker or stronger than a previously applied magnetic field using the flux bias line <NUM>. The method <NUM> includes reducing the temperature of the tunable qubit device to a temperature below the critical temperature of the second superconducting loop <NUM>. The method <NUM> further includes removing the magnetic field applied by the flux bias line <NUM>.

As described above, the tunable qubit device <NUM> schematically illustrated in <FIG> includes a superconducting loop <NUM> that produces a magnetic field that tunes the tunable qubit <NUM>. In some applications, only semi-static tuning is desirable where the system operates in a static state and occasionally re-tuning may be needed. This would for instance be for transmon, charge, or phase qubit systems incorporating a SQUID loop and where the cross-resonance gate is used; a small tunability would be desirable to avoid frequency collisions. The following estimation of performance assumes that the superconducting loop has a radius R<NUM>, and the SQUID loop has a radius R<NUM>, where R<NUM><R<NUM>. The amount of current circulating for one stored flux quantum Φ<NUM> in the top bias loop is given by <MAT> where d=<NUM> is the width of the wire forming the superconducting loop.

The mutual inductance is given by <MAT>, where H is the thickness of the substrate on which the superconducting loop and the SQUID loop are formed. Taking R<NUM>=<NUM>, R<NUM> = <NUM>, H=<NUM> gives a mutual inductance M ≈ <NUM> pH. To suppress the critical current in the qubit SQUID loop by one percent, a current of <NUM> mA is circulated in the bias loop, approximately corresponding to approximately <NUM> flux quanta trapped in the loop. This is sufficient to tune the frequency of a <NUM> transmon qubit by about <NUM>.

A plurality of tunable qubit device may be used to form a quantum computer. <FIG> is a schematic illustration of a quantum computer <NUM> according to an embodiment of the invention. The quantum computer <NUM> includes a refrigeration system under vacuum including a containment vessel <NUM>. The quantum computer <NUM> includes a qubit chip <NUM> contained within a refrigerated vacuum environment defined by the containment vessel <NUM>. The qubit chip <NUM> comprises a plurality of tunable qubit devices <NUM>, <NUM>, <NUM>. The quantum computer <NUM> also includes a plurality of electromagnetic waveguides <NUM>, <NUM> arranged within the refrigerated vacuum environment so as to direct electromagnetic energy to and receive electromagnetic energy from at least a selected one of the plurality of tunable qubit devices <NUM>, <NUM>, <NUM>. The tunable qubit devices <NUM>, <NUM>, <NUM> each include a tunable qubit, a superconducting loop, and a flux bias line, like the tunable qubit device <NUM> in <FIG>. The frequency of each tunable qubit can be individually tuned by trapping a magnetic flux in the corresponding superconducting loop, using the corresponding flux bias line. A different magnetic flux can be trapped in each superconducting loop, allowing for individualized, persistent tuning of each tunable qubit.

Claim 1:
A tunable qubit device (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprising:
a tunable qubit (<NUM>, <NUM>, <NUM>, <NUM>), the tunable qubit (<NUM>, <NUM>, <NUM>, <NUM>) comprising a superconducting quantum interference device (SQUID) loop (<NUM>, <NUM>);
a superconducting loop (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) inductively coupled to the SQUID loop (<NUM>, <NUM>); and
a flux bias line (<NUM>, <NUM>, <NUM>, <NUM>, <NUM><NUM>, <NUM>) inductively coupled to the superconducting loop (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>),
wherein the superconducting loop (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is formed from a superconducting material having a critical temperature that is a lower temperature than a critical temperature of any superconducting material of the tunable qubit (<NUM>, <NUM>, <NUM>, <NUM>), and
wherein, in operation, the superconducting loop (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) provides a persistent bias to the tunable qubit (<NUM>, <NUM>, <NUM>, <NUM>).