Patent ID: 12204995

DETAILED DESCRIPTION

FIG.2illustrates components of a quantum computing system. Some components of the quantum computing system reside in a cryogenically cooled environment, while other components reside in an adjacent room temperature environment. As an example, the cryogenically cooled environment may be located inside a cryostat that itself is located in the room temperature environment. The designation “room temperature” should not be taken as a limitation that would actually require the environmental conditions in the room temperature environment correspond to those in rooms where people live and work. It is more an indication that the conditions to the left inFIG.2do not require cryogenic cooling to the temperatures found in the cryogenically cooled environment. The environmental conditions in the cryogenically cooled environment may include an extremely low temperature, such as only a few kelvin, or even considerably less than one kelvin, such as on the order of only a few millikelvins. The lowest temperature may exist in only a part of the cryogenically cooled environment, as there may be cooled stages of progressively lower temperatures. Indicating that some component of the system is located within the cryogenically cooled environment does not limit that component to any particular stage. The environmental conditions in the cryogenically cooled environment may also include a high vacuum, which is typically needed to provide thermal insulation for maintaining the low temperatures.

The components shown inFIG.2relate in particular to an arrangement for resetting qubits as a part of a quantum computing system. The arrangement comprises a plurality of qubits, of which qubits201,202, and203are shown as examples inFIG.2. A qubit control unit104is provided in the room temperature environment. For the purposes of this description, the nature and operation of the qubit control unit104is not described. The qubit control unit104is merely shown inFIG.2as a reminder that the qubits201-203have some purpose that is related to performing quantum computing operations.

The arrangement ofFIG.2also comprises a QCR block204, which includes one or more quantum circuit refrigerators (QCRs). As described herein, a quantum circuit refrigerator is a circuit element that can be designed as a standalone component. The quantum circuit refrigerator may include a tunneling junction and a control input for receiving a control signal. Such a quantum circuit refrigerator is configured to enable photon-assisted single-electron tunneling across the tunneling junction in response to a control signal received through the control input. The control signal (also referred to as a bias signal) can include a DC bias signal, a pulsed bias signal, or an AC bias signal.

In an example, the tunneling junction of a QCR has a superconducting gap on an order of 400 microelectronvolts (calculated as 96 GHz times h, where h is Boltzmann's constant). In this example, a DC bias value can be calculated as 92 GHz times h per e, where e is the electron charge. This provides a bias voltage of 383 microvolts. Applying such a bias voltage to the exemplary QCR would enable photons having a frequency of 4 GHz to make up for the difference for photon-assisted tunneling to occur. If an excited qubit was appropriately coupled to the QCR, a corresponding portion of the excitation energy may transfer from the qubit to the QCR in the form of a 4 GHz photon, consequently cooling the qubit.

As illustrated inFIG.2, the arrangement comprises couplings205-207between the plurality of qubits201-203and the one or more quantum circuit refrigerators in the QCR block204. The couplings205-207can be made, for example, through capacitive or inductive coupling elements between the plurality of qubits201-203and the one or more quantum circuit refrigerators. Such coupling elements are configured to couple each of the plurality of qubits201-203to one of the one or more quantum circuit refrigerators in the QCR block204.

The terms capacitive coupling and capacitive coupling element cover all possible embodiments that can be used to capacitively couple two elements of a quantum circuit. Examples of capacitive coupling elements include, but are not limited to, parallel plate capacitors, finger capacitors, and lumped element circuitry. Similarly, the terms inductive coupling and inductive coupling element cover all possible embodiments that can be used to inductively couple two elements of a quantum circuit. Inductive coupling may include using e.g. SQUIDs (superconducting quantum interference devices) as the inductive coupling elements.

As shown inFIG.2, the qubits201-203, the quantum circuit refrigerators, and the coupling elements are configured for operation in the cryogenically cooled environment. This means in practice that these components are built as parts of one or more cryogenic integrated circuits or circuit modules, the materials of which are suitable for operation in the extremely low temperatures that prevail in the cryostat during operation. Such cryogenic integrated circuits or circuit modules are also built so that they can be attached, directly or indirectly, to the cryogenically cooled structures inside the cryostat. External connections to and from the components can be made using technologies that enable minimizing the thermal load to the cryostat.

Contrary to the prior art solution shown inFIG.1, the arrangement ofFIG.2comprises a common control signal line208to the control input(s)209of the one or more quantum circuit refrigerators in the QCR block204. The common control signal line208is configured for crossing into the cryogenically cooled environment from the room temperature environment. Concerning the hardware aspects of the arrangement, configuring a signal line for crossing into the cryogenically cooled environment from the room temperature environment means constructing the physical signal line and connectors so that the arrangement can be assembled as a part of the quantum computing system. Any necessary cold anchoring and other measures can be performed to minimize the conduction of heat into the cryostat, while simultaneously ensuring good signal propagation and protection against electromagnetic interference.

In the room temperature environment, a QCR control unit210may be used as a source of the control signals that are delivered to the control input(s)209of the one or more quantum circuit refrigerators. The QCR control unit210may be a standalone unit, or it may be included in the qubit control unit104or other larger signal processing entity within the room temperature environment.

FIGS.3and4illustrate two approaches for constructing the QCR block204and its couplings to the plurality of qubits. According to the approach shown inFIG.3, in the one or more quantum circuit refrigerators of the QCR block204, there is a shared quantum circuit refrigerator that is common to at least a subset of the plurality of qubits201-203. The arrangement includes a resonator301for coupling a tunnel junction302of the shared quantum circuit refrigerator to the subset of the plurality of qubits201-203via at least a respective subset of the coupling elements mentioned above.

To make the most effective use of the coupling, it is advantageous to dimension the resonator301in a particular way. In general, the resonance frequencies of a resonator constitute a harmonic series, in which the resonance frequencies can be numbered as the first, second, third, etc. harmonic frequency. In some sources, the first harmonic frequency is called the base frequency, the basic resonance frequency, or the zeroth harmonic frequency. Certain properties of the harmonic frequencies and the resonator dimensioning may be considered with respect toFIG.5. In the case of a λ/2 (lambda per two) transmission line resonator, the first harmonic frequency is the frequency at which the whole wavelength501of the oscillating electric signal is exactly twice the length502of the resonator. An example of an oscillating electric signal at the first harmonic frequency is shown with the solid line graph ofFIG.5. As shown inFIG.5, the first harmonic frequency involves two voltage antinodes504and505along the length502of the λ/2 transmission line resonator.

The dashed line506inFIG.5illustrates an example of an oscillating electric signal at the second harmonic frequency. As shown inFIG.5, the second harmonic frequency includes three voltage antinodes507,508, and509along the length502of the λ/2 transmission line resonator. In general, the n:th harmonic frequency (of a basic resonance frequency, at which the length of the resonator is one half wavelength) involves n+1 voltage antinodes along the length of the resonator, where n is a positive integer. The voltage antinodes may be called a maxima of an oscillation amplitude of the voltage of the oscillating electric signal. Correspondingly, there are maxima of the oscillation amplitude of the oscillating electric signal. These maxima are located at the nodes of the lines501and506, i.e. at the points where the graphs intersect a horizontal axis. The dotted line510inFIG.5illustrates a combined oscillating electric signal that consists of oscillations at the first and second harmonic frequencies.

Capacitive coupling between an excited quantum circuit element (such as a qubit) and a resonator is strongest when the capacitive coupling is made at or close to an antinode of voltage oscillations along the length of the resonator. Therefore, based on this property, the resonator301inFIG.3has a length dimensioned for a resonance frequency of an oscillating electric signal. InFIG.3, it is assumed that the capacitive coupling elements that make the capacitive couplings to the (subset of) qubits201-203are located at points303,304, and305along the length of the resonator, which correspond to antinodes of the oscillating electric signal at the resonance frequency.

Additionally, inductive coupling between an excited quantum circuit element (such as a qubit) and a resonator is strongest when the inductive coupling is made at or close to a node of voltage oscillations along the length of the resonator. Again, given that the resonator301inFIG.3has a length dimensioned for a resonance frequency of an oscillating electric signal, a corresponding layout could be provided in which the inductive coupling elements that make the inductive couplings to the (subset of) qubits201-203are located at points along the length of the resonator that correspond to nodes of the oscillating electric signal at the resonance frequency.

The excited quantum circuit element accordingly must have a resonant frequency at the frequency at which the antinode occurs in the resonator.

As disclosed herein, the capacitive or inductive coupling elements may be located “at” points, which correspond to a maxima of an oscillation amplitude of an oscillating electric signal. In some embodiments, the capacitive or inductive coupling elements may be located “at or near” the points to have substantially the same efficiency or result. The coupling elements are placed at the antinodes or nodes to utilize the respective maximal amplitudes of voltage or current that occur. The higher the voltage or the larger the current, the better the point can be used for signal coupling. If, for example, the topology of the conductor(s) of the resonator and its relation to the location of the qubits make it impossible or disadvantageous to place a coupling element at the exact known location of an antinode or node, it can be placed so that it is at the closest possible position to the antinode or node, or at the location in which the balance between the aim mentioned above and other design considerations is the best.

FIG.4illustrates another approach in which the QCR block204includes as many quantum circuit refrigerators401-403as there are qubits201-203in the plurality of qubits. Each of the quantum circuit refrigerators401-403is connected to a common reference potential, which inFIG.4is the local ground potential. A common control signal line404is configured to couple a control signal to the respective control inputs of the quantum circuit refrigerators401-403at a common potential. Given the numerical example discussed above, the common potential of the control signal may be, for example, some hundreds of microvolts or some millivolts with respect to the common reference potential.

FIG.6illustrates a portion of a quantum computing circuit that uses the arrangement for resetting qubits according to the approach discussed in connection withFIG.3. Three qubits601,602, and603and one quantum circuit refrigerator604are shown as an example. The qubits601-603may constitute a subset of all qubits in a larger quantum computing system. Capacitive couplings to the qubits601,602, and603is used here as an example, but inductive coupling could be used as well.

As shown inFIG.6, a quantum circuit refrigerator604includes a control input605. As explained above, the quantum circuit refrigerator604includes a tunneling junction (not separately shown inFIG.6), and is configured to enable photon-assisted single-electron tunneling across the tunneling junction in response to a control signal received through the control input605. As the same quantum circuit refrigerator604can have a cooling effect on all three qubits601-603, the control signal line that is coupled to its control input605of the quantum circuit refrigerator604may be referred to as a common control signal line. Similarly, the quantum circuit refrigerator604may be referred to as a shared quantum circuit refrigerator that is common to the plurality of qubits601-603.

The arrangement shown inFIG.6includes a resonator606for coupling the shared quantum circuit refrigerator604to the qubits601-603. The coupling is provided via a set of capacitive coupling elements, of which the capacitive coupling element607is shown as an example. The resonator606has a length that is dimensioned for a resonance frequency of an oscillating electric signal. The capacitive coupling elements607are located at points along the length of the resonator606that correspond to voltage antinodes of the oscillating electric signal at its resonance frequency. Comparing to the lines inFIG.5, it may be assumed that the resonator606is dimensioned for a basic resonance frequency at which its length is one half wavelength. The second harmonic frequency, which is twice the basic resonance frequency, involves three voltage antinodes along that length, of which two are at the ends of the resonator606and the third is at the middle.

FIG.7illustrates a portion of a quantum computing system and an arrangement for resetting qubits. In this example, the principle explained above with reference toFIGS.3and6is generalized as an arbitrary number of qubits. Qubits701,702,703, and704may constitute a subset of all qubits in the quantum computing system. Alternatively, qubits701,702,703, and704may comprise all of the qubits in the quantum computing system. In other words, the subset does not need to be a proper subset but as an extreme case it may be the whole set. Similar toFIG.6, the quantum computing system shown inFIG.7uses capacitive coupling as an example. In the alternative, inductive coupling could be used.

The plurality of qubits701-704share a common quantum circuit refrigerator705. A control input of the quantum circuit refrigerator705is shown as control input706. The arrangement includes a resonator707for coupling the shared quantum circuit refrigerator705to the plurality of qubits701-704via a respective subset of capacitive coupling elements. The resonator707has a length dimensioned for a resonance frequency of an oscillating signal. Additionally, the capacitive coupling elements are located at points along the length of the resonator707that corresponds to antinodes of an oscillating electric signal at a resonance frequency. The resonance frequency may be an n:th harmonic frequency of a basic resonance frequency at which the length of the resonator707is one half wavelength. In other words, the magnitude of the resonance frequency may be n times the basic resonance frequency. There are n+1 qubits in the arrangement, and n+1 points along the resonator707at which the capacitive coupling elements are located. In the example, n may be a positive integer.

To maintain easy comparison between illustrated embodiments, the capacitive coupling is used as an example also inFIGS.8,9, and10. It should be noted, however, that inductive coupling could be used instead.

FIG.8illustrates a portion of a quantum computing circuit that uses an arrangement for resetting qubits according to the approach described in connection withFIG.4. Qubits801,802,803, and804are shown as an example. The qubits801-804may constitute a subset of all qubits in a larger quantum computing system or they may constitute all the qubits in the quantum computing system (in other words, the subset does not need to be a proper subset but as an extreme case it may be the whole set).

The arrangement ofFIG.8includes as many quantum circuit refrigerators805,806,807, and808as there are qubits in the arrangement. Each quantum circuit refrigerator805-808is connected to a common reference potential, which includes a local ground potential. Control inputs of the quantum circuit refrigerators805-808are shown as control inputs809,810,811, and812. A common control signal line813is configured to couple a control signal to the respective control inputs809-812of the quantum circuit refrigerators805-808at a common potential. This disclosed configuration minimizes impedance differences in the connections between the common control signal line813and each of the control inputs809-812of the quantum circuit refrigerators805-808, which could cause a potential difference between the common control signal line813and any of the control inputs809-812.

In the embodiments ofFIGS.6and7, the qubits601-603or701-704of the arrangement should share at least one common resonance frequency, which is the frequency at which the voltage antinodes occur in the resonator606or707at those points where the capacitive couplings to the qubits are located. In a typical quantum computing system, the qubits comprise some frequency tunability, so the required condition of at least one common resonance frequency of suitable magnitude can be achieved by frequency tuning the qubits appropriately for the duration of time when they are to be reset.

In the embodiment ofFIG.8, each of the quantum circuit refrigerators805-808is coupled to a specific resonator, of which the leftmost resonator814is shown as an example. In such a case, each qubit-resonator-pair must have a common resonance frequency of the qubit and the resonator to enable the emission of photons from the qubit to the quantum circuit refrigerator for the purpose of photon-assisted single-electron tunneling. This can be achieved by suitable dimensioning of the qubits and the respective resonators. As the common control signal appears in each of the quantum circuit refrigerators805-808on the same potential with respect to the common reference potential, the superconductive gaps in the respective tunneling junctions are selected so that in each case, an external control signal of just that magnitude summed with the energy of a photon at the resonance frequency enables single-electron tunneling. This too can be achieved by suitable dimensioning.

FIG.9illustrates a portion of a quantum computing circuit that uses an arrangement for resetting qubits according to the approach described in connection withFIG.4, with certain additions described below. Qubits801-803, qubit-specific quantum circuit refrigerators805-807, respective control inputs809-811, and QCR-specific resonators (e.g., resonator814) are similar to the corresponding components inFIG.8. The arrangement ofFIG.9includes a controllable demultiplexer901, which is a device that controllably distributes a common input signal into a selected subset of its outputs or to all outputs. The controllable demultiplexer901is located in the cryogenically cooled environment. A common control signal line902is configured to couple the control signal to the respective control inputs809-811of the quantum circuit refrigerators805-807through the controllable demultiplexer901. A demultiplexing control signal line903is provided and coupled to the controllable demultiplexer for selectively coupling the control signal to the respective control inputs of the selected quantum circuit refrigerators.

The use of a controllable demultiplexer, as shown inFIG.9, enables an operator to select whether all of the plurality of qubits or only a subset of the qubits should be reset at any one time. A selection made by the operator is transmitted as a selection command to the controllable demultiplexer901through the demultiplexing control signal line903, which may originate from the room temperature environment and propagate to the cryogenically cooled environment in a similar way as the common control lines208,404,813, or902described above.

FIG.10illustrates a portion of a quantum computing circuit that uses an arrangement for resetting qubits that combines the approaches described in connection withFIGS.3and4. Each horizontal line of qubits has a shared quantum circuit refrigerator that is common to all of the coupled qubits (e.g., qubits1001,1002,1003, and1004and their shared quantum circuit refrigerator1005). Each horizontal line also includes a resonator for coupling the shared quantum circuit refrigerator to the qubits on that horizontal line via a respective set of capacitive coupling elements (e.g., resonator1007). The resonator has a length that is dimensioned for a resonance frequency of an oscillating signal. The capacitive coupling elements are located at points along the resonator's length that correspond to voltage antinodes of an oscillating electric signal at a resonance frequency.

The arrangement ofFIG.10includes as many quantum circuit refrigerators as there are horizontal lines (or, more generally, subsets of qubits) (including quantum circuit refrigerators1005,1015, and1025). Each of these quantum circuit refrigerators is connected to a common reference potential. A common control signal line1031is configured to couple a control signal to the respective control inputs1006,1016, and1026of the quantum circuit refrigerators1005,1015and1025at a common potential.

In some embodiments, the resonators1007,1017, and1027may be dimensioned for different basic resonance frequencies, and/or there may be different numbers of voltage antinode points along their lengths. This provides more flexibility regarding the resonance frequencies to which the different subsets of qubits1001-1004,1011-1014, and1021-1024need to be tuned for resetting.

A controllable demultiplexer, such as the demultiplexer901ofFIG.9could be added between the common control signal line1031and at least a subset of the quantum circuit refrigerators1005,1015and1025ofFIG.10. This would enable an operator to select a desired number of horizontal qubit lines for resetting at any one time.

In the above description, the common control signal to the one or more quantum circuit refrigerators is generally assumed to carry a DC or quasi-DC control signal. As an alternative, the RF-QCRs can be used, which are controlled with an oscillating control signal having a high frequency. The control signal may also be a combination of a DC (or quasi-DC) signal and an oscillating signal superposed thereupon.

The use of a high-frequency control signal to control one or more QCRs is based on using the control signal to inject “assisting” energy to the QCR tunneling junction. When executed properly, the amount of injected RF energy can be made to correlate with the number of photons that the electrons absorb in order to tunnel across the junction, which in turn means more effective cooling of the quantum circuit element to be cooled.

FIG.11illustrates schematically the use of an RF-QCR as the shared quantum circuit refrigerator204in conformity with the approach described above in connection withFIG.3. The high-frequency control signal is brought through a common control signal line208to the RF-QCR204, where it may be coupled to a tunnel junction1101of the shared quantum circuit refrigerator through a resonator1102, for example. The resonator1102is not mandatory, and the high-frequency control signal could instead be coupled directly to the tunnel junction1101of the shared quantum circuit refrigerator. The frequency of the high-frequency control signal may be significantly higher than the resonance frequency of the resonator301in the RF-QCR. For example, the control signal may be twice or three times the resonance frequency of the resonator301in the RF-QCR.

FIG.12illustrates schematically the use of a plurality of RF-QCRs1201,1202, and1203in conformity with the approach described above in connection withFIG.4. The high-frequency control signal is brought through a common control signal line1204to an RF splitter1205, from which it is provided to a plurality of RF-QCRs1201,1202, and1203. In this case, the RF splitter1205may include, for example, a transmission line. The frequency of the high-frequency control signal brought to the transmission line may be a multiple of the resonant frequencies of the QCR resonators in the RF-QCRs1201,1202, and1203.

It is obvious to a person skilled in the art that with the advancement of technology, the invention disclosed herein may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above, and instead may vary within the scope of the claims.