Patent ID: 12193339

DETAILED DESCRIPTION

Quantum computing entails coherently processing quantum information stored in the quantum bits (qubits) of a quantum computer. Superconducting quantum computing is a promising implementation of solid-state quantum computing technology in which quantum information processing systems are formed, in part, from superconducting materials. To operate quantum information processing systems that employ solid-state quantum computing technology, such as superconducting qubits, the systems are maintained at extremely low temperatures, e.g., in the10sof mK. The extreme cooling of the systems keeps superconducting materials below their critical temperature and helps avoid unwanted state transitions. To maintain such low temperatures, the quantum information processing systems may be operated within a cryostat, such as a dilution refrigerator.

In some implementations, control signals are generated in higher-temperature environments, such as room-temperature, and are transmitted to the quantum information processing system using shielded impedance-controlled GHz-capable transmission lines, such as coaxial cables. The cryostat may step down from room-temperature (e.g., about 300 K) to the operating temperature of the qubits in one or more intermediate cooling stages. For instance, the cryostat may employ a stage maintained at a temperature range that is colder than room temperature stage by one or two orders of magnitude, e.g., about 30-40 K or about 3-4 K, and warmer than the operating temperature for the qubits (e.g., about 10 mK or less).

In some implementations, the state measurement of superconducting qubits is achieved using a dispersive detection scheme. In order to read out or detect the state of any qubit, a probing signal, a travelling microwave, may be excited along a readout transmission line coupled to the qubit via a respective readout resonator. The frequency of the probing signal may be in the vicinity of the resonance frequency of the readout resonator. Depending on the internal quantum mechanical state of the qubit, the intensity and the phase of the probing signal transmitted along the readout transmission line may be altered because the reflectivity of the readout resonator coupled to the qubit changes depending on the state of the qubit. This allows for the state detection of the qubits.

Even at the extremely low qubit operating temperatures, qubits may still suffer from decoherence and gate errors. Therefore, for high fidelity state measurements of superconducting qubits with near quantum-limited noise performance, a Josephson junction parametric amplifier may be constructed and used as a preamplifier for the probing signal. Within the Josephson junction parametric amplifier, a Josephson junction acts as a nonlinear inductor where the inductance is dependent on the intensity of a pump tone received at the Josephson junction. In other words, the inductance of the Josephson junction is dependent on the flux applied through a SQUID loop which includes the Josephson junction. This inductance can be modulated by applying a flux pump tone to the SQUID loop. The Josephson junction parametric amplifier can impart part of the energy of the pump tone to the probing signal, leading to the parametric amplification of the probing signal.

The dispersive detection scheme further requires, in addition to the Josephson junction parametric amplifier as a preamplifier, circulators for isolation of the signals and directional couplers for combining signals. In particular, using circulators for isolation requires impedance matching with a terminating resistor attached one of the ports of the circulators for termination, as will be explained later. Due to the heat dissipated, it is crucial to properly thermalize the termination resistor, such as a 50 Ohm resistor within the cryostat. As the number of qubits increases, the number of the termination resistors attached to the circulators also increases, which may raise a challenge in thermalizing all of the termination resistors properly inside a cryostat. If the termination resistors are not well thermalized, the temperature of the termination resistors may stay higher than that of the base temperature of the cryostat. The energy dissipated at the 50 Ohm resistors may radiate noise which can affect qubit performance and lead to degradation of the coherence via dephasing of the qubits.

Using conventional prototype hardware for circulators, directional couplers, and termination resistors for the dispersive readout scheme may not be suitable for superconducting quantum systems with a large number of qubits to implement error correction algorithms. This is mainly due to the constraint in the available space within the cryostat. Since all of the hardware for the dispersive readout scheme must be mounted on a mix plate at the mK-stage of the dilution refrigerator, space constraints become severe as the number of qubits increases. The number of modules which can fit in the available volume within the dilution refrigerator may be limited if the modules are assembled using conventional prototype hardware. A typical number of readout lines in such a system is 12. Additionally, the conventional prototype hardware does not lend itself easily to efficient thermalization. For example, in case the 50 Ohm resistor is provided by a 50 Ohm SubMiniature version A (SMA) terminator cap, the heat generated at the 50 Ohm SMA terminator cap may be difficult to thermalize properly because of the geometry of the SMA terminator which does not allow a large surface contact for cooling. Furthermore, using multiple SMA connectors in the signal path may lead to signal loss which lowers the signal to noise ratio of the detected signals.

The present disclosure relates to an integrated circuit board on which components of the dispersive readout scheme are mounted together. The integrated circuit board may also be referred to as an “integrated readout card”. The integrated circuit board may include the Josephson junction preamplifiers, circulators and 50 Ohm terminators directly mounted on the circuit board. These components can be connected by signal lines fabricated directly on a conducting layer on the integrated circuit board for low loss connection between components and reduced volume. Directional couplers can be directly fabricated on the conducting layer out of the signal lines to further reduce the volume of the readout circuit. This allows for a compact design in which most components can be surface mounted and soldered or wire bonded directly into the circuit board.

The integrated circuit board may contain multiple conducting layers. A signal layer may be buried within the volume of a dielectric support layer. A front plane layer and a back plane layer may be provided on both sides of the support layer to serve as an electric ground and a thermal anchor. Most of the components can be mounted on the front plane layer to have a large surface contact with the front plane layer. Since the front plane layer and the back plane layer are connected with conducting vias, heat generated from the components may be dissipated efficiently to the back plane layer, which is again connected to a heat sink or directly to the mixing plate of the dilution refrigerator. Such integrated circuit board may allow for a high degree of integration of the readout circuit compact while providing an efficient thermalization.

FIG.1is a schematic that illustrates an exemplary qubit readout circuit.

A qubit readout circuit100may include a first circulator111, a second circulator112, a third circulator113and a fourth circulator114. The qubit readout circuit100further includes a first termination resistor121, a second termination resistor122, a third termination resistor123and a fourth termination resistor124. The qubit readout circuit100further includes a Josephson parametric amplifier130.

A circulator110is a passive device which usually includes three or four ports. A signal entering one of the ports110-1,110-2,110-3is transmitted to another one of the ports110-1,110-2,110-3but only in one direction. As illustrated on the right panel ofFIG.1, a circulator110in this implementation includes three ports110-1,110-2,110-3. As illustrated with dotted lines in the right panel, when a signal enters the first port110-1of the isolator110, the signal is transmitted to the second port110-2of the circulator110and exits the second port110-2of the circulator110. When a signal enters the second port110-2of the circulator110, the signal is transmitted to the third port110-3, but not the first port110-1, and exits at the third port110-3of the circulator110. Throughout this implementation, a first port, a second port, and a third port included in the first to fourth circulators111,112,113,114follow this convention.

In the readout circuit100, an input signal is received at the first port111-1of the first circulator111.

In some implementations, the input signal may be provided by a travelling microwave reflected from a readout resonator coupled to a qubit. The frequency of the travelling microwave, a probe signal, may be at the resonance frequency or in the vicinity of the resonance frequency of the readout resonator. Since the readout resonator is coupled to the qubit, the resonance frequency of the readout resonator changes depending on the state of the qubit. Therefore, depending on the internal quantum mechanical state of the qubit, the intensity or phase of the probing signal may be altered, which allows for the state detection of the qubits.

In some implementations, the input signal may be provided by a travelling microwave reflected from one of a plurality of readout resonators coupled to a plurality of respective qubits. The plurality of readout resonators may be coupled to a common readout transmission line. A travelling microwave, a probe signal, may be excited to travel along the readout transmission line. The frequency of the probe signal, may be at the resonance frequency or in the vicinity of the resonance frequency of one of the readout resonators. Since the plurality of readout resonators are coupled to the plurality of respective qubits, the resonance frequency of the readout resonator changes depending on the state of the qubit. Therefore, depending on the internal quantum mechanical state of the probed qubit, the intensity or phase of the probing signal may be altered, which allows for the state detection of the qubits.

In some implementations, the input signal may be provided by a probe signal comprising multiple tones of a travelling microwave reflected from a respective plurality of readout resonators coupled to a plurality of respective qubits. The plurality of readout resonators may be coupled to a common readout transmission line. The frequency of each of the multiple tones of the probe signal, may be at the resonance frequency or in the vicinity of the resonance frequency of a respective one or the plurality of the readout resonators. Since the plurality of readout resonators are coupled to the plurality of respective qubits, the resonance frequency of the readout resonator changes depending on the state of the qubit. Therefore, depending on the internal quantum mechanical state of the probed qubits, the intensity or phase of each tone of the probing signal may be altered, which allows for the state detection of the qubits.

In the example ofFIG.1, a reflection scheme for measuring the states of the qubits is shown where only one line connects the qubit chip to the readout circuit100via a directional coupler10. The probing signal is input into an input port10-1of the directional coupler10. Most of the power of the probing signal input into the input port10-1is dissipated at the third termination resistor123. A fraction of power, determined by the coupling coefficient of the directional coupler10, of the probing signal is coupled into a coupled port10-2of the directional coupler10and sent to the plurality of readout resonators respectively coupled to the plurality of qubits in the qubit chip. Therefore, the intensity or power of the probing signal input into the input port10-1of the directional coupler10is set to be larger than the level of intensity or power intended for the readout circuit100in view of the coupling coefficient of the directional coupler10. The coupling coefficient of the directional coupler10may be decided by considering parameters including the signal-to-noise ratio of the measurement and heat dissipation at the third termination resistor123. The coupling coefficient of the directional coupler10may be set to be as low as possible in order to minimize the attenuation of the probing signal reflected off the readout resonators at the directional coupler10before the probing signal is input into the first port111-1of the first circulator111. However, this is balanced by the fact that the power of the probing signal10-1needs to be accordingly large to maintain the level of the probing signal in the readout circuit100, which increases heat dissipation at the third termination resistor123.

The probe signal is then transmitted to the second port111-2of the first circulator111and exits the first circulator111through the second port111-2and passes to the second circulator112. The third port111-3of the first circulator111is terminated with the first termination resistor121. The first to fourth termination resistors121,122,123,124are a matched load to the transmission line forming the circulators111,112,113,114and the connections between the circulators111,112,113,114. For example, when the impedance of the transmission lines is 50 Ohms, the resistance of the termination resistors121,122,123,124is 50 Ohms.

If any signal is transmitted back to the second port111-2of the first circulator111reflected from a component in the later stage of the qubit readout circuit100, the reflected signal exits from the third port111-3of the first circulator111and becomes terminated or dissipated at the first termination resistor121. Therefore, between the unterminated ports of the first circulator111, the first port111-1and the second port111-2of the first circulator111, the signal can travel only in one direction, namely from the first port111-1to the second port111-2. Therefore, a circulator111,112,113,114with the third port110-3,111-3,112-3,113-3,114-3terminated with a termination resistor121,122,123acts as an isolator, which is used to shield components coupled to the first port110-1,111-1,112-1,113-1,114-1from any back-propagating microwave signals from the subsequent components.

In the qubit readout circuit100, the third ports111-3,112-3,114-3of the first circulator111, the second circulator112, and the fourth circulator114are terminated with the first termination resistor121, the second termination resistor122, and the fourth termination resistor124, respectively, therefore configured as isolators. These are to protect the qubits connected to the first port111-1of the first circulator111from back-propagating signals, as will be explained in more detail below.

The second port111-2of the first circulator111is electrically connected, via a matched transmission line, to the first port112-1of the second circulator112. The third port112-3of the second circulator112is terminated with the second termination resistor122. Therefore, as discussed above, the second circulator112also forms an isolator from the first port112-1to the second port112-2. When the probe signal outputted from the second port111-2of the first circulator111enters the first port112-1of the second circulator112, the probe signal exits the second port112-2. If any back-reflected spurious signal enters the second port112-2of the second circulator112, it is subsequently transmitted to the third port112-3of the second circulator112and terminated. Therefore, the second circulator112is terminated with the second termination resistor122at the third port112-3which serves as a further shielding of the qubits in addition to the isolator formed by the first circulator111and the first termination resistor121.

The second port112-2of the second circulator112is electrically connected, via a matched transmission line, to the first port113-1of the third circulator113. The probe signal enters the first port113-1of the third circulator113and exits through the second port113-2of the third circulator towards the Josephson junction parametric amplifier130.

Parametric amplifiers are nonlinear devices in which a reactance in the circuit is modulated by a pump tone of frequency fp to facilitate amplification and frequency conversion from a first band of frequencies Δf centered around f1to a second band of frequencies Δf centered around f2, such that fp=f1+f2.

For example, if a pump tone at 11 GHz is provided to the Josephson junction parametric amplifier130and the frequency of the probe signal is 5 GHz, the Josephson junction parametric amplifier130up-converts the frequency of the first signal into 6 GHz. If a pump tone at 10 GHz is provided to the Josephson junction parametric amplifier130and the frequency of the probe signal is 5 GHz, the Josephson junction parametric amplifier130outputs a signal at 5 GHz. In both cases, the intensity of the probe signal can be amplified to a degree dependent on the amplitude of the pump tone. In both cases, the probe signal at 5 GHz is also amplified.

The pump tone is received at the Josephson parametric amplifier130via a pump terminal131.

The Josephson junction parametric amplifier130outputs an amplified probe signal back into the second port113-2of the third circulator113. The amplified probe signal exits through the third port113-3of the third circulator113.

In some implementations, in place of the Josephson junction parametric amplifier130, the Josephson junction parametric converter may be used for the qubit readout circuit100. Parametric converters are nonlinear devices in which a reactance in the circuit is modulated by a pump tone of frequency fp to facilitate amplification and frequency conversion from a first band of frequencies Δf centered around f1to a second band of frequencies Δf centered around f2, such that fp=f2−f1.

The amplified probe signal enters the first port114-1of the fourth circulator114and is outputted at the second port114-2of the fourth circulator114. Since the third port114-3of the fourth circulator114is terminated with the fourth termination resistor124, the fourth circulator also acts as an isolator.

In some implementations, the qubit readout circuit100may correspond to a pre-amplification stage before the signals are combined or multiplexed for further amplification. For example, the output signal from the second port114-2of the fourth circulator114may be amplified by a HEMT (High Electron Mobility Transistor) amplifier before processing.

The number of isolators, which are circulators111,112,114terminated with a termination resistors121,122,124, required for the qubit readout circuit100may vary depending on the requirement or the components attached to the output of the qubit readout circuit100.

The qubit readout circuit100shown inFIG.1includes one isolator114,124between the output terminal114-2and the Josephson junction parametric amplifier130. This is mainly to shield the Josephson junction parametric amplifier from the signals reflected from subsequent components, such as a HEMT amplifier. The degree of isolation may depend on the isolation ratio of each circulator111,112,114, which ranges from 20 to 40 dB rejection. If a higher degree of isolation is required, further isolators, which are circulators terminated with a termination resistor, can be cascaded to the second port114-2of the fourth circulator114.

The qubit readout circuit100shown inFIG.1includes two isolators111,112,121,122between the input terminal111-1and the Josephson junction parametric amplifier130. This is mainly to shield the qubits from either the pump tone propagating from the Josephson parametric amplifier130or amplified noise emitted from the Josephson parametric amplifier130. Since the circulator111,112,113,114have a finite bandwidth, the pump tone may be out of band with the circulators111,112,113,114. For example, when the circulator operates from 4 to 7 GHz and the frequencies of the probe signal and the amplified probe signal are at 5 GHz, the frequency of the pump tone is 10 GHz, which is out of band of the operation bandwidth of the circulator. Therefore, the isolation ratio of the circulators111,112may be lower than specified. Also, when the pump tone is far away from the resonance frequencies of the readout resonators in the qubit chip, it may not affect the performance of the qubits significantly. Therefore, the two isolators111,112,121,122between the input terminal111-1and the Josephson junction parametric amplifier130may be mainly used to suppress the amplified noise from the Josephson junction parametric amplifier from propagating to the qubits. The pump tone whose intensity is 10's of nW, is in general much more intense than the probe signal. For example, the power of the pump tone may be from −70 dBm to −40 dBm whereas the power of the probe signal may be from −130 to −110 dBm. If a higher degree of isolation is required, further isolators, each comprising a circulator terminated with a termination resistor, can be disposed between the input port111-1and the first port113-1of the third circulator113. Therefore, if the readout circuit100may be designed such that more isolators can be added without occupying much volume within the experimental space of the cryostat.

Any unwanted signals propagating the wrong direction through a circulator111,112,113,114will be terminated at the termination resistors121,122,123,124. Properly thermalizing the terminating resistors121,122,123,124may prevent this energy from re-radiating toward the qubit, which can otherwise affect the coherence of the qubits. In addition to the termination resistors,121,122,123,124, which are the major sources of heat dissipation, the circulators111,112,113,114themselves may dissipate heat because the conductors within the circulators111,112,113,114which have finite resistance can dissipate the pump tone. Therefore, the heat dissipated at the qubit readout circuit100needs to be efficiently channeled to a heat sink for the coherence of the qubits.

FIGS.2ato2care schematics that illustrate an exemplary integrated circuit board for a qubit readout circuit shown inFIG.1.

FIGS.2aand2bshow a circulator210, which can be any one of the circulators111,112,113,114shown inFIG.1, mounted on an integrated circuit board200.FIGS.2aand2bshow the circulator210mounted on the integrated circuit board200viewed in two different directions. In particular,FIG.2ashows a cross section of the integrated circuit board200and the circulator210, along the y-z plane andFIG.2bshows a cross section of the integrated circuit board200and the circulator210, along the x-z plane.

As shown inFIG.2a, the circulator210includes a first port210-1and a second port210-2. As shown inFIG.2b, the circulator210further includes a third port210-3which is electrically connected to a termination resistor220.

The integrated circuit board200includes three conducting layers, a front plane layer230, a signal layer240and a back plane layer250and one dielectric layer, a first support layer260-1and a second support layer260-2. The first support layer260-1is disposed between the front plane layer230and the signal layer240. The second support layer260-2is disposed between the signal layer240and the back plane layer250.

In some implementations, the front plane layer230, the signal layer240and the back plane layer250are substantially parallel to one another.

In some implementations, the front plane layer230, the signal layer240and the back plane layer250comprise the same conducting material.

In some implementations, the front plane layer230, the signal layer240and the back plane layer250comprise copper which provides a high thermal conductivity for efficient thermalization and a high electrical conductivity such that a significant signal loss is prevented within the integrated circuit board200. It is not essential for the front plane layer230, the signal layer240or the back plane layer250to be superconducting at the operating temperature of the qubits.

In some implementations, the back plane layer250may comprise aluminum. Aluminum is superconducting at the temperature given by the mixing plate of the dilution refrigerator. Therefore, the signal transmission becomes largely lossless.

The Josephson junction parametric amplifier130may be formed on a separate chip comprising an aluminum layer and electrically connected to the signal layer240via, for example, wire bonding.

The first support layer260-1and the second support layer260-2provide an overall planar shape of the integrated circuit board.

The examples of the material for the first support layer260-1and the second support layer260-2include a dielectric material such as Rogers, which is both compatible with microwave circuits and cryogenic temperatures.

In some implementations, the front plane layer230and the back plane layer250may be disposed on both sides of the plane of the support layer260, as shown inFIGS.2aand2b.

In some implementations, the signal layer240is buried within the support layer260and not exposed to the outside environment.

In some implementations, the signal layer240may comprise one or more stripline waveguides, where the waveguide circuit is defined by strips of metal fabricated within the signal layer240, and the front plane layer230and the back plane layer250act as ground planes.

In some implementations, the front plane layer230may comprise one or more coplanar waveguides, where the waveguide circuit is defined by the strips of metal fabricated within the front plane layer230with return tracks defined on either side of the strips, also fabricated within the front plane layer230.

The stripline waveguides comprised by the signal layer240may form a waveguide circuit within in a plane parallel to the front plane layer230and the back plane layer250.

In some implementations, in case the signal layer240is buried within the support layers260-1,260-2, the plane within which the signal layer240is formed may be equidistant from the front plane layer230and the back plane layer250. For example, if the signal layer240comprises one or more stripline waveguides, in order to ensure 50 Ohm impedance in the stripline waveguides throughout the plane of the signal layer, the signal layer240is positioned equidistant from the plane layer230and the back plane layer250. However, as far as the signal layer240is not exposed from the support layer260, and the waveguides comprised by the signal layer240can maintain the impedance required for the operation, the position of the signal layer240can be anywhere between the front plane layer230and the back plane layer250.

In some implementations, the front plane layer230may be patterned to form one or more solder pads231. The solder pads231can be patterned such that they are electrically disconnected from an electric ground formed in other regions of the front plane layer230, which will form an electric ground.

In some implementations, as shown inFIGS.2aand2b, the terminals of the circulator210, forming respectively the first port210-1, the second port210-2, and the third port210-3of the circulator210, are directly wire bonded to the solder pads231.

Alternatively, the terminals of the circulator210, forming respectively the first port210-1, the second port210-2, and the third port210-3of the circulator210, may include conducting pins. In this case, the conducting pins can be directly soldered to the solder pads231.

The area of the solder pads231are arranged to be large enough for wire bonding or soldering to be possible.

The electrical connection between the ports210-1,210-2,210-3of the circulator210and the solder pads231are not limited to wire bonding or soldering. As long as the connection is compatible with high frequency signals, for example 1 MHz or higher, and the connection can withstand a cryogenic temperature of operation, any connection method can be used.

The integrated circuit board200further includes one or more signal vias241. The signal vias241comprise a conducting material. In some implementations, the signal vias241comprise the same material as the front plane layer230and the signal layer240. In some implementations, the signal vias extend in a direction perpendicular to the plane of the integrated circuit board200and electrically connects the solder pads231to the respective signal lines within the signal layer240. In some implementations, the signal vias241extend in any direction from the front plane layer230to the signal layer240to electrically connect the solder pads231to the respective signal lines within the signal layer240.

In the cross sections shown in the example ofFIGS.2aand2b, only the signal lines connected to the solder pads231connected to the first port210-1and the second port210-2are shown. Therefore, the cross section of the integrated circuit board200shown inFIG.2bdoes not show any part of the signal line240.

Parts of the front plane layer230which are not connected to the solder pads231form an electrical ground.

The integrated circuit board200further includes one or more ground vias251. The ground vias251comprise a conducting material. In some implementations, the ground vias251comprise the same material as the front plane layer230and the back plane layer250. In some implementations, the ground vias251extend in a direction perpendicular to the plane of the integrated circuit board200and electrically connects part of the front plane layer230not electrically connected to the solder pads231to the back plane layer250. In some implementations, the ground vias251extend in any direction from the front plane layer230to the back plane layer250to electrically connect part of the front plane layer230not electrically connected to the solder pads231to the back plane layer250.

The part of the front plane layer230not electrically connected to the solder pads231and the back plane layer250, connected to each other via the ground vias251form an electrical ground.

Heat dissipated into the front plane layer230is transferred to the back plane layer250via the ground vias251. Therefore, the cross section of the ground vias251may be arranged such that the ground vias251can transmit heat from the front plane layer to the back plane layer without heating up significantly at any point of the ground vias251.

In some implementations, the back plane layer230may be arranged to be in direct contact with the mixing plate of the dilution refrigerator such that heat transferred to the back plane layer230is taken from the integrated circuit board200and dissipated.

Alternatively, the back plane layer230is arranged to be in a shape connectable to a heat sink, which can be maintained at a thermal equilibrium with the mixing plate of the dilution refrigerator.

FIGS.2aand2bshow that the circulator210and the termination resistor220are surface mounted on the front plane layer230. In some implementations, the circulator210and the termination resistor220may be constructed as a so-called “drop-in” component, which has a low-profile shape such that the surface contact area with the front plane layer230can be made as large as possible for efficient thermalization.

In some implementations, one or more sunken holes are formed within the support layer260to house the circulator210or the termination resistor220.

The external surface of the circulator210is made of steel to form the field lines within the circulator210as desired for operation. In some implementations, the material of the external surface of the circulator210may further comprise annealed copper layer for thermalizing.

FIG.2bshows that the termination resistor220is mounted on the front plane layer230.FIG.2bfurther shows that one end of the termination resistor220is electrically connected to the third terminal210-3of the circulator210via one of the solder pads231and that the other end of the termination resistor220is connected to the part of the front plane layer230which serves as an electric ground.

In some implementations, the external surface of the termination resistor220, except the part which makes electrical connections with the solder pad231and the front plane layer230, may be formed with a material which is not electrically conducting but has a high thermal conductivity at a cryogenic temperature.

FIG.2cshows a top view of the integrated circuit board200and the first circulator111,211, the second circulator112,212, the fourth circulator114,214from the qubit readout circuit100ofFIG.1, mounted on the front plane layer230. As discussed above inFIG.1, the third ports111-3,211-3,112-3,212-3,114-3,214-3of the first circulator111,211, the second circulator112,212, the fourth circulator114,214are each connected to a termination resistor121,221,122,222,124,224such that the first circulator111,211, the second circulator112,212, the fourth circulator114,214each functions as an isolator.

The front plane layer230is patterned such that the support layer260is exposed around the periphery of the circulators211,212,214, represented inFIG.2cas unshaded areas. Although not shown being blocked by the circulators211,212,214, underneath each circulator211,212,214, part of the front plane layer230is patterned to be in contact with the surface of the circulators211,212,214. As shown inFIGS.2aand2b, these hidden parts of the front plane layer230are connected to the back plane layer250via the ground vias251for efficient thermal transfer.

FIG.2cshows that the first ports211-1,212-1,214-1and the second ports211-2,212-2,214-2of the circulators211,212,214are electrically connected to respective solder pads231. As discussed above, these electrical connections can be achieved either via direct soldering or wire bonding.

FIG.2cshows that the third ports211-3,212-3,214-3of the circulators211,212,214are electrically connected to one end of the first termination resistor221, the second termination resistor222and the fourth termination resistor224, respectively. These connections are made directly without solder pads231, for example, either via direct soldering or wire bonding. The other end of the first termination resistor221, the second termination resistor222and the fourth termination resistor224is electrically connected to the front plane layer230.

Although not shown blocked by the first termination resistor221, the second termination resistor222and the fourth termination resistor224, part of the front plane layer230are in contact with the bottom surfaces of the first termination resistor221, the second termination resistor222and the fourth termination resistor224for thermal contact. These parts of the front plane layer230in contact with the bottom surfaces of the termination resistors221,222,224are integrally formed with the part of the front plane layer230which forms an electric ground and connected to the back plane layer250vias ground vias251.

Compared to the case where connectors are used for electrical connections, such as stainless steel SMA connector, the integrated circuit board200may allow for more compact implementation and transmission of the signals with less loss. In case the front plane layer230, the signal layer240, the back plane layer250, the signal vias241, the ground vias251comprise a metal with high thermal conductivity, such as copper, direct contact of the components with the copper allows for more efficient thermalization.

FIG.3ais a schematic that illustrates an exemplary integrated circuit board for a qubit readout circuit with references toFIGS.1and2. In particular,FIG.3ashows a CAD drawing of a top view plan of the integrated circuit board300described inFIG.2to implement the qubit readout circuit100described inFIG.1.

The integrated circuit board300includes four sites for the circulators111,211,112,212,113,114,214, namely a first site311for the first circulator111,211, a second site312for the second circulator112,212, a third site313for the third circulator113, and a fourth site314for the fourth circulator114,214.

The integrated circuit board300further includes a site for the Josephson junction parametric amplifier370. The Josephson junction parametric amplifier370is implemented on a separate chip from the integrated circuit board300, and can be wire bonded to the integrated circuit board300.

The integrated circuit board300further includes an input terminal301-1, an excite terminal301-2, an output terminal302, a pump terminal303. Referring toFIG.1, the input terminal301-1is the coupled port10-2of the directional coupler10and the excite terminal301-2is the input port10-1of the directional coupler10. As explained inFIG.1, the probe signal is input into the excite terminal301-2. A portion of the probe signal is coupled into input terminal301-1and sent to the plurality of readout resonators coupled respectively to the plurality of qubits. The probe signal reflected from the plurality of readout resonators are transmitted back to the input terminal301-1and enters the integrated circuit board300.

Each site311,312,313,314includes three solder pad areas331. The solder pad areas331are labelled inFIG.3aas1,2,3corresponding to respectively the first ports111-1,211-1,112-1,212-1,113-1,114-1,214-1, the second ports111-2,211-2,112-2,212-2,113-2,114-2,214-2and the third ports111-3,211-3,112-3,212-3,113-3,114-3,214-3of the circulators111,211,112,212,113,114,214.

FIG.3ashows that the solder pad areas331of the sites311,312,313,314are connected as described inFIG.1. For example, the solder pad area331of the first site311which is labelled as ‘1’ is connected to the input terminal301. The solder pad area331of the fourth site314which is labelled as ‘2’ is connected to an output terminal302. The pump port303is connected to the site for the Josephson junction parametric amplifier370.

As discussed above inFIG.2, each of the sites311,312,313,314are arranged such that when the circulators111,211,112,212,113,114,214are mounted, at least part of the front plane layer230are in contact with at least one of the external surface of the circulators111,211,112,212,113,114,214for thermalization. The part of the front plane layer230which is to come in contact with the circulators111,211,112,212,113,114,214when the circulators111,211,112,212,113,114,214are mounted, are electrically and thermally connected to the back plane layer250via ground vias251.

In some implementations, the sites311,312,313,314may be formed by patterning the front plane layer250, as depicted inFIGS.2aand2bsuch that at least one surface of the circulators110,210,111,211,112,212,113,114,214and the termination resistor120,220,121,221,122,222,123,124,224are in contact with the front plane layer230.

In some implementations, the front plane layer230may be configured to have a sunken hole which houses the circulators110,210,111,211,112,212,113,114,214or the termination resistors120,220,121,221,122,222,123,124,224. In this case, the front plane layer230and the signal layer240may be rearranged accordingly to facilitate electrical connections and efficient thermalization.

For example, in some implementations, the first support layer260-1may be formed to have a sunken hole such that the part of the front plane layer230to be in contact with one of the external surfaces of the circulators110,210,111,211,112,212,113,114,214for thermalization. This is to align the front plane layer240with the electrical connections or the ports210-1,210-2,210-3,210-1,211-2,211-3,212-1,212-2,212-3,213-1,213-2,213-3of the circulators210,211,212,213which may be positioned higher than the bottom surfaces of the circulators210,211,212,213.

Since the electrical terminals of the circulators110,210,111,211,112,212,113,114,214or the termination resistors120,220,121,221,122,222,123,124,224may be formed on the side surfaces, along the x-z plane or the y-z plane inFIG.2, the distance between the plane with the solder pads231and the lowered front plane230, or the bottom of the sunken hole, or the plane in contact with the external surfaces of the circulators110,210,111,211,112,212,113,114,214or the termination resistors120,220,121,221,122,222,123,124,224may largely match vertical the distance, along the z-axis, between the bottom surface and the electrical terminals of the circulators110,210,111,211,112,212,113,114,214or the termination resistors120,220,121,221,122,222,123,124,224. These may allow straightforward soldering or wire bonding after the circulators110,210,111,211,112,212,113,114,214or the termination resistors120,220,121,221,122,222,123,124,224are dropped into the sunken hole for mounting.

The configuration of the sites311,312,313,314where the circulators110,210,111,211,112,212,113,114,214are to be mounted are not limited to these implementations. As far as electrical connections can be made to the signal layer240via the solder pads,231,331and an efficient thermal connection to the back plane layer250can be made via the ground vias251, any configuration of the sites311,312,313,314can be used.

FIG.3bis a schematic that illustrates an exemplary integrated circuit board for qubit readout circuit with references toFIGS.1and2. In particular,FIG.3bshows a mechanical drawing of the integrated circuit board300when sunken holes are formed to mount the circulators110,210,111,211,112,212,113,114,214.

FIG.3bshows that each of the sites311,312,313,314is sunken such that part of a front plane layer330is lowered to form a thermal contact area332. As discussed above, most of the thermal contact area332may not be visible from the top once the circulators110,210,111,211,112,212,113,114,214are mounted. Also as discussed above, the thermal contact area332comprises a conducting layer which is electrically and thermally connected to the back plane layer350, not shown inFIG.3bvia the ground vias251, also not shown inFIG.3b.FIG.3balso shows that each of the sites311,312,313,314includes at the level of the thermal contact area332a cylindrical void such that the all of the front plane layer230,330, the signal layer240, the back plane layer250and the support layer260,360are removed. The physical extent of such cylindrical voids may be determined such that the thermalization of the circulators110,210,111,211,112,212,113,114,214are efficiently performed within the sites311,312,313,314. For example, in order to increase the thermal transfer, the area of the thermal contact area332may be increased and the extent of the voids may be decreased.

FIG.3bshows that the first ports111-1,211-1,112-1,212-1,113-1,114-1,214-1, the second ports111-2,211-2,112-2,212-2,113-2,114-2,214-2and the third ports111-3,211-3,112-3,212-3,113-3,114-3,214-3of the circulators111,211,112,212,113,114,214are electrically connected as described inFIGS.1and3a.

The lines inFIG.3brepresenting the electrical connects may be at the level of the signal layer240. In some implementations, the signal layer240may comprise one or more stripline waveguides. As discussed above, in some implementations, the signal layer240and the thermal contact area332may be at the same level within the integrated circuit board300. Also as discussed above, in some implementations, the signal layer240may be at a higher level than the thermal contact area332to allow for convenient electrical connection via soldering or wire bonding.

The design of the integrated circuit board300shown inFIG.3ballows for independent clamping of circuit components on the integrated circuit board300while enabling the circuit board300to act as a good thermal sink.

In some implementations, the integrated circuit board300may further include a magnetic shield tube375, which is described later inFIG.3c.

In some implementations, SMA connectors may be clamped to the input terminal301-1, the excite terminal301-2, the output terminal302, and the pump terminal303of the integrated circuit board300such that a sufficient amount of torque can be applied in fastening the SMA connectors without damaging the board. The four holes around each terminal301-1,301-2,302,303are for mounting the SMA connectors with four pins for alignment and grounding.

In some implementations, SMA connectors may be clamped to the input port301, output port302, and the pump port303of the integrated circuit board300such that a sufficient amount of torque can be applied in fastening the SMA connectors without damaging the board.FIG.3cis a schematic that illustrates a portion of an exemplary integrated circuit board for a qubit readout circuit. In particular,FIG.3cshows a schematic of the magnetic shield tube375and the configuration of the integrated circuit board300around the site for the Josephson junction parametric amplifier370.

The circulators110,210,111,211,112,212,113,114,214may comprise magnetized ferrite materials and the Josephson junction parametric amplifier130may be sensitive to the magnetic field since the operation of the Josephson junction is dependent on the magnetic flux bias applied on it. Therefore, in order to integrate the circulators110,210,111,211,112,212,113,114,214and the Josephson junction parametric amplifier130in a close proximity within one integrated circuit board300, the Josephson junction parametric amplifier130should be shielded from the magnetic field generated by the circulators110,210,111,211,112,212,113,114,214to a degree that it does not affect the operation of the Josephson junction parametric amplifier130.

FIG.3cshows in the left panel the magnetic shield tube375when it is slotted in a slot305formed within the integrated circuit board. The material for the magnetic shield tube375comprises a mu-metal, which is an alloy with a high magnetic permeability, often used for magnetic shielding. The example of the material for the magnetic shield includes Amuneal 4K.

In some implementations, the magnetic shield tube375may be a separate component from the integrated circuit board300and to be assembled by slotting into the slot305formed within the integrated circuit board300, as shown in the left panel ofFIG.3c.

The magnetic shield tube375may comprise a mu metal shaped in the form of a cylinder with at least one of the faces open such that it can be slotted in to the slot305formed on at least one of the side surfaces of the integrated circuit board300.

It is known that for effective magnetic shielding, the aspect ratio is one of the crucial parameters. In other words, the ratio between the length of the magnetic shield tube375along the x-axis inFIG.3c, and the lateral extent of the cross section of the magnetic shield tube375, along the y-axis inFIG.3cdetermines the degree of magnetic shielding. Therefore, if the thickness of the integrated circuit board300, along the z-axis inFIG.3c, is kept small, the volume occupied by the magnetic shield tube375can be correspondingly small while maintaining the aspect ratio of the magnetic shield tube375. Since the thickness of the integrated circuit board300in the z-direction, may be determined by the depth of the sunken holes to mount the circulators110,210,111,211,112,212,113,114,214or the termination resistors120,220,121,221,122,222,123,124,224, if the extent of the circulators110,210,111,211,112,212,113,114,214or the termination resistors120,220,121,221,122,222,123,124,224in the z-direction, is kept small to keep a low-profile, the volume of the magnetic shield tube375can be decreased.

In some implementations, the slot305may be formed from one of the sides, for example in the y-z plane ofFIG.3c, of the integrated circuit board300near the site for the Josephson junction parametric amplifier370. The slot305includes at least two elongated channels within which the walls of the cylinder formed by the magnetic shield tube375fit in.

In some implementations, the slot305may further include a recession along the side of the integrated circuit board in which the blocked end of the cylinder formed by the magnetic shield tube375is disposed such that it is flush with the side surface of the integrated circuit board300.

The right panel ofFIG.3cshows the cross section of the assembly of the integrated circuit board300and the magnetic shield tube375along the dotted line in the left panel ofFIG.3c.

In some implementation, the extent of the interior of the cylinder formed by the magnetic shield tube375, along the z-axis inFIG.3c, may be arranged to fit or to be larger than the thickness of the integrated circuit board300. In this case, the magnetic shield tube375encloses, on one edge of the integrated circuit board300around the site for the Josephson parametric amplifier370, the front plane layer330, the signal layer340, and the back plane layer350around the site for the Josephson parametric amplifier370.

Alternatively, in some implementations, the slots305may be further formed such that the magnetic shield tube375encloses only the signal layer340and the site for the Josephson parametric amplifier370, or such that the magnetic shield tube375encloses only the front plane layer330, the signal layer340and the site for the Josephson parametric amplifier370, or such that the magnetic shield tube375encloses only the signal layer340, the site for the Josephson parametric amplifier370and the back plane layer370. As long as the aspect ratio of the magnetic shield tube375is maintained at a level where the necessary magnetic shielding is obtained, the slot305around the site for the Josephson parametric amplifier370may be arranged accordingly.

Compared to the case where magnetic shielding is achieved by the casings of the circulators110,210,111,211,112,212,113,114,214comprising a mu-metal, the implementations described above and shown inFIGS.3band3care more compact and also more effective for thermalization because only the site for the Josephson parametric amplifier370is magnetically shielded and because the material for the casings of the circulators110,210,111,211,112,212,113,114,214can be chosen to optimize thermalization rather than requiring magnetic shielding. As such, the circulators110,210,111,211,112,212,113,114,214can be disposed close to the Josephson parametric amplifier370, providing a more compact design of the integrated circuit board300. Furthermore, the tube-shape of the magnetic shield tube375and the fact that it is a separate component from the integrated circuit board300provide more freedom in routing the electrical connections into and out of the site for the Josephson parametric amplifier370compared to the case where the magnetic shielding is built into the integrated circuit board300.

FIG.4is a schematic that illustrates an exemplary qubit readout assembly with references toFIGS.1to3.

The qubit readout assembly400includes an expansion board410. The expansion board410is configured to receive one or more the integrated circuit boards or cards200,300described above inFIGS.2and3in a stacked configuration. The integrated circuit boards200,300may further include the circulators110,210,111,211,112,212,113,114,214or the termination resistors120,220,121,221,122,222,123,124,224mounted on the integrated circuit boards200,300such that each of the integrated circuit boards200,300form a qubit readout circuit100described above inFIG.1.

Each of the integrated circuit boards200,300configured to form the qubit readout circuit100may serve as a pre-amplifying stage for a single channel which includes a plurality of qubits coupled to a single readout transmission line via respective readout resonators. In case a plurality of channels of qubits are used for computation, a corresponding number of the integrated circuit boards200,300configured to form the qubit readout circuit100may be used.

Since each of the integrated circuit boards200,300configured to form the qubit readout circuit100is provided in a shape of a planar board with low-profile components mounted on it, the expansion board may further comprise a plurality of sockets411arranged to receive the plurality of the integrated circuit boards200,300configured to form the qubit readout circuit100such that they are mounted on the expansion board410largely parallel to one another. In particular, the plurality of sockets411may be formed such that the front plane layer230,330and the back plane layer250,350are with a large surface contact with the body of the expansion board410.

The expansion board410further includes a connector412which allows connection of the expansion board410with a cold finger10of the cryostat with a large surface contact. In case the cryostat is a dilution refrigerator, the cold finger10may be the mixing plate of the dilution refrigerator which provides around 10 mK temperature.

It is crucial that the plurality of sockets411and the connector412are arranged to allow an efficient thermal transfer between two parts joined by the plurality of sockets411and the connector412.

The expansion board410may comprise a material with a good thermal conductivity at a cryogenic temperature, such as copper. The expansion board410may have a large enough volume and correspondingly a large enough thermal capacity such that the temperature does not rise locally at a certain position within the expansion board due to the heat received via the plurality of sockets411and such that the heat received is transferred to the cold finger10of the cryostat efficiently.

The shape and the types of the plurality of sockets411and the connector412may be determined such that when the dilution refrigerator is in operation and the temperature of the cold finger10is at its base temperature, the plurality of the integrated circuit boards200,300attached to the expansion board410are at a thermal equilibrium with the cold finger10and at a temperature largely equal to the temperature of the cold finger10.

In some implementations, the expansion board410may include electrical connections connected to the input port301, the output port302, and the pump port303of each of the integrated circuit boards200,300configured to form the qubit readout circuit100. For example, a qubit chip containing the plurality of channels of qubits may be mounted on the cold finger10, which is the mixing plate of the dilution refrigerator and a HEMT (High Electron Mobility Transistor) amplifier may be mounted on a 3K stage of the dilution refrigerator. The expansion board410may be arranged such that it contains or mechanically supports the electrical connections from the qubit chip to the input port301and the electrical connections from the output port302to the HEMT amplifier.

In some implementations, the expansion board410may form a tower mount to which the integrated circuit boards200,300are mounted. The expansion board410and the integrated circuit boards200,300may provide a highly modular system such that broken electrical lines can be easily repaired and the circuit components mounted on the integrated circuit boards can be replaced and/or reconfigured.

Using the design of the integrated circuit boards200,300shown inFIGS.2and3and the expansion board shown inFIG.4, the readout lines may be constructed at a fraction of the size of an existing readout line. For example, 30 readout lines may fit within the experimental space near the mixing plate of a dilution refrigerator.

Implementations of the quantum subject matter and quantum operations described in this specification can be implemented in suitable quantum circuitry or, more generally, quantum computational systems, also referred to as quantum information processing systems, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. The terms “quantum computational systems” and “quantum information processing systems” may include, but are not limited to, quantum computers, quantum cryptography systems, topological quantum computers, or quantum simulators.

The terms “quantum information” and “quantum data” refer to information or data that is carried by, held or stored in quantum systems, where the smallest non-trivial system is a qubit, e.g., a system that defines the unit of quantum information. It is understood that the term “qubit” encompasses all quantum systems that may be suitably approximated as a two-level system in the corresponding context. Such quantum systems may include multi-level systems, e.g., with two or more levels. By way of example, such systems can include atoms, electrons, photons, ions or superconducting qubits. In some implementations the computational basis states are identified with the ground and first excited states, however it is understood that other setups where the computational states are identified with higher level excited states are possible. It is understood that quantum memories are devices that can store quantum data for a long time with high fidelity and efficiency, e.g., light-matter interfaces where light is used for transmission and matter for storing and preserving the quantum features of quantum data such as superposition or quantum coherence.

Quantum circuit elements (also referred to as quantum computing circuit elements) include circuit elements for performing quantum processing operations. That is, the quantum circuit elements are configured to make use of quantum-mechanical phenomena, such as superposition and entanglement, to perform operations on data in a non-deterministic manner. Certain quantum circuit elements, such as qubits, can be configured to represent and operate on information in more than one state simultaneously. Examples of superconducting quantum circuit elements include circuit elements such as quantum LC oscillators, qubits (e.g., flux qubits, phase qubits, or charge qubits), and superconducting quantum interference devices (SQUIDs) (e.g., RF-SQUID or DC-SQUID), among others.

In contrast, classical circuit elements generally process data in a deterministic manner. Classical circuit elements can be configured to collectively carry out instructions of a computer program by performing basic arithmetical, logical, and/or input/output operations on data, in which the data is represented in analog or digital form. In some implementations, classical circuit elements can be used to transmit data to and/or receive data from the quantum circuit elements through electrical or electromagnetic connections. Examples of classical circuit elements include circuit elements based on CMOS circuitry, rapid single flux quantum (RSFQ) devices, reciprocal quantum logic (RQL) devices and ERSFQ devices, which are an energy-efficient version of RSFQ that does not use bias resistors.

Fabrication of the quantum circuit elements and classical circuit elements described herein can entail the deposition of one or more materials, such as superconductors, dielectrics and/or metals. Depending on the selected material, these materials can be deposited using deposition processes such as chemical vapor deposition, physical vapor deposition (e.g., evaporation or sputtering), or epitaxial techniques, among other deposition processes. Processes for fabricating circuit elements described herein can entail the removal of one or more materials from a device during fabrication. Depending on the material to be removed, the removal process can include, e.g., wet etching techniques, dry etching techniques, or lift-off processes. The materials forming the circuit elements described herein can be patterned using known lithographic techniques (e.g., photolithography or e-beam lithography).

During operation of a quantum computational system that uses superconducting quantum circuit elements and/or superconducting classical circuit elements, such as the circuit elements described herein, the superconducting circuit elements are cooled down within a cryostat to temperatures that allow a superconductor material to exhibit superconducting properties. A superconductor (alternatively superconducting) material can be understood as material that exhibits superconducting properties at or below a superconducting critical temperature. Examples of superconducting material include aluminum (superconductive critical temperature of about 1.2 kelvin), indium (superconducting critical temperature of about 3.4 kelvin), NbTi (superconducting critical temperature of about 10 kelvin) and niobium (superconducting critical temperature of about 9.3 kelvin). Accordingly, superconducting structures, such as superconducting traces and superconducting ground planes, are formed from material that exhibits superconducting properties at or below a superconducting critical temperature.

While this specification contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various components in the implementations described above should not be understood as requiring such separation in all implementations.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.