CRYOGENIC PARAMETRIC AMPLIFIER CONTROL APPARATUS

Abstract

A cryogenic parametric amplifier control apparatus is disclosed. Methods of implementation and devices incorporated within the whole of the apparatus are disclosed. Methods of reducing the number of signal lines necessary to control a parametric amplifier are disclosed. Schema allowing for control of multiple parametric amplifiers with a single apparatus are disclosed.

Claims

1-17. (canceled)

18. A method for operating a cryogenic refrigerator having one or more cooling stages comprising: multiplexing a DC signal with one or more RF signals using two or more bias tees, to thereby produce a combined signal on a transmission line; and feeding the transmission line through the one or more cooling stages of the cryogenic refrigerator.

19. The method of claim 18 additionally comprising: attenuating the combined signal in-line using a thermalized attenuator to provide an attenuated RF and DC path between the bias tees and the cooling stages.

20. The method of claim 18 wherein a selected bias tee is coupled to a parametric amplifier, and the method additionally comprises: disposing the selected bias tee at a pump and DC inputs of the parametric amplifier, to separate the DC signal from the one or more RF signals.

21. The method of claim 18 additionally comprising: coupling a selected bias tee to an output of a parametric amplifier, to separate the DC signal from the one or more RF signals.

22. The method of claim 18 wherein the RF signals comprise two or more pump signals combined onto the transmission line using a coupler, a power combiner, or other combining device.

23. The method of claim 18 wherein the RF signals comprise one or more RF pump signals combined onto the transmission line with an RF test signal using a coupler, a power combiner, or other combining device.

24. The method of claim 19 further comprising: DC bypassing the attenuation path, using an inductor or other RF choking component disposed between an input and output of the attenuation path, and using capacitors disposed at the input and output of the thermalized attenuator for further blocking the DC from the attenuated path.

25. The method of claim 24 wherein a DC path is attenuated by an amount different from an RF path using a separate attenuator for the DC path.

26. The method of claim 24 wherein the inductor or other RF choking component is thermalized using silver epoxy potting.

27. A method to limit frequencies that pass through a DC path to thereby limit thermal and other noise through the DC path, comprising: constructing a low-pass filter on a thermally conductive substrate; applying a lossy non-superconducting material on top of a planar inductor, the inductor further comprising a meander, a spiral, or high-Z low-Z elements forming a low-pass structure, such that the lossy material attenuates frequencies higher than a cut-off of the low pass filter, and prevents re-entrant high frequency noise; mounting the substrate to a thermally conductive material base; and whereby heat dissipated in the lossy material is conducted through the substrate material to the base.

28. A method for reducing a number of pump signal lines for a plurality of cryogenic parametric amplifiers (PAs) that share pump frequencies, comprising: disposing a power divider and/or combiner within a lower temperature stage that operates at a lower than ambient temperature; and coupling one or more RF signals to the power divider and/or combiner, to provide two or more pump signals, where each pump signal provides attenuated pump power to a corresponding parametric amplifier.

29. The method of claim 28 additionally comprising: multiplexing a DC signal with one or more of the RF signals using two or more bias tees, to thereby produce a combined signal on a transmission line.

30. The method of claim 28 wherein: a DC bias adjustment is applied to each parametric amplifier.

31. The method of claim 28 additionally comprising: combining a test signal with one or more of the pump signals; monitoring the test signal at one or more outputs of the parametric amplifiers; and wherein the monitoring further comprises detecting the test signal via a microwave detector or homodyne receiver, or by switching between parametric amplifier outputs using an RF switch.

32. A method of multiplexing cryogenic amplifier outputs comprising: combining the amplifier outputs via a power divider/combiner that comprises part of a cooling stage that operates at a lower temperature than room temperature to thereby reduce a number of connections through at least one stage of the cryogenic refrigerator.

33. The method of claim 32 further comprising combining a test signal with one or more of the pump signals; monitoring the test signal at one or more outputs of the parametric amplifier; and wherein the monitoring further comprises detecting the test signal via a microwave detector or homodyne receiver, or by selectively enabling a DC bias to the parametric amplifier under test.

34. A method for operating a parametric amplifier comprising: combining one or more pump signals, or at least one pump signal and a test signal; disposing a directional coupler at an input of the parametric amplifier; wherein the directional coupler is mounted in a same package as the amplifier; and wherein the directional coupler is formed on microstrip, strip line or coplanar substrate.

35. An apparatus comprising: two or more bias tees, configured to multiplex a DC signal with one or more RF signals, to produce a combined signal; a transmission line, with one end comprising a center conductor coupled to receive the combined signal; and a cryogenic refrigerator, the cryogenic refrigerator having one or more cooling stages, the cooling stages coupled to thermalize the transmission line.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0031] The disclosed drawings are not intended to be drawn to scale. Every identical or nearly identical component that is illustrated in the various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings:

[0032] FIG. 1 is a block diagram of an example control apparatus for an amplifier requiring both DC and RF signals with one or more stages of attenuation and DC bypass path filtering and or attenuation. The multiplexed DC and RF signals are split by a bias-T at the RF control signal input to the amplifier.

[0033] FIG. 2 is a block diagram of an example control apparatus for an amplifier requiring both DC and RF signals with one or more stages of attenuation and DC bypass path filtering and or attenuation. The DC signal is multiplexed with the output signal from the amplifier and split by a bias-T on the output of the amplifier.

[0034] FIG. 3 is diagram of an example meandered strip line filter.

[0035] FIG. 4 is a block diagram of an example control apparatus whereby multiple amplifiers are controlled using a single line whose signals are divided and then distributed to each amplifier. The output signal may be detected by a series of couplers and a switch to select which amplifier to monitor.

[0036] FIG. 5 is a block diagram of an example control apparatus whereby multiple amplifiers are controlled using a single RF line whose signals are divided and then distributed to each amplifier. The outputs of the amplifiers are combined onto a single line. Separate DC bias lines are run to each amplifier in this diagram.

[0037] FIG. 6 is a block diagram of one possible configuration of components that may be integrated into a single housing.

DETAILED DESCRIPTION

[0038] An object of the invention is to multiplex RF and DC signals onto single coaxial connections to reduce the connections required through the cryostat or cryogenic refrigerator. An example of a control circuit for an amplifier 110 requiring both DC and RF signals is shown in FIG. 1. A bias-T 104 is used at the room temperature to combine or multiplex RF and DC signals onto a single coaxial transmission line 105. Here, ‘bias-T’ refers to any scheme or device with three or more ports and one or more frequency filtering elements that result in at least three port-to-port paths having different passing bandwidths and isolation frequency bandwidths from one another. The signals on the line 105 are then separated again at the final cold temperature stage of the cryostat or cryogenic refrigerator using another bias-T.

[0039] One potential application is for biasing amplifiers such as low noise HEMT or HBT amplifiers that require both DC bias and RF signals to be supplied within a cold stage of a cryostat or cryogenic refrigerator. Another application is for traveling wave parametric amplifiers (TWPAs), a subset of quantum limited parametric amplifiers (QLPAs), that require magnetic field-producing bias connections.

[0040] The TWPA disclosed in U.S. Pat. No. 10,516,375 B2 requires both RF signals and DC signals for operation. For simplification purposes, and generalization to the applications discussed, what are herein referred to as ‘RF signals’ are to be taken as electromagnetic signals with frequencies typically greater than 1 MHz. Likewise, ‘DC signals’ refer to electromagnetic signals with frequencies typically less than 1 MHz. ‘DC’ in the context herein is taken as the low-pass side of a frequency crossover network that includes 0 Hz. In a parametric amplifier, RF signal may provide a ‘pump’, from which power is drawn from through wave mixing on the parametric amplifier's transmission line. This pump signal(s) 102 may be typically injected into the amplifier at its input. The DC signal 108 may be used to generate a magnetic bias field though a superconducting trace or wire in proximity to the TWPA's non-linear elements. A directional coupler 109 is typically used for injection of one or more pumps and provides isolation of those pumps in the signal source direction. The level of isolation is limited by the coupler isolation and the match into the TWPA. In some cases, multiple pump signals can be used, allowing for varied amplification bandwidth of input weak signals. Herein ‘weak signals’ refer to signals incident to the amplifier being controlled, typically from a device under test, such as a qubit, or some other observable source wished to be observed. The use of ‘weak’ is appropriate because in order to see appreciable gain of any incident signals on the output of a parametric amplifier, those incident signals much be significantly lower in power level than any supplied pump signals.

[0041] A further object of the invention is to combine multiple pump signals 102 using one or more couplers, power combiners, or similar combining devices 103, to combine multiple pumps prior to their input to one or more bias-Ts 104 at room temperature. In this arrangement, all pumps and DC are combined on a single transmission line 105. In addition to the pump or pumps, a test signal 101 can also be combined, through similar means, and occupy the same path as the multiplexed DC/pump transmission line 105. The test signal 101 could serve as a surrogate to the input signal to the amplifier and, when detected by warmer temperature electronics 100, could be used for monitoring and optimization of amplifier performance. The test signal could be stepped or swept in frequency to characterize the frequency-gain response of the amplifier. This detection of the test signal can be done at whatever stage in the cryostat or cryogenic refrigerator is most convenient. Currently, this is typically done at room temperature, but detection of the test signal can be easily performed at an intermediate stage, such as the 4K (−268.15° C.) stage, particularly if circuitry is already being used there for signal generation and control.

[0042] A further object of the invention is shown in FIG. 2 where a bias-T is on the RF output signal transmission line 112 instead of the pump transmission line 105 in order to multiplex the DC with the RF output, as an alternative to combining the DC bias with the pump. The option to use this configuration may be desirable given the wide variety of applications and approaches to these cryogenic amplifier technologies. Another reason may be to place the added insertion loss of the bias-Ts 104 on the output line rather than RF control signal line. Typically, this will not be the case as added loss on the output decreases the overall gain of the system. However, in cases where there is ample or excess gain, but the system is starved for RF control signal power, such a configuration may become desirable.

[0043] Depending on the application and hardware, it may be desirable to have different amounts of attenuation used on the DC signal and RF signal. When the DC and RF are combined on a single conductor, both will be attenuated the same amount when passing through a typical resistor element attenuator 106. To overcome this limitation, one embodiment of the invention includes a method of integrating a DC bypass path with an inductor so that the DC signal is not attenuated while the RF is. We further propose a DC blocking capacitor be used to isolate the DC signal from the resistive elements of the attenuator 106 and keep power from being dissipated in those elements. This is especially important at the lower temperature cooling stages both because a hot conductor element will add extra Johnson thermal noise with fewer attenuation stages left to filter it out and because the lower temperature stages of cryostats and cryogenic refrigerator have much smaller heat removal capabilities.

[0044] Cryogenic bias-Ts could also be used to split the RF and DC from the single line prior to attenuation and then recombined with a second bias-T. By adding a DC attenuator or resistor divider in the DC path 107, the DC signal can also have attenuation, but different from than that of the RF path. A compact way of achieving different levels of attenuation on the RF and DC signals is to add attenuation to the DC bypass path with resistive elements or a lossy resistive coating. In addition to this attenuation of the DC signal, it is also desirable to filter off higher in frequency signals in the DC path that may re-enter above the operational frequency range of the bias-Ts. These high frequency signals can add Johnson noise to the DC path that is undesirable.

[0045] Another object of the invention is a non-reentrant low-pass filter on the DC path that is incorporated with the attenuator, in the DC bypass path 107, to provide thermal noise filtering without additionally attenuating the DC signal. In all of the configurations of attenuating and filtering the DC signal(s) separately from the RF signal(s) when they are multiplexed on a single transmission line, the power dissipated in the inductors, used to choke off the RF from the DC bypass, and resistors are a concern and should be well thermalized. To improve the thermal conduction and keep the entire inductor coil as close to the housing temperature as possible, a further object of the inventions is the potting of the inductor with a thermally conductive epoxy, such as a silver-loaded epoxy. The method could vary slightly depending on the choice of inductor, but for a wire-wound inductor the epoxy is spread over the surface of the windings to create a low thermal resistance path between the housing and the inductor windings. Note that wire wound inductors typically have electrically insulating coatings that are very thin and will not add significantly to the thermal resistance while providing necessary electrical insulation of the wire.

[0046] In one embodiment, the filtering device 107 is constructed of a transmission line and includes a two-dimensional series inductor such as a meander, spiral, or Low-z/high-Z structure forming a low-pass filter. The inductor acts to choke microwave energy by presenting a high impedance that increases with frequency according the formula,

[00001]$X_L=2\pi \mathrm{fL},$

[0047] where X.sub.L is the inductive reactance in ohms, f is the frequency in Hertz, and L is the inductance of the inductor in Henries. The reflection coefficient, given by,

[00002]$\Gamma =\frac{\left(\frac{z_L}{z_0}-1\right)}{\left(\frac{z_L}{z_0}+1\right)},$

[0048] approaches unity for large values of f and will act to filter high frequency thermal noise. In practice, the reactance of the inductor will increase above self-resonance due to stray capacitance resulting in diminished reactance. The addition of a lossy material within the series inductor EM fields produces an imaginary propagation constant to the propagating modes resulting in losses that increase with frequency for a constant length of the inductor's transmission line. These losses produce heating of the lossy layer and heat must be carried away through weak electron-photon interaction. More generally, the inductor will produce its own thermal noise proportional to its temperature. For these reasons, the inductor and lossy layer are fabricated in close contact with a thermally conductive transport medium. One possible embodiment shown in FIG. 3 is a 50-Ohm microstrip transmission line 116 fabricated as a gold metallization on a quartz dielectric substrate 114 with TEM propagating modes. A series meander-line inductor 113 is fabricated on the substrate to form an inductor. A thin nichrome layer is deposited over the dielectric to form a lossy layer. A non-functional layer of SiO2 may be added on top to passivate the NiCr layer. The substrate may be mounted onto a heatsink or housing 115 made from a material with good thermal conduction, such as oxygen-free copper, using a thermally conductive adhesive, such as silver-loaded epoxy. Transitions to standard SMA connectors could also be incorporated into the housing and used to interface between the microstrip transmission line 116 and coaxial interfaces.

[0049] An additional object of the invention is a method for further reducing the number of connections required in cryogenic systems that have multiple cryogenic parametric amplifiers. The method is comprised of one or more source at room temperature that may be combined on a single line, and/or multiplexed with a DC signal, and connected through the cooling layers of the cryostat or cryogenic refrigerator. Attenuator components 106 may be used to attenuate the pump signal(s) in the same manner as when controlling a single amplifier. Typically, multiple sources and multiple connections with multiple sets of attenuators have been used. With the addition of cryogenic power divider at a cooled stage 117 within the cryostat or cryogenic refrigerator, a single control line is shared among multiple amplifiers. The power division does not need to be equal and can be adjusted to provide different pump powers to the multitude of amplifiers. One embodiment would be to have un-even power division in the microwave power divider. Another embodiment would be to add trimming attenuator pads to each of the outputs of the power divider to adjust the pump power levels. For amplifiers that that support gain separated from the pump signals, such as the TWPA disclosed by US patent 2018/0034425 A1, multiple frequency-separated amplification channels can operate from the same pump frequency. Many amplifier types require a DC bias for operation. In the case of the asymmetric SQUID-based TWPA disclosed by US patent 2018/0034425 A1, DC bias may be used to supply the magnetic flux required. While not technically required, as an external magnetic field may be applied to the TWPA during operation, the implementation of that external field takes up significant space and is not desirable for applications with high component counts. If DC bias is combined with the microwave pump signal, using the method described previously herein, the DC bias will nominally be constant across all amplifiers. In one embodiment, the amplifiers have different DC bias levels produced by voltage adjustors 118 such as resistor divider networks.

[0050] In addition to pump sharing, the addition of a test signal 101, combined with the shared pump 102, can be shared. The test signal would share the same connection through the cooling layers of the cryostat or cryogenic refrigerator as the shared pump. The test signal is injected along with the pump into each amplifier. By monitoring the output levels at the test frequency, the parametric performance of each amplifier (e.g. gain, gain compression) could be measured. As shown in FIG. 4, the amplified test signals are monitored by a microwave homodyne receiver 119 that can be switched in the room temperature electronics between the various outputs. The test signal can be used to tune the performance of the amplifier. For example, the amplifier gain can be maximized at the test frequency while adjusting the DC bias or pump power to determine optimal values of DC and pump levels. The test frequency could also be stepped or swept across the frequency band of interest so that optimal DC and pump levels can be determined for many frequencies, or globally optimized over a range of frequencies.

[0051] An additional invention comprising an output combiner 120 connected to the outputs of a multiple amplifiers at a cooled stage would reduce the number of output connections through the cooling layers of the cryostat or cryogenic refrigerator. For the application where qubit signals are incident on each amplifier, each qubit would be distinct in frequency or separated in the time domain so that the information associated with each qubit may be separately detected by the detection apparatus. In the case where both the pump signals and amplifier outputs are combined for device characterization at cooled temperatures, a common test signal could still be used to tune the performance in the method described in the previous paragraph, however some means of selecting an individual amplifier under test would need to be used. One such means is to separate DC bias connections 121 for each amplifier, in one embodiment, so that only the DC bias-To the amplifier of interest is enabled. It is generally easier to implement DC bias connections than the output RF connections, so this configuration still results in an overall simpler readout architecture with built-in-test capability than the configuration the previous paragraph describes.

[0052] Another invention is a single component that integrates the: amplifier input 122; output 123; and pump signal, DC bias signal and test signal 124, into one component. A block diagram of one such integrated configuration is shown in FIG. 6. The bias-T 125 separates the pump signal from DC bias. An integrated microwave coupler 126 directionally couples the pump with the input signal. The directivity of the coupler reduces the amount of the pump signal that goes back toward the input and could interfere with the quantum state of the qubit. The pump can be mixed with a test signal when characterizing the performance of the parametric amplifier. In one possible embodiment, an isolator 111 is used at the input of the component to provide additional isolation from the weak signal source. In another embodiment, isolators on both the input and output of the amplifier are used and provide additional isolation from components further down the chain. This is desirable because the parametric amplifier often may not provide sufficient reverse isolation. In another embodiment, an integral DC bias adjustment, possibly in the form of resistor divider, is used to control the DC bias level within the amplifier. In another embodiment, after splitting the DC and RF within the integrated component, the DC signal may be filtered 128 prior to injection to the amplifier. In another embodiment, the RF may be attenuated 127 within the package prior to injection to the amplifier.

[0053] Described above include several examples of control systems and parametric amplifier technologies in various applications. It is not possible to describe every conceivable combination of components, products, and/or parametric amplifier technologies for purposes of describing this disclosure. One skilled in the art can recognize that many further combinations and permutations of this disclosure are possible. The descriptions of the various embodiments have been presented for purposes of instruction but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.