Insulation Amplifier

Abstract

According to an exemplary aspect of the present invention, there is provided an isolation amplification apparatus comprising a first 2×2 hybrid coupler and a second 2×2 hybrid coupler, each 2×2 hybrid coupler having a first input port, a second input port, a first output port and a second output port, the apparatus further comprising a first traveling wave parametric amplifier TWPA having an input connected to the first output port of the first 2×2 hybrid coupler and an output connected to the first input port of the second 2×2 hybrid coupler, and a second traveling wave parametric amplifier TWPA having an input connected to the second output port of the first 2×2 hybrid coupler and an output connected to the second input of the second 2×2 hybrid coupler.

Description

[Technical field]

[0001] The present invention relates to a traveling wave parametric amplifier, TWPA. [Background technology]

[0002] A parametric amplifier is effectively a mixer that can amplify a weaker input signal by mixing it with a stronger pump tone, resulting in a stronger output signal. Parametric amplifiers rely on the nonlinear response of a physical system to generate amplification. Such amplifiers may consist of standing-wave parametric amplifiers or traveling-wave parametric amplifiers, TWPAs, where traveling-wave parametric amplifiers use dispersive nonlinearity along a waveguide, and in the ideal case, the signal travels in one direction. Dispersive nonlinearity can consist of a continuous nonlinear material distributed along a transmission line, such as a coplanar waveguide, or a series of many nonlinear lumped elements. If the nonlinear elements include Josephson junctions, the amplifier may be called a Josephson traveling-wave parametric amplifier, JTWPA. In a JTWPA, the Josephson junctions are maintained in a superconducting state and carry a supercurrent. If a high kinetic inductance material, such as NbTiN, is used to provide the nonlinearity, the amplifier may be called a kinetic inductance traveling-wave amplifier. In the optical domain, nonlinearity is typically provided by nonlinear crystals, and traveling-wave parametric amplifiers are often simply called optical parametric amplifiers, without emphasizing their traveling-wave nature. Summary of the Invention [Means for solving the problem]

[0003] Some aspects provide subject matter of the independent claims. Some embodiments are defined in the dependent claims.

[0004] According to a first aspect of the present disclosure, there is provided an isolated amplification apparatus comprising: a first 2×2 hybrid coupler and a second 2×2 hybrid coupler, each 2×2 hybrid coupler having a first input port, a second input port, a first output port, and a second output port; a first traveling-wave parametric amplifier TWPA having an input connected to the first output port of the first 2×2 hybrid coupler and an output connected to the first input port of the second 2×2 hybrid coupler; and a second traveling-wave parametric amplifier TWPA having an input connected to the second output port of the first 2×2 hybrid coupler and an output connected to the second input of the second 2×2 hybrid coupler.

[0005] According to a second aspect of the present disclosure, there is provided a method comprising using the isolation amplifier apparatus according to the first aspect as an amplifier, the method comprising the steps of: connecting a first input of the first 2×2 hybrid coupler to a signal source; supplying a pump tone to the first input or the second input of the first 2×2 hybrid coupler; extracting an idler tone as an amplified signal from a first output of the second 2×2 hybrid coupler; and dissipating the amplified signal from a second output of the second 2×2 hybrid coupler. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 illustrates an exemplary system useful for explaining how the operating principles of the present invention contrast with interferometers. [Diagram 2] FIG. 1 illustrates an exemplary apparatus in accordance with at least some embodiments of the present invention. [Diagram 3] FIG. 1 illustrates a system according to at least some embodiments of the present invention. [Figure 4] FIG. 1 illustrates a system according to at least some embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] By connecting two TWPAs with a 2×2 hybrid coupler as described herein, an isolated amplifier is obtained that provides low noise amplification in the forward direction, but does not transmit noise or other signals in the reverse direction from the output of the isolated amplifier to the source of the signal being amplified. This provides the effect and beneficial advantage of better isolating the signal source from the surrounding environment while still gaining access to the signal information from the signal source. This is achieved by a combination of phase control using frequency mixing and properly placed interference effects, as described in detail herein below. This disclosure describes a circuit arrangement of two or more individual TWPAs and standard passive components that constitute an isolated amplifier that suppresses noise propagating in the reverse direction without compromising the desirable properties of the constituent TWPAs. In various embodiments, the arrangement also automatically filters pump tones from the output signal that would otherwise have to be done with separate filter components. Additionally, the disclosed arrangement can optionally frequency shift the output signal relative to the input signal.

[0008] FIG. 1 shows an example system useful for explaining how the principles of operation of the present invention compare to an interferometer. The apparatus of FIG. 1 is a mutual interferometer, in which 2×2 hybrid couplers 102, 104 are connected together with waveguides 110, 120 as shown. Although the term "waveguide" is used in this disclosure, this choice of terminology is not intended to limit the applicability of the disclosed technology, even though other terms for the waveguide structure, such as transmission line, hollow core waveguide, integrated optical waveguide, or fiber optic cable, are used in various frequency domains. Furthermore, for the sake of brevity, the term hybrid coupler is used instead of 2×2 hybrid coupler. In particular, a first input 102a of the hybrid coupler 102 is fed from a source P1, and a second input 102b of the hybrid coupler 102 is fed from a source P4. A first input 104a of the hybrid coupler 104 is connected to a first output 102c of the hybrid coupler 102 via a waveguide 110, and a second input 104b of the hybrid coupler 104 is connected to a second output 102d of the hybrid coupler 102 via a waveguide 120. The hybrid coupler 104 provides a first output 104c and a second output 104d connected to loads P2 and P3, respectively. The sources P1, P4 and the loads P2, P3 may be impedance matched.

[0009] The hybrid coupler may be comprised of a 90-degree hybrid, which may also be referred to as a 3 dB coupler, a branch-line coupler, or other terms referring to the physical implementation. The use of the term hybrid coupler, or the use of a 90-degree hybrid coupler in some embodiments, is not intended to limit the applicability of the disclosed technology, even though different phase shifts are more typical for power splitters in certain frequency ranges and different terms such as beam splitters are used in certain frequency ranges. Furthermore, the relative phase of the outputs may depend on the inputs of the coupler, and the relative phase may be significantly different at different frequencies, which is relevant for TWPAs where the frequencies of interest span more than an octave. The defining features of a hybrid coupler include that it is passive and that each input is split equally to the two outputs (hence the 3 dB term). In a 90-degree hybrid coupler, the outputs are given a relative phase shift of 90 degrees. Hybrid couplers may in practice have slight imperfections due to manufacturing variations such that the magnitudes are not divided exactly equally or the phase difference is slightly different from the nominal value, but these effects, although they lead to a decrease in performance, do not prevent the operation of the present invention. For example, the phase difference applied to a 90-degree hybrid coupler may actually be 90 degrees or 90 degrees ±10 degrees, and amplitude imbalances may occur, for example, of 0.5 dB and / or 1 dB of loss or reflection. Practical imperfections of a similar magnitude may also be introduced into the overall isolation amplifier by imperfections of other components, components shown to be identical in the examples are not completely identical, or the presence of additional short segments of passive waveguides not shown in the examples.

[0010] In the illustrated device, the signal coming from P1 and entering the first input 102a propagates through the device as follows: the signal is split in the hybrid coupler 102 into the waveguides 110 and 120, and has a relative phase of 90 degrees in the waveguide 120, if the relative phase in the waveguide 110 is defined as zero. These are each split a second time in the hybrid coupler 104, which adds a further relative phase shift of 90 degrees to the outputs diagonal to the inputs in the figure. The output signal at the first output 104c of the hybrid coupler 104 is therefore the sum of two equal amplitude parts, the first part coming from the waveguide 110 with a relative phase of zero and the second part coming from the waveguide 120 with a relative phase of (90° + 90°) = 180°. The two equal amplitude parts therefore have opposite phases and cancel each other at the output 104c. In contrast, the output signal at the second output 104d of the hybrid coupler 104 is the sum of two equal amplitude parts, a first part originating from the waveguide 120 with a relative phase of 90 degrees, and a second equal amplitude part originating from the hybrid coupler 104 and coming from the waveguide 110 with a relative phase of 90 degrees. The two equal amplitude parts therefore have the same sign and interfere constructively at the output 104d. In other words, the signal provided by P1 is not seen at all at the first output 104c of the waveguide 130, and the signal provided by P1 is directed in its entirety from the first input 102 to the second output 104d and to the waveguide 140.

[0011] Using similar logic, a signal provided to the second input 102b of the hybrid coupler 102 from P4 winds up as an output signal at P2, which is a load connected to the first output 104c of the hybrid coupler 104.

[0012] Thus, energy can flow from P1 to P3, and from P4 to P2. Because the exemplary system of FIG. 1 is symmetric with respect to inputs and outputs, it also operates symmetrically in the other direction. This is called reciprocity. As a result, energy can flow from P3 to P1, and from P2 to P4. In an ideal component, energy would not flow in the opposite direction, for example, from P2 to P1, for the same reason, i.e., destructive interference.

[0013] FIG. 2 illustrates an exemplary apparatus according to at least some embodiments of the present invention. Like numbers indicate similar structures to FIG. 1. The TWPA 210 is coupled to the waveguide 110 such that the first output 102c of the hybrid coupler 102 is fed to the input of the TWPA 210, and the output of the TWPA 210 is transmitted to the first input 104a of the hybrid coupler 104. Similarly, the second output 102d of the hybrid coupler 102 is transmitted via the waveguide 120 to the input of the TWPA 220, and the output of the TWPA 220 is transmitted to the second input 104b of the hybrid coupler 104. The TWPAs 210 and 220 are non-reciprocal elements that provide gain only in the forward direction, but individually do not provide significant isolation beyond bidirectional dissipative losses, i.e., dissipation of the signal due to loss mechanisms within the TPWA that are independent of the direction of propagation. In contrast, the apparatus of FIG. 2 as a whole provides significant additional isolation that is not related to dissipation within the TWPA itself, as will be described below. Bidirectional dissipative losses are generally undesirable because they directly reduce the net gain in the forward direction, increase power dissipation, increase pump power requirements, and typically lead to additional noise at the output. For example, in the JTPWA, the bidirectional losses are low, e.g., in the range of 1-10 dB. In typical optical parametric amplifier implementations, the bidirectional losses are much higher, e.g., tens of dB, although in the future optical parametric amplifiers with lower losses may be developed.

[0014] In a typical application of JTWPA, distributed readout of superconducting qubits, the phase and amplitude of a pulse of a few hundred microwave photons encodes the state of the superconducting qubit being read out. Detecting such a weak signal is difficult due to its small amplitude. Other applications, such as quantum teleportation, may require sensitivity down to the single photon level. Therefore, a suitable amplifier may be used to increase the amplitude of the received signal before feeding it to a subsequent amplifier and detector to recover the information encoded in the weak signal. As another example, communication in the single photon regime may be employed to communicate cryptographic keys in a secure manner using quantum communication such that eavesdropping can be detected. Other embodiments of the invention may be applied to amplify signals that are not as weak as the signals from the qubits.

[0015] At least some embodiments of the present disclosure focus on superconducting realizations of TWPAs, where a significant portion of the inductance of the transmission line is contributed by kinetic inductance or an array of Josephson junction-based elements known as Josephson elements, such as single junctions, Superconducting Quantum Interference Devices, SQUIDs, or Superconducting Nonlinear Asymmetric Inductive Devices, SNAILs. The Josephson junctions in a Josephson element can be of multiple types, and different weak links can be placed between the two superconductors. The Josephson element provides a nonlinearity that enables a mixing process that provides power gain for weak signals (typically in the radio frequency band for JTWPAs) propagating along the same direction as the strong pump tone. The strength of the pump tone may be parameterized by the ratio of the pump current amplitude Ip to the designed value of the critical current Ic of one arbitrarily selected Josephson junction in the Josephson element. The nature of the nonlinearity depends on the placement of the Josephson junctions in the element. The simplest realization is to use a single Josephson junction as the nonlinear element, and the Taylor expansion of the associated inductance is a constant plus a term proportional to (Ip / Ic) ∧ 2, i.e., the Kerr nonlinearity. The Kerr term results in the desired four-wave mixing process, but also changes the wave vector of the rf tone as a function of pump power, an effect that can be compensated for with dispersion engineering. Balancing the wave vectors is also called phase matching, and can exponentially increase the TWPA gain as a function of device length. The present invention is independent of the details of the dispersion engineering used within the TWPA. In addition to four-wave mixing TWPAs based on Kerr nonlinearity, the present invention is also applicable to three-wave mixing TWPAs. Furthermore, JTWPAs are not the only type of TWPA, and the present disclosure is not limited thereto. In fact, TWPAs in the optical field can be implemented without superconducting components and are within the scope of the present invention as well. For example, the solutions disclosed herein can be used in microwave amplifier systems, or optical amplifier systems.

[0016] In general, TWPAs fall into at least two categories: devices based primarily on three-wavelength mixing (3WM) and devices based primarily on four-wavelength mixing (4WM). These mixing concepts are widely used in the field of nonlinear optics. In 3WM, the pump tone at frequency f_p is twice the frequency f_s and is near the center of the frequency band to be amplified. In 4WM, the pump tone is near the center of the frequency band to be amplified.

[0017] TWPAs amplify signals in the forward direction, but do not amplify or attenuate signals propagating backward from the output to the input, except for dissipative losses in both directions. This allows unwanted noise from components connected to the output of the TWPA to leak into the signal source being measured. Such noise may have sufficient amplitude to significantly disturb sensitive elements of the signal source, such as qubits. Additionally, preventing reverse propagation of signals from the output to the input with an isolated amplifier is generally useful in environments with less than ideal impedance matching, because an amplifier with high forward gain and little reverse isolation may exhibit instabilities and undesirable behavior due to low reflections at the output and input. Reverse power propagation can be controlled by adding non-reciprocal components such as isolators and circulators, but in practice this adds loss, reflections, complexity, size and cost to the system as a whole.

[0018] Furthermore, in a TWPA that generates gain, an idler tone f_i is generated at a frequency of f_p-f_s (3WM) or 2f_p-f_s (4WM). The generation of the idler tones is an inherent feature of the operation of the TWPA. Importantly for the present invention, the idler tones are effectively copies of the amplified signal at a different frequency, and as a product of the mixing process, have a phase shift that depends on the phase of the pump tones. In an ideal parametric amplifier, the amplitude of the idler is √(G-1) times the amplitude of the input signal, where G is the power gain at the signal frequency. In the case of four-wave mixing, the relative idler phase shift is twice the relative pump phase, which is 180 degrees in total for the arrangement illustrated in FIG. 2 with a 90 degree hybrid coupler and where the pump is fed from one of the inputs of the hybrid coupler 102. For three-wave mixing, the only relative idler phase shift is the relative pump phase, which is 90 degrees for the arrangement illustrated in Figure 2 with a 90 degree hybrid coupler, where the pump is fed from one of the inputs of the hybrid coupler 102. The amplified signal or pump tones do not experience a phase shift that depends on the phase of the pump. Idler tones are generated only in the forward direction in an ideal TWPA.

[0019] If an input signal to be amplified is provided at P1 to the first input 102a of the hybrid coupler 102, as was the case in the apparatus of Figure 1, an amplified signal at the input frequency will wind up at P3, a load connected to the waveguide 140 at the second output 104d of the hybrid coupler 104. This is because the TWPAs 210, 220 amplify the signal between the hybrid couplers in the same way on both signal paths, adding the same amount of phase delay on both. As a result, the signal in the waveguide 140 is an amplified version of the input signal. Since the TWPA requires a pump tone, this may be conveniently input to, for example, the second input 102b of the hybrid coupler 102, so that an equal amplitude pump signal propagates through both TWPAs 210, 220, triggering amplification in both. 1, the pump tone injected at the second input 102b of the hybrid coupler 102 winds up at the first output 104c of the hybrid coupler 104 within the waveguide 130.

[0020] Alternatively, the pump tone may be introduced into the first input 102a of the hybrid coupler 102 but not into the second input 102b. In this case, the pump tone winds up with the amplified signal into the second output 104d of the hybrid coupler 104. When the pump is introduced through the first input 102a, it may be coupled to the input signal using a passive component, referred to herein as a pump coupler. Examples of pump couplers include weakly coupled directional couplers that allow for the delivery of the pump tone without significantly attenuating the input signal. For example, directional couplers with weak coupling of 10 dB, 20 dB, or even more may be used, as known in the art. The directional coupler delivers the pump tone to the hybrid coupler 102 but not to the signal source. A similar end result can be achieved with a pump coupler that is comprised of a passive but frequency selective component, such as a diplexer, that couples the narrow frequency band required for the pump to a different physical port than the signal. Pump tones may be fed into both input ports of the hybrid coupler 102, providing independent control of the pump tones fed to the TWPAs 210 and 220, at the expense of additional hardware components. Yet another variation is to provide independently controlled pumps to the TWPAs by adding one pump coupler between the hybrid coupler 102 and the TWPA 210 and a second pump coupler between the hybrid coupler 102 and the TWPA 220. Similarly, one or more additional pump couplers may be placed in the circuit after the TWPAs 210 and 220 for the purposes of pump cancellation. Pump cancellation is a term used in the art to describe an arrangement in which an additional tone at the pump frequency is fed into an additional directional coupler at the output of the TWPA, and the amplitude and relative phase of the additional tone are adjusted so that the additional tone and the pump tone coming from the TWPA output destructively interfere at the output of the additional directional coupler.

[0021] Next, we will evaluate how the idler tones generated by both TWPAs 210, 220 behave in the apparatus shown in Figure 2. Both TWPAs 210, 220 generate an idler tone in the forward direction towards the hybrid coupler 104, with the phase of the idler determined by both the phase of the input signal and the phase of the pump tone. Because the pump tones have different relative phases in the waveguides 110, 210 and thus in the TWPAs 210, 220, the idler tone incident on the first input 104a of the hybrid coupler 104 has a phase difference with respect to the idler tone incident on the second input 104b of the hybrid coupler 104. As mentioned above, in a four-wave mixing TWPA, the relative idler phase shift is twice the relative pump phase, and in a three-wave dropping TWPA, the relative idler phase shift is just the relative pump phase. If the pump is fed from one of the inputs of the hybrid coupler 102, these correspond to 180 degrees and ±90 degrees, respectively, if a 90 degree hybrid is used. The signs are by convention. If any of the other pump coupler schemes mentioned above are used, other phase shifts can be designed in, which is particularly useful for alternative hybrid coupler types, for 3WMTWPAs, or when some of the components are asymmetric or otherwise non-ideal.

[0022] As a result, for an embodiment where a 90 degree hybrid supplies a pump from the input 102a of the hybrid coupler 102, the relative phase of the idler is as follows: the relative phase of the idler generated by the TWPA 220 is 90°+2×90°=270° (4WM) or 90°+1×90°=180° (3WM) since the relative phase of the signal is 90° and the relative phase of the pump is also 90°. The relative phase of the pump and the relative phase of the idler in the TWPA 210 are defined as zero. Thus, at the first output 104c of the hybrid coupler 104, the two equal amplitude idlers have a relative phase equal to zero for the first part originating from the TWPA 210 and 270°+90°=360° (4WM) or 180°+90°=270° (4WM) for the second part originating from the TWPA 220. Thus, in the case of 4WM, the equal amplitude idlers constructively interfere and all of the idler power flows into the first output 104c of the hybrid coupler 104. In the case of 3WM, the equal amplitude idlers are ¼ period out of phase, which means that half of the total idler power flows into the first output 104c of the hybrid coupler 104. Additionally, either none of the total idler power flows into the second output 104d of the hybrid coupler 104 (4WM) or half of the total idler power flows into the second output 104d of the hybrid coupler 104 (3WM).

[0023] The device of FIG. 2, in the example with a 90-degree hybrid and pumping from the input 102a of the hybrid coupler 102, therefore directs all (4WM) or half (3WM) of the idler power to the first output 104c of the hybrid coupler 104. This is due to the pump-dependent phase shift that the idler acquires in the TWPA. Since the idler is a virtual copy of the amplified input signal, except that it differs in amplitude by approximately √(G-1) / √G ​​and at a different frequency, it may be used as the true output of the isolation amplifier device. The waveguide 140 may be connected to a termination resistor or other impedance matching component into which the amplified signal is dissipated. In the case of the 3WM and in the example with the pumping from the input 102a of the hybrid coupler 102 with a 90-degree hybrid, half of the idler power also flows into the end of the waveguide 140, but this only reduces the effective gain of the device by 3 dB. If a pump tone is provided via the input 102a, it is also directed to the waveguide 140. When a pump is applied via input 102b, it winds up with the idler into the waveguide 130. All (4WM) or half (3WM) of the idler is taken from the waveguide 130, which is the output of the isolation amplifier in either case. This arrangement provides the technical advantage that noise and other waveforms propagating to the output 104c are guided out of the isolation amplifier via the input 102b of the hybrid coupler 102. The input 102b may be terminated with a resistor or another impedance matching component. In other words, noise impinging on the output 104 of the device does not wind up into the signal source connected to the input 102a of the hybrid coupler signal 102.

[0024] The only noise propagating back to the input 102a of the hybrid coupler 102, and therefore to the signal source, is from the end of the waveguide 140 in the case of ideal components. If the end is an impedance matching resistor, the noise may consist only of thermal noise and quantum mechanical zero point fluctuations. Manufacturing imperfections in all the components in FIG. 2 may result in imperfect interference effects, reflections, and tiny losses within the components, which may result in less than perfect isolation between the output and input of the isolated amplifier device, generating noise within the device itself. Even with these imperfections, the isolation between the output and input of the device may be significant, e.g., 10 dB, 20 dB, or even more.

[0025] Notably, the disclosed mechanism of isolation is different from methods of isolation that use idler tones but where frequency selective filtering is essential to provide isolation. Frequency selective filtering is not required in the present invention. Rather, isolation in the present invention comes from interference effects and is similar to isolators based on Faraday rotation, but the non-reciprocal phase modulation based on the Faraday effect is replaced by the non-reciprocal generation of idler tones in the TWPA, whose phase is controlled by the phase of the pump. It is also possible to use parametric conversion processes to create isolators using resonant parametric converters, but these are also substantially different from the solutions disclosed herein, since TWPAs are not resonant devices. The term traveling wave is, by definition, essentially the opposite of resonance. Furthermore, a distinguishing feature of the present invention is that the isolation amplifier provides power gain, so long as the combination of the correct hybrid type, pump coupling, and mixing process (4WM or 3WM) is such that a significant portion of the idler power comes out of an output different from the output from which the amplified signal originates. A significant portion here means a portion greater than about 1 / √(G-1) such that the output power is higher than or approximately equal to the original input signal power. The opposite example of a non-isolated amplifier would be the arrangement of FIG. 2, but with 180 degree hybrids 102 and 104, 4WMTWPAs 210 and 220, and pumps fed from either input 102a or 102b. In this case, the relative phase of the pumps in the TWPAs is 180 degrees, and the pump-dependent portion of the idler's relative phase is 2×180°=360°. Thus, the idler does not in effect undergo a relative pump-dependent phase shift, modulo 360°, but simply inherits the phase of the signal, and therefore exits from the same port as the amplified signal. Depending on whether output 104c or 104d is used as the true output of the device, the device will either not amplify or not isolate for this particular combination of components selected.

[0026] In the above example, if the compromise of 3 dB reduction in gain for 3WM is not desired, pump signals may be provided to both inputs of the hybrid coupler 102 so that a relative phase shift close to 180 degrees can be designed for the pump tones in the TWPAs 210 and 220. In particular, providing pump tones of equal frequency and amplitude but opposite sign (180 degree relative phase) to the inputs of the hybrid coupler 102 results in a 180 degree relative phase shift of the pump signals propagating through the TWPAs 210 and 220. When pump signals are provided to both inputs of the hybrid coupler 102, the inputs may be provided from a single pump tone generator combined with passive components to select the relative amplitude and relative phase, or each of the two inputs may be provided from a separate pump tone generator. The pump may also be provided to the TWPAs 210 and 220 using separate pump couplers between the TWPAs and the hybrid coupler 102. The idler from the output 104c may also be used as the output of the isolation amplifier for the 3WM case.

[0027] FIG. 3 illustrates a system in accordance with at least some embodiments of the present invention. The system includes two isolation amplifier devices, as shown in FIG. 2, which are shown in the figure as amplifier 310 and amplifier 320. Thus, each of amplifiers 310 and 320 includes two TWPAs and two hybrid couplers, as shown in FIG. 2. The inputs and outputs are depicted separately for clarity. Signal S is provided to input 102a and a pump is provided to input 102b. The combined pump and idler of amplifier 310 is output via output 104c and directed to a first input 310a of amplifier 320 for a second stage of amplification. A second input 310b of the first hybrid coupler of amplifier 320 may also be provided with a pump tone or may be terminated with a matching resistor. The unused amplified signal is terminated in resistors 301, 311 as shown, and the idler from the overall output 320 is taken from output 310c of amplifier 320, which is the first output of the second hybrid coupler of amplifier 320. Isolation amplifiers 310 and 320 may be identical or may have different parameter values, e.g., gain.

[0028] The amplifier sequence is not limited to two stages as shown in FIG. 3, but rather can have any number of stages depending on the application. Noise propagating backwards to output 310c reaches input 310b. Similarly, noise propagating backwards to output 104c reaches input 102b and does not reach the signal source connected to input 102a. Thus, noise impinging on the output of the last stage is suppressed exponentially as it propagates backwards through the amplifier sequence. Advantageously, for use cases with an even number of stages and the same pump frequency, the output idler frequency is also the same as the original signal input frequency. In other applications, it is advantageous to separate the output frequency and the input frequency. For an odd number of stages and all the same pump frequency, the output frequency is equal to the idler frequency of the first set of TWPAs. If different stages are pumped at different frequencies, by appropriately selecting the pump frequency of each stage, almost any output frequency can be obtained from the last stage.

[0029] The forward gain of the amplifier sequence scales with the number of stages, as would be expected for an independent amplifier: the idler power gains of the different stages, √(G-1), are multiplied to give approximately the total gain. As a concomitant benefit, the systems of Figures 2 and 3 provide a 3 dB increase in dynamic range compared to the power handling of an individual TWPA.

[0030] As a packaging option, the components of the system of Figure 3 may be monolithically implemented as a single integrated circuit, or may be implemented, for example, as discrete chips on a common printed circuit board, as separate chips in a flip-chip bonded module, or as separate connectorized components connected by discrete waveguides.

[0031] FIG. 4 illustrates a system according to at least some embodiments of the present invention. Like numbers indicate like structures in FIG. 3. The amplifier sequence in FIG. 4 differs from that of FIG. 3 in the way the pump tones are provided to the amplifier stages. In particular, the pump tones are introduced to the inputs 102a and 310a via directional couplers 315 and 325, respectively. The directional couplers direct the pump tones in the desired forward direction, rather than towards the signal source S. Providing the pump tones from these inputs has the advantage that the pump does not need to be isolated from the output idler tone, as described herein above, since the pump winds up on resistors 301, 311. Furthermore, the pumps of the different stages 310 and 320 are then independent. As in the single stage case described above, it is also possible to provide the pump to both inputs of each stage, further designing the pump signal in each arm of each stage. It is also possible to use pump couplers other than directional couplers, such as diplexers, as described above. Additionally, as also mentioned above, the pump coupler may be located in other locations as well, such as between the hybrid coupler and the TWPA described above.

[0032] In general, an apparatus is provided that includes a first hybrid coupler and a second hybrid coupler, each hybrid coupler having a first input port, a second input port, a first output port and a second output port, comprising a first traveling wave parametric amplifier (TWPA) having an input connected to the first output port of the first hybrid coupler and an output connected to the first input port of the second hybrid coupler, and a second traveling wave parametric amplifier (TWPA) having an input connected to the second output port of the first hybrid coupler and an output connected to the second input of the second hybrid coupler. The first and second TWPAs may be similar, in particular having the same operating parameters, such as gain.

[0033] The apparatus may be used as an isolation amplifier by connecting the first input of the first hybrid coupler to a signal source, by providing a pump tone to the first input or the second input of the first hybrid coupler, or to both, and by extracting the idler tone as an amplified output from the first output of the second hybrid coupler, and by dissipating the amplified signal from the second output of the second hybrid coupler. It should be understood that the pump tones for the first and second TWPAs may be injected in a number of alternative locations, such as between the first hybrid coupler and the first TWPA, or between the first hybrid coupler and the second TWPA, without significantly changing the principle.

[0034] It is to be understood that the disclosed embodiments of the invention are not limited to the particular structures, process steps, or materials disclosed herein, but extend to equivalents thereof that would be recognized by one of ordinary skill in the relevant arts. It is also to be understood that the terminology used herein is used for the purpose of describing particular embodiments only, and is not intended to be limiting.

[0035] Throughout this specification, a reference to an embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, although the phrases "in one embodiment" or "in one embodiment" appear in various places throughout this specification, they do not necessarily all refer to the same embodiment. For example, when a numerical value is referred to using a term such as about or substantially, the exact numerical value is also disclosed.

[0036] As used herein, a plurality of items, structural elements, components, and / or materials may be presented in a common list for convenience. However, these lists should be construed as if each member of the list were individually identified as a separate and unique member. Thus, the individual members of such lists should not be construed as being de facto equivalents to other members of the same list merely based on their appearance in a common list. Furthermore, various embodiments and examples of the present invention may be referred to herein along with alternatives for various parts thereof. It is understood that such embodiments, examples, and alternatives should not be construed as de facto equivalents of one another, but should be considered as separate and autonomous presentations of the present invention.

[0037] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the foregoing description, numerous specific details, such as numbers, types of components, etc., are provided to provide a thorough understanding of embodiments of the present invention. However, one of ordinary skill in the relevant art will recognize that the present invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations have not been shown or described in detail to avoid obscuring aspects of the present invention.

[0038] While the above-described embodiments illustrate the principles of the present invention in one or more particular applications, it will be apparent to those skilled in the art that numerous changes in form, use, and details of implementation may be made without the exercise of inventive faculty and without departing from the principles and concepts of the present invention. Accordingly, it is not intended that the present invention be limited except as by the scope of the claims that follow.

[0039] In this specification, the verbs "comprise" and "include" are used as open limitations that do not exclude or require the presence of any unrecited features. Features recited in the dependent claims may be freely combined with each other, unless otherwise expressly stated. Furthermore, it is to be understood that throughout this specification the use of "a" or "an", i.e. the singular, does not exclude the plural. [Industrial Applicability]

[0040] At least some embodiments of the present invention find industrial application in low noise signal amplification. List of Abbreviations 3WM 3 wave mixing 4WM four wave mixing JWTPA Josephson TWPA TWPA Traveling Wave Parametric Amplifier Code List 102a, 102b First and second inputs to the 2×2 hybrid coupler 102 102c, 102d First and second outputs to the 2×2 hybrid coupler 102 104a, 104b First and second inputs to the 2×2 hybrid coupler 104 104c, 104d First and second outputs to the 2×2 hybrid coupler 104 102, 104 2x2 Hybrid Coupler 110, 120, 130, 140 Waveguide 210, 220 TWPA 310, 320 Isolation amplifier 301, 311 terminating resistor 315, 325 Directional Coupler References list [1] A. Kamal, "Nonreciprocity in active Josephson circuits," Ph.D. thesis, Yale University (2013) https: / / qulab.eng.yale.edu / documents / theses / ArchanaThesis_Nonreciprocity [2] Macklin et al., "A near-quantum-limited Josephson traveling-wave parametric amplifier," Science 350, 307 (2015). http: / / dx.doi.org / 10.1126 / science.aaa8525 [3] KM Sliwa et al., "Reconfigurable Josephson Circulator / Directional Amplifier," Physical Review (Phys. Rev.) X 5, 041020 (2015) https: / / link.aps.org / doi / 10.1103 / PhysRevX.5.041020 [4] MP Westig and TM Klapwijk, "Josephson Parametric Reflection Amplifier with Integrated Directionality," Phys.Rev. Applied, 9, 064010 (2018), DOI:10.1103 / PhysRevApplied.9.064010

Claims

1. An isolated amplifier device, comprising: a first 2×2 hybrid coupler (102) and a second 2×2 hybrid coupler (104), each 2×2 hybrid coupler having a first input port (102a, 104a), a second input port (102b, 104b), a first output port (102c, 104c), and a second output port (102d, 104d); a first superconducting traveling wave parametric amplifier (TWPA) (210) having an input connected to a first output port (102c) of the first 2×2 hybrid coupler (102) and an output connected to a first input port (104a) of the second 2×2 hybrid coupler (104); a second superconducting traveling wave parametric amplifier (TWPA) (220) having an input connected to a second output port (102d) of the first 2×2 hybrid coupler (102) and an output connected to a second input port (104b) of the second 2×2 hybrid coupler (104); a pump tone is coupled to the first input port (102a) or the second input port (102b) of the first 2×2 hybrid coupler (102), an input signal from a superconducting quantum bit to be amplified is coupled to the first input port (102a) of the first 2×2 hybrid coupler (102), the pump tones have different relative phases in the first TWPA and the second TWPA, and the isolation amplifier is configured to transmit an idler tone to the first output port (104c) of the second 2×2 hybrid coupler (104) as an amplified output signal of the isolation amplifier, and to dissipate the input frequency signal from the second output port (104d) of the second 2×2 hybrid coupler (104) into an impedance matching termination resistor.

2. 2. The isolation amplifier according to claim 1, wherein the first and second TWPAs (210, 220) are four-wave mixing TWPAs.

3. 2. The isolation amplifier according to claim 1, wherein the first and second TWPAs (210, 220) are three-wave mixing TWPAs.

4. 2. The isolation amplifier device of claim 1, wherein the first output port (104c) of the second 2x2 hybrid coupler (104) is configured to provide an output signal of the device, and the second output port (104d) of the second 2x2 hybrid coupler (104) is coupled to a termination resistor.

5. 2. The isolation amplifier apparatus of claim 1, wherein the first input port (102a) of the first 2x2 hybrid coupler (102) is coupled to a signal source to be amplified and the second input port (102b) of the first 2x2 hybrid coupler (102) is coupled to a pump tone source.

6. 2. The isolation amplifier of claim 1, wherein the first input port (102a) of the first 2x2 hybrid coupler (102) is coupled to a signal source to be amplified, and a pump tone source is also coupled to the first input port (102a) of the first 2x2 hybrid coupler (102).

7. 2. The isolation amplifier apparatus of claim 1, wherein a first pump tone source is coupled to the first input port (102a) of the first 2×2 hybrid coupler (102) and a second pump tone source is coupled to the second input port (102b) of the first 2×2 hybrid coupler (102).

8. 2. The isolation amplifier of claim 1, wherein the superconducting TWPA is a microwave TWPA based on Josephson junctions.

9. 2. The isolation amplifier according to claim 1, wherein the isolation amplifier is constructed on a single integrated chip.

10. 10. A system comprising an isolation amplifier device according to any one of claims 1 to 9 and at least one second isolation amplifier device according to any one of claims 1 to 9, characterized in that the isolation amplifier device and the at least one second isolation amplifier device form an amplifier chain, in which the signal output of each isolation amplifier device except the last isolation amplifier device is coupled to the signal input of the immediately succeeding isolation amplifier device, and each isolation amplifier device in the chain is coupled to at most one immediately preceding isolation amplifier device and at most one immediately succeeding isolation amplifier device in the chain.

11. 11. The system of claim 10 depending on any one of claims 5, 6 or 7, further comprising a second pump tone source coupled to a second input port (102b) of the first 2x2 hybrid coupler (102) of each isolation amplifier device.

12. The system of claim 10, wherein the system comprises an even number of isolation amplifiers forming an amplifier chain.

13. 11. The system of claim 10, wherein the first and second isolation amplifiers are constructed on a single integrated chip.

14. The isolation amplifier device of claim 1 or the system of claim 10, wherein each of the 2x2 hybrid couplers (102, 104) is a 90 degree hybrid coupler.

15. 10. A method of using the isolation amplifier apparatus according to claim 1 as an amplifier, comprising the steps of: connecting the first input port (102a) of the first 2x2 hybrid coupler (102) to a signal source; supplying a pump tone to the first input port (102a) or the second input port (102b) of the first 2x2 hybrid coupler (102); extracting an idler tone as an amplified signal from the first output port (104c) of the second 2x2 hybrid coupler (104); and dissipating the amplified signal from the second output port (104d) of the second 2x2 hybrid coupler (104).

16. 16. The method of claim 15, wherein the pump tones are provided to the first and second input ports of the first 2x2 hybrid coupler, and the pump tones provided to the first and second input ports of the first 2x2 hybrid coupler have a phase difference of 180 degrees.

Images