T 2651/22 (Impedance matching quantum circuits/RIGETTI) 19-03-2025
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IMPEDANCE-MATCHED MICROWAVE QUANTUM CIRCUIT SYSTEMS
I. The appeal lies from the decision of the Opposition Division to maintain the opposed patent EP 3 217 336 in amended form on the basis of the then second auxiliary request.
II. The Appellant (Opponent) requested that the decision of the Opposition Division be set aside and that the patent be revoked. The Appellant argued that the independent claims of the patent as maintained lacked inventive step over document
D8: T.C. White et al., "Traveling wave parametric amplifier with Josephson junctions using minimal resonator phase matching", arXiv:1503.04364v1 [cond-mat.supr-con] 15 March 2015,
in combination with common general knowledge as evidenced by
D11: David M. Pozar, "Microwave Engineering", 4th ed., 2012,
or in combination with
D2: US 2015/0028970 Al.
III. The Respondent (Proprietor) requested, as a main request, that the appeal be dismissed and that the patent be maintained in the form found allowable by the Opposition Division. The Respondent also filed six auxiliary requests.
IV. Claim 1 of the patent as maintained defines (numbering according to the decision):
l.1 A microwave quantum circuit system comprising:
l.2 a dielectric substrate;
l.3 a quantum circuit device on the dielectric substrate, the quantum circuit device comprising a Josephson junction; and
l.4 an impedance matching circuit device on the dielectric substrate, and coupled to the quantum circuit device,
l.4.1 the impedance matching circuit device comprising a ladder network of inductors and shunt capacitors;
l.8.1 a ground conductor defining an interior clearance area, wherein the impedance matching circuit device comprises inductors and capacitors,
l.8.2 the inductors comprising meandered conductive traces within the interior clearance area, and Fl.8.3 the capacitors comprising conductive traces forming interdigitated gaps between meandered conductive traces;
l.5 wherein the quantum circuit device comprises a parametric amplifier configured to:
1.6 provide a gain of at least four dB to microwave signals over a bandwidth greater than five hundred MHz, and
1.7 provide gain to microwave signals having a nominal frequency between four and ten Ghz.
V. The claims of the other requests are irrelevant for the present decisions.
The opposed patent
1. The patent relates to impedance matched microwave quantum circuit systems, in particular to a system comprising a parametric amplifier (e.g. a Josephson Parametric Amplifier (JPA)) for amplifying a qubit readout (paragraphs 22 to 24). A JPA includes a SQUID loop (two Josephson junctions in a loop) and a capacitor in parallel, i.e. a shunting capacitor (paragraph 37). The amplifying signal is provided by a pump tone source (paragraph 24).
1.1 The patent proposes the addition of an impedance matching circuit in order to improve performance, more specifically to improve the power transfer between the amplifier circuit and the qubit and to improve the bandwidth (paragraph 5).
1.2 This circuit is implemented as a ladder network of inductors and shunt capacitors (paragraph 35), and is connected between the JPA and the input signal (qubit readout signal). It may be implemented in a coplanar waveguide topology, with inductors implemented as meandered conductive traces, and shunt capacitors implemented as interdigitated gaps between the meandered conductive traces (see paragraphs 43-44, figure 7).
1.3 According to the patent, this design allows for an amplification of four dB to microwave signals over a bandwidth greater than five hundred Mhz (see e.g. paragraph 54).
Document D8
2. D8 proposes a travelling wave parametric amplifier (TWPA) comprising (see figure 2, and the left column on the same page)
- amplifier cells, each comprising three Josephson junctions and a shunt capacitor, the cells being grouped in 26 amplifying non-linear sections of 17 cells each, and
- a linear section comprising a resonator placed after each non-linear section, the resonators acting as phase shifters to match the pump tone phase to the signal.
2.1 In each cell, the three Josephson junctions also act as inductors. The capacitors are formed as parallel plate capacitors (ibid.).
2.2 The various design values are chosen so as to provide impedance matching between the sections (ibid.) and to external devices (impedance approaching 50 Ohms, see last paragraph on the first page, and supplementary information page 5).
2.3 According to D8 one such TWPA provides a gain of 6-8 dB (page 3, right column, paragraph 2). A chain of two provides 12 to 14 dB over the full bandwidth of 4-8 GHz (loc. cit., last paragraph).
Differences to D8
3. The Opposition Division found that all the features of claim 1 of the present main request (then the second auxiliary request; see the decision, points II.A.1 and II.C.2) were disclosed in D8, except the implementation of the capacitor as an interdigitated capacitor instead of parallel plate (feature 1.8.3). It maintained the patent on the basis that this new feature was not obvious in view of D8.
4. In its analysis the Opposition Division considered in particular that the amplifying sections of D8 have a double functionality, amplifying and impedance matching. Hence one amplifying section - e.g. left side in figure 2(b) - anticipated the impedance matching circuit as claimed, and another - e.g. right side in figure 2(b) - the amplifying circuit as claimed (see decision, top of page 6).
4.1 It based this conclusion on the fact that the first and last shunt capacitors on each section had half the capacitance of the others in order to improve impedance matching between sections (bottom of page 5, top of page 6). It argued that the claim did not specify which impedances where matched, so that this type of impedance matching was covered by the claim (II.A.1.3).
4.2 It also equated the series of Josephson junctions with inductors comprising meandered conductive traces (II.C.2.1 to II.C.2.3).
5. The Respondent argued inter alia (reply to the appeal, 4.4.1) that D8 did not disclose an impedance matching circuit as claimed.
5.1 For the person skilled in the art this term had a specific meaning, i.e. a circuit with the sole purpose of providing an impedance matching functionality to another device outside the claimed system. The person skilled in the art would not consider a sub-part of the amplifier circuitry in D8 to be an impedance matching circuit, but would see the circuit of D1 as one single amplifier circuit.
5.2 The halved capacitance values, which the Opposition Division found to provide impedance matching, only provided matching between the section, not to an external device.
5.3 Further, one section of the amplifier, identified by the Opposition Division as being the amplifying circuit, did not provide the claimed gain.
6. In its preliminary opinion, the Board remarked that D8 provided for two different types of impedance matching, one between the sections, and one to outside circuitry. The capacitance values in particular were chosen so that overall the impedance seen from the outside approached the standard 50 Ohms.
6.1 So, even if the person skilled in the art would understand impedance matching as claimed to refer to impedance matching to an external device, the first amplifying section of the TWPA of D8, connecting to the input, was functionally also an impedance matching circuit in that sense.
6.2 The remaining part of the circuit according to D8 (the other 25 sections; see section 2 above) was an amplifying circuit which satisfied the claimed gain conditions.
7. During the oral proceedings before the Board the Respondent argued again that the claim could not be interpreted as covering a single integrated circuit.
7.1 Feature 1.3 defined a quantum circuit device. Feature 1.4 defined an impedance matching circuit coupled to the quantum device. For the person skilled in the art it was clear that these were two different circuits.
7.2 The scope of protection did not extend to one integrated circuit with double functionality as that of D8. Had the patentee desired to cover such a circuit it would have worded the claim differently, for instance to define an impedance matched circuit, not an impedance matching circuit.
7.3 The specification was also consistent in describing two different circuits (figures 5, 6, 8A and 8B), one of which being placed in front of the amplifier and providing only impedance matching functionality and no amplification.
7.4 This solution was different to that of D8, and also simpler.
8. The Appellant argued that the Patentee's intention when drafting the claim was irrelevant. If the claimed technical features could be mapped to the prior art then that was sufficient for the features to be disclosed.
8.1 The function of impedance matching was disclosed in D8. Therefore, an impedance matching circuit was disclosed, even if this circuit was a sub-circuit of one larger integrated circuit, and even if it also fulfilled another function.
8.2 In order to establish a difference vis-à-vis D8 the claim should have been limited by inserting features from the specification into the claim, instead of relying on the specification for interpretation.
9. The Board finds the position of both parties to be reasoned and reasonable. The problem lies in the inherent difficulty of defining whether and when circuitry which is part of a larger circuitry can be identified as one circuit separate from the rest, or as fulfilling only one function. In the present case for instance, the presence of an impedance matching circuit as described in the specification also influences the amplifying function; it improves it by reducing losses (see point 1.1 above).
9.1 Considering this context, the Board is of the opinion that the claim must be interpreted in the light of the description. On that basis the Board arrives at the conclusion that the claimed impedance matching circuit is to be interpreted as a circuit designed specifically for the (sole) purpose of impedance matching of a pre-existing quantum device circuit to external circuitry (see points 1.1 and 1.2 above).
10. It is undisputed that D8 does not disclose such a circuit. Further, the Appellant did not argue that under this interpretation the claimed matter was obvious. Accordingly, the Board comes to the conclusion that claim 1 of the patent as maintained is inventive over D8.
For these reasons it is decided that:
The appeal is dismissed.