Patent Publication Number: US-8970018-B2

Title: Differential excitation of ports to control chip-mode mediated crosstalk

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 13/827,977 filed Mar. 14, 2013, the disclosure of which is incorporated by reference in its entirety. 
    
    
     FEDERAL RESEARCH STATEMENT 
     This invention was made with Government support under Contract No.: W911NF-10-1-0324 awarded by the U.S. Army. The Government has certain rights in this invention. 
    
    
     BACKGROUND 
     The present invention relates to a quantum computing chip, and more specifically, to a signal port of the quantum computing chip. 
     In quantum computing, a quantum bit (qubit) is a quantum oscillator that eventually experiences unwanted decoherence in the form of dephasing and relaxation (T1 and T2 relaxation). Longer coherence times (larger values for T1 and T2) correspond with a longer time to perform quantum operations before the system decoheres. Several factors may contribute to the perturbations in the oscillation and hasten the T1 and T2 relaxation. A circuit comprising the qubits, resonators, and signal ports is formed as a thin film on a substrate. The substrate, typically formed of an insulating material with a high dielectric constant, may be viewed as a microwave resonator with chip resonant modes (chip modes). Signal ports are points on the circuit through which voltage may be applied to drive the circuit and output signals from the circuit are received. The chip modes may facilitate cross talk between ports. This cross talk may contribute to a noisy environment that leads to faster decoherence of the qubits. Further, in a multi-qubit architecture, the cross talk may lead to unwanted interactions between the qubits. 
     SUMMARY 
     According to one embodiment of the present invention, a method of arranging a differential port in a quantum computing chip includes arranging a first electrode to receive a drive signal; arranging a second electrode to receive a guard signal, the guard signal having a different phase than the drive signal and the first electrode and the second electrode having a gap therebetween; and disposing a signal line from the first electrode to drive a radio frequency (RF) device. 
     According to another embodiment of the invention, a differential port in a quantum computing chip includes a first electrode to receive a drive signal; a second electrode to receive a guard signal, the first electrode and the second electrode being arranged to have a gap therebetween and the drive signal being out of phase with the guard signal; and a signal line from the first electrode to drive a radio frequency (RF) device. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a three-dimensional view of a chip according to an embodiment of the invention; 
         FIG. 2  depicts a differential port according to an embodiment of the invention; 
         FIG. 3  depicts a differential port according to an embodiment of the invention; 
         FIG. 4  depicts a differential port according to an embodiment of the invention; 
         FIG. 5  depicts a differential port according to an embodiment of the invention; 
         FIG. 6  depicts a differential port according to an embodiment of the invention; 
         FIG. 7  depicts a differential port according to an embodiment of the invention; 
         FIG. 8  depicts a differential port according to an embodiment of the invention; 
         FIG. 9  is a block diagram of a signal generator for a differential port according to an embodiment of the invention; 
         FIG. 10  is a block diagram of a signal readout for a differential port according to an embodiment of the invention; and 
         FIG. 11  is a flow diagram of a method of arranging a differential port in a quantum computing chip according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As noted above, cross talk between ports that is mediated by chip modes (substrate resonant frequencies) may contribute to both unwanted single and two qubit interactions and or decoherence perturbations in the oscillation of the qubits. Embodiments of the invention described herein relate to controlling cross talk by driving the ports without injecting energy into the chip modes. 
       FIG. 1  is a three-dimensional view of a chip  100  according to an embodiment of the invention. The substrate  110  may be a silicon or sapphire wafer. The circuit  120  is patterned as thin metal and insulating films on top of the substrate  110 . The circuit  120  includes qubits  130  that are interrogated by microwave pulses through their interaction with microwave resonators  140 . The circuit  120  also includes ports  150  through which drive signals are introduced and output signals of the circuit  120  are received. Coupling of spurious energy into the qubit  130  may be caused by the resonators  140  or another energy reservoir such as chip modes. Crosstalk between the ports  150 , mediated by the chip modes, may also contribute to accelerated decoherence and or unwanted interactions of the qubits  130 . 
       FIG. 2  depicts a differential port  210  according to an embodiment of the invention. The differential port  210  may be regarded as a combination of two ports that are driven by signals that are out of phase with each other. The signal applied to the inner electrode  220  may be 180 degrees out of phase with the signal applied to the outer electrode  230 . In alternate embodiments discussed further below, the signals applied to the inner electrode  220  and the outer electrode  230  may be out of phase by varying degrees. The signal applied to the inner electrode  220  may have the same amplitude as the signal applied to the outer electrode  230 . In alternate embodiments discussed further below, the signals applied to the inner electrode  220  and the outer electrode  230  may have different amplitudes. Exciting the differential port  210  with out of phase signals controls the energy that is driven into the chip mode (of the substrate  240  on which the differential port  210  is placed) by essentially cancelling out the signals. That is, one of the signals is used to drive a resonator ( 140 ,  FIG. 1 ) while the other signal cancels out the drive signal to some extent, if not completely, to prevent driving the chip mode frequencies. By not energizing chip mode frequencies, the differential port  210  controls crosstalk which occurs when a signal propagates from a port  150  ( FIG. 1 ) and is carried by substrate  110  chip modes to another port  150 . If the two ports were driven by signals that are in phase with each other, the differential port  210  would instead be a common-mode port, which would likely exacerbate the crosstalk. As noted above, the application of signals that are out of phase with each other to the inner electrode  220  and outer electrode  230  of the differential port  210  reduces the energy introduced into the chip mode (and thereby reduces crosstalk). However, the inner electrode  220  and outer electrode  230  are of different sizes and shapes and, therefore, have different capacitance to ground. Further, the inner electrode  220  and outer electrode  230  may not be completely symmetrical (about an intersection of two perpendicular lines  250 ,  260 ) as they are in  FIG. 2 . That is, the inner electrode  220  and the outer electrode  230  may not be in complete alignment with each other, and, as a result, signals that are 180 degrees out of phase with each other may not cancel each other out completely and prevent all energy from being injected into the chip mode (i.e., may not completely eliminate crosstalk). In fact, when a signal line is added to one of the electrodes, as discussed below, the symmetry shown in  FIG. 2 , for example, cannot be maintained. These issues may be addressed as detailed below. 
       FIG. 3  depicts a differential port  310  according to an embodiment of the invention. The inner electrode  320  is adjusted in size to more closely approximate the size of the outer electrode  330  as compared to the relative size of the inner electrode  220  and outer electrode  230  of the differential port  210  shown in  FIG. 2 . A comparison of  FIGS. 2 and 3  shows that the inner electrode  320  and the outer electrode  330  of  FIG. 3  are closer in size to each other than the inner electrode  220  and outer electrode  230  of  FIG. 2 . As such, the signals applied to the inner electrode  320  and to the outer electrode  330  may cancel out more completely to address crosstalk. Alternately or additionally, the signal applied to the inner electrode  320  may have a larger amplitude than the signal applied to the outer electrode  330  to compensate for the relatively smaller size of the inner electrode  320  with respect to the outer electrode  330 . Further, alternately or additionally, the signals applied to the inner electrode  320  and outer electrode  330  may be out of phase by a phase angle other than 180 degrees. An angle other than 180 degrees may account for the lack of symmetry noted above between the inner electrode  320  and the outer electrode  330 . Thus, depending on the degree of misalignment between the inner electrode  320  and the outer electrode  330 , signals that are out of phase by some phase angle other than 180 degrees may prevent energy from being injected into the chip mode and thereby prevent crosstalk that contributes to decoherence and unwanted interactions of the qubits. 
       FIGS. 4-6  depict differential ports  410 ,  510 ,  610  according to embodiments of the invention. Each of the embodiments shown in the figures includes a different arrangement for two electrodes comprising the differential port ( 410 ,  510 ,  610 ). In  FIG. 4 , the first electrode  420  and the second electrode  430  of the differential port  410  ( FIG. 4 ) include parallel elements but one of the electrodes is not placed within the other. This is also the case for the first electrode  620  and the second electrode  630  of the differential port  610  ( FIG. 6 ). For differential port  510  ( FIG. 5 ), the first electrode  520  and the second electrode  530  are formed as parallel bars. The arrangement of the first electrodes ( 420 ,  520 ,  620 ) and second electrodes ( 430 ,  530 ,  630 ) in  FIGS. 4-6  are less symmetric tan the arrangement of the inner electrodes ( 220 ,  320 ) and outer electrodes ( 230 , 330 ) in  FIGS. 2 and 3 . The arrangements shown in  FIGS. 4-6  may pertain to cases in which crosstalk is frequency dependent. That is, only chip modes of the same symmetry as either the first electrodes ( 420 ,  520 ,  620 ) or the second electrodes ( 430 ,  530 , 630 ), whichever are used to drive the resonator ( 140 ,  FIG. 1 ), are excited. Thus, if the signal applied to the other electrode (the one not driving the resonator) sufficiently cancels out the signal driving the resonator, the chip mode excitation may be avoided or reduced, and, consequently, crosstalk may be avoided or reduced. Any of the embodiments of differential ports  210 ,  310 ,  410 ,  510 ,  610  shown in  FIGS. 2-6  may be used on a chip ( 100 ,  FIG. 1 ). The signal applied to one of the electrodes may be considered a drive signal that drives the resonator ( 140 ,  FIG. 1 ), for example. The other signal applied to the other electrode may be considered a guard signal that cancels out the component of the drive signal that would drive chip modes. 
       FIG. 7  depicts a differential port  710  according to an embodiment of the invention. The differential port  710  includes a transition (signal line)  740  configured to drive a coplanar waveguide (CPW). In the embodiment of  FIG. 7 , the outer electrode  730  may be regarded as having the drive signal applied because it connects to the transition  740 . The inner electrode  720  may be regarded as having the guard signal applied thereto. 
       FIG. 8  depicts a differential port  810  according to an embodiment of the invention. In this embodiment, the inner electrode  820  may be regarded as having the drive signal applied because it connects to the transition  840 . The outer electrode  830  may be regarded as having the guard signal applied. The electrodes may have an oval shape, as shown for inner and outer electrodes  720 ,  730  in  FIG. 7 , for example, or a circular shape, as shown for inner and outer electrodes  820 ,  830  in  FIG. 8 , for example. Based on the relative amplitudes and phases of the signals applied and any particular chip mode frequency being targeted, one of the shapes may provide more complete cancellation. The transition  740 ,  840  or signal line may affect the symmetry between the inner electrode  720 ,  820  and the outer electrode  730 ,  830  and, therefore, may also affect the extent to which the drive signal excitation of the chip modes is canceled by the guard signal to some degree. As noted above, any of the previously discussed embodiments of differential ports  210 ,  310 ,  410 ,  510 ,  610  may have a transition added to use them on a chip. 
       FIG. 9  is a block diagram of a signal generator  900  for a differential port ( 210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ) according to an embodiment of the invention. The signal generator  900  includes a radio frequency (RF) signal source  910  that generates a signal at a desired RF frequency. This RF signal is output to a type of signal splitter  920  that outputs a 180 degree out of phase version of the RF signal as the guard line  930  signal and also outputs the RF signal without a phase change to an amplifier  940  to output an amplitude adjusted drive line  950  signal. The amplitude of the drive line  950  signal may be higher (or lower) than the amplitude of the guard line  930  signal to compensate for differences in the size and shape of the electrodes of the differential port ( 210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ), as discussed above. In alternate embodiments, the signal splitter  920  may have an adjustable phase such that the guard line  930  signal is not exactly 180 degrees out of phase with the drive line  950  signal but is, instead, out of phase by a different phase angle. The adjustment in phase difference between the drive line  950  signal and the guard line  930  signal may compensate for misalignment (asymmetry) between the electrodes of the differential port ( 210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ). Embodiments discussed above relate to differentially driving a port on the chip ( 100 ,  FIG. 1 ), but readout of a signal from the port may also be done differentially, as discussed below. 
       FIG. 10  is a block diagram of a signal readout  1000  for a differential port ( 210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ) according to an embodiment of the invention. The signal readout  1000  includes an input for the signal on the drive line  1010 , an input for the signal (noise) on the guard line  1020 , and an output  1030  that results from subtracting the guard line  1020  signal from the drive line  1010  signal. The amplifier  1040  may be included in the guard line  1020 , as shown. In alternate embodiments the amplifier  1040  may be alternately or additionally included in the drive line  1010 . 
       FIG. 11  is a flow diagram of a method of arranging a differential port in a quantum computing chip according to embodiments of the invention. Arranging the first and second electrodes at block  1110  includes determining the relative size and positioning of the two electrodes as discussed above and shown in the several exemplary embodiments discussed above with reference to  FIGS. 2-8 . The two electrodes may not contact each other, but their arrangement is not otherwise limited by the exemplary embodiments shown herein. Including a transition (e.g.,  740 ,  840  shown in  FIGS. 7 and 8 , respectively) from the electrode ( 730 ,  820  shown in  FIGS. 7 and 8 , respectively) to which the drive signal is applied to a coplanar waveguide connects the differential port to the circuit on the chip. That is, a signal line is disposed from one of the electrodes (the one to which the drive signal is applied) so that the differential port may drive an RF component (e.g., resonator  140 ,  FIG. 1 ) at block  1120 . As discussed with reference to several of the exemplary embodiments, in addition to or alternate with adjusting the size and shape of the electrodes, adjusting the amplitude and/or phase of the drive signal and the guard signal at block  1130  may result in more complete cancellation of the differentially applied signals and thereby further reduce crosstalk. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated 
     The flow diagram depicted herein is just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.