CN117891108A - On-chip superconducting microwave frequency comb and preparation method thereof - Google Patents
On-chip superconducting microwave frequency comb and preparation method thereof Download PDFInfo
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Abstract
The invention discloses an on-chip superconducting microwave frequency comb and a preparation method thereof, comprising a Josephson junction and a superconducting microwave resonator, wherein the Josephson junction and the superconducting microwave resonator are coupled to form a nonlinear system, and when direct-current bias voltages are applied to two ends of the Josephson junction, the generated microwave photons can generate up-down frequency conversion in the nonlinear system, so that each resonant frequency corresponding to the superconducting microwave resonator has microwave photon output. And an injection locking technique of the frequency comb is proposed, which can be used to detect unknown signals. The devices of the invention are distributed at equal intervals on the frequency domain, are signals of periodic stable pulse on the time domain, accord with the characteristics of frequency comb signals, have the advantages of low loss, simple manufacturing process, only need of direct current bias and the like, and are very favorable for on-chip high integration.
Description
Technical Field
The invention belongs to the technical field of superconducting devices, and particularly relates to an on-chip superconducting microwave frequency comb and a preparation method thereof.
Background
Frequency comb refers to a microwave or optical signal consisting of a series of frequency components that are equally spaced in frequency and coherent in phase, which are periodically stationary pulses in the time domain. The frequency comb can be used as a high-precision ruler for measuring frequency and time, and has wide and important application in modern technology, such as optical clocks, precision spectroscopy, arbitrary waveform generators and the like. In the past two decades, highly integrated frequency comb devices have been widely studied, and various miniaturized on-chip photonic systems have emerged. However, many microwave frequency combs require relatively complex fabrication processes, and also require relatively expensive optical or microwave sources, such as Kerr frequency combs based on semiconductor mode-locked lasers and microresonators. And the energy loss of the frequency comb based semiconductor device is relatively high, which can lead to a device with a high threshold value of pumping power and a low energy conversion rate, which can greatly limit the high integration of the device.
Disclosure of Invention
The invention solves the technical problems that: the microwave frequency comb signal is formed by superconducting materials, the energy loss is extremely low, the process is simple, and meanwhile, the microwave frequency comb signal can be generated only by direct current bias, so that the high integration is facilitated.
The technical scheme is as follows: in order to solve the technical problems, the invention adopts the following technical scheme:
An on-chip superconductive microwave frequency comb comprises a Josephson junction and a superconductive microwave resonator, wherein the Josephson junction and the superconductive microwave resonator are coupled to form a nonlinear system, and when direct-current bias voltages are applied to two ends of the Josephson junction, generated microwave photons can generate up-down frequency conversion in the nonlinear system, so that each resonant frequency corresponding to the superconductive microwave resonator has microwave photon output.
Further, the Josephson junction is positioned at one end of the superconducting microwave resonator, and two end electrodes of the Josephson junction are respectively lapped on a central conductor and a grounding surface of the superconducting microwave resonator to realize the coupling of the central conductor and the grounding surface.
Further, the superconducting microwave resonator comprises a central conductor and a gap capacitor, and two ends of the superconducting microwave resonator are coupled by the two gap capacitors.
Further, the superconducting microwave resonator is a half-wavelength coplanar waveguide resonator, the length of a central conductor of the superconducting microwave resonator is 10mm, the superconducting microwave resonator has multi-order resonant modes, and the corresponding resonant frequency is n multiplied by 750 MHz, wherein n is a positive integer.
Further, when the direct current bias voltage is applied to the Josephson junction, the frequency of the emitted photons is as follows:
fJ = 2eVdc/h
Wherein 2e is the charge amount of the Cooper pair, V dc is the bias voltage at two ends of the Josephson junction, and h is the Planck constant.
Further, a dc bias voltage is applied across the josephson junction, and the obtained I D is the magnitude of the current passing through the josephson junction, and the magnitude of this current value is related to the number of superconducting cooper pairs tunneled through the barrier layer, and the more the number of cooper pairs tunneled, the larger the current value I D, the more photons are radiated.
Further, the microwave signals radiated by the on-chip superconducting microwave frequency comb are equally spaced in the frequency domain, and the frequencies of the signals of each order are fitted by the following formula:
fm=mfr
Wherein, m is the signal order (m=1, 2,3 …), f r is the frequency interval, f m is the frequency of the m-th order signal, and the frequency signal and the formula fit well, which indicates that the frequency signal is equally spaced, and meets the requirement of the frequency comb.
The preparation method of the on-chip superconducting microwave frequency comb comprises the following steps:
S1, sputtering a layer of niobium film on the surface of a substrate by utilizing a magnetron sputtering technology;
s2, obtaining a superconducting microwave resonator by adopting photoetching and reactive ion etching processes;
s3, obtaining a suspension bridge structure through photoetching;
S4, obtaining the Josephson junction with the Al/AlO x/Al structure through a double-angle evaporation process.
Further, in step S1, a sapphire substrate is used, and the thickness of the niobium film is 120 nm.
The beneficial effects are that: compared with the prior art, the invention has the following advantages:
The signals which are generated by the device and are distributed at equal intervals on the frequency domain and are periodic stable pulses on the time domain are frequency comb signals, and the device has the advantages of low loss, simple manufacturing process, only needing direct current offset and the like, and is very beneficial to high integration on a chip.
The signals which are generated by the device and are distributed at equal intervals in the frequency domain and are periodic stable pulses in the time domain are frequency comb signals, the device is formed by coupling a superconducting microwave resonator and a Josephson junction, and superconducting materials are adopted, so that the device has the advantage of low loss. The superconductive microwave resonator and the Josephson junction are both composed of superconductive films, and the film structure can conveniently process the superconductive microwave resonator and the Josephson junction by adopting a micro-nano processing technology, and the manufacturing technology is simple. The device can generate a frequency comb signal only by driving with a direct current signal, which is very beneficial to on-chip integration.
Drawings
Fig. 1 is a schematic diagram of an on-chip superconducting microwave frequency comb sample.
Fig. 2 is a schematic diagram of a josephson junction.
Fig. 3 is an optical view of an on-chip superconducting microwave frequency comb sample.
Fig. 4 is an optical diagram of a josephson junction.
Fig. 5 is a partial enlarged view of fig. 4.
Fig. 6 is an optical diagram of gap capacitance.
Fig. 7 is a graph of the current-voltage characteristics of the on-chip superconducting microwave frequency comb at different biases for the josephson junction.
A in fig. 8 is a frequency domain plot of the device at V dc = 43 μv; b is the frequency value of the signal of each order (red open dot) and its fitting line (blue broken line).
A in fig. 9 is an IQ mixing schematic; b is a frequency domain diagram of the device when V dc = 244.94 μv, and IQ mixing is performed on the 6 th, 9 th and 12 th order signals (i.e., m = 6, 9 and 12) in the frequency comb signals at the time; c-e are the mixing results for m=6, 9 and 12, respectively.
A in fig. 10 is a frequency domain plot of the device at V dc = 83 μv; b-d are time-domain plots measured at this time; b is a partial enlargement in graph c, and d is the overlapping contrast of the two pulse waveforms in b.
A in fig. 11 is a spectrum diagram of the device at V dc = 237.6 μv; b-d is the frequency variation of the spectrum of each order signal with the frequency of the injected signal (the dotted line is a frequency fitting line), the power of the injected signal is fixed, and P inj = -82dBm; e-g is a comparison graph of each order signal before and after injection locking, black lines are spectrums before injection locking, red lines are spectrums after injection locking, and each graph is a linewidth graph of each order signal after injection locking; h-j is the variation of the frequency spectrum of each order signal with the power of the injected signal (the dotted line is a frequency fitting line), the frequency of the injected signal is fixed and 8MHz lower than the center frequency of the seventh order signal; k-m is the correspondence of 、/>、/> and/> , respectively, to the signal order m sen.
A in fig. 12 is a spectrum diagram of the device at V dc = 36.39 μv, where the frequency of the radiation signal of the josephson junction is 17.6GHz; b-d is the injection locking observed at different orders of the frequency comb, where the injection signal orders are m inj =1, 6 and 54, respectively, and the observed order is m inj =7.
Detailed Description
The invention will be further illustrated with reference to specific examples, which are carried out on the basis of the technical solutions of the invention, it being understood that these examples are only intended to illustrate the invention and are not intended to limit the scope thereof.
The on-chip superconducting microwave frequency comb mainly comprises two parts: josephson junction 1 and superconducting microwave resonator 2. Fig. 1 is a schematic diagram of a sample of the present invention, and fig. 3 is an optical diagram of a sample of an on-chip superconducting microwave frequency comb sample.
The structure of the superconducting microwave resonator 2 adopted by the invention is a half-wavelength coplanar waveguide resonator, the resonator has multi-order resonant frequencies, and the resonant frequencies are equally spaced.
The superconductive microwave resonator 2 is composed of a central conductor 3 and a ground plane 4, two ends of the superconductive microwave resonator are respectively provided with a gap capacitor 5 (the coupling distance is 4 mu m), the gap capacitors 5 are used for coupling with the outside, the lasing signal can be detected and modulated, and an annular inductor 6 is added in the middle of the central conductor 3, so that the superconductive microwave resonator has a low-pass filtering function and prevents microwave energy in the resonator from flowing out along a direct-current bias line. The width of the central conductor 3 of the superconducting microwave resonator 2 is 10-30 mu m, and 16 mu m is adopted in the embodiment; the length of the central conductor 3 is about 10mm, and the distance from the central conductor 3 to the ground plane 4 is 5-15 mu m, in this embodiment 9 mu m is used.
As shown in fig. 1 and 3, a josephson junction 1 is located at one end of a superconducting microwave resonator 2. The size of the josephson junction 1 is about 4 μm 2. The Josephson junction 1 uses an aluminum junction which is of an Al/AlO x/Al three-layer structure, has simple manufacturing process, is one of the most commonly used Josephson junctions at present, and is commonly used in the field of superconducting quantum computing. The two superconducting electrodes of the josephson junction 1 are respectively lapped on the central conductor 3 and the ground plane 4 of the superconducting microwave resonator 2.
Josephson junction 1 is an excellent voltage-to-frequency converter, and when dc bias voltage is applied to josephson junction 1, the frequency of the emitted photons is:
fJ = 2eVdc/h
where 2e is the charge of the cooper pair, V dc is the bias across the josephson junction 1, and h is the planck constant.
Coupling the Josephson junction 1 and the superconductive microwave resonator 2 (electrodes at two ends of the Josephson junction 1 are respectively lapped on a central conductor 3 and a grounding surface 4 of the superconductive microwave resonator 2, as shown in figures 1 and 3), the whole of the Josephson junction and the superconductive microwave resonator forms a nonlinear extremely strong system, and when DC bias voltages are applied to two ends of the Josephson junction, microwave photons generated by the Josephson junction generate up-down frequency conversion in the nonlinear system, so that each resonant frequency corresponding to the superconductive microwave resonator 2 has microwave photon output.
The preparation method of the on-chip superconducting microwave frequency comb comprises the following steps:
s1, selecting sapphire (C direction, thickness is 650 microns) as a substrate, and sputtering a layer of niobium film (Nb) with thickness of 120 nanometers on the surface of the substrate by utilizing magnetron sputtering.
S2, obtaining the superconducting microwave resonator 2 through standard photoetching and reactive ion etching processes, wherein the reactive gas used by the reactive ion etching is CF4. The superconducting microwave resonator 2 is a half-wavelength coplanar waveguide resonator, the length of a central conductor 3 of the superconducting microwave resonator is about 10 mm, the superconducting microwave resonator has multi-order resonant modes, and the corresponding resonant frequency is nx 750MHz (wherein n is a positive integer, and 750MHz is the fundamental frequency of the superconducting microwave resonator).
S3, obtaining a suspension bridge structure through photoetching, wherein the manufacturing process of the suspension bridge comprises the following steps:
Two kinds of photoresist are required to be sequentially spin-coated, the first layer of LOR10B photoresist is spin-coated, the photoresist is extremely sensitive to light and is extremely soluble in developing solution, the photoresist is used for manufacturing a supporting layer when a suspension bridge is manufactured, a baking table is placed after spin coating, and the photoresist is baked at 150 ℃ for 5 minutes, so that redundant solvent in the photoresist is removed. And (3) after baking, standing to room temperature, spin-coating a second layer of AZ5214 photoresist, placing, and then baking at 95 ℃ for 2 minutes at a baking table. And then, carrying out pattern exposure on the sample by using laser direct writing, after the exposure is finished, placing the sample in a developing solution for developing, and forming a suspension bridge because the LOR10B photoresist at the bottom layer is very soluble in the developing solution and a hollowed-out structure can appear at the bridge area part.
S4, obtaining the Josephson junction 1 with the Al/AlO x/Al structure through a double-angle evaporation process (the thickness of aluminum is 40 nanometers/80 nanometers respectively).
In order to ensure good electrical contact between the electrodes at both ends of the josephson junction 1 and the superconducting microwave resonator 2, the sample is surface cleaned by ion milling to remove the oxide layer before double-angle evaporation.
The test process and the test result of the invention:
The on-chip superconducting microwave frequency comb device is arranged in a dilution refrigerator, and the working temperature of the on-chip superconducting microwave frequency comb device is 20mK. The DC offset line passes through a low-pass filter and can add low-noise voltage offset on the sample; the microwave signal radiated by the on-chip superconducting microwave frequency comb device is amplified by a low-temperature microwave amplifier (+42 dB) and two room-temperature microwave amplifiers (both are +32 dB), and then is connected into a frequency spectrograph, so that a frequency domain diagram of the microwave signal can be obtained. When the dc bias voltage V dc across the device is changed, it can be seen that the bias voltage V dc is approximately in the range of 20-300 μv, and a microwave signal frequency is radiated near each order of resonant modes of the resonator (as shown in fig. 7), where I D is the magnitude of the current through the josephson junction, and the magnitude of this current value is related to the number of superconducting cooper pairs tunneling through the barrier layer, and the greater the number of cooper pairs tunneling, the greater the current value I D, and the greater the number of photons radiated.
Fig. 8 is a spectrum diagram of the device offset at V dc = 43 μv, and it can be seen that the radiated microwave signals are equally spaced in the frequency domain. The frequencies of the various order signals are fitted using the following formula:
fm=mfr
Wherein, m is the signal order (m=1, 2,3 …), f r is the frequency interval, f m is the frequency of the m-th order signal, and the frequency signal and the formula are well fitted, which indicates that the frequency signal is really equally spaced, and meets the definition of the frequency comb.
Subsequently, the coherence of the frequency comb order signals was verified by IQ mixing. The test procedure and results are shown in fig. 9. In fig. 9, a shows an IQ mixing schematic diagram, in which each order signal (V dc = 244.94 μv) of the device and a local oscillator signal are respectively subjected to IQ mixing, the frequency of the local oscillator signal and the frequency of the signal to be measured are different by 2mhz, and two signals with pi/2 phase difference (i.e. mutually orthogonal) are obtained after IQ mixing and low-pass filtering. The two signals are recorded by a digital oscilloscope and then are drawn in a coordinate system, so that a circular graph can be obtained, which shows that the phases of the signals of all steps are stable and have time coherence, the radius of the circular graph is related to the intensity of the signals, and the width of the circular graph is related to the line width of the signals. As shown in c, d, e in fig. 9, the mixing results are m=6, 9, and 12, respectively.
Subsequently, in order to demonstrate a stable phase relationship between the signals of the respective orders, the time domain signals of the frequency comb were again acquired. The signal generated by the device is connected to a high-frequency oscilloscope (Keysight MSOV A) to obtain a time domain diagram of the signal. As shown in fig. 10, it can be seen that the signal is a pulse signal having the same periodicity in the time domain, and its waveform is extremely stable in a plurality of periods; this illustrates that the phase between the various orders of signals radiated by the device are coherent.
Subsequently, the present invention again demonstrates the coherent injection locking effect of the frequency comb. As shown by a in fig. 11, which is a frequency comb signal in a normal operation state, the frequency of the injection signal is changed around m inj =7 (the power P inj = -82dBm of the fixed injection signal), then the frequency spectrums of the other order signals are detected, and the detected signal pattern number is denoted as m sen, where m sen =6-10. The frequency of the injection signal f inj is changed as shown in b-d of fig. 11, while the frequency spectra of the 6 th, 7 th and 10 th order signals are tested, and signals of different orders are found to have the same injection locking range as shown in k of fig. 11. But different orders m sen have different frequency offsets/> ,/> proportional to the m sen relationship, as shown by l in fig. 11. When the lasing signal is in the frequency locking range/> , the phase of the signal is completely locked, the line width of the signal is greatly improved, the line width of the signal is narrowed to 1Hz, and 1Hz is the resolution limit of the spectrometer, so that the line width of the signal is possibly less than 1 Hz; because the line width of the signal is narrowed, the energy of the signal originally scattered at other frequency points is also distributed at one place, and the energy of the lasing signal is greatly improved at the moment, as shown by e-g in fig. 11. Then fixing the frequency of the injection signal, changing the power of the injection signal, and performing spectrum test on each order of the lasing signal. Setting the frequency of the injection signal near the center frequency of the 7 th order signal, the frequency of the injection signal is about 8MHz lower than the frequency of the 7 th order signal, changing the injection power P inj (-95 to-70 dBm), finding that as the power of the injection signal increases continuously, a plurality of sideband signals begin to appear near each order lasing signal, and finally at a certain critical power, the lasing signal and the sideband signals near the lasing signal are all in one place, and the signals are completely locked at this time, as shown by h-j in fig. 11. It is noted here that near each order signal it appears that an injection signal f id is induced, the frequency of which is almost the same as the frequency spacing/> of its corresponding lasing signal, as shown by k in fig. 11. Near the critical power, f id is locked together with other signals, the frequency difference/> between the locked and pre-locked lasing signals is also different, and statistics of/> of the respective order signals find that the frequency difference is also proportional to the signal order m sen, as shown by m in fig. 11. In a word, the effect of narrowing the line width of each order signal of the frequency comb is realized by means of injection locking, and simultaneously, each order signal of the frequency comb can be simultaneously locked, which also shows that the phases of each order signal of the frequency comb are coherent; when the lock is injected, the corresponding relation that the frequency offset of each order signal is proportional to the signal order has very important application prospect, for example, we can determine which order of the frequency comb the unknown signal is located near by the phenomenon, thereby detecting the frequency of the unknown signal.
Finally, the present invention verifies the up-down conversion effect of the exciter. The josephson junction radiates a microwave signal when biased at V dc = 36.387V across it, at a frequency of about 17.6GHz, as indicated by a in figure 12. Since the lasing signal at the observation order m sen also responds when signals are injected at other orders m inj in the frequency comb, the present invention uses this phenomenon to prove that signals exist outside the observation frequency (3-10 GHz). Injecting signals near the 1 st and 6 th order signals (corresponding frequencies of 0.727GHz and 4.365GHz, respectively), the m sen =7 th order signals were observed to have responses, as shown by b and c in fig. 12, indicating that there is a signal present at an order 17.6GHz below the radiation frequency of the josephson junction; again, the signal was injected near the 54 th order signal (corresponding to a frequency of about 39.28 GHz) and it was observed that the m sen =7 th order signal also had a response, as indicated by d in fig. 12.
In a word, the signals which are generated by the invention and are distributed at equal intervals on the frequency domain and are periodic stable pulses on the time domain are frequency comb signals, and the device has the advantages of low loss, simple manufacturing process, only needing direct current bias and the like, and is very beneficial to high integration on a chip.
The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention, which are intended to be comprehended within the scope of the present invention.
Claims (9)
1. An on-chip superconducting microwave frequency comb, characterized in that: the microwave photon generation device comprises a Josephson junction (1) and a superconductive microwave resonator (2), wherein the Josephson junction (1) and the superconductive microwave resonator (2) are coupled to form a nonlinear system, and when direct-current bias voltages are applied to two ends of the Josephson junction (1), the generated microwave photon can generate up-down frequency conversion in the nonlinear system, so that each resonant frequency corresponding to the superconductive microwave resonator (2) has microwave photon output.
2. The on-chip superconducting microwave frequency comb of claim 1, wherein: the superconducting microwave resonator (2) comprises a central conductor (3) and a grounding surface (4), and two ends of the superconducting microwave resonator (2) are coupled with the outside through two gap capacitors (5).
3. The on-chip superconducting microwave frequency comb of claim 1, wherein: the Josephson junction (1) is positioned at one end of the superconducting microwave resonator (2), and two end electrodes of the Josephson junction (1) are respectively lapped on a central conductor (3) and a grounding surface (4) of the superconducting microwave resonator (2) to realize the coupling of the two.
4. The on-chip superconducting microwave frequency comb of claim 1, wherein: the superconducting microwave resonator (2) is a half-wavelength coplanar waveguide resonator, the length of a central conductor (3) of the superconducting microwave resonator is 10mm, the superconducting microwave resonator has multi-order resonant modes, and the corresponding resonant frequency is n multiplied by 750 MHz, wherein n is a positive integer.
5. The on-chip superconducting microwave frequency comb of claim 1, wherein: when a direct-current bias voltage is applied to the Josephson junction (1), the frequency of the emitted photons is as follows:
fJ = 2eVdc/h
Wherein 2e is the charge amount of the Cooper pair, V dc is the bias voltage at two ends of the Josephson junction, and h is the Planck constant.
6. The on-chip superconducting microwave frequency comb of claim 5, wherein: the direct current bias voltage is applied to two ends of the Josephson junction (1), the obtained I D is the current passing through the Josephson junction, the current value is related to the number of superconducting Cookies tunneled through the barrier layer, and the more the Cookies tunneled, the larger the current value I D, the more photons are radiated.
7. The on-chip superconducting microwave frequency comb of claim 5, wherein: microwave signals radiated by the on-chip superconducting microwave frequency comb are equally spaced in a frequency domain, and the frequencies of signals of all orders are fitted by the following formula:
fm=mfr
Wherein, m is the signal order (m=1, 2,3 …), f r is the frequency interval, f m is the frequency of the m-th order signal, and the frequency signals are distributed at equal intervals, thereby meeting the requirement of frequency comb.
8. The preparation method of the on-chip superconducting microwave frequency comb is characterized by comprising the following steps of:
S1, sputtering a layer of niobium film on the surface of a substrate by utilizing a magnetron sputtering technology;
S2, obtaining a superconducting microwave resonator (2) by adopting photoetching and reactive ion etching processes;
s3, obtaining a suspension bridge structure through photoetching;
S4, obtaining the Josephson junction (1) with the Al/AlO x/Al structure through a double-angle evaporation process.
9. The method for preparing the on-chip superconducting microwave frequency comb according to claim 8, wherein: in step S1, a sapphire substrate is used, and the thickness of the niobium film is 120 nanometers.
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| WO2022240535A2 (en) * | 2021-04-21 | 2022-11-17 | Massachusetts Institute Of Technology | Methods and apparatus to generate macroscopic fock and other sub-poissonian states of radiation |
| CN115691898A (en) * | 2021-07-21 | 2023-02-03 | 特拉量子股份公司 | High temperature superconducting qubits and methods of fabrication |
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