Within the framework of fundamental research or radio astronomy, it is often necessary to resort to amplifiers designated very low noise amplifiers, that is to say amplifiers of which the noise is as close as possible to the theoretical limit imposed by quantum mechanics, and this is true for frequencies ranging from MHz to THz. It is possible to distinguish two types of amplifiers: phase preserving amplifiers and phase sensitive amplifiers. Phase sensitive amplifiers are not subject to the theoretical limitation but they are difficult to use because the phase of the signal to amplify must correspond perfectly to the amplified quadrature. Phase preserving amplifiers are subject to the quantum limit but on the other hand do not impose any condition regarding the phase of the signal to amplify.
In the field of high frequency and very high frequency amplification it is known to use amplification by reflection. The latter is obtained for example using a transmission line of impedance Z0 terminated by an impedance ZL(f) where f is the frequency of the signal. The voltage reflection coefficient of such a structure is given by
      G    ⁡          (      f      )        =                    (                                            Z              L                        ⁡                          (              f              )                                -                      Z            0                          )                    (                                            Z              L                        ⁡                          (              f              )                                +                      Z            0                          )              .  If it is managed to obtain a negative impedance ZL(f) of which the absolute value is close to Z0, the reflection coefficient G(f) diverges and the reflected signal is thus amplified. Obviously, this condition is only met for a given frequency band which then corresponds to the pass band of the amplifier.
In the field of amplification, it is also known to use semiconductor transistors working at cryogenic temperatures. The transistors are voltage polarised which makes the use of this type of amplifier very easy to implement. On the other hand their noise level, even at cryogenic temperature, still remains high: of the order of 10 photons in the best of cases, i.e. 20 times the quantum limit. To limit noise to the maximum, it has been proposed to use a parametric amplifier in which the negative impedance is obtained for example through four Josephson junctions. The circuit of the amplifier has a first mode at the signal frequency fs and a second mode at the idler frequency fi. The parameters of the circuit are modified using the pump of frequency fp. This amplifier makes it possible to obtain a pass band of several MHz with a noise close to the quantum limit but requires the generation of a pump, which makes it complex to implement.
Another way of understanding this parametric amplification is to envisage the latter in terms of photons, each photon being associated with an energy. For example, a first photon of energy E1 may be converted into a second photon of energy E2 and a third photon of energy E3 if the relation between these three energies is such that E1=E2+E3 (this is a necessary condition but not sufficient). Knowing that it is possible to associate a frequency with each energy, this equality becomes f1=f2+f3. If it is sought to amplify a signal of frequency f2, it is thus possible to resort to a signal of frequency f1 on condition that a mode, that is to say a resonance of the system, at a frequency f3 is also present. Hereafter, the frequency f1 is called pump frequency fp, the frequency f2 is called signal frequency fs and the frequency f3 is called idler frequency fi. It is possible to obtain an amplification of a signal of frequency fs by the generation of other signal photons of frequency fs from pump photons of frequency fp, this generation also leading to the generation of idler photons of frequency fi. In order that this amplification process takes place, it is moreover necessary, in addition to the condition of conservation of energy, that the different frequencies are coupled together. Such a coupling is achieved using a non-linear impedance.
Alternatively, it has been proposed to use an amplifier in which the negative impedance is obtained using a voltage polarised Josephson junction. This voltage polarisation makes it possible to dispense with the pump and contributes to the simplification of the device. Indeed, once the junction polarised, the Cooper pairs responsible for the superconductor current provided with an energy proportional to the voltage applied to the junction are going to have to relax, that is to say return to their fundamental energy state. Yet, the only mechanism enabling this relaxation leads to the emission of one or more photons of which the energy is linked to the applied polarisation voltage. In other words, the applied voltage makes it possible to generate photons in the same way as the pump frequency of the parametric amplifier, the frequency of these photons here being a function of the voltage applied to the junction.
In addition, amplification may be obtained, in the simplest embodiment, with a single Josephson junction. Such a device is illustrated in FIG. 1 and is composed of a matching circuit comprising a first connection port J1 and a second connection port J2, these two ports being connected to a Josephson junction JJ. In order to polarise the Josephson junction JJ, a voltage source V is connected using a T connection constituted of the inductance L3 and the capacitance C2. The matching circuit also comprises a resistance R1 in series with a capacitance C1 and an inductance L1 in parallel with each other. This matching circuit enables a polarisation of the Josephson junction JJ such that the latter has a negative differential impedance over a wide voltage range while maintaining a positive global impedance, that is to say the impedance of the Josephson junction JJ and the matching circuit.
In this device, it is possible to distinguish two situations. For high and low frequencies, the Josephson junction is short-circuited by the matching circuit. The real part of the reflection coefficient is then equal to
      Re    ⁡          (              G        ⁡                  (          f          )                    )        =            (                        R          1                -                  Z          0                    )              (                        R          1                +                  Z          0                    )      with R1 the resistance of the matching circuit. R1 being of the order of several ohms compared to several tens of ohms for Z0, the real part of the reflection coefficient becomes Re(G(f))˜−1 and there is thus no amplification of the signal but only a dephasing of π. On the other hand, at the frequency of the signal fs, the impedance of the matching circuit is high and the impedance of the Josephson junction JJ thus becomes “visible” such that the impedance measured at the level of the input/output port is such that ZL<0 and |ZL|≥Z0, the amplitude of the reflection is thus Re(G(fs))>1 and there is thus amplification of the signal. Moreover, the more ZL approaches −Z0, the greater the amplification. In the circuit evoked previously, the circuit L2C2 makes it possible to achieve impedance matching between the impedance of the Josephson junction JJ at the frequency fs and the input port of the circuit. The inductance L3 makes it possible for its part to isolate the high frequency part of the circuit from the DC part. However, although this device improves the noise compared to an amplifier using transistors, the noise level remains higher than the noise level obtained with a parametric amplifier.
There thus exists a need to manufacture an amplifier making it possible to maintain the noise level similar to that of a parametric amplifier while having the simplicity of an amplifier with voltage polarised Josephson junction.