Switching Dynamics of an Underdamped Josephson Junction Coupled to a Microwave Cavity

Current-biased Josephson junctions are promising candidates for the detection of single photons in the microwave frequency domain. With modern fabrication technologies, the switching properties of the junction can be adjusted to achieve quantum limited sensitivity. Namely, the width of the switching current distribution can be reduced well below the current amplitude produced by a single photon trapped inside a superconducting cavity. However, for an effective detection a strong junction cavity coupling is required, providing nonlinear system dynamics. We compare experimental findings for our prototype device with a theoretical analysis aimed to describe the switching dynamics of junctions under microwave irradiation. Measurements are found in qualitative agreement with our simulations.     

Use of the Josephson junction in fundamental quantum electronics experiments

The Josephson junction provides two strong nonlinearities, the Josephson inductance utilized in parametric amplification and the sharp resistive knee used in SIS mixing. Both of these nonlinearities can be strong compared to a characteristic quantum current or voltage scale. This opens the possibility of using Josephson junctions to carry out fundamental experiments in quantum electronics that would be difficult or impossible to carry out at optical frequencies due to the lack of a correspondingly large nonlinearity in optical media. Here we propose a number of such experiments ranging from the generation of squeezed states with Josephson-parametric amplifiers to the generation of quantum-mechanical superpositions of macroscopically distinguishable states via Josephson-transmission lines. How the properties of such states can be investigated using SIS mixers will also be described. Squeezed states may be technologically useful in sensitive and precision measurement. How such states can be used to enhance interferometer sensitivity or reduce noise in phase sensitive measurement will also be described.     

基于SNS型双路约瑟夫森结阵驱动方法研究

根据SNS型双路约瑟夫森结阵的驱动原理以及结阵分段特点,提出了平衡三进制驱动算法,实现了双路约瑟夫森结阵偏置状态的快速计算。根据约瑟夫森结阵的偏置状态以及组合方式,采用节点电压法,准确合成了双路阶梯波交流量子电压的台阶电压值,最终实现了最小分辨率为2个结,有效位为15位的交流量子电压输出。双路交流量子电压互测实验结果表明,合成交流量子电压的最大误差为0.06 μV,双路信号同步性测试实验中,两个通道的相位差为-0.01 μrad,证明了合成双路交流量子电压具有较高的幅值准确度和相位同步性。     

1V脉冲驱动型交流量子电压源研究

脉冲驱动型交流量子电压标准ACJVS通过高速脉冲序列驱动约瑟夫森结阵芯片的方式实现宽频带交流量子电压的合成,相比于可编程型交流量子电压标准PJVS,具有免台阶切换、频谱纯净、频带宽等优点。搭建的系统主要包括8位高速脉冲码型发生器、微波放大器、直流阻断、约瑟夫森结阵芯片等。通过驱动包含4个子阵列,每个子阵列含12810个约瑟夫森结的结阵芯片,并结合4通道联合低频补偿的方式,成功产生了1V有效值的脉冲驱动型交流量子电压,为进一步建立交流量子电压基准打下了坚实的基础。最后,展望了脉冲驱动型交流量子电压在量子阻抗桥、交流量子功率源、交流量子功率表方面的应用价值。     

Efficient preparation of Greenberger–Horne–Zeilinger state and W state of atoms with the help of the controlled phase flip gates in quantum nodes connected by collective-noise channels

Noise plays a troublesome role for entanglement generation, because the polarization entanglement of photons can be easily disturbed by the noise. In this paper, we propose a protocol to prepare Greenberger–Horne–Zeilinger state and W state of atoms in quantum nodes connected by collective-noise channels assisted by quantum nondemolition detectors (QNDs) and the controlled phase flip gates. The frequency degrees of freedom are exploited in our protocol. The successful probability of our protocol can reach 100% neglecting the photon loss and assuming that the efficiency of QNDs is 1. Moreover, the number of times of using QNDs is limited, and this makes the protocol quite effective when collective noise is small.     

Quantum nondemolition squeezing of a nanomechanical resonator

We show that the nanoresonator position can be squeezed significantly below the ground state level by measuring the nanoresonator with a quantum point contact or a single-electron transistor and applying a periodic voltage across the detector. The mechanism of squeezing is basically a generalization of quantum nondemolition measurement of an oscillator to the case of continuous measurement by a weakly coupled detector. The quantum feedback is necessary to prevent the "heating" due to measurement back-action. We also discuss a procedure of experimental verification of the squeezed state.     

Temperature dependence of Nb penetration depth measured by a resistive method

Besides the conventional method of measuring the penetration depth of Nb superconductors in Nb/AlO_x/Nb Josephson junction, a simple resistive method is applied in this study. With the applied magnetic field parallel to the junction plane, resistive measurement of resistance–temperature characteristics in a given magnetic field or resistance–magnetic field curves at a fixed temperature show resistance peaks whenever the total magnetic flux through the junction equals an integral multiple of the flux quantum. We demonstrate how to determine the penetration depth from such measurements and discuss its temperature dependence in terms of fundamental film properties.     

Quantum dynamics of the charge in Josephson tunnel junctions

It is shown that (quasi) charge of a Josephson tunnel junction can be considered as a macroscopic quantum degree of freedom. In the adiabatic limit we derive the effective action of the tunnel junction in the quasicharge representation and calculate the quantum decay rate of the charge states.     

Optimization of the R-SQUID noise thermometer

The Josephson junction can be used to convert voltage into frequency and thus it can be used to convert voltage fluctuations generated by Johnson noise in a resistor into frequency fluctuations. As a consequence, the temperature of the resistor can be defined by measuring the variance of the frequency fluctuations. Unfortunately, the absolute determination of temperature by this approach is disturbed by several undesirable effects: a rolloff introduced by the bandwidth of the postdetection filter, additional noise caused by rf amplifiers, and a mixed noise effect caused by the nonlinearity of the Josephson junction together with rf noise in the tank circuit. Furthermore, the variance is a statistical quantity and therefore the limited number of frequency counts produces inaccuracy in a temperature measurement. In this work the total inaccuracy of the noise thermometer is analyzed and the optimal choice of the parameters is derived. A practical way to find the optimal conditions for the Josephson junction noise thermometer is discussed. The inspection shows that under the optimal conditions the total error is dependent only on the temperature under determination, the equivalent noise temperature of the preamplifier, the bias frequency of the SQUID, and the total time used for the measurement.     

Phase Transitions and Phase Fluctuations in the Josephson-Junction Arrays

We study the effect of quantum and classical phase fluctuations on the phase transitions in the system of Josephson-junction arrays. We employed a variational method for calculating the Gaussian type fluctuation of the phase in the Josephson-junction array lattice systems without and with an external magnetic field. We investigate the spectrum of collective excitations and the effects of collective excitations on the transport properties of Josephson-junction arrays. We showed that the Hamiltonian for the lattice system of the Josephson junction is the same as the Hamiltonian for the classical or quantum two-dimensional interacting rotators. We also showed that the dynamics of fluctuations of the phase in the lattice system of Josephson junction is very similar to the lattice dynamics of the lattice in crystals. We also showed that in the lattice system of Josephson junctions there is the collective acoustic mode similar to the acoustic mode in the crystal lattice, and this mode may lead to the dissipation of the Josephson current in the superconducting array of Josephson junctions. The speed of sound of the collective acoustic mode of the phase fluctuation depends on the Josephson coupling energy and the Coulomb charging energy. The contribution of the collective acoustic mode to the low temperature specific heat is the same as the contribution of the acoustic phonons to the specific heat of crystals. We also discuss the future development of results and their application.     

Quantum measurements with an amplitude-squeezed-light beam splitter

Quantum measurement of amplitude fluctuation is performed by the injection of 2.5-dB amplitude-squeezed light produced by a quantum-well laser into the dark port of a beam splitter as the meter wave. It is shown that the measurements satisfy the criteria of quantum nondemolition measurement. The measured transfer coefficient and the quantum-state preparation ability are 1.07 and 0.8, respectively.     

Quasiparticles and Polaronic Effects in Crystals,Fermi Gasses and Fermi Liquids

We study the effect of quantum and classical phase fluctuations on the phase transitions in the system of Josephson-junction arrays. We employed a variational method for calculating the Gaussian type fluctuation of the phase in the Josephson-junction array lattice systems without and with an external magnetic field. We investigate the spectrum of collective excitations and the effects of collective excitations on the transport properties of Josephson-junction arrays. We showed that the Hamiltonian for the lattice system of the Josephson junction is the same as the Hamiltonian for the classical or quantum two-dimensional interacting rotators. We also showed that the dynamics of fluctuations of the phase in the lattice system of Josephson junction is very similar to the lattice dynamics of the lattice in crystals. We also showed that in the lattice system of Josephson junctions there is the collective acoustic mode similar to the acoustic mode in the crystal lattice, and this mode may lead to the dissipation of the Josephson current in the superconducting array of Josephson junctions. The speed of sound of the collective acoustic mode of the phase fluctuation depends on the Josephson coupling energy and the Coulomb charging energy. The contribution of the collective acoustic mode to the low temperature specific heat is the same as the contribution of the acoustic phonons to the specific heat of crystals. We also discuss the future development of results and their application.     

Design of an Optical Absorption Cavity for Titanium Transition Edge Sensors

We have developed a TES optical photon detector with a titanium superconducting film showing a very fast response with rise time and fall times of 30 ns and 313 ns, respectively. The fast response is promising for many quantum measurement applications. Increasing the quantum efficiency of this device from the current value of ∼20% makes the detector even more suitable for these applications. Here we report on simulation and experimental results of a cavity designed to improve optical photon absorption of titanium.     

The effect of the resonant mode structure on flux tunneling in SQUIDs

The problem of macroscopic quantum tunneling in SQUIDs is discussed, taking into account the resonant mode structure of typical devices. These are evaluated for the particular case of a SQUID formed from a conical point intersecting a hemispherical cavity, and it is shown that the conventional representation of a SQUID as a Josephson junction in parallel with an inductor and a capacitor is a good first approximation in most cases, provided the inductance and capacitance used are those of the whole device rather than of the weak link alone. The discussion is extended to another case of practical importance, where such a cavity is connected to an outer hole by a flange, and it is found that if the capacitance of the flange is large, the tunneling behavior is largely independent of the presence of the outer hole, apart from the effects of any dc bias.     

Frequency Stabilization of X-Band Sources for Use in Frequency Synthesis into the Infrared

An X-band source of excellent frequency stability is needed in infrared frequency multiplication of high order. Such a source has been used in frequency multiplication by a factor of 401 using a point-contact Josephson junction as a frequency multiplier and mixer. Noise data on three X-band systems are reported. Two of these systems use klystrons as the source of X-band power; the other uses a Gunn oscillator. Each of these three systems employs both cavity and injection stabilization. Injection stabilization, using a quartz-oscillator-driven multiplier chain, provides the second-to-second and minute-to-minute stability needed for the Josephson junction experiment. To our knowledge, this is the first published noise data where cavity and injection stabilization are simultaneously employed. The quality of the best system reported here is much better-both around 1 Hz from the carrier and around 50 kHz from the carrier?than the source used to multiply by a factor of 401 to 3.8 THz.     

Resonant macroscopic quantum tunneling in Josephson junctions

Many theoretical studies and some experiments have shown the possibility of several new secondary quantum effects in small Josephson junctions at low temperatures. We show that, within the well-established quantum picture of the junction, it is possible to experimentally observe sharp voltalge peaks at certain current values. Such peaks are related to a resonant macroscopic quantum tunneling between levels in neighboring wells of the proper washboard potential having close energies. The proposed experiment, with respect to experiments on Coulomb blockade or Block oscillations, requires a larger junction area, reducing the technological difficulties in making the samples.     

Influence of dissipation on the prefactor in the expression of the decay rate of a metastable state

The influence of dissipation on the prefactor in the expression of the decay probability of a metastable state via macroscopic quantum tunneling is discussed. It is shown that under experimental conditions, because of the small number of levels in the well, such a correction is of paramount importance. Moreover, the full coincidence is shown between the expression of the decay probability derived from usual quantum mechanics and that obtained by the functional integral method in the limit of zero viscosity. The whole discussion, though of wide generality, is referred to a Josephson junction as a physical model, for which some conclusions of experimental interest are drawn.     

Two different regimes in a V-type three-level atom trapped in an optical cavity

In this paper two different behaviors of a V-type three-level atom which is driven by a classical field in an optical cavity are shown. We show that the behavior of an atom depends on the values of the critical photon number. The system treatment for large enough values of the critical photon number is described by the semiclassical laser theory. In contrast, for small enough values of the critical photon number, the system shows new quantum properties. In the former case, it is shown that the system performs like a conventional laser and in the latter one, the system acts as an effective two-level model. The behaviors of the system are investigated both in the semiclassical approximation and full quantum theory. For comparing the results, computer simulations are implemented.     

Phase qubit on a superconducting single-junction interferometer

The energy spectrum of a phase quantum bit (qubit) implemented on a superconducting interferometer with a single Josephson junction has been studied in terms of the Hamilton formalism. An analytical formula for ground level splitting in the qubit spectrum is obtained and analyzed.     

Detecting Current Noise with a Josephson Junction in the Macroscopic Quantum Tunneling Regime

We discuss the use of a hysteretic Josephson junction to detect current fluctuations with frequencies below the plasma frequency of the junction. These adiabatic fluctuations are probed by switching measurements observing the noise-affected average rate of macroscopic quantum tunneling of the detector junction out of its zero-voltage state. In a proposed experimental scheme, frequencies of the noise are limited by an on-chip filtering circuit. The third cumulant of current fluctuations at the detector is related to an asymmetry of the switching rates.