A csoport tudományos kutatásainak fókuszában a rezonátoros kvantumelektrodinamika és az ultrahideg atomok állnak. A fényanyag kölcsönhatást vizsgáljuk abban a végső határesetben, amikor mindkét komponens dinamikailag változó kvantumrendszer. Az anyagi összetevő lehet egy atom, kis számú atom, vagy atomok BoseEinstein kondenzátuma. Az ultrahideg atomok, amelyek hőmérséklete az abszolút zérus felett mindössze néhány nanokelvin, fizikája elvezet kritikus jelenségekhez, fázisátalakulásokhoz és a kvantummechanikai soktestprobléma módszereihez. A csoport 2016ban elkezdte egy laboratórium felépítését Rb atomokkal végzett optikai kísérletekhez.
Experiment on Cavity Quantum Electrodynamics with cold Rb atoms. — We developed further our cavity QED experimental setup and achieved the detection of atoms within the resonator. We performed the first measurements with cold trapped Rb atoms interacting with the mode of a high finesse optical resonator. We studied the photoelectron statistics of the probe light transmitted through the resonator for different configuration of the atom cloud in the mode.
Figure 1. First observation of atoms in the cavity. The time evolution of the transmitted intensity of an optical resonator is plotted for various frequency detunings between the probe laser light and the resonator mode. The peaks describe the effect of the atoms on the transmission. The sign of the change (peak vs. dip) is in good agreement with the theoretically expected effect. For negative detuning between the laser and the resonator, the atoms shift the resonance mode frequency closer to that of the laser, therefore the transmission exhibits peaks, as can be seen in the upper row. The measured data allow for a calibration of various parameters of the system, e.g., the atom number in the cavity.
Cavity Quantum Electrodynamics with BoseEinstein condensates. — The interaction of a magnetically trapped Bose–Einstein condensate of Rubidium atoms with the stationary microwave radiation field sustained by a coplanar waveguide resonator (CPW) has been studied. This coupling allows for the measurement of the magnetic field of the resonator by means of counting the atoms that fall out of the condensate due to hyperfine transitions to nontrapped states. We determined the quantum efficiency of this detection scheme and showed that weak microwave fields at the singlephoton level can be sensed.
Figure 2. Detection efficiency of the magnetically levitated BEC in sensing microwave photons of a coplanar resonator. The ultracold atom cloud is situated at the center of the central conductor of the coplanar waveguide resonator. The geometry of the CPW ensures that its magnetic field is polarized in the x direction at the location of the BEC. The magnetic field Bx (t) of the CPW oscillates in time with an angular frequency which is resonant with a magnetic dipole transition of the BEC atoms, and induces transitions from the trapped state into an untrapped one. A onephoton microwave field in the resonator generates an atom current of about 3 atoms per 1.7 millisecond.
Experiment on Cavity Quantum Electrodynamics with cold Rb atoms. ― We set up a single mode optical cavity in the UHV chamber and frequency stabilized one of the fundamental modes (4 MHz linewidth) to the Rubidium 780 nm D2 transition line by piezo feedback and by means of the PonderDreverHall (PDH) method. We created a magnetooptical trap and Rb captured atoms from vapour. The fluorescence of the trapped atom cloud is shown in Fig. 1. We developed the absorption imaging technique to determine the number of atoms (~10^{8}) and the temperature in the trap, which, after polarisation gradient cooling, is about 100 microKelvin. The atoms, optically pumped to a well defined magnetic state, have been loaded into a purely magnetic trap which allows for transporting the atoms, eventually to position them into the volume of the optical cavity. The system is close to realise the atomphoton interface for the first set of planned experiments.
Figure 1. Adiabatic transport of cold Rb atoms in the magnetic trap into the optical cavity.
Cavity Quantum Electrodynamics, quantum critical phenomena. ― We completed and justified the interpretation of the photonblockadebreakdown effect as a firstorder dissipative quantum phase transition. To this end, we introduced the concepts of thermodynamic limit and finitesize scaling for the microscopic system of a driven dissipative JaynesCummings model. The thermodynamic limit is defined for this microscopic system in such a way that the number of relevant degrees of freedom remains fixed. Instead of growing the system size, the scaling of the parameters keeps the form of the stationary solution of the drivendissipative system invariant. At the same time, the proposed finitesize scaling leads to an increasing robustness of the attractor states associated with the bistability signal, until these states reach full stability in the thermodynamic limit. On approaching this limit, at variance with the fluorescence shelving experiment, no single quantum jump or other microscopic event can flip the system from one phase to the other. In the thermodynamic limit, the blinking telegraphlike signal vanishes completely and the state of the system is determined by the initial condition, similarly to the usual hysteresis behaviour in classical critical systems. If such a finite size scaling is possible – and here we show that this is the case for the photonblockade breakdown effect –, then the bistability that can be observed in a given experimental realization of the system with its finite parameters not in the thermodynamic limit, can be considered the finitesize approximation of what is a genuine firstorder phase transition in the thermodynamic limit.
Figure 2. Classical phase diagram of the driven JaynesCumings model in the ultrastrong coupling regime. The photonblockadebreakdown phase transition can be induced by changing the drive amplitude (vertical axis) or the drive frequency (horizontal axis). We studied the finitesize scaling in the vicinity of the blue star, which is in the middle of the bistability region.
Numerical methods in Quantum Optics. — We developed a stepwise adaptivetimestep version of the Quantum Jump (Monte Carlo wavefunction) algorithm. Our method has proved to remain robust even for problems where the integrating implementation of the Quantum Jump method is numerically problematic. The only specific parameter of our algorithm is the single a priory parameter of the Quantum Jump method, the maximal allowed total jump probability per timestep. We studied the convergence of ensembles of trajectories to the solution of the full master equation as a function of this parameter. This study is expected to pertain to any possible implementation of the Quantum Jump method.
Experiment on Cavity Quantum Electrodynamics with cold Rb atoms ― We set up a new laboratory for cavity QED experiments with ultracold Rb atoms (Fig. 1). We realized the UHV system which is operated in the pressure range below 10^{10} mbar, and includes the magnetooptical trap and the invacuo highfinesse singlemode optical resonator. We completed the optical system, which is based on three laser sources that are referenced to Rb resonance line by Dopplerfree nonlinear spectroscopy. By means of acoustooptical modulators, we provide the narrowlinewidth phaselocked laser sources at five different frequencies for atomic manipulation.
Figure 1. Photo of the actual status of the cavity QED laboratory.
Cavity Quantum Electrodynamics, quantum critical phenomena. ― The quantum measurement backaction noise effects on the dynamics of an atomic Bose lattice gas inside an optical resonator have been described by means of a hybrid model consisting of a Bose–Hubbard Hamiltonian for the atoms and a Heisenberg–Langevin equation for the lossy cavity field mode (Fig. 2). Considering atoms initially prepared in the ground state of the lattice Hamiltonian, we calculated the transient dynamics due to the interaction with the cavity mode. We showed that the cavity field fluctuations originating from the dissipative outcoupling of photons from the resonator lead to vastly different effects in the different possible ground state phases, i.e., the superfluid, the supersolid, the Mott and the chargedensitywave phases. In the former two phases with the presence of a superfluid wavefunction, the quantum measurement noise appears as a driving term leading to depletion of the ground state. The time scale for the system to leave the ground state was presented in a simple analytical form. For the latter two incompressible phases, the quantum noise results in the fluctuation of the chemical potential. We derived an analytical expression for the corresponding broadening of the quasiparticle resonances.
Figure 2. Illustration of the coupled cavity BoseHubbard model setup. An atomic cloud is loaded into a square optical lattice, which is inside a singlemode highQ FabryPérot resonator. The period of the cavity mode is approximately equal to that of the optical lattice. The cavity is pumped from the side by light scattering off the atoms. The system is open: together with the external drive, the photons leak out from the cavity, resulting in heating and decoherence, or, in other terms, quantum measurement backaction since the outcoupled photons can be measured by classical detectors.
Ultracold gases, BoseEinstein condensates. ― We studied quasiparticle scattering effects on the dynamics of a homogeneous BoseEinstein condensate of ultracold atoms coupled to a single mode of an optical cavity. The relevant excitations, which are polaritonlike mixed excitations of photonic and atomic densitywave modes, have been identified. All the firstorder correlation functions were presented by means of the Keldysh Green’s function technique. Beyond confirming the existence of the resonant enhancement of Beliaev damping, we found a very structured spectrum of fluctuations. There is a spectral hole burning at half of the recoil frequency, reflecting the singularity of the Beliaev scattering process. The effects of the photonloss dissipation channel and that of the Beliaev damping due to atomatom collisions could be well separated. We showed that the Beliaev process does not influence the properties of the selforganization criticality.


Figure 3. (a) The schematic picture of an atomic ensemble inside a FabryPérot cavity pumped from the side by a laser close to resonance with the cavity. The atomic gas undergoes selforganization for strong enough laser drive: the atoms scatter photons from the laser to the cavity and may create a classical field serving as an optical lattice trapping them even further in the optimal scattering positions. (b) Spectrum of the quasiparticle excitation. Large detuning ∆C = 100 implies little mixing of the condensate quasiparticle with the photon mode. The enhanced Beliaev scattering process is manifested by the large peak, which exhibits a hole burning at 0.5. (All angular frequencies are expressed in units of the recoil frequency.) The correlation function is plotted for different coupling strengths y approaching the critical value y_{c}: y = 0 (solid red), y/y_{c} = 0.5 (shortdashed green), 0.7 (dotted blue), 0.8 (dashdotted orange), 0.9 (dasheddoubledotted brown), 0.95 (longdashed magenta). 
We studied the spin1 bilinear–biquadratic model on the complete graph of N sites, i.e. when each spin is interacting with every other spin with the same strength. Because of its complete permutation invariance, this Hamiltonian can be rewritten as the linear combination of the quadratic Casimir operators SU(3) and SU(2). Using group representation theory, we explicitly diagonalized the Hamiltonian and mapped out the groundstate phase diagram of the model. Furthermore, the complete energy spectrum with degeneracies was obtained analytically for any number of sites.
Cavity Quantum Electrodynamics, quantum critical phenomena. — Nonequilibrium phase transitions exist in dampeddriven open quantum systems, when the continuous tuning of an external parameter leads to a transition between two robust steady states.
In secondorder transitions, this change is abrupt at a critical point, whereas in firstorder transitions, the two phases can coexist in a critical hysteresis domain. In collaboration with the experimental group of the ETH Zürich, we found a firstorder dissipative quantum phase transition in a drivencircuit quantum electrodynamics (QED) system. It takes place when the photon blockade of the driven cavityatom system is broken by increasing the drive power. The observed experimental signature is a bimodal phase space distribution with varying weights controlled by the drive strength. The measurements showed an improved stabilization of the classical attractors up to the millisecond range when the size of the quantum system is increased from one to three artificial atoms. The theoretical work included the fitting of the experimental data as well as it contributed to prove the phasetransition character of the effect. Furthermore it was possible to prove theoretically that the photonblockadebreakdown effect relies on a given range of the parameters of a threelevel atomic system. We showed that the parameters of the actual experimental setup happen to correspond to this range [Phys. Rev. X 7, 011012 (2017) ].
Figure 1. (a) Simulated histogram of the output intensity as a function of the coupling constant g_{2} between the two excited atomic states eñ and fñ at a given driving amplitude h with red representing high probability and blue indicating zero probablility. This plot shows that only a certain range of the ratio g_{2}/g_{1} gives rise to bistability.
(b) Measured vacuum Rabi spectra for various input powers with all three atoms in resonance with the cavity. For better visibility the shown spectra are offset by 1.6 nW from each other. The sharp transmission peak shown in the inset appears stochastically. In this particular measurement (orange line at 4.4 fW input power), we observe only two frequency points with small but finite switching probability and we sample over multiple switching events resulting in a certain mean detected power. At lower drive power, no transmission is observed (no switching). At higher drive powers, the transmission peak approaches the cavity linewidth and scales linearly with input power (no switching again).
(c) Measured histogram of the detected power as a function of the cavity input power for a single transmon (density plot). The most likely photon numbers (line plots) are extracted from this measurement (red) and two similar measurements taken with 2 (orange) and 3 qubits (green) in resonance with the cavity mode. Simulation results for the single qubit case are shown with connected black symbols for comparison. The dashed line is for reference and represents the response of the empty cavity.
Ultracold gases, BoseEinstein condensates. — Modeling the coupling between a trapped BoseEinstein condensate and a current carrying nanowire, we studied, in collaboration with an experimental group at Tübingen, the magnetomechanical interaction by means of classical radiofrequency sources. We performed the spectral analysis and the local measurement of intensity correlations of microwave fields using ultracold quantum gases. The fluctuations of the electromagnetic field induce spin flips in a magnetically trapped quantum gas and generate a multimode atom laser. The output of the atom laser was measured with high temporal resolution on the singleatom level, from which the spectrum and intensity correlations of the generating microwave field have been reconstructed in accordance with our recently proposed scheme. We gave the theoretical description of the atomlaser output and its correlations in response to resonant microwave fields and verified the model with measurements on an atom chip. The measurement technique is applicable for the local analysis of classical and quantum noise of electromagnetic fields, for example, on chips, in the vicinity of quantum electronic circuits [Phys. Rev. A 95, 043603 (2017)].
Figure 2. (a) Coldatom spectrometer (not to scale) consisting of a magnetically trapped BoseEinstein condensate and an ionizationbased singleatom detector. (b) The microwave couples atoms at resonance surfaces given by equipotential surfaces of the atomic Zeeman potential, i.e., magnetic isofield lines (dashed lines). Due to gravity, the BEC is displaced from the magnetic trap center and the resonance surfaces become nearly plane. Without amplitude modulation, the microwave carrier couples atoms from a single resonance surface (red solid thick line) with a position given via ωc. Amplitude modulation at a single frequency generates sidebands to the carrier and outcoupling from two resonance surfaces (green solid thin lines). (c) Normalized spectral response γ(ω)/γmax of a BEC to a single microwave frequency (black dots) and model function (red line).