arXiv subjects

Klaus Mølmer

Publications and source records attributed to Klaus Mølmer.

At least 55 records · Page 3Linked to original sources

arXiv2023

Nonclassical radiation from a nonlinear oscillator driven solely by classical $1/f$ noise

Low-frequency classical $1/f$-noise and quantum noise from low-temperature phonon modes are ubiquitous across various experimental platforms, and are usually considered a hindrance for quantum technological applications. Here we show that the simultaneous action of classical $1/f$ noise and a low-temperature phonon bath on a nonlinear oscillator can result in the generation of nonclassical antibunched radiation without the need for any additional drive. The $1/f$ noise itself provides the source of energy for generation of photons, while the phonon bath prevents heating up to infinite temperature and takes the nonlinear oscillator to a noise-averaged non-equilibrium steady state. The photon current in this non-equilibrium steady state may be detected by a standard wide-band detector. For sufficient nonlinearity and frequency dependence of the effective noise spectrum, the detected radiation can be antibunched. This opens the possibility to turn two of the most ubiquitous intrinsic noises in experimental platforms from a hindrance to a resource. It shows that wasteful heat from unavoidable noises can be converted into useful radiation. These results are based on the Redfield equation, which provides a rigorously derived general approach to treat any type of weak noise in a quantum system, specified only via the noise spectral function, as we discuss in detail.

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arXiv2023

Realization of an extremely anisotropic Heisenberg magnet in Rydberg atom arrays

Strong mutual interactions correlate elementary excitations of quantum matter and plays a key role in a range of emergent phenomena, from binding and condensation to quantum thermalization and many-body localization. Here, we employ a Rydberg quantum simulator to experimentally demonstrate strongly correlated spin transport in anisotropic Heisenberg magnets, where the magnon-magnon interaction can be tuned two orders of magnitude larger than the magnon hopping strength. In our approach, the motion of magnons is controlled by an induced spin-exchange interaction through Rydberg dressing, which enables coherent transport of a single Rydberg excitation across a chain of ground-state atoms. As the most prominent signature of a giant anisotropy, we show that nearby Rydberg excitations form distinct types of magnon bound states, where a tightly bound pair exhibits frozen dynamics in a fragmented Hilbert space, while a loosely bound pair propagates and establishes correlations beyond a single lattice site. Our scheme complements studies using resonant dipole-dipole interactions between Rydberg states, and opens the door to exploring quantum thermodynamics with ultrastrong interactions and kinetic constraints.

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arXiv2023

The Multimode Character of Quantum States Released from a Superconducting Cavity

Quantum state transfer by propagating wave packets of electromagnetic radiation requires tunable couplings between the sending and receiving quantum systems and the propagation channel or waveguide. The highest fidelity of state transfer in experimental demonstrations so far has been in superconducting circuits. Here, the tunability always comes together with nonlinear interactions, arising from the same Josephson junctions that enable the tunability. The resulting non-linear dynamics correlates the photon number and spatio-temporal degrees of freedom and leads to a multi-mode output state, for any multi-photon state. In this work, we study as a generic example the release of complex quantum states from a superconducting resonator, employing a flux tunable coupler to engineer and control the release process. We quantify the multi-mode character of the output state and discuss how to optimize the fidelity of a quantum state transfer process with this in mind.

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arXiv2022

Superradiant Masing with Solid-state Spins at Room Temperature

Steady-state superradiance and superradiant lasing attract significant attentions in the field of optical lattice clocks, but have not been achieved so far due to the technical challenges and atom loss problem. In this article, we propose that their counter-part may be observed in the microwave domain with solid-state spins-microwave resonator systems at room temperature with realistic technical restrictions. To validate our proposal, we investigate systematically the system dynamics and steady-state by solving quantum master equations for the multi-level and multi-process dynamic of trillions of spins. To this end, we employ a mean-field approach, and convert the mean-field dynamics of the spin ensemble into the one in a more intuitive Dicke state picture. Our calculations show that for systems with nitrogen vacancy center spins and pentacene molecular spins the superradiant Rabi oscillations occur firstly due to transitions among different Dicke states, and the subsequent continuous-wave superradiant masing can achieve a linewidth well below millihertz. Our work may guide further exploration of transient and steady-state superradiant masing with the mentioned and other solid-state spins systems, such as silicon vacancy centers in silicon carbide and boron vacancy centers in hexagonal boron nitride, where the coherent radiation with ultra-narrow linewidth may find applications in deep-space communications, radio astronomy and high-precision metrology.

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arXiv2022

Frequency Measurement with Superradiant Pulses of Incoherently Pumped Calcium Atoms: Role of Quantum Measurement Backaction

A recent experiment demonstrated heterodyne detection-based frequency measurements with superradiant pulses from coherently pumped strontium atoms in an optical lattice clock system, while another experiment has analyzed the statistics of superradiant pulses from incoherently pumped calcium atoms in a similar system. In this article, we propose to perform heterodyne detection of the superradiant pulses from the calcium atoms, and analyze theoretically the corresponding atomic ensemble dynamics in terms of the rotation of a collective spin vector and the incoherent quantum jumps among superradiant Dicke states. We examine the effect of quantum measurement backaction on the emitted field and the collective spin vector dynamics, and we demonstrate that it plays an essential role in the modelling of the frequency measurements. We develop a stochastic mean field theory, which is also applicable to model frequency measurements with steady-state superradiance signals, and to explore quantum measurement effects in the dynamics of atomic ensembles.

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arXiv2022

Measuring out-of-time-ordered correlation functions without reversing time evolution

Out-of-time-ordered correlation functions (OTOCs) play a crucial role in the study of thermalization, entanglement, and quantum chaos, as they quantify the scrambling of quantum information due to complex interactions. As a consequence of their out-of-time-ordered nature, OTOCs are difficult to measure experimentally. Here we propose an OTOC measurement protocol that does not rely on the reversal of time evolution and is easy to implement in a range of experimental settings. The protocol accounts for both pure and mixed initial states, and is applicable to systems that interact with environmental degrees of freedom. We demonstrate the application of our protocol by the characterization of scrambling in a periodically-driven spin that exhibits quantum chaos.

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arXiv2022

Creation of non-classical states of light in a chiral waveguide

Creating non-classical states of light from simple quantum systems together with classical resources is a challenging problem. We show how chiral emitters under a coherent drive can generate non-classical photon states. For our analysis, we select a specific temporal mode in the transmitted light field, resulting in a coupled master equation for the relevant mode and the chiral emitters. We characterise the mode's state by its Wigner function and show that the emission from the system predominantly produces mixtures of few-photon-added coherent states. We argue that these non-classical states are experimentally accessible and show their application for quantum metrology.

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arXiv2022

Optical multi-qubit gate operations on an excitation blockaded atomic quantum register

We consider a multi-qubit system of atoms or ions with two computational ground states and an interacting excited state in the so-called blockade regime, such that only one qubit can be excited at any one time. Examples of such systems are rare-earth-ion-doped crystals and neutral atoms trapped in tweezer arrays. We present a simple laser excitation protocol that yields a complex phase factor on any desired multi-qubit product state, and which can be used to implement multi-qubit gates such as the n-bit Toffoli gates. The operation is performed using only two pulses, where each pulse simultaneously address all qubits. By the use of complex hyperbolic secant pulses our scheme is robust and permits complete transfers to and from the excited states despite the variability of interaction parameters. A detailed analysis of the multi-qubit gate performance is provided.

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arXiv2022

Manipulating synthetic gauge fluxes via multicolor dressing of Rydberg-atom arrays

Arrays of highly excited Rydberg atoms can be used as powerful quantum simulation platforms. Here, we introduce an approach that makes it possible to implement fully controllable effective spin interactions in such systems. We show that optical Rydberg dressing with multicolor laser fields opens up distinct interaction channels that enable complete site-selective control of the induced interactions and favorable scaling with respect to decoherence. We apply this method to generate synthetic gauge fields for Rydberg excitations where the effective magnetic flux can be manipulated at the single-plaquette level by simply varying the phase of the local dressing field. The system can be mapped to a highly anisotropic Heisenberg model, and the resulting spin interaction opens the door for explorations of topological phenomena with nonlocal density interactions. A remarkable consequence of the interaction is the emergence of topologically protected long-range doublons, which exhibit strongly correlated motion in a chiral and robust manner.

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arXiv2022

Perfect splitting of two-photon pulses

We employ a cascaded system approach to numerically simulate the interaction of photon pulses with a two-level scatterer in a chiral waveguide QED setup. It is possible to expand any pure state of two photons as a superposition of orthogonal two-photon pulses. We show that the scattering of a two-photon pulse of suitable duration may lead to an entangled output state between predominately two equally populated two-photon states. In a complementary wave packet basis, this state is a product state of two orthogonal single photon wave packets. The time reversal of the above scattering process allows for a perfect combination of distinguishable single-photon wave packet into a single-mode pulse carrying two identical photons.

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arXiv2022

Single-shot determination of quantum phases via continuous measurements

We propose that weak continuous probing may be exploited to determine and define quantum phases of complex many-body systems based on the measurement record alone. We test the resulting phase criterion in numerical simulations of measurements on the Bose-Hubbard model and the quantum Ising chain. This yields a phase transition point in reasonable agreement with the quantum phase transition in the ground state of the closed system in the thermodynamic limit, despite the system being highly excited through the measurement dynamics. At high measurement strengths, the system's response enters a Zeno regime suppressing transitions between eigenstates of the measurement operator.

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arXiv2022

Enhanced metrology at the critical point of a many-body Rydberg atomic system

The spectral properties of an interacting many-body system may display critical character and have potential applications in precision metrology. Here, we demonstrate such many-body enhanced metrology for microwave (MW) electric fields in a non-equilibrium Rydberg atomic gas. Near criticality the high sensitivity of Rydberg atoms to external MW electric fields, combined with many-body enhancement induces significant changes in the optical transmission. We quantify this behavior using the Fisher information. For continuous optical transmission at the critical point, the Fisher information is three orders of magnitude larger than in independent particle systems, the measured data provides an equivalent sensitivity of 49 nV/cm/$\textrm{Hz}^{1/2}$. The reported results constitute a milestone towards the application of many-body effects in precision metrology.

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arXiv2022

Qubit readout and quantum sensing with pulses of quantum radiation

Different hypotheses about a quantum system such as the logical state of a qubit or the value of physical interaction parameters can be investigated by the interaction with a probe field. Such fields may be prepared in particularly sensitive quantum states, and we can use quantum trajectories to model the stochastic measurement record and conditional evolution of the state of the quantum system subject to its interaction with a travelling pulse of radiation. Our analysis applies to different measurement strategies and to arbitrary input quantum states of the probe field pulse and it thus permits direct comparison of their metrological advantages.

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arXiv2022

Random-access quantum memory using chirped pulse phase encoding

As in conventional computing, key attributes of quantum memories are high storage density and, crucially, random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories in a dense, hardware-efficient manner remains a challenge, for example requiring dedicated cavities per qubit or pulsed field gradients. Here we introduce a protocol using chirped pulses to encode qubits within an ensemble of quantum two-level systems, offering both random access and naturally supporting dynamical decoupling to enhance the memory lifetime. We demonstrate the protocol in the microwave regime using donor spins in silicon coupled to a superconducting cavity, storing up to four multi-photon microwave pulses in distinct memory modes and retrieving them on-demand up to 2~ms later. A further advantage is the natural suppression of superradiant echo emission, which we show is critical when approaching unit cooperativity. This approach offers the potential for microwave random access quantum memories with lifetimes exceeding seconds, while the chirped pulse phase encoding could also be applied in the optical regime to enhance quantum repeaters and networks.

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arXiv2022

Optical control of the complex phase of a quantum ground state amplitude

We discuss how coherent driving of a two-level quantum system can be used to induce a complex phase on the ground state and we discuss its geometric and dynamic contributions. While the global phase of a wave function has no physical significance, coherent dynamics in a two-level subspace provides relative phases and is an essential building block for more advanced dynamics in larger systems. In this regard, we note that one must be careful with intuitive accounts of the phase dynamics as it depends on the interaction picture applied. To mitigate ambiguities in practical analyses, we suggest to complement the Bloch sphere picture with the path taken by the ground state amplitude in the complex plane, and we show how the two-level pure state dynamics can serve as a starting point for the study of the dynamics explored in three-level lambda systems, four-level tripod systems, and open quantum systems.

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arXiv2022

Photon-photon interactions in Rydberg-atom arrays

We investigate the interaction of weak light fields with two-dimensional lattices of atoms, in which two-photon coupling establishes conditions of electromagnetically induced transparency and excites high lying atomic Rydberg states. This system features different interactions that act on disparate length scales, from zero-range defect scattering of atomic excitations and finite-range dipole exchange interactions to long-range Rydberg-state interactions that span the entire array. Analyzing their interplay, we identify conditions that yield a nonlinear quantum mirror which coherently splits incident fields into correlated photon-pairs in a single transverse mode, while transmitting single photons unaffected. Such strong photon-photon interactions in the absence of otherwise detrimental photon losses in Rydberg-EIT arrays opens up a promising approach for the generation and manipulation of quantum light, and the exploration of many-body phenomena with interacting photons.

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arXiv2022

Interaction of quantum systems with single pulses of quantized radiation

The interaction of a propagating pulse of quantum radiation with a localized quantum system can be described by a cascaded master equation with a distinct initially populated input and a finally populated output field mode [Phys. Rev. Lett. 123, 123604 (2019), arXiv:1902.09833v3]. By transformation to an appropriate interaction picture, we identify the usual Jaynes-Cummings Hamiltonian between the scatterer and a superposition of the initial and final mode, with a strength given by the travelling pulse mode amplitude. The transformation also identifies a coupling of the scatterer with an orthogonal combination of the two modes. The transformed master equation offers important insights into the system dynamics and it permits numerically efficient solutions.

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arXiv2022

Cavity Quantum Electrodynamics Effects of Optically Cooled Nitrogen-Vacancy Centers Coupled to a High Frequency Microwave Resonator

Recent experiments demonstrated the cooling of a microwave mode of a high-quality dielectric resonator coupled to optically cooled nitrogen-vacancy (NV) spins in diamond. Our recent theoretical study [arXiv:2110.10950] pointed out the cooled NV spins can be used to realize cavity quantum electrodynamics effects (C-QED) at room temperature. In this article, we propose to modify the setup used in a recent diamond maser experiment [Nature 55, 493-496 (2018)], which features a higher spin transition frequency, a lower spin-dephasing rate and a stronger NV spins-resonator coupling, to realize better microwave mode cooling and the room-temperature CQED effects. To describe more precisely the optical spin cooling and the collective spin-resonator coupling, we extend the standard Jaynes-Cumming model to account for the rich electronic and spin levels of the NV centers. Our calculations show that for the proposed setup it is possible to cool the microwave mode from $293$ K (room temperature) to $116$ K, which is about $72$ K lower than the previous records, and to study the intriguing dynamics of the CQED effects under the weak-to-strong coupling transition by varying the laser power. With simple modifications, our model can be applied to, e.g., other solid-state spins or triplet spins of pentacene molecules, and to investigate other effects, such as the operations of pulsed and continuous-wave masing.

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