arXiv subjects

Kater W. Murch

Publications and source records attributed to Kater W. Murch.

36 records · Page 2Linked to original sources

arXiv2023

Speeding up entanglement generation by proximity to higher-order exceptional points

Entanglement is a key resource for quantum information technologies ranging from quantum sensing to quantum computing. Conventionally, the entanglement between two coupled qubits is established at the time scale of the inverse of the coupling strength. In this work, we study two weakly coupled non-Hermitian qubits and observe entanglement generation at a significantly shorter time scale by proximity to a higher-order exceptional point. We establish a non-Hermitian perturbation theory based on constructing a biorthogonal complete basis and further identify the optimal condition to obtain the maximally entangled state. Our study of speeding up entanglement generation in non-Hermitian quantum systems opens new avenues for harnessing coherent nonunitary dissipation for quantum technologies.

quant-ph↗

arXiv2023

Many-body quantum vacuum fluctuation engines

We propose a many-body quantum engine powered by the energy difference between the entangled ground state of the interacting system and local separable states. Performing local energy measurements on an interacting many-body system can produce excited states from which work can be extracted via local feedback operations. These measurements reveal the quantum vacuum fluctuations of the global ground state in the local basis and provide the energy required to run the engine. The reset part of the engine cycle is particularly simple: The interacting many-body system is coupled to a cold bath and allowed to relax to its entangled ground state. We illustrate our proposal on two types of many-body systems: a chain of coupled qubits and coupled harmonic oscillator networks. These models faithfully represent fermionic and bosonic excitations, respectively. In both cases, analytical results for the work output and efficiency of the engine can be obtained. Generically, the work output scales as the number of quantum systems involved, while the efficiency limits to a constant. We prove the efficiency is controlled by the "local entanglement gap" -- the energy difference between the global ground state and the lowest energy eigenstate of the local Hamiltonian. In the qubit chain case, we highlight the impact of a quantum phase transition on the engine's performance as work and efficiency sharply increase at the critical point. In the case of a one-dimensional oscillator chain, we show the efficiency approaches unity as the number of coupled oscillators increases, even at finite work output.

quant-ph↗

arXiv2023

Observing Parity Time Symmetry Breaking in a Josephson Parametric Amplifier

A coupled two-mode system with balanced gain and loss is a paradigmatic example of an open quantum system that can exhibit real spectra despite being described by a non-Hermitian Hamiltonian. We utilize a degenerate parametric amplifier operating in three-wave mixing mode to realize such a system of balanced gain and loss between the two quadrature modes of the amplifier. By examining the time-domain response of the amplifier, we observe a characteristic transition from real-to-imaginary energy eigenvalues associated with the Parity-Time-symmetry-breaking transition.

quant-ph↗

arXiv2022

Energetic cost of measurements using quantum, coherent, and thermal light

Quantum measurements are basic operations that play a critical role in the study and application of quantum information. We study how the use of quantum, coherent, and classical thermal states of light in a circuit quantum electrodynamics setup impacts the performance of quantum measurements, by comparing their respective measurement backaction and measurement signal to noise ratio per photon. In the strong dispersive limit, we find that thermal light is capable of performing quantum measurements with comparable efficiency to coherent light, both being outperformed by single-photon light. We then analyze the thermodynamic cost of each measurement scheme. We show that single-photon light shows an advantage in terms of energy cost per information gain, reaching the fundamental thermodynamic cost.

quant-ph↗

arXiv2022

Efficiently Fuelling a Quantum Engine with Incompatible Measurements

We propose a quantum harmonic oscillator measurement engine fueled by simultaneous quantum measurements of the non-commuting position and momentum quadratures of the quantum oscillator. The engine extracts work by moving the harmonic trap suddenly, conditioned on the measurement outcomes. We present two protocols for work extraction, respectively based on single-shot and time-continuous quantum measurements. In the single-shot limit, the oscillator is measured in a coherent state basis; the measurement adds an average of one quantum of energy to the oscillator, which is then extracted in the feedback step. In the time-continuous limit, continuous weak quantum measurements of both position and momentum of the quantum oscillator result in a coherent state, whose coordinates diffuse in time. We relate the extractable work to the noise added by quadrature measurements, and present exact results for the work distribution at arbitrary finite time. Both protocols can achieve unit work conversion efficiency in principle.

quant-ph↗

arXiv2022

Single electrons on solid neon as a solid-state qubit platform

Progress toward the realization of quantum computers requires persistent advances in their constituent building blocks - qubits. Novel qubit platforms that simultaneously embody long coherence, fast operation, and large scalability offer compelling advantages in the construction of quantum computers and many other quantum information systems. Electrons, ubiquitous elementary particles of nonzero charge, spin, and mass, have commonly been perceived as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits via either motional or spin states depends critically on their material environment. Here we report our experimental realization of a new qubit platform based upon isolated single electrons trapped on an ultraclean solid neon surface in vacuum. By integrating an electron trap in a circuit quantum electrodynamics architecture, we achieve strong coupling between the motional states of a single electron and a single microwave photon in an on-chip superconducting resonator. Qubit gate operations and dispersive readout are implemented to measure the energy relaxation time $T_1$ of $15~μ$s and phase coherence time $T_2$ over $200~$ns. These results indicate that the electron-on-solid-neon qubit already performs near the state of the art as a charge qubit.

quant-ph↗

arXiv2022

Topological quantum state control through exceptional-point proximity

We study the quantum evolution of a non-Hermitian qubit realized as a submanifold of a dissipative superconducting transmon circuit. Real-time tuning of the system parameters to encircle an exceptional point results in non-reciprocal quantum state transfer. We further observe chiral geometric phases accumulated under state transport, verifying the quantum coherent nature of the evolution in the complex energy landscape and distinguishing between coherent and incoherent effects associated with exceptional point encircling. Our work demonstrates an entirely new method for control over quantum state vectors, highlighting new facets of quantum bath engineering enabled through dynamical non-Hermitian control.

quant-ph↗

arXiv2022

Maximal quantum entanglement at exceptional points via unitary and thermal dynamics

Minimal, open quantum systems that are governed by non-Hermitian Hamiltonians have been realized across multiple platforms in the past two years. Here we investigate the dynamics of open systems with Hermitian or anti-Hermitian Hamiltonians, both of which can be implemented in such platforms. For a single system subject to unitary and thermal dynamics in a periodic manner, we show that the corresponding Floquet Hamiltonian has a rich phase diagram with numerous exceptional-point (EP) degeneracy contours. This protocol can be used to realize a quantum Hatano-Nelson model that is characterized by asymmetric tunneling. For one unitary and one thermal qubit, we show that the concurrence is maximized at the EP that is controlled by the strength of Hermitian coupling between them. Surprisingly, the entropy of each qubit is also maximized at the EP. Our results point to the multifarious phenomenology of systems undergoing unitary and thermal dynamics.

quant-ph↗

arXiv2021

Decoherence Induced Exceptional Points in a Dissipative Superconducting Qubit

Open quantum systems interacting with an environment exhibit dynamics described by the combination of dissipation and coherent Hamiltonian evolution. Taken together, these effects are captured by a Liouvillian superoperator. The degeneracies of the (generically non-Hermitian) Liouvillian are exceptional points, which are associated with critical dynamics as the system approaches steady state. We use a superconducting transmon circuit coupled to an engineered environment to observe two different types of Liouvillian exceptional points that arise either from the interplay of energy loss and decoherence or purely due to decoherence. By dynamically tuning the Liouvillian superoperators in real time we observe a non-Hermiticity-induced chiral state transfer. Our study motivates a new look at open quantum system dynamics from the vantage of Liouvillian exceptional points, enabling applications of non-Hermitian dynamics in the understanding and control of open quantum systems.

quant-ph↗

arXiv2021

Quantum jumps in the non-Hermitian dynamics of a superconducting qubit

We study the dynamics of a driven non-Hermitian superconducting qubit which is perturbed by quantum jumps between energy levels, a purely quantum effect with no classical correspondence. The quantum jumps mix the qubit states leading to decoherence. We observe that this decoherence rate is enhanced near the exceptional point, owing to the cube-root topology of the non-Hermitian eigenenergies. Together with the effect of non-Hermitian gain/loss, quantum jumps can also lead to a breakdown of adiabatic evolution under the slow-driving limit. Our study shows the critical role of quantum jumps in generalizing the applications of classical non-Hermitian systems to open quantum systems for sensing and control.

quant-ph↗

arXiv2021

Weak Measurement of Superconducting Qubit Reconciles Incompatible Operators

Traditional uncertainty relations dictate a minimal amount of noise in incompatible projective quantum measurements. However, not all measurements are projective. Weak measurements are minimally invasive methods for obtaining partial state information without projection. Recently, weak measurements were shown to obey an uncertainty relation cast in terms of entropies. We experimentally test this entropic uncertainty relation with strong and weak measurements of a superconducting transmon qubit. A weak measurement, we find, can reconcile two strong measurements' incompatibility, via backaction on the state. Mathematically, a weak value -- a preselected and postselected expectation value -- lowers the uncertainty bound. Hence we provide experimental support for the physical interpretation of the weak value as a determinant of a weak measurement's ability to reconcile incompatible operations.

quant-ph↗

arXiv2020

Floquet exceptional contours in Lindblad dynamics with time-periodic drive and dissipation

The dynamics of an isolated quantum system is coherent and unitary. Weak coupling to the environment leads to decoherence, which is traditionally modeled with a Lindblad equation for the system's density matrix. Starting from a pure state, such a system approaches a steady state (mixed or otherwise) in an underdamped or overdamped manner. This transition occurs at an eigenvalue degeneracy of a Lindblad superoperator, called an exceptional point (EP), where corresponding eigenvectors coalesce. Recent years have seen an explosion of interest in creating exceptional points in a truly quantum domain, driven by the enhanced sensitivity and topological features EPs have shown in their classical realizations. Here, we present Floquet analysis of a prototypical qubit whose drive or dissipator strengths are varied periodically. We consider models with a single dissipator that generate global loss (phase damping) or mode-selective loss (spontaneous emission). In all cases, we find that periodic modulations lead to EP lines at small dissipator strengths, and a rich EP structure in the parameter space. Our analytical and numerical results show that extending Lindblad Liouvillians to the Floquet domain is a new, potentially preferred route to accessing exceptional points in the transient dynamics towards the Lindblad steady state.

quant-ph↗

arXiv2008

Quantum micro-mechanics with ultracold atoms

In many experiments isolated atoms and ions have been inserted into high-finesse optical resonators for the study of fundamental quantum optics and quantum information. Here, we introduce another application of such a system, as the realization of cavity optomechanics where the collective motion of an atomic ensemble serves the role of a moveable optical element in an optical resonator. Compared with other optomechanical systems, such as those incorporating nanofabricated cantilevers or the large cavity mirrors of gravitational observatories, our cold-atom realization offers direct access to the quantum regime. We describe experimental investigations of optomechanical effects, such as the bistability of collective atomic motion and the first quantification of measurement backaction for a macroscopic object, and discuss future directions for this nascent field.

quant-ph↗

arXiv2008

Observation of quantum-measurement backaction with an ultracold atomic gas

Current research on micro-mechanical resonators strives for quantum-limited detection of the motion of macroscopic objects. Prerequisite to this goal is the observation of measurement backaction consistent with quantum metrology limits. However, thermal noise presently dominates measurements and precludes ground-state preparation of the resonator. Here we establish the collective motion of an ultracold atomic gas confined tightly within a Fabry-Perot optical cavity as a system for investigating the quantum mechanics of macroscopic bodies. The cavity-mode structure selects a single collective vibrational mode that is measured by the cavity's optical properties, actuated by the cavity optical field, and subject to backaction by the quantum force fluctuations of this field. Experimentally, we quantify such fluctuations by measuring the cavity-light-induced heating of the intracavity atomic ensemble. These measurements represent the first observation of backaction on a macroscopic mechanical resonator at the standard quantum limit.

quant-ph↗

arXiv2007

Cavity Nonlinear Optics at Low Photon Numbers from Collective Atomic Motion

We report on Kerr nonlinearity and dispersive optical bistability of a Fabry-Perot optical resonator due to the displacement of ultracold atoms trapped within. In the driven resonator, such collective motion is induced by optical forces acting upon up to $10^5$ $^{87}$Rb atoms prepared in the lowest band of a one-dimensional intracavity optical lattice. The longevity of atomic motional coherence allows for strongly nonlinear optics at extremely low cavity photon numbers, as demonstrated by the observation of both branches of optical bistability at photon numbers below unity.

quant-ph↗

arXiv2006

Probing the quantum state of a guided atom laser pulse

We describe bichromatic superradiant pump-probe spectroscopy as a tomographic probe of the Wigner function of a dispersing particle beam. We employed this technique to characterize the quantum state of an ultracold atomic beam, derived from a Rb-87 Bose-Einstein condensate, as it propagated in a 2.5 mm diameter circular waveguide. Our measurements place an upper bound on the longitudinal phase-space area occupied by the 300,000 atom beam of 9(1) $\hbar$ and a lower bound on the coherence length (L > 13 microns). These results are consistent with full quantum degeneracy after multiple orbits around the waveguide.

quant-ph↗

arXiv2005

Bose-Einstein condensation in a mm-scale Ioffe-Pritchard trap

We have constructed a mm-scale Ioffe-Pritchard trap capable of providing axial field curvature of 7800 G/cm$^2$ with only 10.5 Amperes of driving current. Our novel fabrication method involving electromagnetic coils formed of hard anodized aluminum strips is compatible with ultra-high vacuum conditions, as demonstrated by our using the trap to produce Bose-Einstein condensates of 10$^6$ $^87$Rb atoms. The strong axial curvature gives access to a number of experimentally interesting configurations such as tightly confining prolate, nearly isotropic, and oblate spheroidal traps, as well as traps with variable tilt angles with respect to the nominal axial direction.

cond-mat.other↗

arXiv2004

Collimated, single-pass atom source from a pulsed alkali metal dispenser for laser-cooling experiments

We have developed an improved scheme for loading atoms into a magneto-optical trap (MOT) from a directed alkali metal dispenser in < 10^-10 torr ultra-high vacuum conditions. A current-driven dispenser was surrounded with a cold absorbing "shroud" held at < 0 C, pumping rubidium atoms not directed into the MOT. This nearly eliminates background alkali atoms and reduces the detrimental rise in pressure normally associated with these devices. The system can be well-described as a current-controlled, rapidly-switched, two-temperature thermal beam, and was used to load a MOT with 3 x 10^8 atoms.

physics.atom-ph↗