arXiv subjects

K. W. Lehnert

Publications and source records attributed to K. W. Lehnert.

At least 37 records · Page 2Linked to original sources

arXiv2018

Results from phase 1 of the HAYSTAC microwave cavity axion experiment

We report on the results from a search for dark matter axions with the HAYSTAC experiment using a microwave cavity detector at frequencies between 5.6-5.8$\, \rm Ghz$. We exclude axion models with two photon coupling $g_{aγγ}\,\gtrsim\,2\times10^{-14}\,\rm GeV^{-1}$, a factor of 2.7 above the benchmark KSVZ model over the mass range 23.15$\,<\,$$m_a \,$<$\,$24.0$\,μ\rm eV$. This doubles the range reported in our previous paper. We achieve a near-quantum-limited sensitivity by operating at a temperature $T<hν/2k_B$ and incorporating a Josephson parametric amplifier (JPA), with improvements in the cooling of the cavity further reducing the experiment's system noise temperature to only twice the Standard Quantum Limit at its operational frequency, an order of magnitude better than any other dark matter microwave cavity experiment to date. This result concludes the first phase of the HAYSTAC program utilizing a conventional copper cavity and a single JPA.

hep-ex↗

arXiv2017

The HAYSTAC Axion Search Analysis Procedure

We describe in detail the analysis procedure used to derive the first limits from the Haloscope at Yale Sensitive to Axion CDM (HAYSTAC), a microwave cavity search for cold dark matter (CDM) axions with masses above $20\ μ\text{eV}$. We have introduced several significant innovations to the axion search analysis pioneered by the Axion Dark Matter eXperiment (ADMX), including optimal filtering of the individual power spectra that constitute the axion search dataset and a consistent maximum likelihood procedure for combining and rebinning these spectra. These innovations enable us to obtain the axion-photon coupling $|g_γ|$ excluded at any desired confidence level directly from the statistics of the combined data.

astro-ph.IM↗

arXiv2017

Cavity quantum acoustic device in the multimode strong coupling regime

We investigate an acoustical analog of circuit quantum electrodynamics that facilitates compact high-Q (${>}20,000$) microwave-frequency cavities with dense spectra. We fabricate and characterize a device that comprises a flux tunable transmon coupled to a $300\,\mathrm{μm}$ long surface acoustic wave resonator. For some modes, the qubit-cavity coupling reaches $6.5\,\mathrm{MHz}$, exceeding the cavity loss rate ($200\,\mathrm{kHz}$), qubit linewidth ($1.1\,\mathrm{MHz}$), and the cavity free spectral range ($4.8\,\mathrm{MHz}$), placing the device in both the strong coupling and strong multimode regimes. With the qubit detuned from the cavity, we show that the dispersive shift behaves according to predictions from a generalized Jaynes-Cummings Hamiltonian. Finally, we observe that the qubit linewidth strongly depends on its frequency, as expected for spontaneous emission of phonons, and we identify operating frequencies where this emission rate is suppressed.

quant-ph↗

arXiv2017

Optimal operation of a Josephson parametric amplifier for vacuum squeezing

A Josephson parametric amplifier (JPA) can create squeezed states of microwave light, lowering the noise associated with certain quantum measurements. We experimentally study how the JPA's pump influences the phase-sensitive amplification and deamplification of a coherent tone's amplitude when that amplitude is commensurate with vacuum fluctuations. We predict and demonstrate that by operating the JPA with a pump power greater than the value that maximizes gain, the amplifier distortion is reduced and consequently squeezing is improved. Optimizing the JPA's operation in this fashion, we directly observe $3.87 \pm 0.03$ dB of vacuum squeezing.

quant-ph↗

arXiv2017

Breaking Lorentz reciprocity with frequency conversion and delay

We introduce a method for breaking Lorentz reciprocity based upon the non-commutation of frequency conversion and delay. The method requires no magnetic materials or resonant physics, allowing for the design of scalable and broadband non-reciprocal circuits. With this approach, two types of gyrators --- universal building blocks for linear, non-reciprocal circuits --- are constructed. Using one of these gyrators, we create a circulator with > 15 dB of isolation across the 5 -- 9 GHz band. Our designs may be readily extended to any platform with suitable frequency conversion elements, including semiconducting devices for telecommunication or an on-chip superconducting implementation for quantum information processing.

physics.class-ph↗

arXiv2017

Widely tunable on-chip microwave circulator for superconducting quantum circuits

We report on the design and performance of an on-chip microwave circulator with a widely (GHz) tunable operation frequency. Non-reciprocity is created with a combination of frequency conversion and delay, and requires neither permanent magnets nor microwave bias tones, allowing on-chip integration with other superconducting circuits without the need for high-bandwidth control lines. Isolation in the device exceeds 20 dB over a bandwidth of tens of MHz, and its insertion loss is small, reaching as low as 0.9 dB at select operation frequencies. Furthermore, the device is linear with respect to input power for signal powers up to hundreds of fW ($\approx 10^3$ circulating photons), and the direction of circulation can be dynamically reconfigured. We demonstrate its operation at a selection of frequencies between 4 and 6 GHz.

quant-ph↗

arXiv2017

Single-sideband modulator for frequency domain multiplexing of superconducting qubit readout

We introduce and experimentally characterize a superconducting single-sideband modulator compatible with cryogenic microwave circuits, and propose its use for frequency domain multiplexing of superconducting qubit readout. The monolithic single-quadrature modulators that comprise the device are formed with purely reactive elements (capacitors and Josephson junction inductors) and require no microwave-frequency control tones. Microwave signals in the 4 to 8 GHz band, with power up to -85 dBm, are converted up or down in frequency by as much as 120 MHz. Spurious harmonics in the device can be suppressed by up to 25 dB for select probe and modulation frequencies.

quant-ph↗

arXiv2017

Faithful conversion of propagating quantum information to mechanical motion

We convert propagating qubits encoded as superpositions of zero and one photons to the motion of a micrometer-sized mechanical resonator. Using quantum state tomography, we determine the density matrix of both the propagating photons and the mechanical resonator. By comparing a sufficient set of states before and after conversion, we determine the average process fidelity to be $F_{\textrm{avg}} = 0.83\substack{+0.03-0.06}$ which exceeds the classical bound for the conversion of an arbitrary qubit state. This conversion ability is necessary for using mechanical resonators in emerging quantum communication and modular quantum computation architectures.

quant-ph↗

arXiv2017

Improving broadband displacement detection with quantum correlations

Interferometers enable ultrasensitive measurement in a wide array of applications from gravitational wave searches to force microscopes. The role of quantum mechanics in the metrological limits of interferometers has a rich history, and a large number of techniques to surpass conventional limits have been proposed. In a typical measurement configuration, the tradeoff between the probe's shot noise (imprecision) and its quantum backaction results in what is known as the standard quantum limit (SQL). In this work we investigate how quantum correlations accessed by modifying the readout of the interferometer can access physics beyond the SQL and improve displacement sensitivity. Specifically, we use an optical cavity to probe the motion of a silicon nitride membrane off mechanical resonance, as one would do in a broadband displacement or force measurement, and observe sensitivity better than the SQL dictates for our quantum efficiency. Our measurement illustrates the core idea behind a technique known as \textit{variational readout}, in which the optical readout quadrature is changed as a function of frequency to improve broadband displacement detection. And more generally our result is a salient example of how correlations can aid sensing in the presence of backaction.

quant-ph↗

arXiv2017

Design and Operational Experience of a Microwave Cavity Axion Detector for the 20-100 micro-eV Range

We describe a dark matter axion detector designed, constructed, and operated both as an innovation platform for new cavity and amplifier technologies and as a data pathfinder in the $5 - 25$ GHz range ($\sim20-100\: μ$eV). The platform is small but flexible to facilitate the development of new microwave cavity and amplifier concepts in an operational environment. The experiment has recently completed its first data production; it is the first microwave cavity axion search to deploy a Josephson parametric amplifier and a dilution refrigerator to achieve near-quantum limited performance.

physics.ins-det↗

arXiv2017

First results from a microwave cavity axion search at 24 micro-eV

We report on the first results from a new microwave cavity search for dark matter axions with masses above $20~μ\text{eV}$. We exclude axion models with two-photon coupling $g_{aγγ} \gtrsim 2\times10^{-14}~\text{GeV}^{-1}$ over the range $23.55~μ\text{eV} < m_a < 24.0~μ\text{eV}$. These results represent two important achievements. First, we have reached cosmologically relevant sensitivity an order of magnitude higher in mass than any existing limits. Second, by incorporating a dilution refrigerator and Josephson parametric amplifier, we have demonstrated total noise approaching the standard quantum limit for the first time in an axion search.

astro-ph.CO↗

arXiv2016

Accelerating dark-matter axion searches with quantum measurement technology

The axion particle, a consequence of an elegant hypothesis that resolves the strong-CP problem of quantum chromodynamics, is a plausible origin for cosmological dark matter. In searches for axionic dark matter that detect the conversion of axions to microwave photons, the quantum noise associated with microwave vacuum fluctuations will soon limit the rate at which parameter space is searched. Here we show that this noise can be partially overcome either by squeezing the quantum vacuum using recently developed Josephson parametric devices, or by using superconducting qubits to count microwave photons. The recently demonstrated ability of superconducting qubits to make QND measurements of microwave photons offers great advantages over destructive photon counting methods such as those using Rydberg atoms.

hep-ph↗

arXiv2016

General purpose multiplexing device for cryogenic microwave systems

We introduce and experimentally characterize a general purpose device for signal processing in circuit quantum electrodynamics systems. The device is a broadband two-port microwave circuit element with three modes of operation: it can transmit, reflect, or invert incident signals between 4 and 8 GHz. This property makes it a versatile tool for lossless signal processing at cryogenic temperatures. In particular, rapid switching (less than or equal to 15 ns) between these operation modes enables several multiplexing readout protocols for superconducting qubits. We report the device's performance in a two-channel code domain multiplexing demonstration. The multiplexed data are recovered with fast readout times (up to 400 ns) and infidelities less than 0.01 for probe powers greater than 7 fW, in agreement with the expectation for binary signaling with Gaussian noise.

quant-ph↗

arXiv2015

Laser cooling of a micromechanical membrane to the quantum backaction limit

The radiation pressure of light can act to damp and cool the vibrational motion of a mechanical resonator. In understanding the quantum limits of this cooling, one must consider the effect of shot noise fluctuations on the final thermal occupation. In optomechanical sideband cooling in a cavity, the finite Stokes Raman scattering defined by the cavity linewidth combined with shot noise fluctuations dictates a quantum backaction limit, analogous to the Doppler limit of atomic laser cooling. In our work we sideband cool to the quantum backaction limit by using a micromechanical membrane precooled in a dilution refrigerator. Monitoring the optical sidebands allows us to directly observe the mechanical object come to thermal equilibrium with the optical bath.

quant-ph↗

arXiv2015

Generation and efficient measurement of single photons from fixed frequency superconducting qubits

We demonstrate and evaluate an on-demand source of single itinerant microwave photons. Photons are generated using a highly coherent, fixed-frequency qubit-cavity system, and a protocol where the microwave control field is far detuned from the photon emission frequency. By using a Josephson parametric amplifier (JPA), we perform efficient single-quadrature detection of the state emerging from the cavity. We characterize the imperfections of the photon generation and detection, including detection inefficiency and state infidelity caused by measurement backaction over a range of JPA gains from 17 to 33 dB. We observe that both detection efficiency and undesirable backaction increase with JPA gain. We find that the density matrix has its maximum single photon component $ρ_{11} = 0.36 \pm 0.01$ at 29 dB JPA gain. At this gain, backaction of the JPA creates cavity photon number fluctuations that we model as a thermal distribution with an average photon number $\bar{n} = 0.041 \pm 0.003$.

quant-ph↗

arXiv2015

Quantum-enabled temporal and spectral mode conversion of microwave signals

Electromagnetic waves are ideal candidates for transmitting information in a quantum network as they can be routed rapidly and efficiently between locations using optical fibers or microwave cables. Yet linking quantum-enabled devices with cables has proved difficult because most cavity or circuit quantum electrodynamics (cQED) systems used in quantum information processing can only absorb and emit signals with a specific frequency and temporal envelope. Here we show that the temporal and spectral content of microwave-frequency electromagnetic signals can be arbitrarily manipulated with a flexible aluminum drumhead embedded in a microwave circuit. The aluminum drumhead simultaneously forms a mechanical oscillator and a tunable capacitor. This device offers a way to build quantum microwave networks using separate and otherwise mismatched components. Furthermore, it will enable the preparation of non-classical states of motion by capturing non-classical microwave signals prepared by the most coherent circuit QED systems.

quant-ph↗

arXiv2015

On-chip superconducting microwave circulator from synthetic rotation

We analyze the design of a potential replacement technology for the commercial ferrite circulators that are ubiquitous in contemporary quantum superconducting microwave experiments. The lossless, lumped element design is capable of being integrated on chip with other superconducting microwave devices, thus circumventing the many performance-limiting aspects of ferrite circulators. The design is based on the dynamic modulation of DC superconducting microwave quantum interference devices (SQUIDs) that function as nearly linear, tunable inductors. The connection to familiar ferrite-based circulators is a simple frame boost in the internal dynamics' equation of motion. In addition to the general, schematic analysis, we also give an overview of many considerations necessary to achieve a practical design with a tunable center frequency in the 4-8 GHz frequency band, a bandwidth of 240 MHz, reflections at the -20 dB level, and a maximum signal power of approximately order 100 microwave photons per inverse bandwidth.

quant-ph↗

arXiv2015

Generating and verifying entangled itinerant microwave fields with efficient and independent measurements

By combining a squeezed propagating microwave field and an unsqueezed vacuum field on a hybrid (microwave beam-splitter), we generate entanglement between the two output modes. We verify that we have generated entangled states by making independent and efficient single-quadrature measurements of the two output modes. We observe the entanglement witness $E_\mathrm{W}=-0.263^{+0.001}_{-0.036}$ and the negativity $N=0.0824^{+0.01}_{-0.0004}$ with measurement efficiencies at least $26\pm{0.1}\%$ and $41\pm{0.2}\%$ for channel~1 and 2 respectively. These measurements show that the output two-mode state violates the separability criterion and therefore demonstrate entanglement. This shared entanglement between propagating microwaves provides an important resource for building quantum networks with superconducting microwave systems.

quant-ph↗