arXiv subjects

I. Siddiqi

Publications and source records attributed to I. Siddiqi.

At least 37 records · Page 2Linked to original sources

arXiv2019

Extended Search for the Invisible Axion with the Axion Dark Matter Experiment

This paper reports on a cavity haloscope search for dark matter axions in the galactic halo in the mass range $2.81$-$3.31$ $μeV$. This search excludes the full range of axion-photon coupling values predicted in benchmark models of the invisible axion that solve the strong CP problem of quantum chromodynamics, and marks the first time a haloscope search has been able to search for axions at mode crossings using an alternate cavity configuration. Unprecedented sensitivity in this higher mass range is achieved by deploying an ultra low-noise Josephson parametric amplifier as the first stage signal amplifier.

hep-ex↗

arXiv2019

Fast gate-based readout of silicon quantum dots using Josephson parametric amplification

Spins in silicon quantum devices are promising candidates for large-scale quantum computing. Gate-based sensing of spin qubits offers compact and scalable readout with high fidelity, however further improvements in sensitivity are required to meet the fidelity thresholds and measurement timescales needed for the implementation of fast-feedback in error correction protocols. Here, we combine radio-frequency gate-based sensing at 622 MHz with a Josephson parametric amplifier (JPA), that operates in the 500-800 MHz band, to reduce the integration time required to read the state of a silicon double quantum dot formed in a nanowire transistor. Based on our achieved signal-to-noise ratio (SNR), we estimate that singlet-triplet single-shot readout with an average fidelity of 99.7% could be performed in 1 $μ$s, well-below the requirements for fault-tolerant readout and 30 times faster than without the JPA. Additionally, the JPA allows operation at a lower RF power while maintaining identical SNR. We determine a noise temperature of 200 mK with a contribution from the JPA (25%), cryogenic amplifier (25%) and the resonator (50%), showing routes to further increase the read-out speed.

cond-mat.mes-hall↗

arXiv2018

High-efficiency measurement of an artificial atom embedded in a parametric amplifier

A crucial limit to measurement efficiencies of superconducting circuits comes from losses involved when coupling to an external quantum amplifier. Here, we realize a device circumventing this problem by directly embedding a two-level artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson parametric amplifier. Surprisingly, this configuration is able to enhance dispersive measurement without exposing the qubit to appreciable excess backaction. This is accomplished by engineering the circuit to permit high-power operation that reduces information loss to unmonitored channels associated with the amplification and squeezing of quantum noise. By mitigating the effects of off-chip losses downstream, the on-chip gain of this device produces end-to-end measurement efficiencies of up to 80 percent. Our theoretical model accurately describes the observed interplay of gain and measurement backaction, and delineates the parameter space for future improvement. The device is compatible with standard fabrication and measurement techniques, and thus provides a route for definitive investigations of fundamental quantum effects and quantum control protocols.

quant-ph↗

arXiv2016

Resonance fluorescence from an artificial atom in squeezed vacuum

We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.

quant-ph↗

arXiv2015

Quantum Zeno effect in the strong measurement regime of circuit quantum electrodynamics

We observe the quantum Zeno effect -- where the act of measurement slows the rate of quantum state transitions -- in a superconducting qubit using linear circuit quantum electrodynamics readout and a near-quantum-limited following amplifier. Under simultaneous strong measurement and qubit drive, the qubit undergoes a series of quantum jumps between states. These jumps are visible in the experimental measurement record and are analyzed using maximum likelihood estimation to determine qubit transition rates. The observed rates agree with both analytical predictions and numerical simulations. The analysis methods are suitable for processing general noisy random telegraph signals.

quant-ph↗

arXiv2015

Stabilizing entanglement via symmetry-selective bath engineering in superconducting qubits

Bath engineering, which utilizes coupling to lossy modes in a quantum system to generate non-trivial steady states, is a tantalizing alternative to gate- and measurement-based quantum science. Here, we demonstrate dissipative stabilization of entanglement between two superconducting transmon qubits in a symmetry-selective manner. We utilize the engineered symmetries of the dissipative environment to stabilize a target Bell state; we further demonstrate suppression of the Bell state of opposite symmetry due to parity selection rules. This implementation is resource-efficient, achieves a steady-state fidelity $\mathcal{F} = 0.70$, and is scalable to multiple qubits.

quant-ph↗

arXiv2015

Weak Measurement and Feedback in Superconducting Quantum Circuits

We describe the implementation of weak quantum measurements in superconducting qubits, focusing specifically on transmon type devices in the circuit quantum electrodynamics architecture. To access this regime, the readout cavity is probed with on average a single microwave photon. Such low-level signals are detected using near quantum-noise-limited superconducting parametric amplifiers. Weak measurements yield partial information about the quantum state, and correspondingly do not completely project the qubit into an eigenstate. As such, we use the measurement record to either sequentially reconstruct the quantum state at a given time, yielding a quantum trajectory, or to close a direct quantum feedback loop, stabilizing Rabi oscillations indefinitely.

quant-ph↗

arXiv2015

Quantum trajectories of superconducting qubits

In this review, we discuss recent experiments that investigate how the quantum sate of a superconducting qubit evolves during measurement. We provide a pedagogical overview of the measurement process, when the qubit is dispersively coupled to a microwave frequency cavity, and the qubit state is encoded in the phase of a microwave tone that probes the cavity. A continuous measurement record is used to reconstruct the individual quantum trajectories of the qubit state, and quantum state tomography is performed to verify that the state has been tracked accurately. Furthermore, we discuss ensembles of trajectories, time-symmetric evolution, two-qubit trajectories, and potential applications in measurement-based quantum error correction.

quant-ph↗

arXiv2014

Large gain quantum-limited qubit measurement using a two-mode nonlinear cavity

We provide a thorough theoretical analysis of qubit state measurement in a setup where a driven, parametrically-coupled cavity system is directly coupled to the qubit, with one of the cavities having a weak Kerr nonlinearity. Such a system could be readily realized using circuit QED architectures. We demonstrate that this setup is capable in the standard linear-response regime of both producing a highly amplified output signal while at the same time achieving near quantum-limited performance: the measurement backaction on the qubit is near the minimal amount required by the uncertainty principle. This setup thus represents a promising route for performing efficient large-gain qubit measurement that is completely on-chip, and that does not rely on the use of circulators or complex non-reciprocal amplifiers.

cond-mat.mes-hall↗

arXiv2014

Prediction and retrodiction for a continuously monitored superconducting qubit

The quantum state of a superconducting transmon qubit inside a three-dimensional cavity is monitored by reflection of a microwave field on the cavity. The information inferred from the measurement record is incorporated in a density matrix $ρ_t$, which is conditioned on probe results until $t$, and in an auxiliary matrix $E_t$, which is conditioned on probe results obtained after $t$. Here, we obtain these matrices from experimental data and we illustrate their application to predict and retrodict the outcome of weak and strong qubit measurements.

quant-ph↗

arXiv2014

Mapping the optimal route between two quantum states

A central feature of quantum mechanics is that a measurement is intrinsically probabilistic. As a result, continuously monitoring a quantum system will randomly perturb its natural unitary evolution. The ability to control a quantum system in the presence of these fluctuations is of increasing importance in quantum information processing and finds application in fields ranging from nuclear magnetic resonance to chemical synthesis. A detailed understanding of this stochastic evolution is essential for the development of optimized control methods. Here we reconstruct the individual quantum trajectories of a superconducting circuit that evolves in competition between continuous weak measurement and driven unitary evolution. By tracking individual trajectories that evolve between an arbitrary choice of initial and final states we can deduce the most probable path through quantum state space. These pre- and post-selected quantum trajectories also reveal the optimal detector signal in the form of a smooth time-continuous function that connects the desired boundary conditions. Our investigation reveals the rich interplay between measurement dynamics, typically associated with wave function collapse, and unitary evolution of the quantum state as described by the Schrodinger equation. These results and the underlying theory, based on a principle of least action, reveal the optimal route from initial to final states, and may enable new quantum control methods for state steering and information processing.

quant-ph↗

arXiv2013

Single Quasiparticle Trapping in Aluminum Nanobridge Josephson Junctions

We present microwave measurements of a high quality factor superconducting resonator incorporating two aluminum nanobridge Josephson junctions in a loop shunted by an on-chip capacitor. Trapped quasiparticles (QPs) shift the resonant frequency, allowing us to probe the trapped QP number and energy distribution and to quantify their lifetimes. We find that the trapped QP population obeys a Gibbs distribution above 75 mK, with non-Poissonian trapping statistics. Our results are in quantitative agreement with the Andreev bound state model of transport, and demonstrate a practical means to quantify on-chip QP populations and validate mitigation strategies in a cryogenic environment.

cond-mat.supr-con↗

arXiv2013

Observing single quantum trajectories of a superconducting qubit

The length of time that a quantum system can exist in a superposition state is determined by how strongly it interacts with its environment. This interaction entangles the quantum state with the inherent fluctuations of the environment. If these fluctuations are not measured, the environment can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture-a process known as decoherence. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a quantum trajectory conditioned on the measurement outcome. We employ weak measurements to monitor a microwave cavity embedding a superconducting qubit and track the individual quantum trajectories of the system. In this architecture, the environment is dominated by the fluctuations of a single electromagnetic mode of the cavity. Using a near-quantum-limited parametric amplifier, we selectively measure either the phase or amplitude of the cavity field, and thereby confine trajectories to either the equator or a meridian of the Bloch sphere. We perform quantum state tomography at discrete times along the trajectory to verify that we have faithfully tracked the state of the quantum system as it diffuses on the surface of the Bloch sphere. Our results demonstrate that decoherence can be mitigated by environmental monitoring and validate the foundations of quantum feedback approaches based on Bayesian statistics. Moreover, our experiments suggest a new route for implementing what Schrodinger termed "quantum steering"-harnessing action at a distance to manipulate quantum states via measurement.

quant-ph↗

arXiv2013

Suppression of the radiative decay of atomic coherence in squeezed vacuum

Quantum fluctuations of the electromagnetic vacuum are responsible for physical effects such as the Casimir force and the radiative decay of atoms, and set fundamental limits on the sensitivity of measurements. Entanglement between photons can produce correlations that result in a reduction of these fluctuations below the vacuum level allowing measurements that surpass the standard quantum limit in sensitivity. Here we demonstrate that the radiative decay rate of an atom that is coupled to quadrature squeezed electromagnetic vacuum can be reduced below its natural linewidth. We observe a two-fold reduction of the transverse radiative decay rate of a superconducting artificial atom coupled to continuum squeezed vacuum generated by a Josephson parametric amplifier, allowing the transverse coherence time T_2 to exceed the vacuum decay limit of 2T_1. We demonstrate that the measured radiative decay dynamics can be used to tomographically reconstruct the Wigner distribution of the the itinerant squeezed state. Our results are the first confirmation of a canonical prediction of quantum optics and open the door to new studies of the quantum light-matter interaction.

quant-ph↗

arXiv2013

In-Plane Magnetic Field Tolerance of a Dispersive Aluminum Nanobridge SQUID Magnetometer

We characterize the flux sensitivity of a dispersive 3D aluminum nanobridge SQUID magnetometer as a function of applied in-plane magnetic field. In zero field, we observe an effective flux noise of 17 n$Φ_0$/Hz$^{1/2}$ with 25 MHz of bandwidth. Flux noise increased by less than a factor of three with parallel magnetic fields up to 61 mT. Operation in higher fields may be possible by decreasing the dimensions of the shunt capacitor in the magnetometer circuit. These devices are thus well suited for observing high-speed dynamics in nanoscale magnets, even in the presence of moderate bias magnetic fields.

cond-mat.mes-hall↗

arXiv2013

A dispersive nanoSQUID magnetometer for ultra-low noise, high bandwidth flux detection

We describe a dispersive nanoSQUID magnetometer comprised of two variable thickness aluminum weak-link Josephson junctions shunted in parallel with an on-chip capacitor. This arrangement forms a nonlinear oscillator with a tunable 4-8 GHz resonant frequency with a quality factor Q = 30 when coupled directly to a 50 $Ω$ transmission line. In the presence of a near-resonant microwave carrier signal, a low frequency flux input generates sidebands that are readily detected using microwave reflectometry. If the carrier excitation is sufficiently strong then the magnetometer also exhibits parametric gain, resulting in a minimum effective flux noise of 30 n$Φ_0$/Hz$^{1/2}$ with 20 MHz of instantaneous bandwidth. If the magnetometer is followed with a near quantum-noise-limited Josephson parametric amplifier, we can increase the bandwidth to 60 MHz without compromising sensitivity. This combination of high sensitivity and wide bandwidth with no on-chip dissipation makes this device ideal for local sensing of spin dynamics, both classical and quantum.

cond-mat.mes-hall↗

arXiv2012

Cavity-assisted quantum bath engineering

We demonstrate quantum bath engineering for a superconducting artificial atom coupled to a microwave cavity. By tailoring the spectrum of microwave photon shot noise in the cavity, we create a dissipative environment that autonomously relaxes the atom to an arbitrarily specified coherent superposition of the ground and excited states. In the presence of background thermal excitations, this mechanism increases the state purity and effectively cools the dressed atom state to a low temperature.

quant-ph↗

arXiv2012

Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise

We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f ~ 1 GHz and find agreement with values extrapolated from a $1/f^α$ fit to the measured flux noise spectral density below 1 Hz.

cond-mat.mes-hall↗