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Superconducting Circuits for Quantum Metrology with Nonclassical Light

Andrew Eddins

Published Jan 1, 2017• 2 citations
OpenAlex

Abstract

The laws of quantum mechanics imply the existence of an intrinsic uncertainty, or noise, in the electromagnetic field. These noise fluctuations are central to many processes in atomic physics and quantum optics, including spontaneous emission of radiation by an atomic system, backaction on an atomic system during non-demolition measurement by probe light, and intrinsic bounds on the noise performance of an ideal amplifier. While quantum noise cannot be eliminated, the noise of one observable quantity may be reduced provided that of the conjugate observable is increased in accord with the relevant Heisenberg uncertainty relation; this process is known as squeezing.Recently, superconducting circuits have emerged as a powerful platform for studying the interaction of squeezed light and matter, leveraging the low-dimensionality of the circuit environment to efficiently couple atomic systems to squeezed radiation. Beyond enabling the verification of canonical predictions of quantum optics, these experiments explore the potential utility of squeezing for the state readout of quantum bits, or qubits, used for quantum information processing.In this thesis, we present three experiments probing the interaction of a superconducting qubit with squeezed radiation. First, we observe how the fluorescence spectra emitted by a two-level atomic system are modified by squeezing of a resonant drive. The subnatural linewidths of the resulting spectra provide the first successful verification in any system of predictions from nearly three decades prior, and provide a tool for characterization of microwave squeezed states. Second, we combine injected squeezed noise with a stroboscopic measurement scheme to demonstrate the first improvement of the signal-to-noise ratio of qubit state readout due to input squeezing. This study includes a characterization of the effect of squeezing on measurement backaction, exhibiting the first use of squeezing to slow measurement-induced dephasing. Finally, we develop a circuit incorporating the qubit inside of a squeezed-microwave source and extensively study the measurement physics of this hybrid system. This device enables the transfer of quantum information from the qubit at $\\sim$30 milliKelvin to a room temperature detector with a marked increase in steady-state efficiency.

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Superconducting quantum circuits provide a versatile and powerful platform for quantum computing as well as for the study of fundamental physics and quantum technologies in general, such as radiation sensors. The development of novel detectors and measurement techniques is important for advancing quantum technologies by enabling detection schemes that are faster, more accurate, or less complex than the current state of the art. In this thesis, we improve upon the existing measurement techniques in superconducting quantum circuits by employing novel methods on a radiation sensor based on superconductor–normal-metal–superconductor (SNS) junctions, in addition to superconducting quantum bits (qubits). The presented methods are experimentally demonstrated in the five publications of this thesis. First, we demonstrate that an SNS sensor, essentially a power or a photon detector, has the capability to directly measure the variance of itinerant microwave photons. Experimentally, we show the measurement of the zero-delay second-order correlation function of interfering thermal and coherent photons displaying bunching with increasing proportion of thermal photons. We combine the signal field with a homodyne field and carry out Wigner function tomography by utilizing techniques from quantum optics. Second, we show a new readout technique for superconducting qubits by multichannel driving which utilizes effectively longitudinal coupling of the qubit and a readout cavity. We find that applying this technique reduces the readout error by 50%, while being roughly 20% faster than the conventional dispersive readout. We finish the thesis by displaying methods for making quantum circuits more compact. We demonstrate a rigorous process for the design of lumped-element resonators without utilizing any computer simulations. The fabricated resonators have a small physical footprint compared to that of coplanar waveguide resonators ubiquitous in superconducting circuits. We achieve an accuracy of 99% in the designed resonance frequency with a standard deviation of 1%, demonstrating the applicability of the model. The final publication demonstrates multiplexed readout of the SNS sensors, reducing the number of required control and measurement lines.

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The laws of quantum mechanics imply the existence of an intrinsic uncertainty, or noise, in the electromagnetic field. These noise fluctuations are central to many processes in atomic physics and quantum optics, including spontaneous emission of radiation by an atomic system, backaction on an atomic system during non-demolition measurement by probe light, and intrinsic bounds on the noise performance of an ideal amplifier. While quantum noise cannot be eliminated, the noise of one observable quantity may be reduced provided that of the conjugate observable is increased in accord with the relevant Heisenberg uncertainty relation; this process is known as squeezing.Recently, superconducting circuits have emerged as a powerful platform for studying the interaction of squeezed light and matter, leveraging the low-dimensionality of the circuit environment to efficiently couple atomic systems to squeezed radiation. Beyond enabling the verification of canonical predictions of quantum optics, these experiments explore the potential utility of squeezing for the state readout of quantum bits, or qubits, used for quantum information processing.In this thesis, we present three experiments probing the interaction of a superconducting qubit with squeezed radiation. First, we observe how the fluorescence spectra emitted by a two-level atomic system are modified by squeezing of a resonant drive. The subnatural linewidths of the resulting spectra provide the first successful verification in any system of predictions from nearly three decades prior, and provide a tool for characterization of microwave squeezed states. Second, we combine injected squeezed noise with a stroboscopic measurement scheme to demonstrate the first improvement of the signal-to-noise ratio of qubit state readout due to input squeezing. This study includes a characterization of the effect of squeezing on measurement backaction, exhibiting the first use of squeezing to slow measurement-induced dephasing. Finally, we develop a circuit incorporating the qubit inside of a squeezed-microwave source and extensively study the measurement physics of this hybrid system. This device enables the transfer of quantum information from the qubit at sim\\simsim30 milliKelvin to a room temperature detector with a marked increase in steady-state efficiency.
Microwave squeezing represents the ultimate sensitivity frontier for superconducting qubit measurement. However, measurement enhancement has remained elusive, in part because integration with standard dispersive readout pollutes the signal channel with antisqueezed noise. Here we induce a stroboscopic light-matter coupling with superior squeezing compatibility, and observe an increase in the final signal-to-noise ratio of 24%. Squeezing the orthogonal phase slows measurement-induced dephasing by a factor of 1.8. This scheme provides a means to the practical application of squeezing for qubit measurement.
Real-time monitoring of a quantum state provides powerful tools for studying the backaction of quantum measurement and performing quantum feedback. Historically, this monitoring capability has been the exclusive province of atomic and optical physics. This thesis describes the implementation of the first such high-fidelity readout scheme in a solid state circuit, a superconducting quantum bit (qubit) coupled to a microwave cavity in the circuit quantum electrodynamics (circuit QED) architecture. The qubit-state-dependent resonance frequency of the cavity is probed with a microwave drive tone, and the resulting signal amplified using a fast, ultralow-noise superconducting parametric amplifier. This arrangement enables the observation of quantum jumps between the qubit states in real time. The ability to monitor the qubit continuously with high fidelity and resolve quantum jumps can be used to investigate the backaction of the measurement process on the qubit. This thesis examines the quantum Zeno effect--where strong measurement inhibits the evolution of a quantum system-- as well as the transition to non-ideal measurement with increasing measurement strength in the circuit QED architecture, a phenomenon shown to be due to the upconversion of low-frequency dephasing noise. These data allow probes of ``universal'' flux noise in previously inaccessible frequency ranges. The work presented here opens the door for quantum feedback and error correction in solid-state quantum systems using continuous weak measurement.
Fault-tolerant quantum information processing with flawed qubits and gates requires highly efficient, quantum nondemolition qubit readout. In superconducting circuits, qubit readout using coherent light with fidelity above 99% has been achieved by using quantum limited parametric amplifiers such as the Josephson parametric converter (JPC). However, further improvement of such measurement faces many challenges, including the vacuum fluctuation of the coherent light that limits measurement fidelity. In this work we demonstrate dispersive qubit readout with a two-mode squeezed light interferometer formed by two JPCs. The first JPC generates two-mode squeezed vacuum at its output, which is coherently recombined by the second JPC after one branch is phase shifted and displaced by its interaction with the qubit-cavity system on that arm of the interferometer. We observe a 31% improvement in the power signal-to-noise ratio (SNR) of projective readout after replacing vacuum fluctuation in the readout cavity with two-mode squeezed vacuum containing on average one photon. This demonstrates a more efficient way of improving SNR than by increasing readout power. Furthermore, we investigate the quantum properties of the two-mode squeezed light in the system through weak measurement. Surprisingly, we find that tuning the interferometer to be less projective increases the measurement efficiency relative to the optimum setting for projective measurement. These enhancements may enable remote entanglement with lower efficiency components in a system with qubits in both arms of the interferometer.
Nonclassical states of light are of increasing interest due to their applications in the emerging field of quantum information processing and communication. Squeezed light is such a state of the electromagnetic field in which the quantum noise properties are altered compared with those of coherent light. Squeezed light and squeezed vacuum states are potentially useful for quantum information protocols as well as optical measurements, where sensitivities can be limited by quantum noise. We experimentally study a source of squeezed vacuum resulting from the interaction of near-resonant light with both cold and hot Rb atoms via the nonlinear polarization self-rotation effect (PSR). We investigate the optimal conditions for noise reduction in the resulting squeezed states, reaching quadrature squeezing levels of up to 2.6 dB below shot noise, as well as observing noise reduction for a broad range of detection frequencies, from tens of kHz to several MHz. We use this source of squeezed vacuum at 795 nm to further study the noise properties of these states and how they are affected by resonant atomic interactions. This includes the use of a squeezed light probe to give a quantum enhancement to an optical magnetometer, as well as studying the propagation of squeezed vacuum in an atomic medium under conditions of electromagnetically induced transparency (EIT). We also investigate the propagation of pulses of quantum squeezed light through a dispersive atomic medium, where we examine the possibilities for quantum noise signals traveling at subluminal and superluminal velocities. The interaction of squeezed light with resonant atomic vapors finds various potential applications in both quantum measurements and continuous variable quantum memories.
Microscopic systems exhibit many intriguing quantum mechanical phenomena which cannot be explained by classical physics. Quantum mechanics has succesfully explained the behaviour of subatomic particles, atoms, and electromagnetic waves where entanglement, wave-particle duality and superposition of particles has been observed. In macroscopic scales, quantum mechanics explains the microscopic origin of electronic properties of solids, and quantum coherence effects are routinely observed in superfluids and superconductors. However, most macroscopic objects behave according to the laws of classical physics due to decoherence effects caused by interaction with the environment. Mechanical systems are ideal for studying quantum behavior of macroscopic objects as they can be measured with high accuracy, and their interaction with the environment can be controlled. Cavity optomechanics studies the interaction between light and mechanical objects through radiation pressure force. Recent developments in the field have made it possible to study quantum behaviour in macroscopic mechanical objects. For example, mechanical oscillators coupled to either microwave or optical light have been cooled to their quantum ground state. This thesis studies a cavity optomechanical system in which micromechanical aluminium membrane resonator is coupled to a superconducting microwave circuit resonator. This work focuses on the phenomena relating to the Heisenberg's uncertainty principle which sets a fundamental limit to the precision in which certain pairs of properties of a system can be measured. Due to uncertainty principle, even if a harmonic oscillator is cooled to the ground state, it still has random motion called the zero-point motion. This sets a fundamental limit on how accurately one can measure the motion of the resonator. In a squeezed quantum state, fluctuations of a quantity can be reduced below this quantum limit, at the cost of increased fluctuations in the pair variable. In this work, motion of a nearly macroscopic object is measured in a squeezed state. Due to uncertainty principle, a measurement amplifying an oscillatory signal always adds noise. However, when information in one part of the signal is lost, one can amplify and measure the other part perfectly. This kind of nearly noiseless phase-sensitive measurement, and squeezing of microwave fields is demonstrated in this work. An optomechanical device that simultaneously performs high-gain amplification and frequency conversion is created. Collective dynamics of a multimode optomechanical system is studied. When measuring the oscillator position continuously, the system is perturbed by the measuring. This can be avoided in quantum back-action evading measurement, which is demonstrated for collective motion of two mechanical resonators. Finally, an entanglement of centre-of-mass motion of two nearly macroscopic objects is created and stabilised.
Superconducting quantum circuits provide a versatile and powerful platform for quantum computing as well as for the study of fundamental physics and quantum technologies in general, such as radiation sensors. The development of novel detectors and measurement techniques is important for advancing quantum technologies by enabling detection schemes that are faster, more accurate, or less complex than the current state of the art. In this thesis, we improve upon the existing measurement techniques in superconducting quantum circuits by employing novel methods on a radiation sensor based on superconductor–normal-metal–superconductor (SNS) junctions, in addition to superconducting quantum bits (qubits). The presented methods are experimentally demonstrated in the five publications of this thesis. First, we demonstrate that an SNS sensor, essentially a power or a photon detector, has the capability to directly measure the variance of itinerant microwave photons. Experimentally, we show the measurement of the zero-delay second-order correlation function of interfering thermal and coherent photons displaying bunching with increasing proportion of thermal photons. We combine the signal field with a homodyne field and carry out Wigner function tomography by utilizing techniques from quantum optics. Second, we show a new readout technique for superconducting qubits by multichannel driving which utilizes effectively longitudinal coupling of the qubit and a readout cavity. We find that applying this technique reduces the readout error by 50%, while being roughly 20% faster than the conventional dispersive readout. We finish the thesis by displaying methods for making quantum circuits more compact. We demonstrate a rigorous process for the design of lumped-element resonators without utilizing any computer simulations. The fabricated resonators have a small physical footprint compared to that of coplanar waveguide resonators ubiquitous in superconducting circuits. We achieve an accuracy of 99% in the designed resonance frequency with a standard deviation of 1%, demonstrating the applicability of the model. The final publication demonstrates multiplexed readout of the SNS sensors, reducing the number of required control and measurement lines.