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Bulletin of the American Physical Society

Remote entanglement of transmon qubits

Michael Hatridge, Katrina Sliwa, A. Narla, Shyam Shankar, Zaki Leghtas, Mazyar Mirrahimi, S. M. Girvin, Robert J. Schoelkopf, Michel Devoret

Published Mar 3, 2014• 0 citations
OpenAlex

Abstract

An open challenge in quantum information processing with superconducting circuits is to entangle arbitrary pairs of qubits, not simply nearest neighbors. This can be accomplished by first entangling the qubits with flying microwave oscillators in the form of traveling pulses and then performing joint manipulation and measurement on a pair of such flying oscillators at a distant site. This method has the advantage of avoiding the overhead required to move information over large distances via only nearest neighbor coupling while also possessing an extremely high on/off ratio of coupling. Remarkably, such a process is naturally embedded in the act of phase-preserving amplification, which transforms two input modes, termed signal and idler, into a two-mode squeezed output state. For an ideal system, this operation generates heralded, perfectly entangled states between remote qubits with a fifty percent success rate. For an inefficient system, the partial loss of information from the flying states degrades the purity of the entanglement. Data will be shown on such a protocol involving two transmon qubits embedded in superconducting cavities connected to the signal and idler inputs of a Josephson Parametric Converter (JPC) operated as a nearly-quantum limited phase-preserving amplifier. I will also discuss new devices which will allow us to improve the process and achieve high success rates with imperfect hardware. Light refreshments will be served.

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An open challenge in quantum information processing with superconducting circuits is to entangle arbitrary pairs of qubits, not simply nearest neighbors. This can be accomplished by first entangling the qubits with flying microwave oscillators in the form of traveling pulses and then performing joint manipulation and measurement on a pair of such flying oscillators at a distant site. This method has the advantage of avoiding the overhead required to move information over large distances via only nearest neighbor coupling while also possessing an extremely high on/off ratio of coupling. Remarkably, such a process is naturally embedded in the act of phase-preserving amplification, which transforms two input modes, termed signal and idler, into a two-mode squeezed output state. For an ideal system, this operation generates heralded, perfectly entangled states between remote qubits with a fifty percent success rate. For an inefficient system, the partial loss of information from the flying states degrades the purity of the entanglement. Data will be shown on such a protocol involving two transmon qubits embedded in superconducting cavities connected to the signal and idler inputs of a Josephson Parametric Converter (JPC) operated as a nearly-quantum limited phase-preserving amplifier. I will also discuss new devices which will allow us to improve the process and achieve high success rates with imperfect hardware. Light refreshments will be served.
Large-scale quantum information processing networks will most probably require the entanglement of distant systems that do not interact directly. This can be done by performing entangling gates between standing information carriers, used as memories or local computational resources, and flying ones, acting as quantum buses. We report the deterministic entanglement of two remote transmon qubits by Raman stimulated emission and absorption of a traveling photon wave packet. We achieve a Bell state fidelity of 73%, well explained by losses in the transmission line and decoherence of each qubit.
The distribution of entanglement across distant qubits is a central challenge for the operation of scalable quantum computers and large-scale quantum networks. Existing approaches rely on deterministic state transfer, or probabilistic protocols that require active control or measurements and postselection. Here, we demonstrate a fundamentally different, fully autonomous process, where two remote qubits are entangled through their coupling to a quantum-correlated photonic reservoir. In our experiment, a Josephson parametric converter produces a Gaussian, continuous-variable entangled state of propagating microwave fields that drives two spatially separated superconducting transmon qubits into a stationary, discrete-variable entangled state. We also show how qubit tomography unlocks a direct and sensitive verification of two-mode squeezing in the microwave domain. These results establish networks of qubits interfaced with distributed continuous-variable entangled states as a powerful platform for foundational studies and quantum-technology applications.
Feedback control in classical systems is an indispensable, ubiquitous tool. The theoretical basis for achieving optimal classical control is well understood, and crucially relies on a very classical assumption: that measurements of the state of a system under control need not perturb that state. In a quantum context this assumption is fundamentally invalid. Although many aspects of the theory of quantum feedback control are relatively well developed, the technological basis for feedback control of a single quantum system has only very recently matured. We demonstrate the experimental realization of a quantum feedback control protocol, perpetually stabilizing the coherent Rabi oscillations of a superconducting qubit. This is the first utilization of quantum feedback control for stabilizing a dynamical process, and the first application of quantum feedback in a solid-state system of any kind. This demonstration comprises the first half of this thesis. The feedback protocol is predicated on the ability to make high-fidelity quantum measurements, which are enabled by quantum-limited Josephson parametric amplifiers (JPAs). The design and realization of the novel Josephson traveling-wave parametric amplifier (JTWPA) comprises the second half of this thesis. The JTWPA achieves order-of-magnitude improvements over state of the art JPAs in bandwidth and signal power handling while providing quantum-limited noise performance, potentially enabling the simultaneous readout of dozens of superconducting qubits and the generation of broadband multi-mode squeezing in the microwave domain.
The precise control, entanglement, and measurement of quantum systems are central challenges in realizing scalable quantum information processing. This dissertation investigates how parametric modulation and engineered nonlinearity can be harnessed to both manipulate and measure superconducting qubits with high fidelity. In one part, controlled parametric drives are used to generate entanglement between qubit pairs and employ their joint states as probes of environmental dynamics. This approach enables the exploration of the non-Markovian nature of quantum environments, revealing how memory effects influence qubit coherence and energy relaxation. In a separate but conceptually related study, the same principle of parametric driving is utilized to design and analyze quantum-limited amplifiers based on three-wave mixing in weakly nonlinear Josephson circuits. An amplifier with a higher coupling quality factor than conventional JPAs is employed, allowing the two modes of a parity-time (PT) symmetric dimer to be mapped onto the in-phase (I) and quadrature (Q) components of the Josephson parametric amplifier. This mapping enables controlled exploration of non-Hermitian dynamics and mode coalescence within the amplifier’s quadrature space, offering new insights into PT-symmetric behavior and quantum-limited detection. Collectively, these investigations unify the themes of parametric control, entanglement, and amplification, advancing the broader understanding of quantum measurement and open-system dynamics.
Feedback control in classical systems is an indispensable, ubiquitous tool. The theoretical basis for achieving optimal classical control is well understood, and crucially relies on a very classical assumption: that measurements of the state of a system under control need not perturb that state. In a quantum context this assumption is fundamentally invalid. Although many aspects of the theory of quantum feedback control are relatively well developed, the technological basis for feedback control of a single quantum system has only very recently matured. We demonstrate the experimental realization of a quantum feedback control protocol, perpetually stabilizing the coherent Rabi oscillations of a superconducting qubit. This is the first utilization of quantum feedback control for stabilizing a dynamical process, and the first application of quantum feedback in a solid-state system of any kind. This demonstration comprises the first half of this thesis. The feedback protocol is predicated on the ability to make high-fidelity quantum measurements, which are enabled by quantum-limited Josephson parametric amplifiers (JPAs). The design and realization of the novel Josephson traveling-wave parametric amplifier (JTWPA) comprises the second half of this thesis. The JTWPA achieves order-of-magnitude improvements over state of the art JPAs in bandwidth and signal power handling while providing quantum-limited noise performance, potentially enabling the simultaneous readout of dozens of superconducting qubits and the generation of broadband multi-mode squeezing in the microwave domain.
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.