Phonon-mediated quantum state transfer and remote qubit entanglement

May 2, 2019 7:56 PM by admin

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Experimental setup (A to C) Micrograph of the assembled flip-chip device (A), with two superconducting bits (Q1 and Q2, blue), connected to two tunable couplers (G1 and G2, purple), made on sapphire (B ). These are connected via two superposed inductances (green) to a SAW resonator (C) made of lithium niobate. The SAW resonator comprises two Bragg mirrors (orange), spaced 2 mm apart, defining a Fabry-Perot acoustic cavity probed by an interdigital transducer (red). The red and blue lines in (A) represent the locations of (B) and (C), respectively. (D) Simplified diagram, with the gray box indicating the elements on the returned lithium niobate chip. (E) Population in the excited state Pe for bit Q1, with coupler G1 set to maximum and G2 deactivated. Q1 is prepared in | e⟩ using a pulse π, its frequency being set on Q1 (vertical scale) for a time t (horizontal scale), before the dispersive reading of its excited population Pe (28). Q1 is released by the emission of phonons via the IDT and, if its frequency is included in the mirror stopband between 3.91 and 4.03 GHz, the transmitted phonon is reflected and generates excitation resuscitation. of qubit at times τ (orange line) and 2τ. The inset shows the sequence of pulses. (F) Measured qubit energy decay time T1 for ωQ, i / 2π = 3.95 GHz as a function of the Josephson junction phase of the δi coupler, showing that the qubit emission can be considerably faster than the transit time of the phonon (orange line), for both Q1 (circles) and Q2 (squares). Credit: Science, doi: 10.1126 / science.aaw8415

Quantum information platforms rely on talking qubits and photons (optical and microwave) are the medium of choice – until now to transfer quantum states between qubits. However, in some solid-state systems, the acoustic vibration properties of the material itself, called phonons, may be advantageous. In a recent study published on Progress of scienceB. Bienfait and colleagues from the interdisciplinary departments of molecular engineering, physics, and materials science in the United States have described the deterministic transmission and capture of traveling (traveling) phonons via an acoustic communication channel to allow a coherent transfer of quantum states from phonons.

Scientists have facilitated phonon transfer from one superconducting qubit (artificial atom) to another and observed quantum entanglement (the quantum state of each particle can not be described regardless of the state of the particle). Other) of the two qubits of an acoustic channel during the course of the study. Bienfait et al. has provided a new way to couple quantum hybrid solid state systems using surface acoustic waves as "good vibrations" in quantum communication for future phonon applications.

Phonons, or more specifically surface acoustic wave phonons, are proposed as a method for coherently coupling distant quantum systems in the solid state. For example, individual phonons in a resonant structure can be monitored and detected using superconducting qubits (described as macroscopically defined artificial atoms by lithography) to generate and measure complex stationary phonon states in a coherent manner. In this book, Bienfait et al. have reported the deterministic emission and capture of moving surface acoustic wave phonons to allow the quantum entanglement of two superconducting qubits in an experimental setup.

They used an acoustic quantum communication channel 2 mm long in the experiments, which allowed a delay line of about 500 nanoseconds to demonstrate phonon emission and recapture. Scientists observed a quantum state transfer between the two superconducting qubits with an efficiency of 67% and using the partial transfer of a phonon, they generated a couple of Bell entangled with a fidelity of 84% .

Electromagnetic waves have played a singular role as carriers of quantum information between distant quantum nodes for the processing of distributed quantum information. Previous quantum experiments used microwave photons to demonstrate the generation of deterministic and probabilistic distance entanglement between superconducting qubits in order to achieve fidelity rates ranging from 60 to 95%. For some quantum systems in the solid state, such as quantum dots or electrostatically defined electron spins, a quantum electron property (also called spintronics), strong interactions with the host material have made acoustic vibrations (or phonons) ) a superior alternative to photon candidates.

For example, surface acoustic wave (SAW) phonons are proposed as a universal means for coupling distant quantum systems. These phonons can also efficiently convert between microwave and optical frequencies, connecting microwave qubits to optical photons. As a result, many propositions have followed experiments to show the coherent emission and detection of SAW phonons in motion by a superconducting qubit, the sound then playing the role of light. Scientists have used roaming SAW phonons to transfer electrons between quantum dots to transport transport of simple electrons, coupled to nitrogen vacuum centers and even rotations of silicon carbide. In earlier work, researchers had also developed stationary wave SAW phonons coherently coupled to superconducting qubits for the creation, detection and on-demand control of quantum acoustic states.

LEFT: Simplified diagram, with the gray box indicating the elements on the returned lithium niobate chip. RIGHT: (A-B) Scanning electron micrographs detailing the IDT and Bragg mirrors. (C) Rate of decay in qubit extracted, measured at maximum coupling. The disintegration is dominated by the phonon emissions of the IDT. The blue circles are extracted from an exponential decay adjustment; The red dotted line corresponds to the predicted circuit model. Credit: Science, doi: 10.1126 / science.aaw8415

Therefore, in this work, Bienfait et al. used roaming (traveling) SAW phonons to perform quantum state transfer between two superconducting qubits experimentally. In the acoustic part of the device, they used a SAW resonator with an effective spacing of the Fabry-Perot 2 mm mirror, to generate a single-pass traveling phonon with a travel time of about 0.5 microseconds (μs ). By design, the coupling between the qubit and Fabry-Perot modes in the system has allowed to completely inject the phonon into the acoustic channel. Bienfait et al. and then coupled the frequency-tunable superconducting two-quencher "Xmon" resonator, Q1 and Q2 (where the "Xmon qubits" were introduced for the first time by Barends et al.), while controlling their electronically coupling to the using two other tunable couplers, G1 and G2. Scientists could turn off each coupler in a few nanoseconds to isolate the qubits.

Scientists have developed tunable couplers, qubits, and their respective control and reading lines on a sapphire substrate, while building the SAW resonator on a separate lithium niobate substrate. For the SAW resonator, they used two acoustic mirrors with two Bragg mirrors (dielectric mirrors) on each side of the central acoustic transceiver configuration. For the acoustic transmitter, they used an interdigital transducer (IDT) connected to a common electrical port.

Scientists applied an electrical pulse to the IDT to form two symmetrical SAW pulses, which traveled in opposite directions, mirroring on the mirrors to make a round trip in 508 nanoseconds. Bienfait et al. Have controlled the coupling of the qubits to the IDT in order to facilitate the emission of phonons moving in the time domain to the resonator. To characterize the emission in the experiments, they first excited the qubit and monitored its population in the excited state before taking into account the state of excitation in decline in as a product of the emission of phonons.

(A) Calibrated control pulses (inlay) ensure the release of a temporally symmetrical phonon and its efficient capture. The circles represent the population measured in the excited state of Q1 when the sequence is interrupted after a time t. (B) Measured excited state population of Q1 by sweeping the delay between the emission control and capture pulses, highlighting a geometrically decreasing population with the number of transits (gray line). (C) Quantum process tomography at the point of maximum efficiency of (B), with a process fidelity F1 = 0.83 ± 0.002. (I) represents the operator of identity and X, Y and Z Pauli operators. In (A) to (C), the dashed lines indicate the results of a master equation simulation including finite transfer efficiency and quarter-day imperfections. Credit: Science, doi: 10.1126 / science.aaw8415

Scientists then experimentally demonstrated the emission and capture of a moving phonon using a single-bit and single-phonon ping-pong experiment using Q1. In the experiment, they tuned the G1 coupler to the maximum while turning it off to monitor the excited state population (P_e) of Q1. They showed that the emission took about 150 ns, after which P_e remained close to zero during phonon transit in the experimental setup. After about 0.5 μs, Bienfait et al. were able to recover the phonons returned with a capture efficiency of 67%.

During successive transits, scientists observed a geometric decrease in capture efficiency, attributed to losses in the acoustic channel. They then proceeded with a quantum process tomography of the release and capture operation of a qubit by reconstructing the process matrix over time. The quantum process tomography technique is the most appropriate and efficient scheme for analyzing quantum systems when two-body interactions are not naturally available.

With Q1 initially prepared in | e⟩, a control signal on G1 releases then recapture half a phonon in the resonator. Simultaneously, a detuning pulse of 20 MHz of variable duration is applied to Q1 to modify its phase of. (A) Population in the excited state Q1 measured during the interruption of the sequence after a time t, with a phase difference Δφ = 0 (squares) or π (circles). The inset shows the control sequence. (B) Final state Q1 Pe (t = tf) for tf = 0.65 μs as a function of the phase difference Δφ between the half-photon and the half-phonon. Circles are experimental points. The dashed lines are simulations based on a theoretical input-output model. Credit: Science, doi: 10.1126 / science.aaw8415