Kevin Chou

A Schrodinger cat living in two boxes

Quantum cats here and there The story of Schrödingers cat being hidden away in a box and being both dead and alive is often invoked to illustrate the how peculiar the quantum world can be. On a twist of the dead/alive behavior, Wang et al. now show that the cat can be in two separate locations at the same time. Constructing their cat from coherent microwave photons, they show that the state of the “electromagnetic cat” can be shared by two separated cavities. Going beyond common-sense absurdities of the classical world, the ability to share quantum states in different locations could be a powerful resource for quantum information processing. Science, this issue p. 1087 A quantum cat can be both alive and dead and in two places at once. Quantum superpositions of distinct coherent states in a single-mode harmonic oscillator, known as “cat states,” have been an elegant demonstration of Schrödinger’s famous cat paradox. Here, we realize a two-mode cat state of electromagnetic fields in two microwave cavities bridged by a superconducting artificial atom, which can also be viewed as an entangled pair of single-cavity cat states. We present full quantum state tomography of this complex cat state over a Hilbert space exceeding 100 dimensions via quantum nondemolition measurements of the joint photon number parity. The ability to manipulate such multicavity quantum states paves the way for logical operations between redundantly encoded qubits for fault-tolerant quantum computation and communication.

A quantum memory with near-millisecond coherence in circuit QED

Significant advances in coherence render superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherent quantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by Josephson-junction-based artificial atoms, while maintaining superior coherence. We demonstrate a superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for storage of quantum superpositions in a resonator on the millisecond scale, while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. This extends the maximum available coherence time attainable in superconducting circuits by almost an order of magnitude compared to earlier hardware. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing in Josephson-junction-based quantum circuits.

Optical control of millimeter wave high Tc superconducting quasi‐optical bandpass filters

The optical response of YBa2Cu3O7 high Tc superconducting quasi‐optical millimeter wave bandpass filters operating at W band (75–110 GHz) has been investigated under various conditions of illumination. Radiation from a cw Ar+ laser (514.5 nm) and a frequency‐doubled Nd:YAG laser (532.8 nm, 120 ps) was used to induce a shift in the resonant frequency of the filter. A shifted Lorentzian line shape function model was used to estimate the magnitude of the light‐induced changes. Shifts of the filter’s resonance frequency on the order of 0. 1 MHz were induced by the laser effects on the superconductor pair population.

On-demand quantum state transfer and entanglement between remote microwave cavity memories

Coupling isolated quantum systems through propagating photons is a central theme in quantum science1,2, with the potential for groundbreaking applications such as distributed, fault-tolerant quantum computing3–5. To date, photons have been used widely to realize high-fidelity remote entanglement6–12 and state transfer13–15 by compensating for inefficiency with conditioning, a fundamentally probabilistic strategy that places limits on the rate of communication. In contrast, here we experimentally realize a long-standing proposal for deterministic, direct quantum state transfer16. Using efficient, parametrically controlled emission and absorption of microwave photons, we show on-demand, high-fidelity state transfer and entanglement between two isolated superconducting cavity quantum memories. The transfer rate is faster than the rate of photon loss in either memory, an essential requirement for complex networks. By transferring states in a multiphoton encoding, we further show that the use of cavity memories and state-independent transfer creates the striking opportunity to deterministically mitigate transmission loss with quantum error correction. Our results establish a compelling approach for deterministic quantum communication across networks, and will enable modular scaling of superconducting quantum circuits.Sending quantum states as shaped microwave photonic wavepackets realizes on-demand, high-fidelity quantum state transfer and entanglement between two superconducting cavity quantum memories.

Quasi-optical millimeter-wave band-pass filters using high-T/sub c/ superconductors

Quasioptical millimeter-wave band-pass filters using YBa/sub 2/Cu/sub 3/O/sub 7/ high-T/sub c/ superconducting films were fabricated on MgO and LaAlO/sub 3/ substrates. Transmitted power through the filter was investigated in the 75 GHz to 110 GHz frequency range at temperatures ranging from 15 to 300 K. At 15 K the measured center frequency and the bandwidth of the superconducting filter were 92 GHz and 0.85 GHz, respectively. Measurements of YBa/sub 2/Cu/sub 3/O/sub 7/ filters were compared with similar filters fabricated using gold. At 15 K and 92 GHz, an improvement of 75% in the quality factor of the superconducting filter was obtained compared with a similar gold filter. >

Superconducting films grown by activated reactive evaporation for high frequency device applications

123 films were grown on MgO substrates by activated reactive evaporation (ARE). In situ post deposition cooldown was optimized at low oxygen pressure to yield films with mirror-smooth surfaces with Tc(O)=86 K, transition width <2 K and Jc at 77 K=105 A cm-2. Surface resistance (Rs) at 60 K and 100 GHz was extremely low and comparable to Rs of films grown by laser ablation on LaA103. A film patterned in to a bandpass filter showed a Q that was nearly an order of magnitude higher than that for a gold film at 92 GHz and <50 K.

Screening of remote charge scattering sites from the oxide/silicon interface of strained Si two-dimensional electron gases by an intermediate tunable shielding electron layer

We report the strong screening of the remote charge scattering sites from the oxide/semiconductor interface of buried enhancement-mode undoped Si two-dimensional electron gases (2DEGs), by introducing a tunable shielding electron layer between the 2DEG and the scattering sites. When a high density of electrons in the buried silicon quantum well exists, the tunneling of electrons from the buried layer to the surface quantum well can lead to the formation of a nearly immobile surface electron layer. The screening of the remote charges at the interface by this newly formed surface electron layer results in an increase in the mobility of the buried 2DEG. Furthermore, a significant decrease in the minimum mobile electron density of the 2DEG occurs as well. Together, these effects can reduce the increased detrimental effect of interface charges as the setback distance for the 2DEG to the surface is reduced for improved lateral confinement by top gates.

Implementing and Characterizing Precise Multiqubit Measurements

Multiqubit measurements will play a vital role in quantum information processing. A new experiment constructs complex measurements on three superconducting qubits and develops important tools toward characterizing them.

Growth of large-area YBa2Cu3O7−x films by activated reactive evaporation☆

Abstract High-Tc superconductor thin films have many important properties for microwave frequency passive device applications. However, for device fabrication it is necessary to deposit uniform defect-free films of YBa2Cu3O7−x (YBCO) over large wafer-size areas. The activated reactive evaporation process has been used successfully to grow YBCO films at relatively low temperatures over large areas. The unique features of the process will be discussed over other popular processes with emphasis on film composition, structure and thickness uniformity.

Erratum: Micromachined Integrated Quantum Circuit Containing a Superconducting Qubit [Phys. Rev. Applied 7 , 044018 (2017)]

We present a device demonstrating a lithographically patterned transmon integrated with a micromachined cavity resonator. Our two-cavity, one-qubit device is a multilayer microwave integrated quantum circuit (MMIQC), comprising a basic unit capable of performing circuit-QED (cQED) operations. We describe the qubit-cavity coupling mechanism of a specialized geometry using an electric field picture and a circuit model, and finally obtain specific system parameters using simulations. Fabrication of the MMIQC includes lithography, etching, and metallic bonding of silicon wafers. Superconducting wafer bonding is a critical capability that is demonstrated by a micromachined storage cavity lifetime