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 entanglinggates 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 wavepacket. We achieve a Bell state fidelity of 73 %, well explained by losses in the transmission line and decoherence of each qubit.
We convert propagating qubits encoded as superpositions of zero and one photons to the motion of a micrometer-sized mechanical resonator. Using quantum state tomography, we determinethe density matrix of both the propagating photons and the mechanical resonator. By comparing a sufficient set of states before and after conversion, we determine the average process fidelity to be Favg=0.83+0.03−0.06 which exceeds the classical bound for the conversion of an arbitrary qubit state. This conversion ability is necessary for using mechanical resonators in emerging quantum communication and modular quantum computation architectures.
We present a device demonstrating a lithographically patterned transmon integrated with a micromachined cavity resonator. Our two-cavity, one-qubit device is a multilayer microwaveintegrated 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 34.3 μs, corresponding to a quality factor of 2 million at single-photon energies. The transmon coherence times are T1=6.4 μs, and TEcho2=11.7 μs. We measure qubit-cavity dispersive coupling with rate χqμ/2π=−1.17 MHz, constituting a Jaynes-Cummings system with an interaction strength g/2π=49 MHz. With these parameters we are able to demonstrate cQED operations in the strong dispersive regime with ease. Finally, we highlight several improvements and anticipated extensions of the technology to complex MMIQCs.
Entangling two remote quantum systems which never interact directly is an essential primitive in quantum information science. In quantum optics, remote entanglement experiments providesone approach for loophole-free tests of quantum non-locality and form the basis for the modular architecture of quantum computing. In these experiments, the two qubits, Alice and Bob, are each first entangled with a traveling photon. Subsequently, the two photons paths interfere on a beam-splitter before being directed to single-photon detectors. Such concurrent remote entanglement protocols using discrete Fock states can be made robust to photon losses, unlike schemes that rely on continuous variable states. This robustness arises from heralding the entanglement on the detection of events which can be selected for their unambiguity. However, efficiently detecting single photons is challenging in the domain of superconducting quantum circuits because of the low energy of microwave quanta. Here, we report the realization of a novel microwave photon detector implemented in the circuit quantum electrodynamics (cQED) framework of superconducting quantum information, and the demonstration, with this detector, of a robust form of concurrent remote entanglement. Our experiment opens the way for the implementation of the modular architecture of quantum computation with superconducting qubits.
As experimental quantum information processing (QIP) rapidly advances, an emerging challenge is to design a scalable architecture that combines various quantum elements into a complexdevice without compromising their performance. In particular, superconducting quantum circuits have successfully demonstrated many of the requirements for quantum computing, including coherence levels that approach the thresholds for scaling. However, it remains challenging to couple a large number of circuit components through controllable channels while suppressing any other interactions. We propose a hardware platform intended to address these challenges, which combines the advantages of integrated circuit fabrication and long coherence times achievable in three-dimensional circuit quantum electrodynamics (3D cQED). This multilayer microwave integrated quantum circuit (MMIQC) platform provides a path toward the realization of increasingly complex superconducting devices in pursuit of a scalable quantum computer.
Superconducting enclosures will be key components of scalable quantum computing devices based on circuit quantum electrodynamics (cQED). Within a densely integrated device, they canprotect qubits from noise and serve as quantum memory units. Whether constructed by machining bulk pieces of metal or microfabricating wafers, 3D enclosures are typically assembled from two or more parts. The resulting seams potentially dissipate crossing currents and limit performance. In this Letter, we present measured quality factors of superconducting cavity resonators of several materials, dimensions and seam locations. We observe that superconducting indium can be a low-loss RF conductor and form low-loss seams. Leveraging this, we create a superconducting micromachined resonator with indium that has a quality factor of two million despite a greatly reduced mode volume. Inter-layer coupling to this type of resonator is achieved by an aperture located under a planar transmission line. The described techniques demonstrate a proof-of-principle for multilayer microwave integrated quantum circuits for scalable quantum computing.