I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
29
Jan
2016
New class of quantum error-correcting codes for a bosonic mode
We construct a new class of quantum error-correcting codes for a bosonic mode which are advantageous for applications in quantum memories, communication, and scalable computation. These
`binomial quantum codes‘ are formed from a finite superposition of Fock states weighted with binomial coefficients. The binomial codes can exactly correct errors that are polynomial up to a specific degree in bosonic creation and annihilation operators, including amplitude damping and displacement noise as well as boson addition and dephasing errors. For realistic continuous-time dissipative evolution, the codes can perform approximate quantum error correction to any given order in the timestep between error detection measurements. We present an explicit approximate quantum error recovery operation based on projective measurements and unitary operations. The binomial codes are tailored for detecting boson loss and gain errors by means of measurements of the generalized number parity. We discuss optimization of the binomial codes and demonstrate that by relaxing the parity structure, codes with even lower unrecoverable error rates can be achieved. The binomial codes are related to existing two-mode bosonic codes but offer the advantage of requiring only a single bosonic mode to correct amplitude damping as well as the ability to correct other errors. Our codes are similar in spirit to `cat codes‘ based on superpositions of the coherent states, but offer several advantages such as smaller mean number, exact rather than approximate orthonormality of the code words, and an explicit unitary operation for repumping energy into the bosonic mode. The binomial quantum codes are realizable with current superconducting circuit technology and they should prove useful in other quantum technologies, including bosonic quantum memories, photonic quantum communication, and optical-to-microwave up- and down-conversion.
28
Jan
2016
Coherent dynamics and decoherence in a superconducting weak link
We demonstrate coherent dynamics of quantized magnetic fluxes in a superconducting loop with a weak link – a nanobridge patterned from the same thin NbN film as the loop. The
bridge is a short rounded shape constriction, close to 10 nm long and 20 – 30 nm wide, having minimal width at its center. Quantum state control and coherent oscillations in the driven time evolution of the tunnel-junctionless system are achieved. Decoherence and energy relaxation in the system are studied using a combination of microwave spectroscopy and direct time-domain techniques. The effective flux noise behavior suggests inductance fluctuations as a possible cause of the decoherence.
27
Jan
2016
Efficient single sideband microwave to optical conversion using an electro-optical whispering gallery mode resonator
Linking classical microwave electrical circuits to the optical telecommunication band is at the core of modern communication. Future quantum information networks will require coherent
microwave-to-optical conversion to link electronic quantum processors and memories via low-loss optical telecommunication networks. Efficient conversion can be achieved with electro-optical modulators operating at the single microwave photon level. In the standard electro-optic modulation scheme this is impossible because both, up- and downconverted, sidebands are necessarily present. Here we demonstrate true single sideband up- or downconversion in a triply resonant whispering gallery mode resonator by explicitly addressing modes with asymmetric free spectral range. Compared to previous experiments, we show a three orders of magnitude improvement of the electro-optical conversion efficiency reaching 0.1% photon number conversion for a 10GHz microwave tone at 0.42mW of optical pump power. The presented scheme is fully compatible with existing superconducting 3D circuit quantum electrodynamics technology and can be used for non-classical state conversion and communication. Our conversion bandwidth is larger than 1MHz and not fundamentally limited.
26
Jan
2016
Holonomic quantum computation with all resonant control in circuit QED
The implementation of holonomic quantum computation generally requires controllable and complicated interaction among addressable multi-level systems, which is challenging on superconducting
circuit. Here, we propose a scalable architecture for non-adiabatic holonomic quantum computation on a circuit QED lattice with hybrid transmon and photon encoding of the logical qubits in a decoherence-free subspace. With proper driven on the transmon, we can obtain tunable resonate interaction between the transmon and each of the resonators, which leads to arbitrary single-qubit operation on the encoded logical qubit. Meanwhile, for a nontrivial two-qubit gate, we only need resonate interactions among the three resonators from the two logical qubits, which can be induced by commonly coupled to a grounding SQUID with ac magnetic driven. More importantly, our scheme is achieved with all resonate interactions among the involved elements, and thus leads to quantum gates with very high fidelity. Therefore, our scheme opens up the possibility of realizing high fidelity universal holonomic quantum computation in solid-state system.
25
Jan
2016
Displacement of propagating squeezed microwave states
Displacement of propagating quantum states of light is a fundamental operation for quantum communication. It enables fundamental studies on macroscopic quantum coherence and plays an
important role in quantum teleportation protocols with continuous variables. In our experiments we have successfully implemented this operation for propagating squeezed microwave states. We demonstrate that, even for strong displacement amplitudes, there is no degradation of the squeezing level in the reconstructed quantum states. Furthermore, we confirm that path entanglement generated by using displaced squeezed states stays constant over a wide range of the displacement power.
21
Jan
2016
A Schrodinger Cat Living in Two Boxes
Quantum superpositions of distinct coherent states in a single-mode harmonic oscillator, known as „cat states“, have been an elegant demonstration of Schrodinger’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 non-demolition measurements of the joint photon number parity. The ability to manipulate such multi-cavity quantum states paves the way for logical operations between redundantly encoded qubits for fault-tolerant quantum computation and communication.
Single microwave-photon detector using an artificial Λ-type three-level system
Single photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five
orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here, we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an „impedance-matched“ artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. We attain a single-photon detection efficiency of 0.66±0.06 with a reset time of ∼400~ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.
20
Jan
2016
Unexpectedly allowed transition in two inductively coupled transmons
We present experimental results in which the unexpected zero-two transition of a circuit composed of two inductively coupled transmons is observed. This transition shows an unusual
magnetic flux dependence with a clear disappearance at zero magnetic flux. In a transmon qubit the symmetry of the wave functions prevents this transition to occur due to selection rule. In our circuit the Josephson effect introduces strong couplings between the two normal modes of the artificial atom. This leads to a coherent superposition of states from the two modes enabling such transitions to occur.
Polariton states in circuit QED for electromagnetically induced transparency
Electromagnetically induced transparency (EIT) has been extensively studied in various systems. However, it is not easy to observe in superconducting quantum circuits (SQCs), because
the Rabi frequency of the strong controlling field corresponding to EIT is limited by the decay rates of the SQCs. Here, we show that EIT can be achieved by engineering decay rates in a superconducting circuit QED system through a classical driving field on the qubit. Without such a driving field, the superconducting qubit and the cavity field are approximately decoupled in the large detuning regime, and thus the eigenstates of the system are approximately product states of the cavity field and qubit states. However, the driving field can strongly mix these product states and so-called polariton states can be formed. The weights of the states for the qubit and cavity field in the polariton states can be tuned by the driving field, and thus the decay rates of the polariton states can be changed. We choose a three-level system with Λ-type transitions in such a driven circuit QED system, and demonstrate how EIT and ATS can be realized in this compound system. We believe that this study will be helpful for EIT experiments using SQCs.
19
Jan
2016
Fock-state stabilization and emission in superconducting circuits using dc-biased Josephson junctions
In this article we propose a protocol to autonomously stabilize a Fock state within a circuit QED architecture that doesn’t recourse to any ac-drive nor pulse. This protocol is
realized with a microwave cavity driven by a Josephson junction and coupled to quantum engineered reservoir. It relies on two important ingredients. First, a high-impedance resonator to enable photon-dependent driving used as the cavity. Such device has recently been implemented in circuit QED platforms and removes the need for periodic driving. Second, on an auxiliary, low-quality factor cavity used as a engineered environment. Its damping rate must be scaled with respect to the Josephson energy and the main cavity quality factor in order to stabilize a quantum state. A second protocol, based on the same platform, is used to produce a flying qubit.