Scheme for parity-controlled multi-qubit gates with superconducting qubits

  1. Kasper Sangild Christensen,
  2. Nikolaj Thomas Zinner,
  3. and Morten Kjaergaard
Multi-qubit parity measurements are at the core of many quantum error correction schemes. Extracting multi-qubit parity information typically involves using a sequence of multiple two-qubit
gates. In this paper, we propose a superconducting circuit device with native support for multi-qubit parity-controlled gates (PCG). These are gates that perform rotations on a parity ancilla based on the multi-qubit parity operator of adjacent qubits, and can be directly used to perform multi-qubit parity measurements. The circuit consists of a set of concatenated Josephson ring modulators and effectively realizes a set of transmon-like qubits with strong longitudinal nearest-neighbor couplings. PCGs are implemented by applying microwave drives to the parity ancilla at specific frequencies. We investigate the scheme’s performance with numerical simulation using realistic parameter choices and decoherence rates, and find that the device can perform four-qubit PCGs in 30 ns with process fidelity surpassing 99%. Furthermore, we study the effects of parameter disorder and spurious coupling between next-nearest neighboring qubits. Our results indicate that this approach to realizing PCGs constitute an interesting candidate for near-term quantum error correction experiments.

Quantum thermal transistor in superconducting circuits

  1. Marco Majland,
  2. Kasper Sangild Christensen,
  3. and Nikolaj Thomas Zinner
Logical devices based on electrical currents are ubiquitous in modern society. However, digital logic does have some drawbacks such as a relatively high power consumption. It is therefore
of great interest to seek alternative means to build logical circuits that can either work as stand-alone devices or in conjunction with more traditional electronic circuits. One direction that holds great promise is the use of heat currents for logical components. In the present paper, we discuss a recent abstract proposal for a quantum thermal transistor and provide a concrete design of such a device using superconducting circuits. Using a circuit quantum electrodynamics Jaynes-Cummings model, we propose a three-terminal device that allows heat transfer from source to drain, depending on the temperature of a bath coupled at the gate modulator, and show that it provides similar properties to a conventional semiconductor transistor.

Native three-body interaction in superconducting circuits

  1. Simon Panyella Pedersen,
  2. Kasper Sangild Christensen,
  3. and Nikolaj Thomas Zinner
We show how a superconducting circuit consisting of three identical, non-linear oscillators in series considered in terms of its electrical modes can implement a strong, native three-body
interaction among qubits. Because of strong interactions, part of the qubit-subspace is coupled to higher levels. The remaining qubit states can be used to implement a restricted Fredkin gate, which in turn implements a CNOT-gate or a spin transistor. Including non-symmetric contributions from couplings to ground and external control we alter the circuit slightly to compensate, and find average fidelities for our implementation of the above gates above 99.5% with operation times on the order of a nanosecond. Additionally we show how to analytically include all orders of the cosine contributions from Josephson junctions to the Hamiltonian of a superconducting circuit.

Hybrid Quantum Error Correction in Qubit Architectures

  1. Lasse Bjørn Kristensen,
  2. Morten Kjaergaard,
  3. Christian Kraglund Andersen,
  4. and Nikolaj Thomas Zinner
Noise and errors are inevitable parts of any practical implementation of a quantum computer. As a result, large-scale quantum computation will require ways to detect and correct errors
on quantum information. Here, we present such a quantum error correcting scheme for correcting the dominant error sources, phase decoherence and energy relaxation, in qubit architectures, using a hybrid approach combining autonomous correction based on engineered dissipation with traditional measurement-based quantum error correction. Using numerical simulations with realistic device parameters for superconducting circuits, we show that this scheme can achieve a 5- to 10-fold increase in storage-time while using only six qubits for the encoding and two ancillary qubits for the operation of the autonomous part of the scheme, providing a potentially large reduction of qubit overhead compared to typical measurement-based error correction schemes. Furthermore, the scheme relies on standard interactions and qubit driving available in most major quantum computing platforms, making it implementable in a wide range of architectures.