Quantum superposition of three macroscopic states and superconducting qutrit detector

Abstract

Superconducting quantum coherent circuits have opened up a novel area of fundamental lowtemperature science since they could potentially be the element base for future quantum computers. Here we report a quasi-three-level coherent system, the so-called superconducting qutrit, which has some advantages over a two-level information cell (qubit), and is based on the qutrit readout circuit intended to measure individually the states of each qubit in a quantum computer. The designed and implemented radio-frequency superconducting qutrit detector (rf SQUTRID) with atomic-size ScStype contact utilizes the coherent-state superposition in the three-well potential with energy splitting ∆E01/kB ≈ 1.5 K at the 30th quantized energy level with good isolation from the electromagnetic environment. The reason why large values of ∆E01 (and thus using atomic-size Nb-Nb contact) are required is to ensure an adiabatic limit for the quantum dynamics of magnetic flux in the rf SQUTRID.

A Tunable Coupler with ScS Quantum Point Contact to Mediate Strong Interaction Between Flux Qubits

“…The critical current of ScS atomic-size contact is quantized in consequence of quantization of the contact conductance R −1 N in units of G 0 = 2e 2 /h, that was observed experimentally 18 . Note that such an effective quantum Hamiltonian (2) peculiarity and the dissipation vanishing at zero temperature, satisfactorily describes the experiments both (i) on macroscopic quantum tunneling phenomena in the loop with the clean ScS contact 22,23 and (ii) on coherent quantum superposition of macroscopically distinct states in the three-well potential of the superconducting qutrit 17 .…”

“…Similarly to stability of the base superposition state of the qutrit toward relaxation to underlying states 17 , the coupler operation level will be the same stable under certain conditions. It is possible due to designing of the coupler loop in the form of a high-quality three-dimensional toroidal superconducting cavity with no resonant modes at frequencies corresponding to that of transitions from the operation level to underlying ones.…”

“…At J 0 = 0 (a = b = 1/2, a ′ = −b ′ = 1/2), i.e. at vanishing interaction strength between the qubits, the states |Ψ 1 , |Ψ 4 decompose into the product of the form (17) so that all the entanglement measures vanish, the qubits are disentangled. Note, the reduced density matrices ρ 1 and ρ 2 in Eq.…”

“…Suppose, the interaction J is turned on between the qubits which are initially disentangled, specifically, in the product state |Ψ 1 from Eq. (17). Applying eigendecomposition of the Hamiltonian, we get the time-varying state vector…”

“…In this paper we propose a tunable coupler to provide strong ZZ interaction between flux qubits using the quantum inductance of a superconducting loop closed by ScS quantum point contact 17 . A quantum inductance coupler (QUINC) is a nonlinear quantum system being in some of its eigenstates, such that additional unwanted entanglement between this state and states of the coupled qubits could be excluded.…”