Rusko Ruskov

Quantum feedback control of a solid-state qubit

We have studied theoretically the basic operation of a quantum feedback loop designed to maintain a desired phase of quantum coherent oscillations in a single solid-state qubit. The degree of oscillations synchronization with external harmonic signal is calculated as a function of feedback strength, taking into account available bandwidth and coupling to environment. The feedback can efficiently suppress the dephasing of oscillations if the qubit coupling to the detector is stronger than the coupling to the environment.

Squeezing of a nanomechanical resonator by quantum nondemolition measurement and feedback

We analyze squeezing of the nanoresonator state produced by periodic measurement of position by a quantum point contact or a single-electron transistor. The mechanism of squeezing is the stroboscopic quantum nondemolition measurement generalized to the case of continuous measurement by a weakly coupled detector. The magnitude of squeezing is calculated for the harmonic and stroboscopic modulations of measurement, taking into account detector efficiency and nanoresonator quality factor. We also analyze the operation of the quantum feedback, which prevents fluctuations of the wave packet center due to measurement back-action. Verification of the squeezed state can be performed in almost the same way as its preparation; a similar procedure can also be used for the force detection with sensitivity beyond the standard quantum limit.

Electron spin resonance and spin-valley physics in a silicon double quantum dot

Silicon quantum dots are a leading approach for solid-state quantum bits. However, developing this technology is complicated by the multi-valley nature of silicon. Here we observe transport of individual electrons in a silicon CMOS-based double quantum dot under electron spin resonance. An anticrossing of the driven dot energy levels is observed when the Zeeman and valley splittings coincide. A detected anticrossing splitting of 60 MHz is interpreted as a direct measure of spin and valley mixing, facilitated by spin-orbit interaction in the presence of non-ideal interfaces. A lower bound of spin dephasing time of 63 ns is extracted. We also describe a possible experimental evidence of an unconventional spin-valley blockade, despite the assumption of non-ideal interfaces. This understanding of silicon spin-valley physics should enable better control and read-out techniques for the spin qubits in an all CMOS silicon approach.

Entanglement of solid-state qubits by measurement

Prospective solid-state realizations of quantum computers may have significant advantages due to natural scalability, simple electrical control of parameters, and use of well developed technology. A number of theoretical proposals have been put forward 1 and interesting experimental results have been achieved, including demonstrations of charge qubits 2 using single-Cooper-pair boxes, flux qubits 3,4 using superconducting loops interrupted by Josephson junctions, and combined charge-flux qubits 5 with the quality factor as high as 5 25 000. Obviously, the next important experimental step is the demonstration of entangled solid-state qubits. Entanglement of qubits can be produced using their direct interaction. In this paper, we discuss an alternative way, when two solid-state qubits are made entangled just by their simultaneous measurement with one detector, which thus provides an indirect coupling between qubits. A somewhat similar idea of entanglement via indirect dissipative coupling has been discussed earlier in quantum optics for the preparation of entangled atoms in an optical cavity by monitoring the cavity decay. 6 Moreover, it has been shown that some entanglement can be produced just by coupling to a common environment. 7 However, in this case the degree of entanglement is very small, while in our setup the full 100% entanglement of qubits can be achieved. The stability of the entangled state is due to equal coupling of the qubits with the detector, so that this state is essentially a decoherence-free subspace. 8 Our procedure works with a probability less than unity, and in this respect it is somewhat similar to the operation of conditional quantum gates 9 based on linear optical

On-chip cavity quantum phonodynamics with an acceptor qubit in silicon

We describe a chip-based, solid-state analogue of cavity-QED utilizing acoustic phonons instead of photons. We show how long-lived and tunable acceptor impurity states in silicon nanomechanical cavities can play the role of a matter non-linearity for coherent phonons just as, e.g., the Josephson qubit plays in circuit-QED. Both strong coupling (number of Rabi oscillations ~ 100) and strong dispersive coupling (0.1-2 MHz) regimes can be reached in cavities in the 1-20 GHz range, enabling the control of single phonons, phonon-phonon interactions, dispersive phonon readout of the acceptor qubit, and compatibility with other optomechanical components such as phonon-photon translators. We predict explicit experimental signatures of the acceptor-cavity system.

Mesoscopic Quadratic Quantum Measurements

We develop a theory of quadratic quantum measurements by a mesoscopic detector. It is shown that the quadratic measurements should have nontrivial quantum information properties, providing, for instance, a simple way of entangling two noninteracting qubits. We also calculate the output spectrum of a detector with both linear and quadratic response, continuously monitoring two qubits.

Spectrum of qubit oscillations from generalized Bloch equations

We have developed a formalism suitable for calculation of the output spectrum of a detector continuously measuring quantum coherent oscillations in a solid-state qubit, starting from microscopic generalized Bloch equations. The results coincide with those obtained using Bayesian and master equation approaches. The previous results are generalized to the cases of arbitrary detector response and finite detector temperature.

Qubit State Monitoring by Measurement of Three Complementary Observables

We consider the evolution of a qubit (spin 1/2) under the simultaneous continuous measurement of three noncommuting qubit operators σ(x), σ(y), and σ(z). For identical ideal detectors, the qubit state evolves by approaching a pure state with a random direction in the Bloch vector space and by undergoing locally isotropic diffusion in the perpendicular directions. The quantum state conditioned on the complete detector record is used to assess the fidelity of classically inspired estimates based on running time averages and discrete time bin detector outputs.

Observation of Autler-Townes effect in a dispersively dressed Jaynes-Cummings System

We report on the spectrum of a superconducting transmon device coupled to a planar superconducting resonator in the strong dispersive limit where discrete peaks, each corresponding to a different number of photons, are resolved. A thermal population of 5.474 GHz photons at an effective resonator temperature of T = 120 mK results in a weak n = 1 photon peak along with the n = 0 photon peak in the qubit spectrum in the absence of a coherent drive on the resonator. Two-tone spectroscopy using independent coupler and probe tones reveals an Autler–Townes splitting in the thermal n = 1 photon peak. The observed effect is explained accurately using the four lowest levels of the dispersively dressed qubit–resonator system and compared to results from numerical simulations of the steady-state master equation for the coupled system.

Continuous quantum feedback of coherent oscillations in a solid-state qubit

We have analyzed theoretically the operation of the Bayesian quantum feedback of a solid-state qubit, designed to maintain perfect coherent oscillations in the qubit for arbitrarily long time. In particular, we have studied the feedback efficiency in presence of dephasing environment and detector nonideality. Also, we have analyzed the effect of qubit parameter deviations and studied the quantum feedback control of an energy-asymmetric qubit.