Coherent Oscillations in a Superconducting Multilevel Quantum System (original) (raw)
Related papers
Coherent oscillations in a superconducting tunable flux qubit manipulated without microwaves
New Journal of Physics, 2009
We experimentally demonstrate the coherent oscillations of a tunable superconducting flux qubit by manipulating its energy potential with a nanosecond-long pulse of magnetic flux. The occupation probabilities of two persistent current states oscillate at a frequency ranging from 6 GHz to 21 GHz, tunable via the amplitude of the flux pulse. The demonstrated operation mode allows to realize quantum gates which take less than 100 ps time and are thus much faster compared to other superconducting qubits. An other advantage of this type of qubit is its insensitivity to both thermal and magnetic field fluctuations.
A double SQUID qubit (Superconducting Quantum Interference Device) can be handled by applying microwave trains, but also by using fast flux pulses. In this second case the manipulation is based on the fast and radical modification of the qubit potential shape that induces non-adiabatic transitions between the computational states (the two lowest energy eigenstates), still avoiding transitions to upper levels. This modality is interesting because it allows faster operations with respect to other techniques, but also because it gives access to interesting nontrivial physical features, concerning in particular decoherence and adiabaticity. About decoherence, we observed experimentally the existence of an “optimal” bias region and the transition between two distinct decoherence regimes. These results can be explained by considering the effect of first and second order slow fluctuations which dominate on high frequency noise contributions. This allows a deep insight in the qubit decoherence mechanisms.
SQUID systems for macroscopic quantum coherence and quantum computing
Applied …, 2001
Among the various devices proposed as elements of a quantum computer, the rf-SQUID is a very promising candidate. In fact, systems based on this element can be adjusted in situ, can be coupled by means of superconducting transformers, can be prepared individually and measured with superconducting electronics. Moreover, many progresses were made in these years which showed quantum effects in this system. The present paper describes a complete device developed in order to get a direct measurement of the quantum coherent oscillation. The knowledge of this time, together with its limiting factors, is a prerequisite for fabricating a qubit based on rf- SQUIDS.
2011
A particular superconducting quantum interference device (SQUID)qubit, indicated as double SQUID qubit, can be manipulated by rapidly modifying its potential with the application of fast flux pulses. In this system we observe coherent oscillations exhibiting non-exponential decay, indicating a non trivial decoherence mechanism. Moreover, by tuning the qubit in different conditions (different oscillation frequencies) by changing the pulse height, we observe a crossover between two distinct decoherence regimes and the existence of an "optimal" point where the qubit is only weakly sensitive to intrinsic noise. We find that this behaviour is in agreement with a model considering the decoherence caused essentially by low frequency noise contributions, and discuss the experimental results and possible issues.
Coherent dynamics of a flux qubit coupled to a harmonic oscillator
Nature, 2004
In the emerging field of quantum computation 1 and quantum information, superconducting devices are promising candidates for the implementation of solidstate quantum bits or qubits. Single-qubit operations 2−6 , direct coupling between two qubits 7−10 , and the realization of a quantum gate 11 have been reported. However, complex manipulation of entangled states − such as the coupling of a two-level system to a quantum harmonic oscillator, as demonstrated in ion/atom-trap experiments 12,13 or cavity quantum electrodynamics 14 − has yet to be achieved for superconducting devices. Here we demonstrate entanglement between a superconducting flux qubit (a two-level system) and a superconducting quantum interference device (SQUID). The latter provides the measurement system for detecting the quantum states; it is also an effective inductance that, in parallel with an external shunt capacitance, acts as a harmonic oscillator. We achieve generation and control of the entangled state by performing microwave spectroscopy and detecting the resultant Rabi oscillations of the coupled system.
Detection of a Schroedinger's Cat State in an rf-SQUID
Nature, 2000
We present experimental evidence for a coherent superposition of macroscopically distinct flux states in an rf-SQUID. When the external flux Phi_x applied to the SQUID is near 1/2 of a flux quantum Phi_0, the SQUID has two nearly degenerate configurations: the zero- and one-fluxoid states, corresponding to a few microamperes of current flowing clockwise or counterclockwise, respectively. The system is modeled as a particle in a double-well potential where each well represents a distinct fluxoid state (0 or 1) and the barrier between the wells can be controlled in situ. For low damping and a sufficiently high barrier, the system has a set of quantized energy levels localized in each well. The relative energies of these levels can be varied with Phi_x. External microwaves are used to pump the system from the well-localized ground state of one well into one of a pair of excited states nearer the top of the barrier. We spectroscopically map out the energy of these levels in the neighborhood of their degeneracy point by varying Phi_x as well as the barrier height. We find a splitting between the two states at this point, when both states are below the classical energy barrier, indicating that the system attains a coherent superposition of flux basis states that are macroscopically distinct in that their mean fluxes differ by more than 1/4 Phi_0 and their currents differ by several microamperes.
Quantum State Engineering With the Rf-SQUID: A Brief Introduction
Arxiv preprint quant-ph/0307101, 2003
The SQUID, or superconducting quantum interference device, is a highly sensitive instrument employed for the nondestructive measurement of magnetic fields, with a host of applications in both biophysics and materials technology. It is composed of a cooled superconductive metal ring separated by a thin insulating barrier of non-superconducting metal. Electrons tunnel across the barrier to form a Josephson junction; an rf SQUID is essentially a Josephson junction with tunable current and energy. Quantum computers take advantage of the superpositional logic of quantum mechanics to allow for dramatic increases in computational efficiency. rf SQUIDs show potential for quantum computing applications by forming the qubit component of a quantum computer, through simply treating the direction of current, clockwise or counterclockwise, as the value of the bit.
Quantum dynamics of a microwave-driven SQUID
Superconductor Science & Technology, 2003
In recent years, the quantum behaviour of Josephson devices has been the object of thorough experimental investigation, mainly with the objective of studying quantum mechanics in macroscopic objects. At present, the same quantum properties are being exploited with the aim of the physical implementation of quantum bits and quantum registers. Here we present measurements on a system containing potential qubits, namely a rf SQUID and a hysteretic dc SQUID, magnetically coupled, under microwave excitation; the devices are realized on a single chip with trilayer Nb/AlO x /Nb technology. On this system we have performed a set of measurements to test the dc SQUID response to short pulses of microwaves ranging from 2 to 32 GHz. A first analysis of our results indicates the presence of population oscillations in the hysteretic dc SQUID. This result is very promising in view of using SQUIDs for more complex qubits systems.