Showing papers on "Qubit published in 2010"
TL;DR: This work shows that conventional cryogenic refrigeration can be used to cool a mechanical mode to its quantum ground state by using a microwave-frequency mechanical oscillator—a ‘quantum drum’—coupled to a quantum bit, which is used to measure the quantum state of the resonator.
Abstract: Quantum mechanics provides a highly accurate description of a wide variety of physical systems. However, a demonstration that quantum mechanics applies equally to macroscopic mechanical systems has been a long-standing challenge, hindered by the difficulty of cooling a mechanical mode to its quantum ground state. The temperatures required are typically far below those attainable with standard cryogenic methods, so significant effort has been devoted to developing alternative cooling techniques. Once in the ground state, quantum-limited measurements must then be demonstrated. Here, using conventional cryogenic refrigeration, we show that we can cool a mechanical mode to its quantum ground state by using a microwave-frequency mechanical oscillator—a ‘quantum drum’—coupled to a quantum bit, which is used to measure the quantum state of the resonator. We further show that we can controllably create single quantum excitations (phonons) in the resonator, thus taking the first steps to complete quantum control of a mechanical system. The bizarre, often counter-intuitive predictions of quantum mechanics have been observed in atomic-scale optical and electrical systems, but efforts to demonstrate that quantum mechanics applies equally to a mechanical system, especially one large enough to be seen with the naked eye, have proved challenging. The difficulty is cooling a mechanical system to its quantum ground state, where all classical noise is eliminated. A team at the Department of Physics at the University of California, Santa Barbara, has overcome this obstacle. Using conventional cryogenic refrigeration, they cool a mechanical resonator with a very high oscillation frequency to one-fortieth of a degree above absolute zero. This resonator, called a 'quantum drum', is coupled to a superconducting quantum bit that acts as a quantum thermometer to detect whether there are any excitations left in the resonator. When it is confirmed there are none, it is further shown that a single quantum of excitation, a phonon, can be introduced in this system and exchanged between resonator and qubit many times, thereby taking the first steps towards complete quantum control of a mechanical system. Quantum mechanics provides an accurate description of a wide variety of physical systems but it is very challenging to prove that it also applies to macroscopic (classical) mechanical systems. This is because it has been impossible to cool a mechanical mode to its quantum ground state, in which all classical noise is eliminated. Recently, various mechanical devices have been cooled to a near-ground state, but this paper demonstrates the milestone result of a piezoelectric resonator with a mechanical mode cooled to its quantum ground state.
1,800 citations
TL;DR: In this paper, the quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond is verified using the quantum eraser technique, and demonstrates that a high degree of control over interactions between a solid state qubit and the quantum light field can be achieved.
Abstract: Quantum entanglement is among the most fascinating aspects of quantum theory. Entangled optical photons are now widely used for fundamental tests of quantum mechanics and applications such as quantum cryptography. Several recent experiments demonstrated entanglement of optical photons with trapped ions, atoms and atomic ensembles, which are then used to connect remote long-term memory nodes in distributed quantum networks. Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond. Our experimental entanglement verification uses the quantum eraser technique, and demonstrates that a high degree of control over interactions between a solid-state qubit and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks.
920 citations
TL;DR: High-fidelity single-shot spin readout in silicon opens the way to the development of a new generation of quantum computing and spintronic devices, built using the most important material in the semiconductor industry.
Abstract: The size of silicon transistors used in microelectronic devices is shrinking to the level at which quantum effects become important. Although this presents a significant challenge for the further scaling of microprocessors, it provides the potential for radical innovations in the form of spin-based quantum computers and spintronic devices. An electron spin in silicon can represent a well-isolated quantum bit with long coherence times because of the weak spin-orbit coupling and the possibility of eliminating nuclear spins from the bulk crystal. However, the control of single electrons in silicon has proved challenging, and so far the observation and manipulation of a single spin has been impossible. Here we report the demonstration of single-shot, time-resolved readout of an electron spin in silicon. This has been performed in a device consisting of implanted phosphorus donors coupled to a metal-oxide-semiconductor single-electron transistor-compatible with current microelectronic technology. We observed a spin lifetime of ∼6 seconds at a magnetic field of 1.5 tesla, and achieved a spin readout fidelity better than 90 per cent. High-fidelity single-shot spin readout in silicon opens the way to the development of a new generation of quantum computing and spintronic devices, built using the most important material in the semiconductor industry.
669 citations
TL;DR: The application of the latest photonic quantum computer technology to calculate properties of the smallest molecular system: the hydrogen molecule in a minimal basis is reported and the complete energy spectrum is calculated to 20 bits of precision.
Abstract: Exact first-principles calculations of molecular properties are currently intractable because their computational cost grows exponentially with both the number of atoms and basis set size. A solution is to move to a radically different model of computing by building a quantum computer, which is a device that uses quantum systems themselves to store and process data. Here we report the application of the latest photonic quantum computer technology to calculate properties of the smallest molecular system: the hydrogen molecule in a minimal basis. We calculate the complete energy spectrum to 20 bits of precision and discuss how the technique can be expanded to solve large-scale chemical problems that lie beyond the reach of modern supercomputers. These results represent an early practical step toward a powerful tool with a broad range of quantum-chemical applications.
654 citations
TL;DR: The dispersive energy-level shift of an LC resonator magnetically coupled to a superconducting qubit is measured, which clearly shows that the system operates in the ultrastrong coupling regime.
Abstract: We measure the dispersive energy-level shift of an LC resonator magnetically coupled to a superconducting qubit, which clearly shows that our system operates in the ultrastrong coupling regime. The large mutual kinetic inductance provides a coupling energy of ?0.82??GHz, requiring the addition of counter-rotating-wave terms in the description of the Jaynes-Cummings model. We find a 50 MHz Bloch-Siegert shift when the qubit is in its symmetry point, fully consistent with our analytical model.
634 citations
Journal Article•
618 citations
TL;DR: In this paper, a spin-orbit quantum bit (qubit) is implemented in an indium arsenide nanowire, where the spinorbit interaction is so strong that spin and motion can no longer be separated.
Abstract: Motion of electrons can influence their spins through a fundamental effect called spin–orbit interaction This interaction provides a way to control spins electrically and thus lies at the foundation of spintronics Even at the level of single electrons, the spin–orbit interaction has proven promising for coherent spin rotations Here we implement a spin–orbit quantum bit (qubit) in an indium arsenide nanowire, where the spin–orbit interaction is so strong that spin and motion can no longer be separated In this regime, we realize fast qubit rotations and universal single-qubit control using only electric fields; the qubits are hosted in single-electron quantum dots that are individually addressable We enhance coherence by dynamically decoupling the qubits from the environment Nanowires offer various advantages for quantum computing: they can serve as one-dimensional templates for scalable qubit registers, and it is possible to vary the material even during wire growth Such flexibility can be used to design wires with suppressed decoherence and to push semiconductor qubit fidelities towards error correction levels Furthermore, electrical dots can be integrated with optical dots in p–n junction nanowires The coherence times achieved here are sufficient for the conversion of an electronic qubit into a photon, which can serve as a flying qubit for long-distance quantum communication
593 citations
TL;DR: In this paper, the nitrogen-vacancy (NV-1) center is identified as a quantum-mechanical defect in diamond and a list of physical criteria that these centers and their hosts should meet and explain how these requirements can be used in conjunction with electronic structure theory to intelligently sort through candidate defect systems.
Abstract: Identifying and designing physical systems for use as qubits, the basic units of quantum information, are critical steps in the development of a quantum computer. Among the possibilities in the solid state, a defect in diamond known as the nitrogen-vacancy (NV-1) center stands out for its robustness—its quantum state can be initialized, manipulated, and measured with high fidelity at room temperature. Here we describe how to systematically identify other deep center defects with similar quantum-mechanical properties. We present a list of physical criteria that these centers and their hosts should meet and explain how these requirements can be used in conjunction with electronic structure theory to intelligently sort through candidate defect systems. To illustrate these points in detail, we compare electronic structure calculations of the NV-1 center in diamond with those of several deep centers in 4H silicon carbide (SiC). We then discuss the proposed criteria for similar defects in other tetrahedrally coordinated semiconductors.
562 citations
TL;DR: This work reports the cooling of the motion of a radio-frequency nanomechanical resonator by parametric coupling to a driven, microwave-frequency superconducting resonator, and expects the mechanical resonator to be found with probability 0.21 in the quantum ground state of motion.
Abstract: Cold, macroscopic mechanical systems are expected to behave contrary to our usual classical understanding of reality; the most striking and counterintuitive predictions involve the existence of states in which the mechanical system is located in two places simultaneously. Various schemes have been proposed to generate and detect such states, and all require starting from mechanical states that are close to the lowest energy eigenstate, the mechanical ground state. Here we report the cooling of the motion of a radio-frequency nanomechanical resonator by parametric coupling to a driven, microwave-frequency superconducting resonator. Starting from a thermal occupation of 480 quanta, we have observed occupation factors as low as 3.8 ± 1.3 and expect the mechanical resonator to be found with probability 0.21 in the quantum ground state of motion. Further cooling is limited by random excitation of the microwave resonator and heating of the dissipative mechanical bath. This level of cooling is expected to make possible a series of fundamental quantum mechanical observations including direct measurement of the Heisenberg uncertainty principle and quantum entanglement with qubits.
516 citations
TL;DR: This work proposes an intuitive and predictive physical frame to describe the DSC regime where photon number wave packets bounce back and forth along parity chains of the Hilbert space, while producing collapse and revivals of the initial population.
Abstract: We study the quantum dynamics of a two-level system interacting with a quantized harmonic oscillator in the deep strong coupling regime (DSC) of the Jaynes-Cummings model, that is, when the coupling strength g is comparable or larger than the oscillator frequency ω (g/ω≳1). In this case, the rotating-wave approximation cannot be applied or treated perturbatively in general. We propose an intuitive and predictive physical frame to describe the DSC regime where photon number wave packets bounce back and forth along parity chains of the Hilbert space, while producing collapse and revivals of the initial population. We exemplify our physical frame with numerical and analytical considerations in the qubit population, photon statistics, and Wigner phase space.
494 citations
TL;DR: A new scheme to interconvert stationary and photonic qubits which is based on indirect qubit-light interactions mediated by a mechanical resonator is described, which enables optical quantum interfaces for a wide range of solid state spin and charge based systems.
Abstract: We describe a new scheme to interconvert stationary and photonic qubits which is based on indirect qubit-light interactions mediated by a mechanical resonator. This approach does not rely on the specific optical response of the qubit and thereby enables optical quantum interfaces for a wide range of solid state spin and charge based systems. We discuss the implementation of state transfer protocols between distant nodes of a quantum network and show that high transfer fidelities can be achieved under realistic experimental conditions.
TL;DR: It is shown that QD spotlight the critical points associated with quantum phase transitions (QPT) for this model even at finite T, which may have important implications for experimental characterization of QPTs when one is unable to reach temperatures below which a QPT can be seen.
Abstract: We compute the quantum correlation [quantum discord (QD)] and the entanglement (EOF) between nearest-neighbor qubits (spin-1/2) in an infinite chain described by the Heisenberg model (XXZ Hamiltonian) at finite temperatures. The chain is in the thermodynamic limit and thermalized with a reservoir at temperature T (canonical ensemble). We show that QD, in contrast to EOF and other thermodynamic quantities, spotlight the critical points associated with quantum phase transitions (QPT) for this model even at finite T. This remarkable property of QD may have important implications for experimental characterization of QPTs when one is unable to reach temperatures below which a QPT can be seen.
TL;DR: A review of various approaches to the implementation of quantum memories, with an emphasis on activities within the quantum memory sub-project of the EU integrated project “Qubit Applications”, is presented in this article.
Abstract: We perform a review of various approaches to the implementation of quantum memories, with an emphasis on activities within the quantum memory sub-project of the EU integrated project “Qubit Applications”. We begin with a brief overview over different applications for quantum memories and different types of quantum memories. We discuss the most important criteria for assessing quantum memory performance and the most important physical requirements. Then we review the different approaches represented in “Qubit Applications” in some detail. They include solid-state atomic ensembles, NV centers, quantum dots, single atoms, atomic gases and optical phonons in diamond. We compare the different approaches using the discussed criteria.
TL;DR: It is shown that it is possible to construct self-contained refrigerators (i.e., not requiring external sources of work) consisting of only a small number of qubits and/or qutrits and to cool towards absolute zero.
Abstract: We investigate the fundamental dimensional limits to thermodynamic machines. In particular, we show that it is possible to construct self-contained refrigerators (i.e., not requiring external sources of work) consisting of only a small number of qubits and/or qutrits. We present three different models, consisting of two qubits, a qubit and a qutrit with nearest-neighbor interactions, and a single qutrit, respectively. We then investigate the fundamental limits to their performance; in particular, we show that it is possible to cool towards absolute zero.
TL;DR: Quantum coherence in a semiconductor charge qubit formed from a GaAs double quantum dot containing a single electron is studied using a quantum point contact charge detector.
Abstract: We study quantum coherence in a semiconductor charge qubit formed from a GaAs double quantum dot containing a single electron. Voltage pulses are applied to depletion gates to drive qubit rotations and noninvasive state readout is achieved using a quantum point contact charge detector. We measure a maximum coherence time of ∼7 ns at the charge degeneracy point, where the qubit level splitting is first-order insensitive to gate voltage fluctuations. We compare measurements of the coherence time as a function of detuning with numerical simulations and predictions from a 1/f noise model.
TL;DR: In this paper, an all-optical spin echo technique was used to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds.
Abstract: Many proposed photonic quantum networks rely on matter qubits to serve as memory elements1,2. The spin of a single electron confined in a semiconductor quantum dot forms a promising matter qubit that may be interfaced with a photonic network3. Ultrafast optical spin control allows gate operations to be performed on the spin within a picosecond timescale4,5,6,7,8,9,10,11,12,13,14, orders of magnitude faster than microwave or electrical control15,16. One obstacle to storing quantum information in a single quantum dot spin is the apparent nanosecond-timescale dephasing due to slow variations in the background nuclear magnetic field15,16,17. Here we use an ultrafast, all-optical spin echo technique to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 105, suggesting strong promise for future photonic quantum information processors18 and repeater networks1,2. An ultrafast, all-optical spin echo technique is used to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 105, suggesting strong promise for future photonic quantum information processors and repeater networks.
TL;DR: In this article, it was shown that up to ten-qubit states can be encoded in five photons, using both their polarization and momentum degrees of freedom, using only five photons.
Abstract: Creating entangled photon states becomes technologically ever more difficult as the number of particles increases, and the current record stands at six entangled photons. However, using both their polarization and momentum degrees of freedom, up to ten-qubit states can be encoded in ‘only’ five photons, as has now been demonstrated.
TL;DR: Couple quantum modes can be strongly sensitive at the single quantum level, even if nonlinear interactions are modest, and solid-state implementations based on the tunneling of polaritons between quantum boxes or their parametric modes in a microcavity are considered.
Abstract: Single photon emitters often rely on a strong nonlinearity to make the behavior of a quantum mode susceptible to a change in the number of quanta between one and two. In most systems, the strength of nonlinearity is weak, such that changes at the single quantum level have little effect. Here, we consider coupled quantum modes and find that they can be strongly sensitive at the single quantum level, even if nonlinear interactions are modest. As examples, we consider solid-state implementations based on the tunneling of polaritons between quantum boxes or their parametric modes in a microcavity. We find that these systems can act as promising single photon emitters.
TL;DR: In this paper, a two-level system composed of a qubit and a harmonic oscillator in the ultrastrong-coupling regime is considered, where the coupling strength is comparable to the qubit energy scales.
Abstract: We consider a system composed of a two-level system (i.e., a qubit) and a harmonic oscillator in the ultrastrong-coupling regime, where the coupling strength is comparable to the qubit and oscillator energy scales. Special emphasis is placed on the possibility of preparing nonclassical states in this system. These nonclassical states include squeezed states, Schr\"odinger-cat states, and entangled states. We start by comparing the predictions of a number of analytical methods that can be used to describe the system under different assumptions, thus analyzing the properties of the system in various parameter regimes. We then examine the ground state of the system and analyze its nonclassical properties. We finally discuss some questions related to the possible experimental observation of the nonclassical states and the effect of decoherence.
TL;DR: A qubit readout scheme that exploits the Jaynes-Cummings nonlinearity of a superconducting cavity coupled to transmon qubits is demonstrated and there is the unexpected onset of a high-transmission "bright" state at a critical power which depends sensitively on the initial qubit state.
Abstract: We demonstrate a qubit readout scheme that exploits the Jaynes-Cummings nonlinearity of a superconducting cavity coupled to transmon qubits. We find that, in the strongly driven dispersive regime of this system, there is the unexpected onset of a high-transmission "bright" state at a critical power which depends sensitively on the initial qubit state. A simple and robust measurement protocol exploiting this effect achieves a single-shot fidelity of 87% using a conventional sample design and experimental setup, and at least 61% fidelity to joint correlations of three qubits.
TL;DR: In this article, a superconducting chip containing a regular array of flux qubits, tunable interqubit inductive couplers, an XY-addressable readout system, on-chip programmable magnetic memory, and a sparse network of analog control lines has been studied.
Abstract: A superconducting chip containing a regular array of flux qubits, tunable interqubit inductive couplers, an XY-addressable readout system, on-chip programmable magnetic memory, and a sparse network of analog control lines has been studied. The architecture of the chip and the infrastructure used to control it were designed to facilitate the implementation of an adiabatic quantum optimization algorithm. The performance of an eight-qubit unit cell on this chip has been characterized by measuring its success in solving a large set of random Ising spin-glass problem instances as a function of temperature. The experimental data are consistent with the predictions of a quantum mechanical model of an eight-qubit system coupled to a thermal environment. These results highlight many of the key practical challenges that we have overcome and those that lie ahead in the quest to realize a functional large-scale adiabatic quantum information processor.
TL;DR: In this article, a microwave scheme was proposed to generate fast, tunable universal two-qubit gates for a superconducting quantum information processor, where one qubit (the control) was irradiated at the transition frequency of another (the target) and the effective coupling between them was switched on by tuning only the frequency of this single drive tone.
Abstract: A register of quantum bits with fixed transition frequencies and weakly coupled to one another through simple linear circuit elements is an experimentally minimal architecture for a small-scale superconducting quantum information processor. Presently, the known schemes for implementing two-qubit gates in this system require microwave signals having amplitudes and frequencies precisely tuned to meet a resonance condition, leaving only the signal phases as free experimentally adjustable parameters. Here, we report a minimal and robust microwave scheme to generate fast, tunable universal two-qubit gates: simply irradiate one qubit (the ``control'') at the transition frequency of another (the ``target''). The effective coupling between them is then switched on by tuning only the frequency of this single drive tone; the drive amplitude adjusts the effective coupling strength; and the drive phase selects the particular two-qubit gate implemented. This cross-resonance effect turns on linearly with the ratio of the drive amplitude $\ensuremath{\Omega}$ to the qubit-qubit detuning $\ensuremath{\Delta}$, as compared with earlier proposals that turn on as ${(\ensuremath{\Omega}/\ensuremath{\Delta})}^{4}$.
TL;DR: The remote preparation of two-qubit "hybrid" entangled states, including a family of vector-polarization beams, are reported, encode in the photon spin and orbital angular momentum, and reconstructed by spin-orbit state tomography and transverse polarization tomography.
Abstract: Quantum teleportation faces increasingly demanding requirements for transmitting large or even entangled systems. However, knowledge of the state to be transmitted eases its reconstruction, resulting in a protocol known as remote state preparation. A number of experimental demonstrations to date have been restricted to single-qubit systems. We report the remote preparation of two-qubit "hybrid" entangled states, including a family of vector-polarization beams. Our single-photon states are encoded in the photon spin and orbital angular momentum. We reconstruct the states by spin-orbit state tomography and transverse polarization tomography. The high fidelities achieved for the vector-polarization states opens the door to optimal coupling of down-converted photons to other physical systems, such as an atom, as required for scalable quantum networks, or plasmons in photonic nanostructures.
TL;DR: Here it is introduced a heralded qubit amplifier based on single-photon sources and linear optics that provides a realistic solution to overcome the problem of channel losses in Bell tests.
Abstract: In device-independent quantum key distribution (DIQKD), the violation of a Bell inequality is exploited to establish a shared key that is secure independently of the internal workings of the QKD devices. An experimental implementation of DIQKD, however, is still awaited, since hitherto all optical Bell tests are subject to the detection loophole, making the protocol unsecured. In particular, photon losses in the quantum channel represent a fundamental limitation for DIQKD. Here we introduce a heralded qubit amplifier based on single-photon sources and linear optics that provides a realistic solution to overcome the problem of channel losses in Bell tests.
TL;DR: A novel method for storing and retrieving binary geometrical shapes in quantum mechanical systems is introduced and it is shown that the employment of maximally entangled qubits allows to reconstruct images without using any additional information.
Abstract: We introduce a novel method for storing and retrieving binary geometrical shapes in quantum mechanical systems. In contrast to standard procedures in classical computer science in which image reconstruction requires not only the storage of light parameters (like light frequency) but also the storage and use of additional information like correlation and pixel spatial disposition, we show that the employment of maximally entangled qubits allows to reconstruct images without using any additional information. Moreover, we provide a concrete application of our proposal in the field of image recognition and briefly explore potential experimental realizations. Our proposal could be employed to enable emergent quantum technology to be used in high-impact scientific disciplines in which extensive use of image processing is made.
TL;DR: In this paper, the authors extend the input-output formalism of quantum optics to analyze few-photon transport in waveguides with an embedded qubit and provide explicit analytical derivations for one-and two-phase scattering matrix elements based on operator equations in the Heisenberg picture.
Abstract: We extend the input-output formalism of quantum optics to analyze few-photon transport in waveguides with an embedded qubit. We provide explicit analytical derivations for one- and two-photon scattering matrix elements based on operator equations in the Heisenberg picture.
TL;DR: In this paper, a parametric amplifier based on Josephson junctions was proposed to reach the quantum limit at microwave frequencies, where the minimum noise energy added by a phase-preserving amplifier to the signal it processes amounts at least to half a photon at the signal frequency.
Abstract: Amplifiers are crucial in every experiment carrying out a very sensitive measurement. However, they always degrade the information by adding noise. Quantum mechanics puts a limit on how small this degradation can be. Theoretically, the minimum noise energy added by a phase-preserving amplifier to the signal it processes amounts at least to half a photon at the signal frequency. Here we propose a practical microwave device that can fulfil the minimal requirements to reach the quantum limit. The availability of such a device is of importance for the readout of solid-state qubits, and more generally for the measurement of very weak signals in various areas of science. We discuss how this device can be the basic building block for a variety of practical applications, such as amplification, noiseless frequency conversion, dynamic cooling and production of entangled signal pairs. The minimum noise energy that a phase-preserving amplifier adds to the signal is fundamentally limited to half a photon. A proposed parametric amplifier based on Josephson junctions should be able to reach this limit at microwave frequencies.
TL;DR: In this paper, the authors demonstrate the use of quantum process tomography (QPT) to characterize the performance of a universal entangling gate between two superconducting qubits.
Abstract: Quantum process tomography provides a means of benchmarking the components and algorithms of a quantum computer in a quantitative fashion, independent of the particular architecture used. Such a procedure has now been demonstrated for a universal entangling gate in a solid-state system. Quantum gates must perform reliably when operating on standard input basis states and on complex superpositions thereof. Experiments using superconducting qubits have validated truth tables for particular implementations of, for example, the controlled-NOT gate1,2, but have not fully characterized gate operation for arbitrary superpositions of input states. Here we demonstrate the use of quantum process tomography3,4 (QPT) to fully characterize the performance of a universal entangling gate between two superconducting qubits. Process tomography permits complete gate analysis, but requires precise preparation of arbitrary input states, control over the subsequent qubit interaction and ideally simultaneous single-shot measurement of output states. In recent work, it has been proposed to use QPT to probe noise properties5 and time dynamics6 of qubit systems and to apply techniques from control theory to create scalable qubit benchmarking protocols7,8. We use QPT to measure the fidelity and noise properties5 of an entangling gate. In addition to demonstrating a promising fidelity, our entangling gate has an on-to-off ratio of 300, a level of adjustable coupling that will become a requirement for future high-fidelity devices. This is the first solid-state demonstration of QPT in a two-qubit system, as QPT has previously been demonstrated only with single solid-state qubits9,10,11.
TL;DR: In this paper, the authors investigated how quantum correlations of a two-qubit one-dimensional Heisenberg chain in thermal equilibrium depend on the temperature of the bath and also on an external magnetic field.
Abstract: We investigate how quantum correlations [quantum discord (QD)] of a two-qubit one-dimensional $\mathit{XYZ}$ Heisenberg chain in thermal equilibrium depend on the temperature $T$ of the bath and also on an external magnetic field $B$. We show that the behavior of thermal QD differs in many unexpected ways from thermal entanglement. For example, we show situations where QD increases with $T$ when entanglement decreases, cases where QD increases with $T$ even in regions with zero entanglement, and that QD signals a quantum phase transition even at finite $T$. We also show that by properly tuning $B$ or the interaction between the qubits we get nonzero QD for any $T$ and we present an effect not seen for entanglement, the ``regrowth'' of thermal QD.
TL;DR: In this article, the authors present a circuit design that effectively eliminates spontaneous emission due to the Purcell effect while maintaining strong coupling to a low-Q cavity, using fast (nanosecond time-scale) flux biasing of the qubit, demonstrating in situ control of qubit lifetime over a factor of 50.
Abstract: Spontaneous emission through a coupled cavity can be a significant decay channel for qubits in circuit quantum electrodynamics. We present a circuit design that effectively eliminates spontaneous emission due to the Purcell effect while maintaining strong coupling to a low-Q cavity. Excellent agreement over a wide range in frequency is found between measured qubit relaxation times and the predictions of a circuit model. Using fast (nanosecond time-scale) flux biasing of the qubit, we demonstrate in situ control of qubit lifetime over a factor of 50. We realize qubit reset with 99.9% fidelity in 120 ns.