Quantum Coherence In Systems Research Articles

Quantum memory with millisecond coherence in circuit QED

Significant advances in coherence have made superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherent quantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by qubits, while maintaining superior coherence. We demonstrate a novel superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for near-millisecond storage of quantum states in a resonator while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. The observed coherence times constitute an improvement of almost an order of magnitude over those of the best available superconducting qubits. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing with Josephson junction-based quantum circuits.

Quantum enhanced metrology

Quantum Physics Exploiting the quantum-mechanical properties of quantum systems offer the possibility of developing devices for enhanced precision measurement and sensing applications. These devices have, however, required low-noise detection capabilities that have hampered their development. Hosten et al. describe a method that manipulates a coherent cloud of cold rubidium atoms in a way that relaxes the ultrasensitive detection requirements. The general method may be applied to other coherent quantum systems. Science , this issue p. [1552][1] [1]: /lookup/doi/10.1126/science.aaf3397

Quantum phase magnification.

Quantum metrology exploits entangled states of particles to improve sensing precision beyond the limit achievable with uncorrelated particles. All previous methods required detection noise levels below this standard quantum limit to realize the benefits of the intrinsic sensitivity provided by these states. We experimentally demonstrate a widely applicable method for entanglement-enhanced measurements without low-noise detection. The method involves an intermediate quantum phase magnification step that eases implementation complexity. We used it to perform squeezed-state metrology 8 decibels below the standard quantum limit with a detection system that has a noise floor 10 decibels above the standard quantum limit.

Phase Space Framework for Coherence: Towards Quantum Computing System Design

This work aims to propose a framework for the phase space representation of quantum coherence in matter. The central thesis of this work is that quantum coherence could result from weak time-periodic forcing in the absence of other types of interactions (or measurements) from the environment. Therefore the Floquet formalism is employed to depict the coherent quantum system. A phase space representation for coherent quantum systems is formulated and the resulting three uncertainty principles are presented. The mathematical structure for quantum coherent transport is constructed via a non-equilibrium thermodynamic approach. The framework presented in this paper is targeted towards the development of nanosystems and other elements; employed to design systems for quantum computing.

Embedding NbN Nanowires Into Quantum Circuits With a Neon Focused Ion Beam

Parasitic two-level systems are generally present in superconducting circuits as a result of conventional fabrication and processing, and these lead to noise and loss of coherence in quantum systems. We examine the use of a Ne focused ion beam for producing nanowires integrated into superconducting circuits with a minimized density of parasitic two-level systems. We report measurements of nanowires produced by Ne focused-ion-beam milling in NbN resonators. The resonator losses increase after the nanowire is fabricated, with the Q factor decreasing by ~30% to ~10 5 , which is a factor 10-100 higher than in the most closely comparable circuits previously measured. This indicates that the Ne focused ion beam is a promising route for creating superconducting-nanowire-based devices with low levels of decoherence.

Band structure of topological insulators from noise measurements in tunnel junctions

The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunneling and noise spectroscopy utilizing TI/Al2O3/Co tunnel junctions with bottom TI electrodes of either Bi2Te3 or Bi2Se3. We demonstrate that features related to the band structure of the TI materials show up in the tunneling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems that can further lead to the manipulation of their spin-polarized properties for technological purposes.

Gauge Nonlocality in Planar Quantum-Coherent Systems

It is shown that a system with quantum coherence can be nontrivially affected by adjacent magnetic or adjacent time-varying electric field regions, with this proximity (or remote) influence having a gauge origin. This is implicit (although overlooked) in numerous works on extended systems with inhomogeneous magnetic fields (with either conventional or Dirac materials) but is generally plagued with an apparent gauge ambiguity. The origin of this annoying feature is explained and it is shown how it can be theoretically removed, leading to macroscopic quantizations (quantized Dirac monopoles, integral quantum Hall effect, quantized magnetoelectric phenomena in topological insulators). Apart however from serving as a theoretical probe of macroscopic quantizations, there are cases (experimental conditions, clarified here) when this "gauge nonlocality" does not really suffer from any ambiguity: an apparently innocent gauge transformation corresponds to real change in physics of a companion system in higher dimensionality, that leads to physical momentum transfers to our own system. This nonlocality, together with the associated "proximity" or remote effects are then real and lead to the remarkable possibility of inducing topological phenomena from outside our system (which always remains field-free and can even reside in simply-connected space). Specific procedures are then proposed to experimentally detect such types of nonlocal effects and exploit them for novel applications. General consequences in solid state physics (such as the first violation of Bloch theorem in a field-free quantum periodic system) are pointed out, and formal analogies with certain high energy physics phenomena (axions, {\theta}-vacua and some types of Gribov ambiguities), as well as with certain largely unexplored phenomena in mechanics and in thermodynamics, are noted.

Robust quantum control using smooth pulses and topological winding.

The greatest challenge in achieving the high level of control needed for future technologies based on coherent quantum systems is the decoherence induced by the environment. Here, we present an analytical approach that yields explicit constraints on the driving field which are necessary and sufficient to ensure that the leading-order noise-induced errors in a qubit’s evolution cancel exactly. We derive constraints for two of the most common types of noise that arise in qubits: slow fluctuations of the qubit energy splitting and fluctuations in the driving field itself. By theoretically recasting a phase in the qubit’s wavefunction as a topological winding number, we can satisfy the noise-cancelation conditions by adjusting driving field parameters without altering the target state or quantum evolution. We demonstrate our method by constructing robust quantum gates for two types of spin qubit: phosphorous donors in silicon and nitrogen-vacancy centers in diamond.

Strong mechanical driving of a single electron spin

The efficient and robust manipulation of single spins is an essential requirement for successful quantum devices. The manipulation of a single nitrogen–vacancy spin centre is now demonstrated by means of a mechanical resonator approach. Quantum devices for sensing and computing applications require coherent quantum systems, which can be manipulated in fast and robust ways1. Such quantum control is typically achieved using external electromagnetic fields, which drive the system’s orbital2, charge3 or spin4,5 degrees of freedom. However, most existing approaches require complex and unwieldy gate structures, and with few exceptions6,7 are limited to the regime of weak coherent driving. Here, we present a novel approach to coherently drive a single electronic spin using internal strain fields8,9,10 in an integrated quantum device. Specifically, we employ time-varying strain in a diamond cantilever to induce long-lasting, coherent oscillations of an embedded nitrogen–vacancy (NV) centre spin. We perform direct spectroscopy of the phonon-dressed states emerging from this drive and observe hallmarks of the sought-after strong-driving regime6,11, where the spin rotation frequency exceeds the spin splitting. Furthermore, we employ our continuous strain driving to significantly enhance the NV’s spin coherence time12. Our room-temperature experiments thereby constitute an important step towards strain-driven, integrated quantum devices and open new perspectives to investigate unexplored regimes of strongly driven multilevel systems13 and exotic spin dynamics in hybrid spin-oscillator devices14.

Spin resonance without spin splitting

We predict that a single-level quantum dot without discernible splitting of its spin states develops a spin-precession resonance in charge transport when embedded into a spin valve. The resonance occurs in the generic situation of Coulomb blockaded transport with ferromagnetic leads whose polarizations deviate from perfect antiparallel alignment. The resonance appears when electrically tuning the interaction-induced exchange field perpendicular to one of the polarizations -- a simple condition relying on vectors in contrast to usual resonance conditions associated with energy splittings. The spin resonance can be detected by stationary dI/dV spectroscopy and by oscillations in the time-averaged current using a gate-pulsing scheme. The generic noncollinearity of the ferromagnets and junction asymmetry allow for an all-electric determination of the spin-injection asymmetry, the anisotropy of spin relaxation, and the magnitude of the exchange field. We also investigate the impact of a nearby superconductor on the resonance position. Our simplistic model turns out to be generic for a broad class of coherent few-level quantum systems.

Nodal areas in coherent beams

Whereas nodal points having zero amplitude are well-known wave phenomena, destructive interference across a continuous area is rare. In fact, these strange states seem to defy nature’s “abhorrence of nothingness.” Philosophizing aside, we report general schemes for constructing a nodal area in a spatially coherent beam of light by use of a lossless phase mask. Theoretical, numerical, and experimental results are presented. Nodal areas are important for high contrast imaging, laser surfacing, radiation shielding, and other structured light applications. Moreover, these results transcend optics and open the possibility of achieving nodal area wave functions in quantum, acoustic, and other coherent wave systems.

Topological Proximity Effect: A Gauge Influence from Distant Fields on Planar Quantum-Coherent Systems

A quantum system that lies nearby a magnetic or time-varying electric field region, and that is under periodic boundary conditions parallel to the interface, is shown to exhibit a “hidden” Aharonov-Bohm effect (magnetic or electric), caused by fluxes that are not enclosed by, but are merely neighboring to our system − its origin being the absence of magnetic monopoles in 3D space (with corresponding spacetime generalizations). Novel possibilities then arise, where a field-free system can be dramatically affected by manipulating fields in an adjacent or even distant land, provided that these nearby fluxes are not quantized (i.e. they are fractional or irrational parts of the flux quantum). Topological effects (such as Quantum Hall types of behaviors) can therefore be induced from outside our system (that is always field-free and can even reside in simply-connected space). Potential novel applications are outlined, and exotic consequences in solid state physics are pointed out (i.e. the possibility of field-free quantum periodic systems that violate Bloch’s theorem), while formal analogies with certain high energy physics phenomena and with some rather under-explored areas in mechanics and thermodynamics are noted.

Dephasing-assisted transport in linear triple quantum dots

Environmental noise usually hinders the efficiency of charge transport through coherent quantum systems; an exception is dephasing-assisted transport (DAT). We show that linear triple quantum dots in a transport configuration and subjected to pure dephasing exhibit DAT if the coupling to the drain reservoir exceeds a threshold. DAT occurs for arbitrarily weak dephasing and the enhancement can be directly controlled by the coupling to the drain. Moreover, for specific settings, the enhanced current is accompanied by a reduction of the relative shot noise. We identify the quantum Zeno effect and long-distance tunnelling as underlying dynamic processes involved in dephasing-assisted and -suppressed transport. Our analytical results are obtained by using the density matrix formalism and the characteristic polynomial approach to full counting statistics.

Experimental noise filtering by quantum control

Instabilities due to extrinsic interference are routinely faced in systems engineering, and a common solution is to rely on a broad class of $\textit{filtering}$ techniques in order to afford stability to intrinsically unstable systems. For instance, electronic systems are frequently designed to incorporate electrical filters composed of, $\textit{e.g.}$ RLC components, in order to suppress the effects of out-of-band fluctuations that interfere with desired performance. Quantum coherent systems are now moving to a level of complexity where challenges associated with realistic time-dependent noise are coming to the fore. Unfortunately, standard control solutions involving feedback are generally impossible due to the strictures of quantum mechanics, and existing error-suppressing gate constructions generally rely on unphysical bang-bang controls or quasi-static error models that do not reflect realistic laboratory environments. In this work we use the theory of quantum control engineering and experiments with trapped $^{171}$Yb$^{+}$ ions to demonstrate the construction of novel $\textit{noise filters}$ which are specifically designed to mitigate the effect of realistic time-dependent fluctuations on qubits \emph{during useful operations}. Starting with desired filter characteristics and the Walsh basis functions, we use a combination of analytic design rules and numeric search to construct time-domain noise filters tailored to a desired state transformation. Our results validate the generalized filter-transfer function framework for arbitrary quantum control operations, and demonstrate that it can be leveraged as an effective and efficient tool for developing novel robust control protocols.

An exactly soluble model of a shallow double well

Shallow one-dimensional double well potentials appear in atomic and molecular physics and other fields. Unlike the "deep" wells of macroscopic quantum coherent systems, shallow double wells need not present low-lying two-level systems. We argue that this feature, the absence of a low-lying two-level system in certain shallow double wells, may allow the finding of new test grounds for quantum mechanics in mesoscopic systems. We illustrate the above ideas with a family of shallow double wells obtained from Bargman potentials through the Darboux-B\"{a}cklund transform.

Experimental bath engineering for quantitative studies of quantum control

We develop and demonstrate a technique to engineer universal unitary baths in quantum systems. Using the correspondence between unitary decoherence due to ambient environmental noise and errors in a control system for quantum bits, we show how a wide variety of relevant classical error models may be realized through In-Phase/Quadrature modulation on a vector signal generator producing a resonant carrier signal. We demonstrate our approach through high-bandwidth modulation of the 12.6 GHz carrier appropriate for trapped $^{171}$Yb$^{+}$ ions. Experiments demonstrate the reduction of coherent lifetime in the system in the presence of an engineered bath, with the observed $T_{2}$ scaling as predicted by a quantitative model described herein. These techniques form the basis of a toolkit for quantitative tests of quantum control protocols, helping experimentalists characterize the performance of their quantum coherent systems.

Manipulation and coherence of ultra-cold atoms on a superconducting atom chip

The coherence of quantum systems is crucial to quantum information processing. Although superconducting qubits can process quantum information at microelectronics rates, it remains a challenge to preserve the coherence and therefore the quantum character of the information in these systems. An alternative is to share the tasks between different quantum platforms, for example, cold atoms storing the quantum information processed by superconducting circuits. Here we characterize the coherence of superposition states of (87)Rb atoms magnetically trapped on a superconducting atom chip. We load atoms into a persistent-current trap engineered next to a coplanar microwave resonator structure, and observe that the coherence of hyperfine ground states is preserved for several seconds. We show that large ensembles of a million of thermal atoms below 350 nK temperature and pure Bose-Einstein condensates with 3.5 × 10(5) atoms can be prepared and manipulated at the superconducting interface. This opens the path towards the rich dynamics of strong collective coupling regimes.

Universal features of spin transport and breaking of unitary symmetries

When time-reversal symmetry is broken, quantum coherent systems with and without spin rotational symmetry exhibit the same universal behavior in their electric transport properties. We show that spin transport discriminates between these two cases. In systems with large charge conductance, spin transport is essentially insensitive to the breaking of time-reversal symmetry. However, in the opposite limit of a single exit channel, spin currents vanish identically in the presence of time-reversal symmetry, but are turned on by breaking it with an orbital magnetic field.

Nonequilibrium mesoscopic Fermi-reservoir distribution and particle current through a coherent quantum system

We study particle current and occupation distribution in a recently proposed model for coherent quantum transport. In this model a system connected to mesoscopic Fermi reservoirs (mesoreservoir) is driven out of equilibrium by the action of superreservoirs with prescribed temperatures and chemical potentials described by a simple dissipative mechanism with the Lindblad equation. We compare exact (numerical) results for the nonequilibrium steady-state particle current with theoretical expectations based on the Landauer formula and show that the model reproduces the behavior of coherent quantum systems in the expected parameter region. We also obtain the mesoreservoir occupation distribution in the nonequilibrium steady-state and compare them with the occupation distribution in the leads in usual description of coherent quantum transport.

A toy model of a macroscopic quantum coherent system

This paper deals with macroscopic quantum coherence while using only basic quantum mechanics. A square double well is used to illustrate Leggett–Caldeira oscillations. The effect of thermal radiation on two-level systems is discussed. The concept of decoherence is introduced at an elementary level. Reference values are deduced for the energy, temperature and time scales involved in macroscopic quantum coherence.