## Physics

### Time-optimal selective pulses of two uncoupled spin 1/2 particles. (arXiv:1810.07134v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

We investigate the time-optimal solution of the selective control of two uncoupled spin 1/2 particles. Using the Pontryagin Maximum Principle, we derive the global time-optimal pulses for two spins with different offsets. We show that the Pontryagin Hamiltonian can be written as a one-dimensional effective Hamiltonian. The optimal fields can be expressed analytically in terms of elliptic integrals. The time-optimal control problem is solved for the selective inversion and excitation processes. A bifurcation in the structure of the control fields occurs for a specific offset threshold. In particular, we show that for small offsets, the optimal solution is the concatenation of regular and singular extremals.

Categories: Journals, Physics

### Scaled Quantum Circuits Emulated with Room Temperature p-Bits. (arXiv:1810.07144v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

Exploiting a well-established mapping from a d-dimensional quantum Hamiltonian to a d+1-dimensional classical Hamiltonian that is commonly used in software quantum Monte Carlo algorithms, we propose a scalable hardware emulator where quantum circuits are emulated with room temperature p-bits. The proposed emulator operates with probabilistic bits (p-bit) that fluctuate between logic 0 and 1, that are suitably interconnected with a crossbar of resistors or conventional CMOS devices. One particularly compact hardware implementation of a p-bit is based on the standard 1 transistor/1 Magnetic Tunnel Junction (1T/1MTJ) cell of the emerging Embedded Magnetoresistive RAM (eMRAM) technology, with a simple modification: The free layer of the MTJ uses a thermally unstable nanomagnet so that the resistance of the MTJ fluctuates in the presence of thermal noise. Using established device models for such p-bits and interconnects simulated in SPICE, we demonstrate a faithful mapping of the Transverse Ising Hamiltonian to its classical counterpart, by comparing exact calculations of averages and correlations. Even though we focus on the Transverse Ising Hamiltonian, many other "stoquastic" Hamiltonians - avoiding the sign problem - can be mapped to the hardware emulator. For such systems, large scale integration of the eMRAM technology can enable the intriguing possibility of emulating a very large number of q-bits by room temperature p-bits. The compact and low-level representation of the p-bit offers the possibility of greater efficiency and scalability compared to standard software implementations of quantum Monte Carlo methods.

Categories: Journals, Physics

### Chip-integrated voltage sources for control of trapped ions. (arXiv:1810.07152v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

Trapped-ion quantum information processors offer many advantages for achieving high-fidelity operations on a large number of qubits, but current experiments require bulky external equipment for classical and quantum control of many ions. We demonstrate the cryogenic operation of an ion-trap that incorporates monolithically-integrated high-voltage CMOS electronics ($\pm 8\mathrm{V}$ full swing) to generate surface-electrode control potentials without the need for external, analog voltage sources. A serial bus programs an array of 16 digital-to-analog converters (DACs) within a single chip that apply voltages to segmented electrodes on the chip to control ion motion. Additionally, we present the incorporation of an integrated circuit that uses an analog switch to reduce voltage noise on trap electrodes due to the integrated amplifiers by over $50\mathrm{dB}$. We verify the function of our integrated electronics by performing diagnostics with trapped ions and find noise and speed performance similar to those we observe using external control elements.

Categories: Journals, Physics

### Sisyphus Optical Lattice Decelerator. (arXiv:1810.07157v1 [physics.atom-ph])

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

We experimentally demonstrate a variation on a Sisyphus cooling technique that was proposed for cooling antihydrogen. In our implementation, atoms are selectively excited to an electronic state whose energy is spatially modulated by an optical lattice, and the ensuing spontaneous decay completes one Sisyphus cooling cycle. We characterize the cooling efficiency of this technique on a continuous beam of Sr, and compare it with the case of a Zeeman slower. We demonstrate that this technique provides similar atom number for lower end temperatures, provides additional cooling per scattering event and is compatible with other laser cooling methods.

Categories: Journals, Physics

### Measurement based coupled quantum heat engine without feedback control. (arXiv:1810.07161v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

Role of entanglement is yet to be fully understood in quantum thermodynamics. We shed some light upon that direction by considering the role of entanglement for a single temperature quantum heat engine without feedback, introduced recently by J. Yi, P. Talkner and Y. W. Kim (Phys. Rev. E 96, 022108 (2017)). We take the working medium of the engine to be a 1-dim Heisenberg model of two spins. We calculate the efficiency of the engine undergoing a cyclic process at a single temperature and show that for a coupled working medium the efficiency can be higher than that of an uncoupled one.

Categories: Journals, Physics

### Generalized HydroDynamics on an Atom Chip. (arXiv:1810.07170v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

The emergence of a special type of fluid-like behavior at large scales in one-dimensional (1d) quantum integrable systems, theoretically predicted in 2016, is established experimentally, by monitoring the time evolution of the in situ density profile of a single 1d cloud of $^{87}{\rm Rb}$ atoms trapped on an atom chip after a quench of the longitudinal trapping potential. The theory can be viewed as a dynamical extension of the thermodynamics of Yang and Yang, and applies to the whole range of repulsion strength and temperature of the gas. The measurements, performed on weakly interacting atomic clouds that lie at the crossover between the quasicondensate and the ideal Bose gas regimes, are in very good agreement with the 2016 theory. This contrasts with the previously existing 'conventional' hydrodynamic approach---that relies on the assumption of local thermal equilibrium---, which is unable to reproduce the experimental data.

Categories: Journals, Physics

### Generation of ideal thermal light in warm atomic vapor. (arXiv:1801.10555v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

We present the experimental generation of light with directly observable close-to ideal thermal statistical properties. The thermal light state is prepared using a spontaneous Raman emission in a warm atomic vapor. The photon number statistics is evaluated by both the measurement of second-order correlation function and by the detailed analysis of the corresponding photon number distribution, which certifies the quality of the Bose-Einstein statistics generated by natural physical mechanism. We further demonstrate the extension of the spectral bandwidth of the generated light to hundreds of MHz domain while keeping the ideal thermal statistics, which suggests a direct applicability of the presented source in a broad range of applications including optical metrology, tests of robustness of quantum communication protocols, or quantum thermodynamics.

Categories: Journals, Physics

### Quantum probe spectroscopy for cold atomic systems. (arXiv:1804.09237v2 [cond-mat.quant-gas] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

We study a two-level impurity coupled locally to a quantum gas on an optical lattice. For state-dependent interactions between the impurity and the gas, we show that its evolution encodes information on the local excitation spectrum of gas at the coupling site. Based on this, we design a nondestructive method to probe the system's excitations in a broad range of energies by measuring the state of the probe using standard atom optics methods. We illustrate our findings with numerical simulations for quantum lattice systems, including realistic dephasing noise on the quantum probe, and discuss practical limits on the probe dephasing rate to fully resolve both regular and chaotic spectra.

Categories: Journals, Physics

### Using and reusing coherence to realize quantum processes. (arXiv:1805.04045v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

Coherent superposition is a key feature of quantum mechanics that underlies the advantage of quantum technologies over their classical counterparts. Recently, coherence has been recast as a resource theory in an attempt to identify and quantify it in an operationally well-defined manner. Here we study how the coherence present in a state can be used to implement a quantum channel via incoherent operations and, in turn, to assess its degree of coherence. We introduce the robustness of coherence of a quantum channel---which reduces to the homonymous measure for states when computed on constant-output channels---and prove that: i) it quantifies the minimal rank of a maximally coherent state required to implement the channel; ii) its logarithm quantifies the amortized cost of implementing the channel provided some coherence is recovered at the output; iii) its logarithm also quantifies the zero-error asymptotic cost of implementation of many independent copies of a channel. We also consider the generalized problem of imperfect implementation with arbitrary resource states. Using the robustness of coherence, we find that in general a quantum channel can be implemented without employing a maximally coherent resource state. In fact, we prove that \textit{every} pure coherent state in dimension larger than $2$, however weakly so, turns out to be a valuable resource to implement \textit{some} coherent unitary channel. We illustrate our findings for the case of single-qubit unitary channels.

Categories: Journals, Physics

### Open quantum systems with local and collective incoherent processes: Efficient numerical simulation using permutational invariance. (arXiv:1805.05129v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

The permutational invariance of identical two-level systems allows for an exponential reduction in the computational resources required to study the Lindblad dynamics of coupled spin-boson ensembles evolving under the effect of both local and collective noise. Here we take advantage of this speedup to study several important physical phenomena in the presence of local incoherent processes, in which each degree of freedom couples to its own reservoir. Assessing the robustness of collective effects against local dissipation is paramount to predict their presence in different physical implementations. We have developed an open-source library in Python, the Permutational-Invariant Quantum Solver (PIQS), which we use to study a variety of phenomena in driven-dissipative open quantum systems. We consider both local and collective incoherent processes in the weak, strong, and ultrastrong-coupling regimes. Using PIQS, we reproduced a series of known physical results concerning collective quantum effects and extended their study to the local driven-dissipative scenario \cite{Johansson12,Johansson13}. Our work addresses the robustness of various collective phenomena, e.g., spin squeezing, superradiance, quantum phase transitions, against local dissipation processes.

Categories: Journals, Physics

### Time travel as a Hilbert-space problem. (arXiv:1805.12129v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

The issue of time travel can be reduced in quantum theory to an appropriate Hilbert-space description of feedback loops. I show how to do it in a way that automatically eliminates problems with chronology protection, provided all input-output relations are given by unitary maps. Examples of elementary loops and a two-loop time machine illustrate the construction.

Categories: Journals, Physics

### The boundaries and twist defects of the color code and their applications to topological quantum computation. (arXiv:1806.02820v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

The color code is both an interesting example of an exactly solved topologically ordered phase of matter and also among the most promising candidate models to realize fault-tolerant quantum computation with minimal resource overhead. The contributions of this work are threefold. First of all, we build upon the abstract theory of boundaries and domain walls of topological phases of matter to comprehensively catalog the objects realizable in color codes. Together with our classification we also provide lattice representations of these objects which include three new types of boundaries as well as a generating set for all 72 color code twist defects. Our work thus provides an explicit toy model that will help to better understand the abstract theory of domain walls. Secondly, we discover a number of interesting new applications of the cataloged objects for quantum information protocols. These include improved methods for performing quantum computations by code deformation, a new four-qubit error-detecting code, as well as families of new quantum error-correcting codes we call stellated color codes, which encode logical qubits at the same distance as the next best color code, but using approximately half the number of physical qubits. To the best of our knowledge, our new topological codes have the highest encoding rate of local stabilizer codes with bounded-weight stabilizers in two dimensions. Finally, we show how the boundaries and twist defects of the color code are represented by multiple copies of other phases. Indeed, in addition to the well studied comparison between the color code and two copies of the surface code, we also compare the color code to two copies of the three-fermion model. In particular, we find that this analogy offers a very clear lens through which we can view the symmetries of the color code which gives rise to its multitude of domain walls.

Categories: Journals, Physics

### Position-dependent mass momentum operator and minimal coupling: point canonical transformation and isospectrality. (arXiv:1806.02983v3 [math-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

The classical and quantum mechanical correspondence for constant mass settings is used, along with some point canonical transformation, to find the position-dependent mass (PDM) classical and quantum Hamiltonians. The comparison between the resulting quantum PDM-Hamiltonian and the von Roos PDM-Hamiltonian implied that the ordering ambiguity parameters of von Roos are strictly determined. Eliminating, in effect, the ordering ambiguity associated with the von Roos PDM-Hamiltonian. This, consequently, played a vital role in the construction and identification of the PDM-momentum operator. The same recipe is followed to identify the form of the minimal coupling of electromagnetic interactions for the classical and quantum PDM-Hamiltonians. It turned out that whilst the minimal coupling may very well inherit the usual form in classical mechanics, it admits a necessarily different and vital form in quantum mechanics. Under our point transformation settings, only one of the two commonly used vector potentialsis found eligible and is considered for our Illustrative examples.

Categories: Journals, Physics

### Implementation of a stable, high-power optical lattice for quantum gas microscopy. (arXiv:1806.08997v2 [cond-mat.quant-gas] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

We describe the design and implementation of a stable high-power 1064 nm laser system to generate optical lattices for experiments with ultracold quantum gases. The system is based on a low-noise laser amplified by an array of four heavily modified, high-power fiber amplifiers. The beam intensity is stabilized and controlled with a nonlinear feedback loop. Using real-time monitoring of the resulting optical lattice, we find the stability of the lattice site positions to be well below the lattice spacing for several hours. The pointing stability of the optical lattice beams is around one lattice spacing and the long-term (six month) relative stability of the lattice spacing itself is 0.5% RMS.

Categories: Journals, Physics

### No-Key Semi-Quantum Direct Communication Protocol with Low Quantum Resource Requirements. (arXiv:1810.06337v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

Full quantum capability devices can provide secure communications, but they are challenging to make portable given the current technology. Besides, classical portable devices are unable to construct communication channels resistant to quantum computers. Hence, communication security on portable devices cannot be guaranteed. Semi-Quantum Key Distribution (SQKD) and Semi-Quantum Direct Communication (SQDC) attempt to break the quandary by lowering the receiver's required quantum capability so that secure communications can be implemented on a portable device. However, all SQKD and SQDC protocols have low qubit efficiency and complex hardware implementations. The protocols involving quantum entanglement require linear Entanglement Preservation Time (EPT) and linear quregister size. In this paper, we propose two new no-key SQDC protocols that address the aforementioned weaknesses. They are named Economic No-key SQDC (ENKSQDC) and Rate Estimation ENKSQDC (RENKSQDC). They achieve theoretically constant minimal EPT and quregister size, regardless of message length. We show that the new protocols, with low overhead, can detect Measure and Replay Attacks (MRAs). RENKSQDC is tolerant to transmission impairments and environmental perturbations. The protocols are based on a new quantum message transmission operation termed Tele-Conjure. Like QKD, their strength depends on physical principles rather than mathematical complexity.

Categories: Journals, Physics

### Classical simulation of quantum-coherent thermal machines. (arXiv:1810.04174v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

The performance enhancements observed in various models of continuous quantum thermal machines have been linked to the buildup of coherences in a preferred basis. But, is this connection always an evidence of 'quantum-thermodynamic supremacy'? By force of example, we show that this is not the case. In particular, we compare a power-driven three-level quantum refrigerator with a four-level combined cycle, partly driven by power and partly by heat. We focus on the weak driving regime and find the four-level model to be superior since it can operate in parameter regimes in which the three-level model cannot, it may exhibit a larger cooling rate, and, simultaneously, a better coefficient of performance. Furthermore, we find that the improvement in the cooling rate matches the increase in the stationary quantum coherences exactly. Crucially, though, we also show that the thermodynamic variables for both models follow from a classical representation based on graph theory. This implies that we can build incoherent stochastic-thermodynamic models with the same steady-state operation or, equivalently, that both coherent refrigerators can be simulated classically. More generally, we prove this for any $N$-level weakly driven device with a 'cyclic' pattern of transitions. Therefore, even if coherence is present in a thermal machine, it is often unnecessary for the underlying energy conversion process.

Categories: Journals, Physics

### Probabilistic Eigensolver with a Trapped-Ion Quantum Processor. (arXiv:1809.10435v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

Quantum simulation of complex quantum systems and their properties often requires the ability to prepare initial states in an eigenstate of the Hamiltonian to be simulated. In addition, to compute the eigenvalues of a Hamiltonian is in general a non-trivial problem. Here, we propose a hybrid quantum-classical probabilistic method to compute eigenvalues and prepare eigenstates of Hamiltonians which are simulatable with a trapped-ion quantum processor.

Categories: Journals, Physics

### High-fidelity conditional two-qubit swapping gate using tunable ancillas. (arXiv:1809.09049v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

Scalable quantum computing relies crucially on high-fidelity entangling operations. Here we demonstrate that four coupled qubits can operate as a high-fidelity two-qubit entangling gate that swaps two target qubits and adds a relative sign on the $\lvert 11 \rangle$ state (ZSWAP). The gate operation is controlled by the state of two ancilla (control) qubits. The system is readily implementable with superconducting qubits, using capacitively coupled qubits arranged in a diamond-shaped architecture. By using realistic device and noise parameters from state-of-the-art superconducting qubits, we show that the conditional ZSWAP operation can be implemented with a fidelity above 0.99 in a time $t_g \sim 65$ ns.

Categories: Journals, Physics

### Large-alphabet Quantum Key Distribution using spatially encoded light. (arXiv:1808.02823v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

Most Quantum Key Distribution protocols use a two-dimensional basis such as HV polarization as first proposed by Bennett and Brassard in 1984. These protocols are consequently limited to a key generation density of 1 bit per photon. We increase this key density by encoding information in the transverse spatial displacement of the used photons. Employing this higher-dimensional Hilbert space together with modern single-photon-detecting cameras, we demonstrate experimentally large-alphabet Quantum Key Distribution with 1024 symbols and a shared information between sender and receiver of 7 bit per photon.

Categories: Journals, Physics

### A Unified and Strengthened Framework for the Uncertainty Relation. (arXiv:1803.08720v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-10-17 23:34

We provide a unified and strengthened framework for the product form and the sum form variance-based uncertainty relations by constructing a unified uncertainty relation. In the unified framework, we deduce that the uncertainties of the incompatible observables are bounded by not only the commutator of themselves, but also the quantities related with the other operator. This operator can provide information so that we can capture the uncertainty of the measurement result more accurately, and thus is named as the information operator. The introduction of the information operator can fix the deficiencies in both the product form and the sum form uncertainty relations, and provides a more accurate description of the quantum uncertainty relation. The unified framework also proposes a new interpretation of the uncertainty relation for non-Hermitian operators; i.e., the "observable" second-order origin moments of the non-Hermitian operators cannot be arbitrarily small at the same time when they are generalized-incompatible on the new definition of the generalized commutator.

Categories: Journals, Physics