Publications


Cost-function embedding and dataset encoding for machine learning with parametrized quantum circuits

PHYSICAL REVIEW A 101 (2020) ARTN 052309

S Cao, L Wossnig, B Vlastakis, P Leek, E Grant


Efficient Hamiltonian programming in qubit arrays with nearest-neighbor couplings

Physical Review A American Physical Society (APS) 102 (2020) 32405

T Tsunoda, G Bhole, SA Jones, JA Jones, PJ Leek

We consider the problem of selectively controlling couplings in a practical quantum processor with always-on interactions that are diagonal in the computational basis, using sequences of local NOT gates. This methodology is well-known in NMR implementations, but previous approaches do not scale efficiently for the general fully-connected Hamiltonian, where the complexity of finding time-optimal solutions makes them only practical up to a few tens of qubits. Given the rapid growth in the number of qubits in cutting-edge quantum processors, it is of interest to investigate the applicability of this control scheme to much larger scale systems with realistic restrictions on connectivity. Here we present an efficient scheme to find near time-optimal solutions that can be applied to engineered qubit arrays with local connectivity for any number of qubits, indicating the potential for practical quantum computing in such systems.


Rescaling Interactions for Quantum Control

PHYSICAL REVIEW APPLIED 13 (2020) 34002

G Bhole, T Tsunoda, PJ Leek, JA Jones

© 2020 American Physical Society. A powerful control method in experimental quantum computing is the use of spin echoes, employed to select a desired term in the system's internal Hamiltonian, while refocusing others. Here, we address a more general problem, describing a method to not only turn on and off particular interactions but also to rescale their strengths so that we can generate any desired effective internal Hamiltonian. We propose an algorithm based on linear programming for achieving time-optimal rescaling solutions in fully coupled systems of tens of qubits, which can be modified to obtain near-time-optimal solutions for rescaling systems with hundreds of qubits.


Development and characterization of a flux-pumped lumped element Josephson parametric amplifier

EPJ Web of Conferences EDP Sciences 198 (2019)

M Esposito, J Rahamim, A Patterson, M Mergenthaler, J Wills, G Campanaro, T Tsunoda, P Spring, S Sosnina, S Jebari, K Ratter, G Tancredi, B Vlastakis, P Leek

Josephson parametric amplification is a tool of paramount importance in circuit-QED especially for the quantum-noise-limited single-shot readout of superconducting qubits. We developed a Josephson parametric amplifier (JPA) based on a lumped-element LC resonator, in which the inductance L is composed by a geometric inductance and an array of 4 superconducting quantum interference devices (SQUIDs). We characterized the main figures of merit of the device, obtaining a −3 dB bandwidth BW = 15 MHz for a gain G = 21 dB and a 1 dB compression point P1dB = −115 dBm. The obtained results are promising for the future use of such JPA as the first stage of amplification for single-shot readout of superconducting qubits.


Double-sided coaxial circuit QED with out-of-plane wiring

APPLIED PHYSICS LETTERS 110 (2017) ARTN 222602

J Rahamim, T Behrle, MJ Peterer, A Patterson, PA Spring, T Tsunoda, R Manenti, G Tancredi, PJ Leek


Generation and detection of squeezed phonons in lattice dynamics by ultrafast optical excitations

NEW JOURNAL OF PHYSICS 19 (2017) ARTN 023032

F Benatti, M Esposito, D Fausti, R Floreanini, K Titimbo, K Zimmermann


Probing the Fluctuations of Optical Properties in Time-Resolved Spectroscopy.

Physical review letters 119 (2017) 187403-187403

F Randi, M Esposito, F Giusti, O Misochko, F Parmigiani, D Fausti, M Eckstein

We show that, in optical pump-probe experiments on bulk samples, the statistical distribution of the intensity of ultrashort light pulses after interaction with a nonequilibrium complex material can be used to measure the time-dependent noise of the current in the system. We illustrate the general arguments for a photoexcited Peierls material. The transient noise spectroscopy allows us to measure to what extent electronic degrees of freedom dynamically obey the fluctuation-dissipation theorem, and how well they thermalize during the coherent lattice vibrations. The proposed statistical measurement developed here provides a new general framework to retrieve dynamical information on the excited distributions in nonequilibrium experiments, which could be extended to other degrees of freedom of magnetic or vibrational origin.


Optimal control of two qubits via a single cavity drive in circuit quantum electrodynamics

PHYSICAL REVIEW A 95 (2017) ARTN 042325

JL Allen, R Kosut, J Joo, P Leek, E Ginossar


Circuit quantum acoustodynamics with surface acoustic waves

Nature Communications Springer Nature 8 (2017) 975-

R Manenti, AF Kockum, A Patterson, T Behrle, J Rahamim, G Tancredi, F Nori, P Leek

The experimental investigation of quantum devices incorporating mechanical resonators has opened up new frontiers in the study of quantum mechanics at a macroscopic level. It has recently been shown that surface acoustic waves (SAWs) can be piezoelectrically coupled to superconducting qubits, and confined in high-quality Fabry-Perot cavities in the quantum regime. Here we present measurements of a device in which a superconducting qubit is coupled to a SAW cavity, realising a surface acoustic version of cavity quantum electrodynamics. We use measurements of the AC Stark shift between the two systems to determine the coupling strength, which is in agreement with a theoretical model. This quantum acoustodynamics architecture may be used to develop new quantum acoustic devices in which quantum information is stored in trapped on-chip acoustic wavepackets, and manipulated in ways that are impossible with purely electromagnetic signals, due to the 105 times slower mechanical waves.In this work, Manenti et al. present measurements of a device in which a tuneable transmon qubit is piezoelectrically coupled to a surface acoustic wave cavity, realising circuit quantum acoustodynamic architecture. This may be used to develop new quantum acoustic devices.


Strong coupling of microwave photons to antiferromagnetic fluctuations in an organic magnet

Physical Review Letters American Physical Society 119 (2017) 147701-

M Mergenthaler, J Liu, J Le Roy, N Ares, A Thompson, L Bogani, F Luis, S Blundell, T Lancaster, A Ardavan, GAD Briggs, PJ Leek, E Laird

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium (DPPH) radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (cQED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multi-spin modes in organic crystals are suitable for cQED, offering a platform for their coherent manipulation. They also utilize the cQED architecture as a way to probe spin correlations at low temperature.


Phase separation in the nonequilibrium Verwey transition in magnetite

PHYSICAL REVIEW B 93 (2016) ARTN 054305

F Randi, I Vergara, F Novelli, M Esposito, M Dell'Angela, VAM Brabers, P Metcalf, R Kukreja, HA Duerr, D Fausti, M Grueninger, F Parmigiani


Quantum Acoustics with Surface Acoustic Waves

Springer International Publishing (2016) 217-244

T Aref, P Delsing, MK Ekström, AF Kockum, MV Gustafsson, G Johansson, PJ Leek, E Magnusson, R Manenti


Surface acoustic wave resonators in the quantum regime

Physical Review B American Physical Society 93 (2016)

R Manenti, MJ Peterer, A Nersisyan, EB Magnusson, A Patterson, P Leek

We present systematic measurements of the quality factors of surface acoustic wave (SAW) resonators on ST-X quartz in the gigahertz range at a temperature of 10mK. We demonstrate an internal quality factor Qi approaching 0.5 million at 0.5GHz and show that Qi≥4.0×104 is achievable up to 4.4GHz. We show evidence for a polynomial dependence of propagation loss on frequency, as well as a weak drive power dependence of Qi that saturates at low power, the latter being consistent with coupling to a bath of two-level systems. Our results indicate that SAW resonators are promising devices for integration with superconducting quantum circuits.


Erratum: Photon number statistics uncover the fluctuations in non-equilibrium lattice dynamics (Nature Communications (2015) 6:10249 DOI: 10.1038/ncomms10249)

Nature Communications 7 (2016)

M Esposito, K Titimbo, K Zimmermann, F Giusti, F Randi, D Boschetto, F Parmigiani, R Floreanini, F Benatti, D Fausti


Quantum interferences reconstruction with low homodyne detection efficiency

EPJ Quantum Technology 3 (2016)

M Esposito, F Randi, K Titimbo, G Kourousias, A Curri, R Floreanini, F Parmigiani, D Fausti, K Zimmermann, F Benatti

© 2016 Esposito et al. Optical homodyne tomography consists in reconstructing the quantum state of an optical field from repeated measurements of its amplitude at different field phases (homodyne data). The experimental noise, which unavoidably affects the homodyne data, leads to a detection efficiency η < 1. The problem of reconstructing quantum states from noisy homodyne data sets prompted an intense scientific debate about the presence or absence of a lower homodyne efficiency bound (η > 0.5) below which quantum features, like quantum interferences, cannot be retrieved. Here, by numerical experiments, we demonstrate that quantum interferences can be effectively reconstructed also for low homodyne detection efficiency. In particular, we address the challenging case of a Schrödinger cat state and test the minimax and adaptive Wigner function reconstruction technique by processing homodyne data distributed according to the chosen state but with an efficiency η < 0.5. By numerically reproducing the Schrödinger’s cat interference pattern, we give evidence that quantum state reconstruction is actually possible in these conditions, and provide a guideline for handling optical tomography based on homodyne data collected by low efficiency detectors.


Photon number statistics uncover the fluctuations in non-equilibrium lattice dynamics.

Nature communications 6 (2015) 10249-

M Esposito, K Titimbo, K Zimmermann, F Giusti, F Randi, D Boschetto, F Parmigiani, R Floreanini, F Benatti, D Fausti

Fluctuations of the atomic positions are at the core of a large class of unusual material properties ranging from quantum para-electricity to high temperature superconductivity. Their measurement in solids is the subject of an intense scientific debate focused on seeking a methodology capable of establishing a direct link between the variance of the atomic displacements and experimentally measurable observables. Here we address this issue by means of non-equilibrium optical experiments performed in shot-noise-limited regime. The variance of the time-dependent atomic positions and momenta is directly mapped into the quantum fluctuations of the photon number of the scattered probing light. A fully quantum description of the non-linear interaction between photonic and phononic fields is benchmarked by unveiling the squeezing of thermal phonons in α-quartz.


Quantum optics for studying ultrafast processes in condensed matter

Conference on Lasers and Electro-Optics Europe - Technical Digest 2015-August (2015)

M Esposito, F Randi, F Giusti, D Boschetto, F Parmigiani, D Fausti

© 2015 OSA. Quantum state reconstruction techniques are used to address non equilibrium matter states. Proof of principle experiments reveal that few photons coherent pulses can be squeezed by the interaction with impulsively excited coherent phonons in quartz.


Quantum optics for studying ultrafast processes in condensed matter

CLEO: QELS - Fundamental Science, CLEO_QELS 2015 (2015) 1551p-

M Esposito, F Randi, F Giusti, D Boschetto, F Parmigiani, D Fausti

Quantum state reconstruction techniques are used to address non equilibrium matter states. Proof of principle experiments reveal that few photons coherent pulses can be squeezed by the interaction with impulsively excited coherent phonons in quartz. © 2014 Optical Society of America.


Surface acoustic wave devices on bulk ZnO crystals at low temperature

Applied Physics Letters AIP Publishing 106 (2015) 063509-063509

EB Magnusson, BH Williams, R Manenti, M-S Nam, A Nersisyan, MJ Peterer, A Ardavan, P Leek

Surface acoustic wave (SAW) devices based on thin films of ZnO are a well established technology. However, SAW devices on bulk ZnO crystals are not practical at room temperature due to the significant damping caused by finite electrical conductivity of the crystal. Here, by operating at low temperatures, we demonstrate effective SAW devices on the (0001) surface of bulk ZnO crystals, including a delay line operating at SAW wavelengths of λ = 4 and 6 μm and a one-port resonator at a wavelength of λ = 1.6 μm. We find that the SAW velocity is temperature dependent, reaching v ≈ 2.68 km/s at 10 mK. Our resonator reaches a maximum quality factor of Qi ≈ 1.5 × 105, demonstrating that bulk ZnO is highly viable for low temperature SAW applications. The performance of the devices is strongly correlated with the bulk conductivity, which quenches SAW transmission above 200 K.


Coherence and decay of higher energy levels of a superconducting transmon qubit

Physical Review Letters American Physical Society 114 (2015) 010501-

MJ Peterer, SJ Bader, X Jin, F Yan, A Kamal, TJ Gudmundsen, P Leek, TP Orlando, WD Oliver, S Gustavsson

We present measurements of coherence and successive decay dynamics of higher energy levels of a superconducting transmon qubit. By applying consecutive π pulses for each sequential transition frequency, we excite the qubit from the ground state up to its fourth excited level and characterize the decay and coherence of each state. We find the decay to proceed mainly sequentially, with relaxation times in excess of 20  μs for all transitions. We also provide a direct measurement of the charge dispersion of these levels by analyzing beating patterns in Ramsey fringes. The results demonstrate the feasibility of using higher levels in transmon qubits for encoding quantum information.

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