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Previous projects

Quantum bath decoherence

Hybrid electron-nuclear qubits

Super-Bloch Oscillations

Quantum Chaotic Ratchets

AC-driven BECs

Double-Kicked Atoms


 

Earlier Work 1990-99

Rydberg Atoms in strong fields

Rydberg Molecules in external fields

Chaos and Tunnelling Diodes


 

We investigate quantum systems relevant to the development of future quantum technologies, such as quantum computers and quantum sensors. We are part of the wider UCL optomechanics group which aims to cool a small mechanical oscillator down to its quantum ground state. We also investigate quantum spin dynamics, with a view to understand and control their quantum coherence. One motivation is the possibility to extend powerful techniques such as NMR and MRI down to the single spin level with single-spin sensing techniques. We investigate also how quantum spins qubits interact with the surrounding environment. Typically, the environment are other quantum spins so the back-action between environment and target spin, and entanglement between them makes the behaviour very different from the situation where a quantum particle loses coherence through its interaction with classical environmental noise.

There are potential PhD opportunities in the following areas:
(1) Optomechanics: towards sensing at the quantum limit
(2) Quantum spin dynamics: single spin sensing and decoherence.
We welcome enquiries from prospective PhD students or other researchers and visitors.
For further information contact: Prof. T S Monteiro: TEL.+44 (020)7679 3504 t.monteiro-AT-ucl.ac.uk

Single-spin
NV centres in diamond.

Sensing

Coherence 'diamonds'
Single-spin sensing
for an NV center in diamond

Coherence Map
PRX15
for bismuth in silicon

We introduced and are currently further developing an approach which allows physical insight on the quantum processes underlying single-spin sensing experiments. The experimental features arising from periodically repeated sequences of microwave control pulses, are analysed in terms of the underlying quantum Floquet spectrum. This allows greater flexibility in understanding effects of many-body interactions and quantum strong coupling regimes which are less amenable to usual classical signal-processing methods of analysis. (See introductory Floquet spectroscopy paper by J. E. Lang, Renbao Liu and T. S. Monteiro Phys. Rev. X 5, 041016 (2015))


Our new paper on imaging heterodyne correlations is accepted in Phys.Rev.Lett.(2018)

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Heterodyne and homodyne are techniques used to amplify the tiny quantum fluctuations in the output of a cavity- they are the mainstays of quantum displacement sensing, enabling (e.g. with LIGO) detection of motions on scales of a fraction of an atomic nucleus. However they both lose some important quantum dynamical correlations. We introduce and verify experimentally a method to restore the lost correlations in heterodyne detection and to permit a "best of both worlds" hybrid spectrum that exposes simultaneously quantum correlations in the light and in the mechanical motion. See A. Pontin, J.E. Lang, A. Chowdhury, P. Vezio, F. Marino, B. Morana, E. Serra, F. Marin, T.S. Monteiro arXiv:1708.03294 (2017)


Our paper on so-called "spurious" resonances in NV spin sensing with diamond.

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Recently new and unexpected spectral features in the coherence of NV spins in diamond, associated with control pulses of finite duration, were identified experimentally. Here we analyse them theoretically in terms of an underlying landscape of Floquet avoided crossings which are usually closed and inactive; but are activated whenever quantum evolution under control pulses is not "sudden". See J. E. Lang, J. Casanova, Z.-Y. Wang, M. B. Plenio, T. S. Monteiro Physical Review Applied 7, 054009 (2017) We argue these ultra narrow features can offer a valuable sensing resource and show how to calculate their strength and position analytically.


We develop theory and models
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for the hybrid trap experiment

The UCL Hybrid Trap
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aims to cool a levitated nano particle to its quantum ground state

Schlumberger Fellow
Erika
Ms Erika Aranas has joined the project.

From our recent papers "Cavity cooling a single charged nanosphere" Phys.Rev.Lett. 114 123602 (2015). see also Physics Focus and "Nonlinear dynamics and strong cavity cooling of levitated particles" Phys.Rev.Lett. 117 173602 (2016).


Our new paper on the theory of the hybrid trap has appeared in the New Journal of Physics

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In a 3 minute video online with NJP 18,113602(2016), Erika Aranas and group members explain the work.


Our paper "Keeping a qubit alive in natural silicon: Optimal working points or dynamical decoupling?"

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Our full quantum bath calculation of decoherence of bismuth quits in silicon by flip-flop noise from surrounding nuclei: S J Balian, Renbao Liu and T.S.Monteiro Phys.Rev.B 91, 245416 (2015).
We provide the first demonstration of the only fully "many-body" coherence decay mechanism: in other words, the Hahn echo decay at optimal working points is caused almost entirely by environmental flip-flops from 3-body clusters, since the pair contribution is completely eliminated at an optimal working point. All other coherence decays investigated previously for any silicon donors are largely accounted for by pair flip-flops, with higher orders providing only a quantitative correction. We also showed dynamical decoupling fully restores the pair contribution which is once again dominant at or away from optimal working points for all donor qubits.
This was the last of our papers aiming understand dramatic suppression of coherence in bismuth donor quits at Optimal Working Points starting with our early quantum bath calculation S Balian et al Phys.Rev.B 86, 104428 (2012) with the first ever theoretical demonstration of the orders of magnitude coherence enhancements at OWPs.


Spinning (out)!
Bismuth donors were pioneered as spin qubits in 2010 at UCL in a paper with (then PhD student) Marc Warner G. W. Morley, Marc Warner et al Nat. Mat. 9, 725 (2010). Marc left for the US and returned to the UK to launch a tech start up ASI Data Science. Seto Balian has now joined Marc at The ASI!! We wish Marc and all the brilliant people at the ASI all the best for the future!!


2015-2016 GALLERY


PhD students Jacob Lang and Erika Aranas took part in the 2016 Physics Hackathon

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Jacob Lang & team building remote detonation device; Jacob presenting the scheme; receiving bit-coin prizes.

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Erika Aranas and Dan Golberg of the opto group hard at work with their 2016 hackathon team.


Seto Balian and Jacob Lang
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visiting Renbao Liu in Hong Kong.

Dr S. J. Balian!!
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and colleagues August 2015 viva

Setrak has graduated
“Setoplane
and gone to California UCSD.

Goodbye Seto!!! After 4 great years, Seto Balian has graduated and left for a postdoc in California.




In the past we have investigated a range of nonlinear and quantum dynamical behaviours in atomic, molecular and solid-state systems, including quantum behaviour of systems which exhibit chaos in the classical limit.

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Classical vs Quantum phase space (Wigner functions) of atomic electron in a strong magnetic field: (a)chaotic(b)mixed(c)regular

 

Resonant
Bifurcations of electronic orbits in the RTD.

Tunnelling Diodes

Rydberg Atoms in
Atoms in external fields
external fields

Rydberg Molecules in
Wavefunctions
of H2 in a
magnetic fields