"Openness to international talent must remain one of this country's most distinctive assets", declared Theresa May in January. Outside the EU, she promised, we will "continue to attract the brightest and the best".
Today the Government hasn't even begun to set out how it expects to ensure this. But even though "free movement" rules are still in place, the number of EU nationals coming to the UK has already fallen off sharply, confirmed by official figures last month.
There are a number of reasons Britain has become a less attractive place for Europeans to come to live and work. The collapse in the value of the pound since the referendum means our wages are worth much less in international terms. Another is the toxic debate over immigration since the referendum and the rise in racist attacks.
But perhaps the most important has been the deep uncertainty facing EU nationals - and their families - now in the UK.
In Parliament this week, Unions21, a campaigning group of unions, will present new research showing the risk to the economy if we lose out on the skills of EU nationals working here.
Three million jobs are currently done by EU nationals living in the UK. Natural turnover in these roles means big skills gaps would soon open if people stopped coming to work in the UK or were forced out by Brexit - with serious implications for our economy and way of life.
According to a recent KPMG survey of EU nationals working in the UK, those with higher skills and qualifications are more likely to be thinking of leaving.
Prospect union represents scientists, engineers and technical specialists across the public and private sectors. This includes thousands of EU nationals, many at the cutting edge of British research and industry.
At the Culham Centre for Fusion Energy in Oxfordshire, for example, dedicated specialists from across the world are working together at the forefront of a pan-European project to unlock a source of abundant low carbon energy.
But the Centre already has problems bringing in the workers it needs from outside the EU under current "Tier 2" visa rules, partly because its low public sector pay rates often fall below the minimum threshold. Imposing similar restrictions to EU nationals would cut it off from the talent it needs to keep going.
Prospect members' reports from the frontline tell us the same story in science and technology intensive sectors across the economy.
Can't the UK just train up its own workforce? Of course we need to do much more to improve science, technology, engineering and maths (STEM) education, and raise the appeal and esteem of STEM professions - particularly to young women.
But even if we delivered that revolution tomorrow - with the serious taxpayer investment that would take - it would take a generation for the benefits to feed through.
And Britain's advantages in areas like renewable energy, life sciences, or advanced manufacturing would still disappear. Scientific and technological progress is driven by the sharing of skills, experience, and ultimately people. Closed societies cannot in the end compete with those working in an open and collaborative way.
The Government needs a plan, and fast.
It should include, first, a unilateral guarantee of rights for EU nationals already working in the UK to remain.
Second, we need to know that any new rules will allow continued international mobility for scientists and engineers - including for UK citizens to work in the EU.
Third, concrete commitments to international professional networks are needed to stop the UK being frozen out of collaborative proposals.
Despite a flurry of position papers from government, our members still have no idea what will happen to their work when the UK leaves the EU in just 18 months' time. Uncertainty is not neutral - every extra day damages relationships and perceptions.
In the US they used to talk about the "giant sucking sound" of manufacturing jobs moving abroad under pressure of global competition.
In the UK it will be the sound of talent, skills and experience leaving these shores if we don't act now to secure our place in the global scientific and technological advances that are remaking our world.